- 193 - EN 50341-1:2012

Calculation method:

tF Fault duration.

⇓
UTp = f (tF) According to Table G.4 and Table G.6 using interpolation or directly
from curve, UD1 in Figure 6.1
⇓
ZB = f (UTp) According to Table G.4 and Table G.6 using interpolation.
IB = UTp / ZB Per definition.
⇓
UD (tF) = UTp (tF) + (Ra1 + Ra2) IB = UTp (tF) + Ra UTp (tF) / ZB = UTp (tF) (1 + Ra / ZB)
The diagram in Figure 6.1 shows curves UD = f (tF) for 4 values of Ra:

Ra = 0 Ω ;
Ra = 1 750 Ω , Ra1 = 1 000 Ω , ρE = 500 Ω m;
Ra = 4 000 Ω , Ra1 = 1 000 Ω , ρE = 2 000 Ω m;
Ra = 7 000 Ω , Ra1 = 1 000 Ω , ρE = 4 000 Ω m.
EN 50341-1:2012 - 194 -

Annex H
(informative)

Installation and measurements of earthing systems

H.1 Definition of symbols used in this annex
Symbol Signification

D Diameter of the ring earth electrode

d Diameter of the stranded earth electrode or half width of an earth strip / Diameter
of the earth rod

I0 Zero sequence current during fault

IE Current to earth during fault

IEW Current in the earth wire (in balanced stage)

Im Measured test current

L Length of the earth strip/length of the earth rod

RE Resistance to earth

Rt Tower footing resistance

r Reduction factor of earth wires

UE Earth potential rise

Uem Measured voltage between the earthing system and a probe in the area of the
reference earth

ZE Impedance to earth

ZEW-E Self impedance of the earth wire

ZML-EW Mutual impedance between phase conductors and earth wire

ZS Earth wire impedance of one span

ρE Soil resistivity

3 Io Sum of zero sequence currents

H.2 Basis for the verification

H.2.1 Soil resistivity
The soil resistivity ρE varies considerably at different locations according to the type of soil, grain size,
density and moisture (see Table H.1).
 - 195 - EN 50341-1:2012

Table H.1  Soil resistivities for alternating frequency currents (ranges of values,
which were frequently measured)

Type of soil Soil resistivity ρE

Ωm
Marshy soil 5 to 40
Loam, clay, humus 20 to 200
Sand 200 to 2 500
Gravel 2 000 to 3 000
Weathered rock mostly below 1 000
Sandstone 2 000 to 3 000
Granite up to 50 000
Moraine up to 30 000

Changes of moisture can cause temporary variations of the soil resistivity for a depth of some metres .
Furthermore, it has to be considered that the soil resistivity can change considerably with the depth
because of distinctly different layers of soil which are usually present.

H.2.2 Resistance to earth

The resistance to earth, RE of an earth electrode depends on the soil resistivity, ρE as well as on the
dimensions and the arrangement of the earth electrode. It depends mainly on the length of the earth
electrode and less on the cross-section. Figures H.1 and H.2 shows the values of the resistance to
earth for surface earth electrodes or earth rods, respectively, relative to the total length.

In the case of very long surface earth electrodes (for example cables with earth electrode effect) the
resistance to earth decreases with the length, but approaches a final value. Foundation earth
electrodes may be regarded as earth electrodes buried in the surrounding soil.

The resistance to earth, RE of a meshed earth electrode is approximately:

ρE
RE =
2D

where

D is the diameter in m of a circle with the same area as the meshed earth electrode;

ρE is the soil resistivity in Ω m.

EN 50341-1:2012 - 196 -

D
4 6 8 10 12 15 20 m 30
60 0
Ω
40 0
30 0

20 0

15 0
12 0
10 0
80

60

40
R
E 30

20

15
12
10
8
6

4
4 6 8 10 12 15 20 30 40 60 8 0 m 10 0

L
Figure H.1 - Resistance to earth RE of surface earth electrodes
(made from strip, round material or stranded conductor)
for straight or ring arrangement in homogenous soil

D
4 6 8 10 12 15 20 m 30
60 0
Ω
40 0
30 0

20 0

15 0
12 0
10 0
80

60

40
R
E 30

20

15
12
10
8
6

4
4 6 8 10 12 15 20 30 40 60 8 0 m 10 0

Figure H.2 - Resistance to earth RE of earth rods,

vertically buried in homogeneous soil
 - 197 - EN 50341-1:2012

Calculated values according to the following formula:

RE = (ρE / 2 π L) ln (4 L / d)

where

L is the length of the earth rod in m;

d is the diameter of the earth rod in m (here 20 mm assumed);

ρE is the soil resistivity in Ω m.

H.3 Installation of earth electrodes and earthing conductors

H.3.1 Installation of earth electrodes
H.3.1.1 Earth electrodes
An earthing system is generally composed of one or more horizontal, vertical or inclined earth
electrodes, buried or driven into the soil by force. It can also consist of the direct embedded tower
itself.

The use of chemicals to reduce soil resistivity is not recommended, because it increases corrosion,
needs periodical maintenance and is not long lasting. However in special circumstances the use of
chemicals may be justified.

Horizontal earth electrodes shall usually be buried to a depth of 0,5 m to 1 m below ground level. This
gives sufficient mechanical protection. It is recommended that the earth electrode is situated below the
frost line.

In the case of vertically driven rods, the top of each rod will usually be situated below ground level.
Vertical or inclined driven rods are particularly advantageous when the soil resistivity decreases with
increasing depth.

H.3.1.2 Horizontal earth electrodes

Horizontal earth electrodes are usually laid at the bottom of a trench or a foundation excavation.

It is recommended that:

• they are surrounded with lightly tamped soil,

• stones or gravel should not be in direct contact with the buried earth electrodes,
• indigenous soil, which is corrosive to the electrode metal used, should be replaced by a
suitable backfill.
H.3.1.3 Vertical or inclined driven rods
Vertical or inclined driven rods are driven into the soil by force and should be separated by a distance
not less than the length of the rod.

Appropriate tools should be used to avoid any damage to the electrodes whilst they are being driven
into the soil.

H.3.1.4 Jointing the earth electrodes

The joints used to connect conductive parts of an earth electrode network (grid) within the network
itself, shall have adequate dimensions to ensure an electrical conductance and mechanical and
thermal strength equivalent to the electrodes themselves.

The earth electrodes shall be resistant to corrosion and should not be liable to contribute to galvanic
cells.

The joints used to assemble rods shall have the same mechanical strength as the rods themselves
and shall resist mechanical stresses during driving. When different metals, which form galvanic cells
possibly causing galvanic corrosion, have to be connected, joints shall be protected by durable means
against contact with electrolytes in their surroundings.
EN 50341-1:2012 - 198 -

H.3.2 Installation of earthing conductors

H.3.2.1 General
In general the path of the earthing conductors should be as short as possible.

H.3.2.2 Installing the earthing conductors

The following installation methods may be considered:

• buried earthing conductors: only protection against mechanical damage is required,

• accessible installed earthing conductors: above the ground the earthing conductors should be
installed in such a way that they remain accessible. If there is a risk of mechanical damage,
the earthing conductor shall be adequately protected,
• concrete embedded earthing conductors: earthing conductors may also be embedded in
concrete. Easily accessible terminals shall be available at both ends.
Special attention shall be taken to avoid corrosion where the bare earthing conductor enters the soil or
concrete.

H.3.2.3 Jointing the earthing conductors

The joints shall have good electrical continuity to prevent any unacceptable temperature rise under
fault current conditions.

Joints shall not become loose and shall be protected against corrosion. When different metals, which
form galvanic cells and can cause galvanic corrosion, have to be connected, joints shall be protected
by durable means against any contact with electrolytes in their surroundings.

Suitable connectors shall be used to connect the earthing conductor to the earth electrode, to the main
earth terminal and to any metallic part.

It shall be impossible to disassemble joints without tools.

H.4 Measurements for and on earthing systems

H.4.1 Measurement of soil resistivities
Measurements of the soil resistivity, ρE for the pre-determination of the resistance to earth, RE or the
impedance to earth, ZE should be undertaken according to a four probe method (for example Wenner-
method), whereby the soil resistivity for different depths can be determined.

H.4.2 Measuring touch voltages

For touch voltage measurements a current injection method shall be used (see H.4.3).

There are two alternative aceptable methods as follows:

1) The touch voltage is determined by assuming the human body resistance as 1 kΩ.
2
The measuring electrode(s) for simulation of the feet shall have a total area of 400 cm and shall
be pressed on the earth with a minimum total force of 500 N. Alternatively, a probe, driven at least
20 cm into the earth, may be used instead of the measuring electrode. For the measurement of the
touch voltage in any part of the installation the electrode shall be placed at a distance of 1 m from
the exposed part of the installation: for concrete or dried soil it shall be on a wet cloth or water film.
A tip-electrode for the simulation of the hand shall be capable of piercing a paint coating (not acting
as insulation) reliably. One terminal of the voltmeter is connected to the hand electrode, the other
terminal to the foot electrode. It is sufficient to carry out such measurements as a sampling test.
NOTE In order to get a quick indication of the upper limit of touch voltages, measurement by a voltmeter with a
high internal resistance and a probe driven 10 cm deep is often sufficient.

2) The touch voltage is determined by measuring the driving voltage, UD (Figure G.3) using a high
impedance voltmeter and calculating the touch voltage as described in G.4.2. For the
measurement of the driving voltage in any part of the installation the electrode shall be placed at a
distance of 1 m from the exposed part of the installation.

One terminal of the voltmeter is connected to the exposed part and the other terminal to the foot
electrode, which will be a probe driven at least 20 cm into earth.
 - 199 - EN 50341-1:2012

H.4.3 Measurement of resistances to earth and impedances to earth

The resistances to earth, RE and impedances to earth, ZE can be determined in different ways. Which
method is suitable depends on the extent of the earthing system and the degree of interference and
disturbance voltages.

Attention should be given to the fact that while the measurements and preparations are carried out,
even when disconnected, but especially during the measurement on and between earthed parts (for
example between tower and detached earth wire), dangerous touch voltages can occur.

Examples of suitable methods of measurements and types of instruments are:

a) Fall-of-potential method with the earth tester

This instrument is used for earth electrodes and earthing systems of small or medium extent,
for example single rod earth electrodes, strip earth electrodes, earth electrodes of overhead
line towers with detached or attached earth wires, high voltage earthing systems and
separation of the low-voltage earthing systems. The frequency of the alternating voltage used
shall not exceed 150 Hz.

The earth electrode under test, probe and auxiliary electrode shall lie in a straight line as far
apart as possible. The distance of the probe from the earth electrode under test shall be at
least 2,5 times the maximum extension of the earth electrode under test (in the measuring
direction), but not less than 20 m; the distance of the auxiliary electrode shall be at least 4
times, but not less than 40 m.

b) High frequency earth tester

This instrument facilitates, without removing the earth wire, the measurement of the resistance
to earth of a single tower. The frequency of the measuring current shall be so high that the
chain impedance of the earth wire and the neighbouring towers becomes high, representing a
practically negligible shunt circuit to the earthing of the single overhead line tower.

c) Heavy-current injection method

This method is used particularly for the measurement of the impedance to earth of large
earthing systems, but also if transferred potentials (i.e. metallic pipes) are to be taken into
account and therefore greater distances between the earthing system of the relevant tower
and the remote earth electrode are necessary.

By applying an alternating voltage of approximately system frequency between the earthing

system and a remote earth electrode, a test current Im is injected into the earthing system,
leading to a measurable potential rise of the earthing system.

Earth wires and cable sheaths with earth electrode effect, which are operationally connected
to the earthing system, shall not be disconnected for the measurement.

The modulus of the impedance to earth, ZE is given by:

Uem
ZE =
Im r

where

Uem is the measured voltage between the earthing system and a probe in the area
of the reference earth (remote earth) in V;

Im is the measured test current in A;

r is the reduction factor of earth wires.

The reduction factor may be determined by calculation (see H.4.5) or by measurement.

For overhead lines without earth wires r = 1.

EN 50341-1:2012 - 200 -

Earth wires of lines which run on separate supports parallel to the test line between earthing
system and remote earth electrode, shall be taken into account, if they are connected to the
earthing system under test.

The distance between the tested earthing system and the remote earth electrode shall, as far
as possible be not less than 5 km. The test current should, as far as possible, be sufficiently
high such that the measured voltages are greater than possible interference and disturbance
voltages. This is generally ensured for test currents above 50 A. The internal resistance of the
voltmeter should be at least 10 times the probe resistance to earth.

For small earthing systems smaller distances and test currents can be sufficient. Possible
interference and disturbance voltages should be taken into account.

H.4.4 Determination of the earth potential rise

The earth potential rise, UE is given by:

UE=ZE IE

where

IE is the current to earth;

ZE is the impedance to earth, for example from the measurement or by calculation.

The approximate calculation of ZE taking into account earth wires and effect of the the neighbouring
towers can be undertaken using the following formula:

(
Z E = 0,25 Z S + Z S (4 R t + Z S ) )
where

ZS is the earth wire impedance of one span;

Rt is the tower footing resistance.

The current to earth during fault is given by:

IE = r 3 I0

where

r is the reduction factor of earth wires;

I0 is the zero sequence current during fault.

The reduction factor may be determined by calculation (see H.4.5) or by measurement.

H.4.5 Reduction factor related to earth wires of overhead lines

H.4.5.1 General
Earth wires of overhead lines carry or transmit part of the fault currents of a corresponding circuit. A
high voltage earthing system installation will be more efficient during the discharge of an earth fault
current. The extent of this efficiency gain is described by the reduction factor, r.

For earth wire(s) of a 3-phase overhead line, the reduction factor, r is the ratio of the current to earth to
the sum of the zero sequence currents of the 3-phase circuit.

r = IE / 3 I0 = (3 I0 – IEW) / 3 I0

where

IEW is the current in the earth wire (in balanced stage);

IE is the current to earth;

3 Io is the sum of zero sequence currents.

 - 201 - EN 50341-1:2012

For the balanced current distribution of an overhead line, the reduction factor, r of earth wire(s) can be
calculated on the basis of the self impedance of the earth wire, ZEW-E and the mutual impedance
between phase conductors and earth wire, ZML-EW :

r = (ZEW-E – ZML-EW) / ZEW-E = 1 - (ZML-EW / ZEW-E)

The most influential characteristic for ZML-EW, is the mean distance between phase conductors and
earth wire, and for ZEW-E, the resistance of the earth wire. It follows that, the reduction effect of earth
wire(s) in respect of the earth current increases (r tending to be small) with lower distance between
phase conductors and earth wire and with lower resistance of the earth wire.

H.4.5.2 Values of reduction factor of overhead lines

The values of reduction factors, r vary within the range 0,2 to 1 and are dependent on several
parameters, e.g: line geometry, location of earth wire(s) to phase conductors, soil resistivity, number of
earth wires and their resistance.
EN 50341-1:2012 - 202 -

Annex J
(normative)

Angles in lattice steel towers

J.1 Definition of symbols used in this annex
Symbol Signification

A Gross cross section area

Aeff Effective cross section area
Agv Gross cross section area for block tearing resistance calculation
Anet Net cross section area at holes
Ant Net cross section area for block tearing resistance calculation
As Tensile stress area of bolt
c Distance between batten plates
d Bolt diameter
d0 Hole diameter
E Modulus of elasticity
e1 End distance from centre of hole to adjacent end in angle
e2 Edge distance from centre of hole to adjacent edge in angle
F Concentrated horizontal load
Fb,Rd Bearing resistance per bolt
Ft,Rd Tension resistance per bolt
Fv,Rd Shear resistance per shear plane
fu Ultimate tensile strength
fub Ultimate tensile strength for bolt
fy Yield strength
i Radius of gyration about the relevant axis
k Reduction coefficient
L Buckling length
L Member length
Leff Effective reduced length
Lth Length of horizontal member
m Number of angles
NEd Design value of the compression force
Nb,Rd Design buckling resistance
Nd Compression force
Nu,Rd Design ultimate resistance
n Number of bolts
P1 Spacing of 2 holes in the direction of load transfer
P1 Compressive force
 - 203 - EN 50341-1:2012

P2 Tensile force
Sd Tension force
Sd Force in the supporting member (tension or compression)
t Thickness
Veff,i,Rd Block shearing resistance

α Imperfection factor

ηi Reduction factor

γM1 Partial factor for resistance of member in bending or tension or to buckling

γM2 Partial factor for resistance of net section at bolt holes

γMb Partial factor for resistance of bolted connections

λ Slenderness for the relevant buckling load

λ eff Effective non-dimensional slenderness

λ Non-dimensional slenderness

λ1 Slenderness of one sub-member

λz Slenderness of one full-member

χ Reduction factor for the relevant buckling mode

J.2 General
The calculation methods for angle members in lattice steel towers with bolted connections proposed in
this annex are mainly based on the ECCS Publication: "Recommendations for angles in lattice
transmissions towers" (publication n°39: 1985).

According to 7.3.6.2, tension resistance of angles connected through one leg should be calculated
using the provisions of EN 1993-1-8 or Annex J.3.

According to 7.3.6.4, Annex J.4 on buckling resistance of members in compression is applicable only
for lattice steel tower designs which are validated by tower tests. For tower designs not validated by
tower tests, requirements of EN 1993-3-1 do apply.
According to 7.3.8, bolted connections in lattice steel towers shall be designed using the provisions of
Annex J.5 or EN 1993-1-8.

Flowchart J.1 summarises the structure of Annex J.

EN 50341-1:2012 - 204 -

J Angles in lattice steel towers

J.3 Tension member J.4 Compression member J.5 Bolted connections

Connected through
one leg ?
Full-scale tests acc. J.5 Design resistance
to 7.3.9 ? or
EN 1993-1-8 (see 7.3.8)

NO YES NO YES

EN 1993-1-1 (see 7.3.6.2) EN 1993-3-1 (see 7.3.6.4)

J.3 Design ultimate resistance J.4.1 Design flexural buckling

or EN 1993-1-8 (see 7.3.6.2) resistance

Flowchart J.1  Structure of Annex J on Angles in Lattice Steel Towers

J.3 Tension resistance of angles connected through one leg (see 7.3.6.2)
A single angle in tension connected by a single row of bolts in one leg, see Figure J.1, may be treated
as concentrically loaded over an effective net cross section area, Anet for which the design ultimate
resistance, Nu,Rd should be determined as follows:

a) Case of one leg connected with 1 bolt

Nu,Rd = (b1 – d0) t fu / γM2

b) Case of one leg connected with 2 or more bolts

Nu,Rd = (b1 – d0 + b2/2) t fu / γM2

where

b1, b2 are defined as in Figure J.1;

d0 is the hole diameter, see Figure J.1;

t is the thickness;

fu is the ultimate tensile strength;

γM2 is the partial factor for resistance of net section as defined in 7.3.6.1.

Flowchart J.2 summarises the structure of this Annex J.3.

 - 205 - EN 50341-1:2012

Figure J.1  Angle with one leg connected

J.3 Design ultimate resistance Effective net section area 7.3.6.2

Ultimate tensile strength, fu

Partial factor, γM2 7.3.6.1

Flowchart J.2  Structure of Annex J.3 on Tension Members

J.4 Buckling resistance of angles in compression (see 7.3.6.4)

J.4.1 Flexural buckling resistance
A compression angle (hot rolled or cold formed angle) should be verified against buckling as follows:

NEd / Nb,Rd ≤ 1

where

NEd is the design value of the compression force;

Nb,Rd is the design buckling resistance of the compression member.

The design flexural buckling resistance of a compression member should be taken as:

Nb,Rd = χ A fy / γM1 for Class 1, 2 and 3 cross-sections

Nb,Rd = χ Aeff fy / γM1 for Class 4 cross-sections

where

χ is the reduction factor for the relevant buckling mode;

A is the gross cross-section area;

Aeff is the effective cross-section area;

fy is the yield strength;

γM1 is the partial factor for resistance of member in bending or tension or to buckling as
defined in 7.3.6.1.
In determining A and Aeff holes for fasteners at the column ends need not to be taken into account.
NOTE Angles are considered to be class 3 or 4 according to 5.5 of EN 1993-1-1:2005.

For axial compression in angles the value of χ should be determined according to:

1
χ= but χ ≤1
2
Φ + Φ² − λ eff
EN 50341-1:2012 - 206 -

where

[ (
Φ = 0,5 × 1 + α × λ eff − 0,2 + λ eff ) 2
]
λ eff is the effective non-dimensional slenderness as defined in J.4.2.4;

α is the imperfection factor, which should be taken as equal to 0,13.

The choice of this imperfection factor value corresponds to buckling curve a0 according to
EN 1993-1-1. The choice of a more conservative value from Table 6.1 of EN 1993-1-1:2005 may be
specified in the NNA's.

Flowchart J.3 summarises the structure of this Annex J.4.1 on the design flexural buckling resistance
of a compression member.

J.4.1 Design flexural buckling Effective cross section area 7.3.6.2

resistance Aeff

Yield strength, fy

Partial factor, γM1

Reduction factor for the Imperfection factor.a0

relevant buckling mode, χ (EN 1993-1-1) or NNA

J.4.2 Effective non-

dimensional slenderness,
λ eff

Flowchart J.3  Structure of Annex J.4.1 on design flexural buckling resistance of a

compression member

J.4.2 Effective non-dimensional slenderness for flexural buckling

J.4.2.1 General

The effective non-dimensional slenderness, λ eff , used for the calculation of the design flexural
buckling resistance of a compression member in Annex J.4.1, is a linear transformation of the non-
dimensional slenderness, λ as described in Annex J.4.2.4.

The non-dimensional slenderness, λ depends on the slenderness ratio, λ as described in Annex

J.4.2.3.

The slenderness, λ is defined in Annex J.4.2.2.

Flowchart J.4 summarises the structure of Annex J.4.2 on the effective non-dimensional slenderness.
 - 207 - EN 50341-1:2012

J.4.2 Slenderness for flexural

buckling

J.4.2.2 Slenderness, λ J.4.3.1 Buckling length for

the relevant buckling mode,
L

Radius of gyration (about

the relevant axis), i J.4.3.2 Legs and chords

J.4.3.3 Primary bracing

J.4.3.4 Compound members

J.4.3.5 Secondary bracing

J.4.2.3 Non-dimensional Yield strength, fy

slenderness, λ

Effective cross section area, 7.3.6.2

Aeff

J.4.2 Effective non-

Gross cross section area, A EN 1993-1-1
dimensional slenderness,
λ eff

Primary
bracing ?

NO YES

J.4.2.4 Effective non- Load eccentricity

dimensional slenderness, λ eff

Continuity of member

Number of bolts at non-

continuous end

Flowchart J.4  Structure of Annex J.4.2 on effective non-dimensional slenderness

J.4.2.2 Slenderness, λ
The slenderness, λ is:

L
λ=
i
with
EN 50341-1:2012 - 208 -

L the buckling length in the buckling plane considered, taken as the distance between
centre line intersections;

i the radius of gyration about the relevant axis, determined using the properties of the
gross cross-section.

The appropriate slenderness, λ is determined according to the various bracing configurations

described in J.4.3 for:

• Leg members and chords (J.4.3.2);

• Primary bracing patterns (J.4.3.3);
• Compound members (J.4.3.4);
• Secondary (or redundant) bracing members (J.4.4).
The recommended maximum slenderness is also given.

J.4.2.3 Non-dimensional slenderness, λ

λ
λ= for Class 3 cross-section
λ1

λ Aeff
λ= × for Class 4 cross-section
λ1 A

where

A is the gross cross-section area;

Aeff is the effective cross-section area;

E
λ1 = π
fy

with

fy the yield strength;

E the modulus of elasticity (210 000 N/mm²).

J.4.2.4 Effective non-dimensional slenderness, λ eff

The effective non-dimensional slenderness, λ eff is determined as follows:

a) For leg members: λ eff = λ

b) For bracing members, 5 cases should be considered. The choice of a case depends for each
member on the slenderness, the load eccentricity, the continuity of the member and the number
of bolts at the non-continuous end. The choice should be made according to Table J.1. The 5
cases (5 linear transformations) are given in Table J.2.
 - 209 - EN 50341-1:2012

Table J.1  Choice of the buckling case

Buckling axis Non-dimensional Load Member Number of bolts Case

slenderness eccentricity continuity at non-
condition condition condition continuous end
1 end - - 2
VV λ ≤ √2
2 end - - 3
- 2 ends - 3
- 1 end 2 bolts 3

λ > √2 - 1 end 1 bolt 1

- 0 end 2 bolts 3
- 0 end 1 bolt 1
1 end - - 3
YY or ZZ λ ≤ √2
2 end - - 4
- 2 ends - 1
- 1 end 2 bolts 3

λ > √2 - 1 end 1 bolt 1

- 0 end 2 bolts 4
- 0 end 1 bolt 5

NOTE Member continuity conditions are:

2 ends = the member is continuous at both ends
1 end = the member is continuous at one end only
0 end = single span member

Table J.2  Buckling cases

Case Linear transformation

1 λ eff = λ

2 λ eff = 0,25 + 0,82 λ

3 λ eff = 0,5 + 0,65 λ

4 λ eff = 0,71 + 0,65 λ

5 λ eff = 0,4 + 0,86 λ

J.4.3 Slenderness of members

J.4.3.1 General
There are several different bracing configurations, which are commonly used in lattice towers, and
each requires separate consideration.
EN 50341-1:2012 - 210 -

The buckling length of a member and its radius of gyration depend on the type of bracing used to
stabilise the member.

The appropriate slenderness, λ for the relevant buckling mode should be determined from Annex
J.4.3.2 to Annex J.4.3.5.
J.4.3.2 Leg members and chords
The recommended maximum slenderness for leg members and chords should not exceed 120.

The cross section of members usually consists of one profile. For compound members reference
should be made to Annex J.4.3.4.

Several cases need to be considered, as shown in Figure J.2, and the slenderness for angles should
be taken as:

• Leg with symmetrical bracing (a) (b) λ = 1,0 L / ivv

• Leg with intermediate transverse support (c) λ = 1,0 L / iyy
• Leg with staggered bracing (d) λ = 1,2 L / iyy

1,0 ivv 1,0 ivv 1,0 iyy 1,2 iyy

z z

Figure J.2  Symmetrical and staggered bracing to legs

J.4.3.3 Primary bracing patterns

J.4.3.3.1 General
The following rules should be used for the typical primary bracing patterns shown in Figure H.1 of
EN 1993-3-1:2006. Secondary, or redundant, bracings may be used to subdivide the primary bracing
or main leg members as shown, for example, in Figures H.1 (IA, IIA, IIIA, IVA) and H.2 of EN 1993-3-
1:2006.

The slenderness, λ for bracing members should be taken as:

Ldi
λ=
ivv

for angles where, Ldi is specified in Figure H.1 of EN 1993-3-1:2006.

The slenderness, λ for primary bracing members should generally be not more than 180 and for
secondary bracing not more than 250. For multiple lattice bracing (Figure H.1(V) of
EN 1993-3-1:2006) the overall slenderness should generally be not more than 350.
 - 211 - EN 50341-1:2012

The cross section of bracing members usually consists of one profile. For compound members
reference is made to Annex J.4.3.4.

In case of long members, it may be appropriate to take account of bending stresses induced by wind
acting on members, in addition to the axial load.

The angle between a main member and a bracing should not be less than 15°.

J.4.3.3.2 Single lattice (Figure H.1(I) of EN 1993-3-1:2006)

The provisions given in H.3.2 of EN 1993-3-1:2006 should be applied.

J.4.3.3.3 Cross bracing (Figure H.1(II) of EN 1993-3-1:2006)

The provisions given in H.3.3 of EN 1993-3-1:2006 should be applied.
NOTE The load can be considered as nearly equally split into tension and compression as long as Sd / Nd > 2/3
with Sd = force in the supporting member in tension,
Nd = force in the compression member.

Another, more accurate method may be proposed in the NNAs.

J.4.3.3.4 Tension bracing (Figure H.1(VI) of EN 1993-3-1:2006)

The provisions given in H.3.4 of EN 1993-3-1:2006 should be applied.

J.4.3.3.5 Cross bracing with redundant members (Figures H.1(IIA and IVA) and H.2(a) of
EN 1993-3-1:2006)
The provisions given in H.3.5 of EN 1993-3-1:2006 should be applied.

J.4.3.3.6 Discontinuous cross bracing with a continuous horizontal member at centre

intersection (Figure H.1(IV) of EN 1993-3-1:2006)
The provisions given in H.3.6 of EN 1993-3-1:2006 should be applied.

J.4.3.3.7 Cross bracing with diagonal corner stays (Figure H.2(b) of EN 1993-3-1:2006)
The provisions given in H.3.7 of EN 1993-3-1:2006 should be applied.

J.4.3.3.8 K bracing (Figures H.1(III), H.1(IIIA) and H.2.(c) of EN 1993-3-1:2006)

The provisions given in H.3.8 of EN 1993-3-1:2006 should be applied.

J.4.3.3.9 Horizontal edge members with horizontal plan bracing

The provisions given in H.3.9 of EN 1993-3-1:2006 should be applied.

The following method may be chosen as an alternative to H.3.9 (5): the horizontal plan bracing needs
to be stiff enough to prevent partial buckling. In case of doubt a good practice design rule is as follows:

- the horizontal plan bracing, as indicated in Figure J.3, has to resist a concentrated horizontal load F
= 1,5 L, in kN, placed in the middle of the horizontal member, where: L = length of the horizontal edge
member in m.

- the deflection of the horizontal bracing under this load is limited to L / 1 000.

Figure J.3  Typical plan bracing

EN 50341-1:2012 - 212 -

More details about plan bracing design can be found in the CIGRE Technical Brochure n°196
"Diaphragms for lattice steel supports".

J.4.3.3.10 Horizontal edge members without horizontal plan bracing

The provisions given in H.3.10 of EN 1993-3-1:2006 should be applied.

For buckling transverse to the frame and when the horizontal member has compression in one half of
its length and tension in the other, an effective reduced length, Leff may be used instead of Lth to
determine λ according to the following formula:

Leff = k x Lth
with

Lth length of the horizontal member (see Figure H.4(a) of EN 1993-3-1:2006).

k reduction coefficient depending on the ratio of the compressive force P1 to the tensile force
P2 as given by the formula:
k = 0,085 x (|P2/P1|) ² - 0,316 x (|P2/P1|) + 0,730
That formula is consistent with the values of Table G.3 of EN 1993-3-1:2006.

The rectangular radius of gyration (iyy) should be used for buckling transverse to the frame over this
effective length, Leff.

J.4.3.3.11 Cranked K bracing

The provisions given in H.3.11 of EN 1993-3-1:2006 should be applied.

J.4.3.3.12 Portal frame

The provisions given in H.3.12 of EN 1993-3-1:2006 should be applied.

J.4.3.3.13 Multiple lattice bracing (Figure H.1(V) of EN 1993-3-1:2006)

The provisions given in H.3.13 of EN 1993-3-1:2006 should be applied.

J.4.3.4 Compound members

J.4.3.4.1 General
Compound members may be built up with two back-to-back angle sections (Figure J.4) or with two,
three or four angles in cruciform section (Figure J.5).

If welded continuously (Figure J.5.(a)), they may be taken as fully composite.

For laced compression members reference should be made to 6.4.2 of EN 1993-1-1:2006.

J.4.3.4.2 Details
The slenderness of a sub-member should be, λ1 ≤ 50

If batten plates are adopted they should be arranged at least at the third points of the total buckling
length and at the ends of the members.

If members comprising two angle sections are connected to a common gusset plate, separate batten
plates at the ends of the members are not necessary.

Every batten plate should be connected to each sub-member by means of bolts or by an equivalent
welded seam. At the ends of the members one additional connecting element should be provided for
each of these connections.

In the case of a cruciform compound member, a minimum of two bolts for each member are required
at each batten plate.

J.4.3.4.3 Design
When the structural design complies with the requirements given previously the members may be
calculated according to the following rules:
 - 213 - EN 50341-1:2012

Compound members, which consist of m sub-members and have a material principal axis, yy, may be
calculated against buckling transversely to this material axis as a single compression member.

As far as buckling transversely to the non-material principal axis, zz, is concerned, the member can be
treated as a single compression member with a virtual slenderness of:

m
λzi = λ z 2 + λ 12
2
where

m is the number of angles;

λz is the slenderness of the full members as defined in J.4.3.2 and J.4.3.3 respectively;

λ1 is the slenderness of one sub-member and equal to c / ivv;

c is the distance between batten plates according to Figure J.4 and Figure J.5.

Figure J.4  Back to back angle section

Figure J.5  Cruciform angle sections

EN 50341-1:2012 - 214 -

J.4.4 Secondary (or redundant) bracing members

The provisions given in H.4 of EN 1993-3-1:2006 should be applied.

The angle between the redundant and the main member should be not less than 15°.,The percentage
p of H.4(2) may be determined according to the alternative following formula:

p = ( λ + 32 ) / 60 with 1 ≤ p ≤ 3,5

J.5 Design resistance of bolted connections (see 7.3.8)

J.5.1 General
The design resistance for an individual fastener subjected to shear and/or tension is given in
Table J.3.

Figure J.6  Location of bolts in angle member connected by one leg

When filler plate is used for packing, the design shear resistance of bolts should be reduced according
to 3.6.1 (12) and (13) of EN 1993-1-8:2005.
 - 215 - EN 50341-1:2012

Table J.3  Design resistance for individual fasteners subjected to shear and/or tension
Shear resistance per shear plane:
If the shear plane passes through the unthreaded portion of the bolt:
Fv,Rd = 0,6 fub A / γM2
If the shear plane passes through the threaded portion of the bolt:
Fv,Rd = 0,6 fub AS / γM2 for classes 4.6 - 5.6 - 6.6 - 8.8
Fv,Rd = 0,5 fub AS / γM2 for classes 4.8 - 5.8 - 6.8 - 10.9
Bearing resistance per bolt:
Fb,Rd = α fu d t / γM2
where α is the smallest value of:
η1 3; η2 1,20 (e1/d0); η3 1,85 (e1/d0 – 0,5); η4 0,96 (P1/d0 – 0,5); η5 2,3 (e2/d0 – 0,5)
and ηi are reduction factors.
The default value for each ηi is 1, but a smaller (more conservative) value may be defined in the NNA.
The value of α is still valid in the case of bolts layout on two or more rows if P1, e1 and e2 are defined as:

P1 is the minimum center-to-center distance between two consecutive holes, on the same row;
e1 is the minimum distance of the bolt nearest to the end;
e2 is the minimum distance of the bolt nearest to the edge.
The design resistance of a group of fasteners may be taken as the sum of the design bearing resistances, Fb,Rd of the individual
fasteners provided that the design shear resistance, Fv,Rd of each individual fastener is greater than or equal to the design
bearing resistance, Fb,Rd. Otherwise the design resistance of a group of fasteners should be taken as the number of fasteners
multiplied by the smallest design resistance of any of the individual fasteners.

Tension resistance per bolt:

Ft,Rd = 0,9 fub AS / γM2
fu is the ultimate tensile strength
fub is the ultimate tensile strength for bolt
A is the gross cross-section area of bolt
AS is the tensile stress area of bolt
d is the bolt diameter
t, d0, e1, e2, P1 are as defined in Figure J.6
γM2 is as defined in 7.3.6.1

J.5.2 Block tearing resistance of bolted connections

Block tearing consists of failure in shear at the row of 2 or more bolts along the shear face of the hole
group accompanied by tensile rupture along the line of bolt holes on the tension face of the bolt group.
Figure J.7 shows block tearing for an angle section.

For gussets plates with a bolt group subject to concentric loading, the design block shearing
resistance Veff,1,Rd is given by:

f A 0,5 f u Agv 
Veff ,1, Rd = 0,95  u nt + 
 γM2 γ M 2 

For an angle section with a bolt group subject to eccentric loading, the design block shearing
resistance Veff,2,Rd is given by:

f A 0,5 f u Agv 
Veff , 2, Rd = 0,80  u nt + 
 γM2 γ M 2 

The symbols in the above formulae are:

fu is the ultimate tensile strength of the plate or the angle section;

EN 50341-1:2012 - 216 -

Ant is the net cross section area subjected to tension.

For an angle section with one bolt row, and using the symbols of Figure J.6, the net area subject to
tension is to be calculated as follows:

d0
Ant = t (e2 − )
2
Agv is the gross cross section area subjected to shear.

For an angle section with one bolt row, and using the symbols of Figure J.6, the gross cross section
area subject to shear is to be calculated as follows:

Agv = t [e1 + P1 (n − 1)]

where

n is the number of bolts.

Figure J.7  Block tearing of angle sections

 - 217 - EN 50341-1:2012

Annex K
(normative)

Steel poles

K.1 Definition of symbols used in this annex

Symbol Signification
A Gross cross section area
Aeff Effective cross section area
As Tensile stress area of holding-down bolt
b Nominal width
beff Effective width
d Outside diameter ; outside diameter across angles of polygon
Ft,Sd Design tensile force per bolt for the ultimate limit state
fbd Bonding stress of steel into concrete
fck Characteristic strength of concrete in compression
fctm Average strength of concrete in tension
fctk0,05 Characteristic strength of concrete in tension
fub Ultimate tensile strength for holding-down bolt
fy Yield strength
Msd Bending moment at cross section
Nsd Axial force at cross section
n Number of sides of the polygon
t Thickness
W eff Effective cross section modulus
W el Elastic section modulus
∆M Additional moment
σ com , Ed Maximum calculated compressive stress
σx, Ed Actual maximal longitudinal stress
γc Partial factor on bonding
γM1 Partial factor for resistance
γMb Partial factor for resistance of holding-down bolt
λp Plate slenderness
ρ Reduction factor
ψ Stress ratio

K.2 Classification of cross sections (EN 1993-1-1:2005 – 5.5)

Cross sections shall be considered as class 3 if the thinness of the wall allows the calculated stress in
the extreme compression fibre of the tube to reach its yield strength. All other sections, in which it is
necessary to make explicit allowances for the effects of local buckling when determining their moment
resistance or compression resistance, shall be considered as class 4 according to the criteria given in
Table K.1.
EN 50341-1:2012 - 218 -

Table K.1  Classification of tubular cross sections in bending

Type of section Criteria for class 4

d/t > 176 ε²

t for n equal 6 to 18 sides

n sides b/t > 42 ε

0,5 2
where ε = (235/fy) and fy is the nominal value of the yield strength in N/mm

K.3 Class 4 cross-sections (EN 1993-1-1:2005 – 6.2.2.5 and EN 1993-1-5:2006 – 4)

The effective cross-section properties of Class 4 cross-sections shall be based on the effective widths
(areas in black) of the compression elements as shown in Figure K.1.

Aeff under axial force Weff under bending moment

Figure K.1 - Class 4 effective cross-sections characteristics

The effective widths of flat compression internal elements shall be designed using Table 5.2 of
EN 1993-1-1:2005 and Clause 4 of EN 1993-1-5:2006. The stress ratio, ψ used in Table 5.2 of
EN 1993-1-1 and Clause 4 of EN 1993-1-5:2006 may be based on the properties of the gross cross-
section.

However, for greater economy, the plate slenderness, λ p of each element may be determined using
the maximum calculated compressive stress, σ com , Ed in that element in place of the yield strength, fy ,
provided that σ com , Ed is determined using the effective widths, beff of all the compression elements.
 - 219 - EN 50341-1:2012

This procedure generally requires an iterative calculation in which ψ is determined again at each step
from the stresses calculated on the effective cross-section defined at the end of the previous step,
including the stresses from the additional moment, ∆M.

K.4 Resistance of circular cross sections

The resistance of a circular cross section, without opening, under preponderant bending moment is
ensured if the actual maximal longitudinal stress, σx, Ed (including the simultaneous axial force),
calculated on the gross section, satisfies the following criteria:

σ x , Ed ≤ ρ f y / γ M 1

with
for class 3 sections : ρ = 1,0
53 ε 2
( )
0 ,5
for class 4 sections : ρ = 0,70 + ≤ 1,0 , with ε = 235 / f y
d /t
Figure K.2 gives directly the reduction factor, ρ as a function of the ratio, d/t .

1
0,99
0,98
0,97
0,96 Steel S235
0,95
0,94
0,93
Steel S355
ρ 0,92
0,91
0,90
0,89
0,88
0,87
0,86
0,85
0,84
0,83

100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250

d/t

Figure K.2  Reduction factor ρ

K.5 Resistance of polygonal cross sections

K.5.1 Class 3 cross-sections (EN 1993-1-1:2005 – 6.2.9.2)

The resistance of a class 3 polygonal cross section will be satisfactory if the maximum longitudinal
stress, σx, Ed, calculated on the gross section, under preponderant bending moment and simultaneous
axial force, satisfies the criterion:

σ x , Ed ≤ f y / γ M 1

For cross sections without an opening, the above criterion becomes:

N Sd M Sd fy
+ ≤
A Wel γ M1
where
EN 50341-1:2012 - 220 -

A is the gross cross-section area;

W el is the elastic section modulus.

K.5.2 Class 4 cross-sections (EN 1993-1-1:2005 – 6.2.9.3)

Class 4 polygonal cross section, without opening, will be satisfactory if the maximum longitudinal
stress, σx, Ed, calculated on the effective widths of the compression elements, under preponderant
bending moment and simultaneous axial force, satisfies the criterion:

σ x , Ed ≤ f y / γ M 1

For cross sections without an opening, the above criterion becomes:

N Sd M Sd fy
+ ≤
Aeff Weff γ M1

where

Aeff is the effective area of the cross-section when subject to uniform compression;

W eff is the effective section modulus of the cross-section when subject only to moment
about the relevant axis.
NOTE The detailed method for calculation of effective cross-section properties of Class 4 cross-sections is given in 6.2.2.5 of
EN 1993-1-1:2005. The curves shown in Figures K.3 and K.4 allow a quick determination of Aeff and Weff for polygonal
cross section, without opening.

K.6 Design of holding-down bolts

The design of the anchorage length of holding-down bolts into concrete is given in Table K.2.
Combined design resistance of bolts in shear and tension or compression is given in EN 1992-1-1.
 - 221 - EN 50341-1:2012

t n sides

d
Wel

Figure K.3  Class 4 polygonal cross-sections

Effective section modulus W eff

EN 50341-1:2012 - 222 -

t n sides

d
A

1. 84 110 162 215 242

n=18
.95
.9 n=16
.85
.8 n=12
.75

A eff .7 n=8
.65
A n=6
.6
.55
.5
.45 f y = 235 N/mm2
.4
.35
.3
50 75 100 125 150 175 200 225 250

d/t

68 89 131 174 196

1.
.95
.9 n=18
.85 n=16
.8
.75 n=12
.7
A eff
.65 n=8
A .6 n=6
.55
.5
.45 f y = 355 N/mm2
.4
.35
.3
50 75 100 125 150 175 200 225 250

d/t

Figure K.4  Class 4 polygonal cross-sections

Effective area Aeff

 - 223 - EN 50341-1:2012

Table K.2  Design of holding-down bolts

Straight anchor Anchor with bend Anchor with plate

v = min ( l ; d1) l0
Lb t > 0,3 r
Φ
Φ
t

r d1

Fa , Rd = π Φ Lb f bd Fa , Rd = π Φ Lb f bd Fa , Rd = π Φ Lb f bd
with with
Lb = (l1 + 3,2 D + 3,5 l2 ) f cd  r ²  r
Lb = 2,45 Φ  − 0,25  1 −  + l0
f bd  Φ ²  ν 
f bd is the bonding stress of steel into concrete

0,36 f ck
with f bd = for plain bars
γc
2,25 f ctk 0, 05
f bd = for deformed bars
γc
2/3
with f ctk 0, 05 = 0,7 f ctm and f ctm = 0,3 f ck
where f ck is the characteristic strength of concrete in compression
f ctm is the average strength of concrete in tension
f ctk 0,05 is the characteristic strength of concrete in tension
γc is the partial factor on bonding = 1,50
for example : with C 20/25 concrete f ck = 20 N/mm², f ctm = 2,2 N/mm², f ctk 0.05 = 1,55 N/mm²,

and f bd = 1,1 N/mm² for plain bars or f bd = 2,3 N/mm² for deformed bars
The anchoring length shall be such that : Fa , Rd = π Φ Lb f bd ≥ Ft , Sd
where Ft ,Sd is the design tensile force per bolt for the ultimate limit state

The size of the bolt shall be such that : Ft , Sd ≤ Ft , Rd = 0,9 f ub As / γ Mb

where f ub is the ultimate tensile strength of holding-down bolt

As is the tensile stress area of holding-down bolt

γ Mb is the partial factor on resistance of holding-down bolt = 1,25

According to 6.5.5 (6) of EN 1993-1-1:2005, when threads are cut by a non-specialist bolt manufacturer, the
relevant value of Ft , Rd shall be reduced by multiplying it by a factor of 0,85.
EN 50341-1:2012 - 224 -

Annex L
(informative)

Design requirements for supports and foundations

L.1 Structural requirement
The following information is required for the design of supports and foundations:

• applied loads, including partial coefficients for actions, at attachment point of insulators
/conductors / earth-wires (in the form of the arrangement of transverse (T), vertical (V) and
longitudinal (L) loads);
• wind loads on supports;
• load combinations;
• ultimate limit state for each load combination;
• serviceability limit state for each load combination (allowable deflections);
• preferred failure sequence;
• maintenance and construction loadings.

L.2 Configuration requirements: types of supports and uses

The support type, outline, disposition of phase conductors, inter-phase spacing, electrical clearances
and disposition of earth wires shall be as specified in the Project Specification.

The following tables may be used as a guide.

Table L.1  Support type and use

Support Type Description Angles of deviation or Type of

line entry insulator

...... ...... ...... ......

Table L.2  Range of extensions

Support type Range of extension Description

Minimum height
Maximum height x
incremental extension
(m)

...... ...... ......

It should be stated in the description column whether extensions are either individual extensions or
combinations of body and leg extensions. The latter height of body extension and range of leg
extensions should be stated. In addition, limitations of use, interchangeability, connection levels
and maximum permitted height differences between individual leg extensions should be clearly
stated.
 - 225 - EN 50341-1:2012

Table L.3  Line design particulars

Number of sub-conductors per phase

Size and type of sub-conductors
Arrangement of sub-conductors
Spacing of sub-conductors (horizontally and vertically)
Number and type of earth conductors
Size of earth conductors
Standard span length for standard height support
Standard height support
Maximum earth wire shielding angle of top/outside
Phase conductor, still air (degrees)
Maximum single span length
Maximum sum of adjacent span lengths
Maximum weight span, normal conditions
Maximum weight span, unbalanced conditions
Minimum weight span under normal conditions with maximum wind span
Minimum weight span, unbalanced conditions
Maximum weight span, for terminal supports

Table L.4  Insulator string details

Minimum/maximum insulator set lengths

Suspension set
Post insulator set
Pilot suspension set
Tension set inner string
Tension set outer string
Low duty set, with or without adjustment
Number of strings per phase
Suspension
Tension
Low duty
Minimum clearance from live metal to support steel or earthed fittings
Assumed maximum swing of suspension set (degrees)
Suspension insulator strings:
(a) Inclined between 0° and .... degrees from vertical
(b) Inclined between ....degrees and maximum from vertical

Tension insulator string:

(a) Jumper loop hanging vertically
(b) Jumper loop inclined .... degrees from vertical

Weighted pilot suspension string:

(a) Assumed initial deflection under still air conditions .... degrees
(b) Maximum deflection .... degrees with jumper at maximum deflected position

Where possible, drawing of insulator set should be provided, complete with arcing devices, sag
adjustment and support attachment details.
If V-strings are used, length of string between attachments or included angle and whether the V-string
is capable of withstanding compressive loading should be specified. If post insulators are used,
inclination of post to the horizontal should be specified.
EN 50341-1:2012 - 226 -

Table L.5  Spatial distance

Arrangement of phase conductors, vertical

Arrangement of phase conductors, horizontal
Arrangement of phase conductors, delta
Minimum height of phase conductors at the support on standard height ... type supports
Maximum swing of earth conductors from vertical (degrees)
Minimum vertical spacing between adjacent phase conductors of one circuit
Minimum projected horizontal spacing between adjacent phase conductors of one circuit
Minimum vertical spacing between phase and earth conductors

L.3 Phase conductor and earth wire attachment

Details of the phase conductor and earth wire insulator attachments to the support crossarms/body
shall be as stated in the Project Specification or agreed with the client prior to detail design
commencing.

L.4 Foundation steelwork

Details of the proposed method of interconnection between the support and the foundation, e.g. stubs
and cleats, anchor bolts or embedded sections shall be as stated in the Project Specification or
agreed with the client.

L.5 Erection/maintenance facilities

Provision of erection and subsequent maintenance facilities, which have design implications, should
be clearly stated in the Project Specification or in accordance with 7.12 of this standard, e.g:

• provision for maintenance facilities ;

• provision for attachment facilities ;
• provision for terrain circumstances in respect to erection ;
• provision for transportation possibilities ;
• provision for marking facilities ;
• provision for grounding requirements.

L.6 Mass-length restrictions

Any special restrictions on either the overall support configuration or fabrication process, which have
design implications, should be clearly stated in the Project Specification, e.g:

• restrictions on overall base width of support ;

• restrictions on overall dimensions of panels ;
• restrictions on overall dimensions or masses of individual members ;
• restrictions on site welding ;
• restrictions on proposed erection methods.
 - 227 - EN 50341-1:2012

Annex M
(informative)

Geotechnical and structural design of foundations

M.1 Typical values of the geotechnical parameters of soils and rocks
M.1.1 General
The values of the geotechnical parameters given hereafter should be used when results of a
geotechnical investigation are not available. They shall not take the place of a soil investigation, and
the values quoted should be confirmed during construction.

If any doubt arises as to the determination of a given soil to one of the categories appearing in the
following tables, the more pessimistic value shall be adopted.

In Table M.1, some of the most commonly encountered soils are described according to their origin
and evaluated as to their suitability as a foundation layer.

The next two Tables M.2 and M.3 give, for the main categories, cohesive and non-cohesive soils and
rocks, the ranges of values of geotechnical parameters needed for the foundation design formulae.

M.1.2 Definitions
Soil classification according to particle size

Particle size in mm Definition

d> 200 Boulders

200 >d> 20 Pebbles, Cobbles

20 >d> 2 Gravel

2 >d> 0,2 Sand (coarse)

0,2 >d> 0,06 Sand (fine)

0,06 >d> 0,002 Silt

d< 0,002 Clay

M.1.3 Symbols, definitions and units of some ground parameters

Soils

γ Weight density kN/m³

γ’ Effective weight density (under ground-water) kN/m³

φ’ Angle of internal friction or shearing resistance degree

c’ Effective cohesion kN/m²

cu Undrained cohesion or undrained shear strength kN/m²

qu Unconfined compressive strength kN/m²

According to EN 1997-2:2007 (Subclause 5.8.4 Unconfined compression test), the undrained shear
strength, cu may be determined as one half of the measured unconfined compressive strength, qu.

Rocks

Rc : Crushing strength MN/m²

Rt : Tensile strength MN/m²

E: Young’s modulus MN/m²

EN 50341-1:2012 - 228 -

Table M.1  Commonly encountered soils

Technical features and

Soil type Mode of formation Description aptitude as foundation
course
Lateral Sandy-gravelly deposit of Sandy-gravelly material, Medium to high compaction,
1 moraine, glacial origin, deposited at with a wide range of low compressibility,
gravelly the edge of the glacier particle sizes. Very pervious. Very good
heterogeneous foundation layer.
Glacial till Unsorted glacial deposit Gravelly material in a High compaction, low
2 (unsorted) from clay to gravel, usually silty-clayey matrix, with compressibility, impervious.
in dense state. Usually wide range of particle Good foundation layer.
covers molassic layers or sizes.
bedrock.
Glacial drift, Sandy-gravelly layer from Sandy-gravelly material, Mean compaction, mean to
3 sorted by rivers morainic aluvium without large pebbles, high compressibility,
and clay, with little silt pervious. Good foundation
layer.
Very fine grained material Varied clays with layers Low compaction, medium to
4 Glacial clay from morainic eluvion and of silt and fine sands. high plasticity,
deposited in lakes Possible presence of compressible, impervious.
peat and mud Poor foundation soil.
Deposits in flood plains and Alternating silty-sandy Variable compaction and
5 Alluvial soil estuaries. and gravelly deposits. permeability,
Possible presence of inhomogeneous soil. Poor
peat and mud to good foundation soil.
Boulders heap at the toe of Detached angular rock Low compaction, high
6 Boulders a cliff fragments of varying permeability. Acceptable for
sizes foundations, although
unstable.
Over Sedimentary soils Clays Generally acceptable for
7 consolidated subjected to greater Sands foundations.
soils overburden than at present. Silts

Soft rocks Sedimentary soils etc. Mud-stone (incl. Marl) Weathered rocks shall be
8 (weathered to subjected to greater Sandstone evaluated from case to
un-weathered) overburden pressure than Chalk case. Otherwise generally
over consolidated soils. good for foundations.
 - 229 - EN 50341-1:2012

Table M.2  Geotechnical characteristics of some standard soils

(Definitions as given in M.1.2 and M.1.3)

γ γ' φ' c’ cu
Soil kN/m³ kN/m³ degree kN/m² kN/m²
Marl, compact 20 ± 2 11 ± 2 25 ± 5 30 ± 5 60 ± 20
Marl, altered 19 ± 2 11 ± 2 20 ± 5 10 ± 5 30 ± 10
Gravel, graded 19 ± 2 10 ± 2 38 ± 5 - -
loose 18 ± 2 10 ± 2 30 ± 5
Sandsemi-dense 19 ± 2 11 ± 2 32 ± 5 - -
dense 20 ± 2 12 ± 2 35 ± 5
Sandy silt 18 ± 2 10 ± 2 25 ± 5 10 ± 5 30 ± 10
Clayey silt 19 ± 2 11 ± 2 20 ± 5 20 ±10 40 ± 10
Loam, Silt, malleable 17 ± 2 7±2 20 ± 5 - 20 ± 10
soft 17 ± 2 7±2 12 ± 5 6±2 19 ± 5
Clay semi-stiff 19 ± 2 9±2 15 ± 5 11 ± 3 37 ± 12
stiff 20 ± 2 10 ± 2 20 ± 5 22 ± 8 75 ± 25
Clay Till 20 ± 2 10 ± 2 30 ± 5 12 ± 7 400 ± 350
Clay, with organic addition 15 ± 2 5±2 15 ± 5 - -
Peat, Marsh 12 ± 2 2±2 - - -
Backfill, Embankment, 19 ± 2 10 ± 2 25 ± 5 - 15 ± 5
medium compaction

Table M.3  Mechanical properties of some common rocks

(Definitions as given in M.1.2 and M.1.3)

Rock designation Rc Rt E
MN/m² MN/m² MN/m²
Granite-Gneiss- Basalt 100 - 200 4 -10 20 000 - 70 000
Clay - Shale 15 - 100 0 - 10 7 000 - 50 000
Limestone, compact 50 - 100 5-7 30 000 - 60 000
Limestone, soft 10 - 20 1-3 4 000 - 20 000
Marl, not altered 10 -20 1-2 200 - 1 000
Sandstone 10 - 100 1-6 10 000 - 40 000
Molasse 2 - 10 0,2 - 1 1 500 - 5 000
Gypsum 3 - 10 0,3 - 1 2 000 - 5 000
NOTE 1 Poisson’s coefficient, µ lies generally between 0,25 and 0,35.
NOTE 2 The angle of internal friction, φ' lies generally between 35° and 45° and is strongly dependent upon the
degree and the direction of fissuration.

M.2 Sample analytical models for uplift resistance calculation

M.2.1 General
Uplift capacity is commonly the controlling design case of spread-type foundations.

The analytical models presented hereafter are intended for concrete stepped block footings in soils
(not in rocks) with undercut (case a) or without undercut (case b) as shown on Figure M.1.
EN 50341-1:2012 - 230 -

D D

S (lower surface area) S (lower surface area)

a - with undercut b - without undercut

Figure M.1  Concrete stepped block footings

The total uplift resistance of such foundations is the sum of two uplift resistances.

RW is the weight of the foundation itself and the weight of the enclosed soil within the
volume S x D (kN):

S is the lower surface area of the foundation (m²);

D is the foundation depth (m);

RS is the uplift resistance (side resistance) (kN).

M.2.2 Calculation of R W
The weight of the foundation itself is the concrete volume, Vc (m³) multiplied by the concrete weight
density, γc (kN/m³).
3 3
Typical values of the weight density, γc are 22 kN/m for concrete, and 24 kN/m for reinforced
concrete. Other values may be specified in the NNAs or Project Specification.
3
Values of γ (kN/m ) are given by geotechnical investigations according to EN 1997-2.

The soil density, γ considers in a multi-layer soil the weighted average of the individual soil densities
occurring throughout the foundation depth.

The weight of the soil is the volume of the soil multiplied by the weighted average of the soil density.

The volume of the soil, Vs to be considered is given by the following formula:

Vs = S x D – Vbc

where

Vbc is the volume of the buried concrete (m³).

Finally, RW (in kN) is given by the following formula:

RW = Vc γc + (S x D – Vbc) γ

The density of the soil backfill should be considered as being equivalent to the density of the native
soil as long as it has been composed of the same soil and is well compacted.

If ground-water is present, the density of the soil and the concrete should be reduced by the density of
3
water (10 kN//m ) assuming the most unfavourable ground-water level.

M.2.3 Calculation of R S
The calculation of the side resistance, RS depends on the type of foundation.

Case a: concrete stepped block footings with undercut

 - 231 - EN 50341-1:2012

Figure M.2  Concrete stepped block footings with undercut

Shearing surface in native soil

The side resistance is due to the shearing resistance of the native soil, which is governed by the
cohesion, c and the angle of shearing resistance, φ (also called angle of internal friction) of that soil.

Values of cohesions, c and angles of shearing resistance, φ are given by geotechnical investigations
according to EN 1997-2.

The geotechnical parameters consider in a multi-layer soil the weighted average of the cohesions and
the weighted average of the angles of shearing resistance occurring throughout the foundation depth.

In case a, the side resistance, RS (in kN) is given by the following formula:

RS = P D [ c + ½ K0 γ D tan φ ]

where

P is the perimeter of the lower surface of the foundation (m);

D is the foundation depth (m);

c is the cohesion of the soil (kPa);

K0 is the coefficient of earth pressure at rest, generally 0,5;

Another value of K0, found in the literature, may be specified in the NNAs or Project
Specification.
3
γ is the density of the soil (kN/m );

φ is the angle of the shearing resistance of the soil.

In case of layers with different properties along the foundation depth, the weighted average values of c
and γ tan φ are considered.

Case b: concrete stepped block footings without undercut

The shearing surface is assumed to be vertical along the backfill interface with the native soil (see
Figure M.3).
EN 50341-1:2012 - 232 -

Figure M.3  Concrete stepped block footings without undercut

Shearing surface along the backfill interface with the native soil

The side resistance is the sum of two components:

• the shearing resistance along the concrete slab interface with the native soil: Rslab;
• the shearing resistance along the backfill interface with the native soil: Rbackfill.
Values of cohesions, c and angles of shearing resistance, φ are given by geotechnical investigations
according to EN 1997-2.

For the calculation of Rslab, the geotechnical parameters consider in a multi-layer soil the weighted
average of the cohesions and the weighted average of the angles of shearing resistance along the
slab thickness, h.

Rslab is given by the formula:

Rslab = P h [ cs + ½ K0 γ ( 2 D – h ) tan φs ]

where

P is the perimeter of the lower surface of the foundation (m);

h is the thickness of the slab (m);

cs is the cohesion of the soil along the slab (kPa);

K0 is the coefficient of earth pressure at rest, generally 0,5;

Another value of K0, found in the literature, may be specified in the NNAs or Project
Specification.
3
γ is the density of the soil occurring throughout the foundation depth (kN/m );
D is the foundation depth (m);

φs is the angle of the shearing resistance of the soil along the slab.

In case of layers with different properties along the foundation depth, the weighted average values of
cs and γ tan φs are considered.
For the calculation of Rbackfill, the geotechnical parameters of the backfill considers in a multi-layer soil
the weighted average of the angles of shearing resistance along the foundation depth D.

Rbackfill is given by the formula:

Rbackfill = P ( D - h ) [ ½ Ka γ ( D – h ) tan φb ]

where

P is the perimeter of the lower surface of the foundation (m);

 - 233 - EN 50341-1:2012

D is the foundation depth (m);

h is the thickness of the slab (m);

Ka is the coefficient of active lateral earth pressure given by the formula: Ka = tan² ( π/4 –
φ/2 );
3
γ is the density of the soil occurring throughout the foundation depth (kN/m );
φb is the angle of the shearing resistance of the backfill.

In case of layers with different properties along the foundation depth, the weighted average value of
γ tan φb is considered.

The angle of shearing resistance of the backfill shall be considered as being equivalent to the angle of
shearing resistance of the native soil as long as it has been composed of the same soil and well
compacted.
NOTE In the backfill, the cohesion is conservatively assumed to be negligible.

In case b, the side resistance RS (in kN) is given by the following formula:

RS = Rslab + Rbackfill

or:

RS = P h [ cs + ½ K0 γ ( 2 D – h ) tan φs ] + P ( D - h ) [ ½ Ka γ ( D – h ) tan φb ]

In case of layers with different properties along the foundation depth, the weighted average values of
cs , γ tan φs and γ tan φb are considered.
M.2.4 Analytical evaluation of R d
As written in EN 1997-1:2004, section 6 (spread foundations), an analytical evaluation of the short-
term and long-term values of Rd shall be considered, particularly in fine-grained soils.

Long-term evaluation

The geotechnical parameters to be used in that case are the effective cohesion, c' and the effective
angle of shearing resistance, φ'.

Ice loading should be considered as generating long-term uplift forces on foundation

Design Approach 2 according to EN 1997-1 (partial factors applied to resistance)

Rd is evaluated with the formula:

Rd = ( RW + RS ) / γR

where

γR is the partial factor to be applied to foundation uplift resistance (see 8.2.2)

Design Approach 3 according to EN 1997-1 (partial factors applied to ground properties)

Rd is evaluated with the formula:

Rd = RW + RS / 1,25

NOTE 1 According to Table 8.1: γc' = γφ' = 1,25 and γγ = 1

EN 50341-1:2012 - 234 -

Short-term evaluation

The geotechnical parameters to be used in that case are the apparent cohesion, cu (undrained) and
the apparent angle of shearing resistance, φu.

Wind loading should be considered as generating short-term uplift forces on foundation

Design Approach 2 according to EN 1997-1 (partial factors applied to resistance)

Rd is evaluated with the formula:

Rd = ( RW + RS ) / γR

where

γR is the partial factor to be applied to foundation uplift resistance (see 8.2.2)

Design Approach 3 according to EN 1997-1 (partial factors applied to ground properties)

Rd is evaluated with the formula:

Rd = RW + RS / 1,4

NOTE 2 According to Table 8.1: γCu = 1,4 and γγ = 1

M.3 Sample semi-empirical models for resistance estimation

M.3.1 Geotechnical design by calculation
M.3.1.1 General
The foundation sub-face which transfers vertical loads to the subsoil shall be bedded at a frost-proof
depth, but at least 0,8 m below ground level.

The chataracteristic soil pressures given in Table M.4 (bearing capacity, Prd at 1,5 m) apply to a depth
of not more than 1,5 m and to a width of the foundation base of more than 1 m. If the depth of
embedment is more than 1,5 m at all sides of the foundation body, the design soil pressure may be
increased by a value which results from the surcharge of the soil associated with the additional depth
multiplied by the factor, κ (see Table M.4 column 6):

PRd = PRd at 1,5 m + γ κ (t – 1,5)

M.3.1.2 Monoblock foundations

Monoblock foundations can be designed with or without a step.

Assumptions for design

When designing monoblock foundations the loadings resulting from the tower, as well as the dead
load of the foundation and the vertical surcharge due to soil resting upon the foundation base shall be
taken into account. Additionally, the dead load of an earth frustum, the limiting faces of which start at
all sides at the lower edges of the foundation base and are inclined at an angle, ß outwards from the
vertical may be considered. The magnitude of the angle, ß depends above all on the angle of internal
friction as well as on the consistency of cohesive soils, on the compaction of soil and on the adhesion
and bond between foundation block and soil (for standard values see Table M.4, column 10).

When rating monoblock foundations the lateral resistance of soil may be taken into account according
to the compaction and characteristics of the soil. It is essential, therefore, that the soil will be neither
permanently nor temporarily removed as long as the external loads apply.

Stability conditions

The inclination of the foundation body under design load shall not exceed 1,5 %. If the resisting
moment due to lateral soil pressure exceeds the resisting moment due to the pressure in the
foundation sub-face, the theoretical proof of a stability of 1,0 will be sufficient. The decreasing
proportion of the lateral soil resistance on the total carrying capacity of the foundation necessitates a
 - 235 - EN 50341-1:2012

progressive increase in stability requirement which shall achieve 1,2 when the lateral soil resistance
falls to zero. The NNA may specify other conditions.

The soil pressure shall be verified. If no other values result from the soil investigations the design soil
pressures may be taken from Table M.4.
EN 50341-1:2012 - 236 -

Table M.4  Soil characteristics for design of foundations according to M.3

1 2 3 4 5 6 7 8 9 10
Specific weight Angle of earth frustum
Bearing
force Angle βo β
capacity
of Factor
pRd Foundation type acc. to
internal k
Type of soils naturally with at a depth Figure 8.5.2
friction
humid buoyancy ≤ 1,5 m Mono-
B A S
bloc
(Charact. values) (Design values)
kN/m3 kN/m3 Degree kN/m² - Degree
UNDISTURBED
SOIL
Non-cohesive soils

Sand, loose 17 9 30 270 4,7  18 to 21 16 to 18 5 to 10

Sand, semi loose 18 10 32,5 405 5,4 38 to 49 20 to 23 18 to 20 5 to 10
Sand, dense 19 11 35 540 6,7 41 to 53 22 to 25 20 to 22 8 to 10
Gravel, bolder, 17 9 35 540 6,7 41 to 53 22 to 25 20 to 22 8 to 12
uniform 18 10 35 540 6,7 41 to 53 22 to 25 20 to 22 8 to 12
Gravel-sand, graded
bolder, stones, 18 10 35 540 8,1 22 to 25 20 to 22 8 to 12
macadam, graded
Cohesive soils

very soft 16 8 0 0 1,3 0 0 0

soft (easy to kneed), 18 9 15 54 2,7 9 to 10 6 to 8 4

purely cohesive
soft, with non- 19 10 17,5 54 3,4 11 to 13 8 to 10 4
cohesive additions
firm (difficult to kneed), 18 9 17,5 135 3,4 21 to 27 11 to 15 8 to 11 6
purely cohesive
firm, with non- 19 10 22,5 135 4,0 26 to 34 13 to 17 10 to 13 6
cohesive additions
stiff, purely cohesive 18 10 22,5 270 4,0 26 to 34 15 to 23 11 to 19 8

stiff, with non-cohesive 19 11 25 270 4,7 29 to 38 17 to 26 13 to 21 8

additions
hard, purely cohesive 18 27,5 540 4,7 32 to 42 23 to 28 19 to 23 10
hard, with non-
cohesive additions 19 30 540 5,4 35 to 46 26 to 28 21 to 23 10
Organic soils
and soils with organic 5 to 16 0 to 7 15 1,6 0 0
additions
Rock independent
of depth
with considerable
fissuring or up to 1350
unfavourable 20
stratification

in sound, not-
decomposed up to 4050
condition with minor 25
fissuring or
favourable
stratification
MADE UP GROUND Depending on condition and thickness of foundation strata as well as compactness and uniformity of
AND FILL their stratification values defined above may be used.
Uncompacted
embankment 12 to 16 6 to 10 10 to 25 40 to 135 2,7 6 to 13 4 to 10
Compacted
Classification according to type of soil, density of stratification and consistency, respectively
embankment
 - 237 - EN 50341-1:2012

M.3.1.3 Slab foundations

Assumptions for design

If the body of a tower or pole is supported by a foundation block formed by a slab, whereby the lateral
restraint of the soil can be neglected, the tower loads as well as the dead load of the foundation block
and the vertical surcharge of the soil resting upon the foundation block shall be taken into account.

Reliability against Tilting

The proof of reliability against tilting is carried out by limiting the eccentricity of the resulting total
vertical load in the foundation sub-face.

The eccentricity of the resulting total vertical load may become so large that the foundation sub-face is
still loaded with pressure up to the centre of gravity.

The proof is considered as furnished if the eccentricity of the resulting total vertical load fulfills the
following conditions:

For rectangular sub-faces (see Figure M.4):

( ex / bx )2 + ( ey / by )2 ≤ 1/ 9

where

ex = Myd / Nd ; ey = Mxd / Nd

Figure M.4  Permissible area of a rectangular foundation sub-face

for the position ex , ey of the force N resulting from total vertical load

Reliability against bearing capacity failure of the soil

A sufficient reliability against bearing capacity failure may be regarded as proven, if the theoretical soil
pressure, p does not exceed the design bearing capacity.

p = Nd / A ≤ PRd
EN 50341-1:2012 - 238 -

If the soil investigations do not provide other values, the design bearing capacity, PRd may be taken
from Table M.4.

For the determination of the theoretical soil pressure only that part of the foundation sub-face shall be
taken into account, for which the resulting total vertical load acts in the centre of gravity.

In case of rectangular slabs with the width, bx and by and the dedicated eccentricities, ex and, ey the
effective area of soil pressure is (see Figure M.4):

A = ( bx – 2 |ex| ) ( by – 2 |ey| )

M.3.1.4 Grillage-type slab foundations

If a grillage-type foundation is designed such that all leg members are connected by one grillage made
of sleepers the stability may be proved according to the method described before. In this case, the
gross area of the grillage may be taken into account if the intermediate space between the sleepers
does not exceed 1/3 of the width of the sleepers.

The compaction of the backfill shall be carried out thoroughly.

Members of the tower embedded in earth and inclined by more than 15° from the vertical shall be
assumed as additionally loaded by the earth resting upon them. The additional load to be assumed
shall at least correspond to the load of a prismatic earth body of three times the member width and
with vertical faces vertically above the member.

M.3.1.5 Single-pile foundations

If the body of a pole is provided with a foundation body consisting of a single pier or pile, the tower
loads, the dead load of the foundation, as well as the lateral restraint of the pile according to the
compactness or consistency and to the characteristics of the soil shall be taken into account when
rating the foundation.

The loadings to be assumed are transferred to the subsoil essentially by lateral soil resistance. The
performance of the subsoil as well as the displacement of the pile in a horizontal direction shall be
considered.

The analysis of a single pile foundation may be carried out according to a qualified method.

M.3.1.6 Separate stepped block foundations, pad and chimney foundations

Assumptions for design

As far as the method of installation and the performance under loading are concerned the stepped
block foundations (Figure M.5) are classified as:

- Foundation type A: Lowermost step concreted to undisturbed subsoil;

- Foundation type S: Lowermost step concreted to shuttering.

If the base slab projects on all sides by at least 0,20 m then, in addition to the dead load of the
foundation block to act against the uplift force, the dead load of earth enclosed by the angle, ßd of
earth frustum according to Figure M.5 may be taken into account. The resistance calculated using the
angle, ßd represents a design value.

The angle, ßd may be calculated according to the formula:

ßd = ß0 (b/t )

where

ß0 is the angle of earth frustum for b/t=1 according to Table M.4, columns 8 and 9;

b is the width of the lower most step (Figure M.5);

t is the depth of earth frustum (Figure M.5).

In the case of foundations with circular sub-face the diameter of the base shall be inserted for the
width. In the case of a rectangular sub-face, the geometric mean
 - 239 - EN 50341-1:2012

b= ( b1 b2 )
shall be assumed as the theoretical width. This applies, when b1 / b2 ≤ 1,4 where b1 is the larger width.

The method explained above only applies to those stepped concrete foundations the ratio b/t of which
is more than 0,6.

If b/t exceeds the value 1, then ß = ß0 shall be assumed for calculation. The angle of earth frustum, ß
shall be limited to 35°.

As a rule, the above mentioned value, ß0 applies to foundation types A and S to widths of the
foundations between 1,5 m and 5,0 m. Within the ranges assigned to the individual types of soil, the
lower values of ß0 given in Table M.4 may be taken together with large foundation widths and the
upper values, ß0 with small foundation widths. Values in between may be linearly interpolated.
EN 50341-1:2012 - 240 -

Figure M.5  Assumptions for design of stepped concrete foundations, auger-bored and
excavated foundations as well as separate grillage foundations

Stability conditions in case of loading by compression

In case of stepped block foundations loaded by compression it shall be proven, that the soil pressures,
which can be assumed to be equally distributed over the foundation sub-face do not exceed the
 - 241 - EN 50341-1:2012

design soil pressures according to Table M.4. The dead load of the soil resting vertically upon the
foundation base shall be considered as a surcharge. The effect of a horizontal load on the soil
pressure may be neglected compared with the prevalent effect of the vertical load.

Stability conditions in case of loading by uplift

In case of stepped block foundations under a design uplift load, a partial factor, γR of 1,1 against being
pulled out shall be proven. The total margin of stability is a result of the partial factor and the design
value, ßd, which includes additional strength reserves.

Additional conditions

In addition to the stipulated stability it shall be proven that the following condition is met:

- For foundation type A: G / Z > 0,67;

- For foundation type S: G / Z > 0,80,

where

G is the dead load of the foundation block and of the soil resting vertically upon the
foundation base;

Z is the vertical component of the uplift force acting on the foundation.

The ultimate capacity of the foundations against uplift is essentially governed by the compactness and
the consistency of the surrounding subsoil. The beneficial results of an intensive artificial compaction
of the surrounding subsoil (compaction by vibration process or similar methods) may be taken into
account.

The virtual point of penetration of the leg member through the foundation sub-face may deviate from
the centre of the foundation sub-face at maximum by the amount, e specified in Figure M.5.

M.3.1.7 Auger-bored and excavated foundations

Assumptions for design

Auger-bored and excavated foundations (foundation type B according to Figure M.5) are column-type
foundations made of reinforced concrete with an expanded base. As a rule, they do not only carry the
loads and moments acting at the top of the foundation axially, but also transfer the loads resulting from
horizontal forces and bending moments by lateral bearing of the shaft onto the subsoil.

The angle of earth frustum, ßd may be evaluated using the formula:

ßd = ß0 ( b/t )

where

ß0 is the angle of earth frustum for b/t=1 according to Table M.4, column 7;

b is the width of foundation (see Figure M.5);

t is the depth of foundation (see Figure M.5).

The angle of earth frustum, ßd shall be limited to 35°.

The soil characteristics may be taken from Table M.4. As a rule, the values, ß0 given in Table M.4,
column 7, for foundation type B apply to foundation widths between 1,2 m and 2,1 m. Within the
ranges mentioned for the individual types of soil the lower values of ß0 apply to large foundation widths
and the upper values of ß0 to small foundation widths. Values in between may be interpolated linearly.

In case of auger-bored and excavated foundations the transfer of the horizontal forces to the subsoil
(lateral bearing), as well as the bending loading need to be proven by an accepted method.
EN 50341-1:2012 - 242 -

Stability conditions in case of loading by compression

In case of foundations loaded by compression it shall be proven that the soil pressures, which may be
assumed to be equally distributed within the foundation sub-face, do not exceed the design soil
pressures according to Table M.4. The dead load of the foundation body as well as the dead load of
the soil resting vertically upon the foundation sub-face may be neglected when calculating the soil
pressure.

Stability conditions in case of loading by uplift

In case of foundations loaded by design uplift load, a partial factor, γR of 1,1 against being pulled out
shall be proven. The analytical proof of stability may be carried out using the earth frustum method.
Thereby, additional to the load of the foundation body counteracting the uplift, the dead load of a soil
body formed by an angle of frustum, ß applied to the edge of the foundation sub-face may be taken
into consideration (see Figure M.5).

Additional conditions

The formula for the determination of the angle of earth frustum, ßd is validated for foundations with
dimensions complying with the following boundary conditions:

• Depth of foundation between 1,8 and 7,0 m

• Diameter of concrete shaft between 0,7 and 1,5 m
• Width of foundation between 1,2 and 2,1 m
• Projection of foundation sub-face ≥ 0,2 m
• Ratio of foundation width to foundation depth 0,25 ≤ b / t ≤ 0,7
With regard to construction, the ratio of the projection of foundation sub-face to the height of the
foundation base should be about 0,5 in case of cohesive soils, and about 0,33 in case of non-cohesive
soils.

M.3.1.8 Separate grillage foundations

The verification of stability of the separate grillage foundations may be carried out using the earth
frustum method according to M.3.1.6. The angle of earth frustum complies with that of stepped block
foundations, with the lowermost step concreted to shuttering (Type S, Figure M.5).

In case of loading by the design uplift load a partial factor, γR of 1,35 against being pulled out shall be
proven.

In case of loading by compression a proof according to that stipulated for stepped block foundations
shall be carried out (see M.3.1.6). The total area of the foundation sub-face may be taken into
account, if the spacing between the individual sleepers does not exceed 1/3 of the width of the
sleepers. The compaction of the backfill shall be carried out thoroughly.

For rating of tower members embedded in the subsoil see M.3.1.4.

M.3.1.9 Pile foundations

As a rule, pile foundations shall be designed such that the loadings resulting from the towers are
exclusively transferred to the subsoil by the piles. Pile foundations shall be designed in accordance
with the requirements of EN 1997-1:2004, Clause 7.

Significant horizontal components of loads may be counteracted by a bending resistant design of the
piles in addition to battered arrangement of piles (raked piles, pile groups).

Foundation piles should be loaded essentially in direction of their axes. The transfer of the loading
from the structure into the piles shall be proven. Floating-pile foundations should be avoided as far as
possible. They may be adopted if the resilient layers at increasing depths are progressively more solid
i. e. less compressible, so that lesser settlements would occur than in the case of a wide shallow
foundation.

Within a separated pile foundation, for the same static function (for example, transfer of uplift or
compression forces) piles shall be used which by their method of installation, their arrangement and
materials provide approximately the same performance in respect of deformation and settlement.
 - 243 - EN 50341-1:2012

If, over an extended area, a wide-spread loading (for example due to a fill) acts upon a soft layer of
soil above good bearing subsoil in the vicinity of a pile foundation, horizontal movements of the soft
soil can occur. The piles will then be additionally loaded by bending.

The external pile loads result from the loads acting on the towers. When rating the piles the effects of
buoyancy and other effects which reduce the stability, shall be considered. In case of foundations
loaded by compression, the releasing effect of buoyancy may not be taken into account.

The piles shall be installed with a minimum length of 6 m and shall be embedded as required to fulfil
the requirements of EN 1997-1:2004, 7.6.2

Parallel, as well as raked piles, shall be provided with sufficient spacing between their axes such that
neither during installation nor after loading, adverse reactions can occur on adjacent piles. This
requirement is met if the distance of the pile axes at the pile point in the soil reaches at least three
times the maximum cross-sectional dimension of the pile.

The strength capacity of a pile depends on the structure of the subsoil and its properties, on the
ground-water conditions, on the depth of penetration into bearing soil layers and on their thickness, on
the shape of the pile and its cross-sectional area, on the material of the pile, on the nature of the
circumferential surface and on design of the pile point, on the arrangement of the pile and on the
distance of piles as well as on the installation procedure. Additionally, the thickness and the strength
of overburden soil layers are significant. Moreover, the effects of ageing, of negative skin friction and
of superimposed lateral loading shall be considered.

Where the skin friction provides an essential portion of total capacity the strength capacity of driven
piles may even increase over longer periods after driving especially in fine-sandy, silty and clayey
soils.

A compression pile may be loaded additionally by negative skin friction if the upper layers of soil settle.
The effect of negative skin friction on the structure can be reduced by a suitable design of piles and by
choice of larger spacing between piles. In case of uplift loaded piles, the releasing effect may not be
considered.

The strength capacity of pile groups may be determined by summation of the strength capacities of
the individual piles.

The theoretical determination of the ultimate tensile load of piles may be carried out by means of skin
friction. The values of skin friction shall be deduced for the given soil conditions and the selected type
of pile based on experience with the particular type of soil. As an approximation, in case of layers of
soil with varying skin friction, the friction forces may be determined separately for each individual layer
and the ultimate tensile load may be calculated by summation of the individual values considering the
thickness of the layers and sequence of layers as well as the ground-water table.

Since for piles a wide scatter of the values of skin friction has to be expected, the theoretical proof of
stability of a pile under design uplift loading shall be carried out for a partial factor, γR of 1,5. When
carrying out the proof by loading tests according to 8.2.4 a partial factor, γR of 1,1 will be sufficient.

When rating compression-loaded piles, at least those values of skin friction adopted for uplift-loaded
piles and the resistance of the pile point may be taken into consideration. A partial factor, γR of 1,1
should apply.

The buckling stability of free-standing piles shall be analysed considering the buckling length and the
restraining conditions. Piles embedded in soil are not normally prone to buckling even in very soft
layers of soil. However, buckling may need to be considered for slender piles installed in layers of very
soft soil depending on the characteristic value of undrained shear resistance of the surrounding soil. If
not otherwise specified in the NNAs, the shear resistance specified in EN 1997-1:2004 (7.8 (5)) may
be used as a guide.

M.3.2 Structural design of concrete foundations

Rating

The rating and the calculation of forces and bending moments and the installation of foundation blocks
shall be carried out according to EN 1992-1-1 if not stipulated otherwise in the following clauses. The
concrete used for foundations shall have a compressive strength of at least C20/25.
EN 50341-1:2012 - 244 -

In case of stepped foundations made of non-reinforced concrete, the ratio n of the height of steps to
the width of the projection shall be at minimum of 1,0. Overhang with ratio n < 1,4 shall be reinforced
and checked.

The specifications for materials used in the construction of the foundation, e.g. concrete and its
constituent materials, structural and reinforcing steel, shall be in accordance with EN 1992-1-1,
EN 1993-1-1 and/or NNAs. For steel and anchor bolts, the recommendations given in 7.2 should be
considered.

Interface between support and foundation

Details of the proposed method of interface between the support and the foundation shall be as stated
in NNAs and/or in the Project Specification.

Due consideration should be given to the design of the interface where fatigue has an influence.

Embedment of steel members into the concrete by means of anchoring elements

If the total tensile or compression load of steel members anchored in concrete is transferred to the
concrete by anchor cleats, anchor plates, lugs or the like, then it shall be proven that the compression
stresses between the anchoring elements and the concrete do not exceed the values given in Table
M.5, and the shearing stress in the contour surface of the anchoring elements does not exceed the
values in Table M.5. If these values are exceeded, the resistance against splitting tensile forces shall
be proved.

The minimum envelope of the anchoring elements shall be taken as the contour surface.

The embedment of steel elements without anchoring elements is not permitted.

Steel elements in concrete shall be designed in accordance with EN 1993-1-1. The bending stress of
welding seams of anchor cleats and anchor plates need not be verified.

Table M.5  Design values for shearing and compressive stress in case of anchoring of steel
members in concrete

Strength quality class of Shearing stress Compression stress

2 2
concrete MN/m MN/m
C 20/25 2,3 14,0
C 25/30 2,7 17,5
C 30/37 3,0 21,0
 - 245 - EN 50341-1:2012

Annex N
(informative)

Conductors and overhead earth wires

N.1 Specification of conductors and earth wires
N.1.1 Factors influencing the specification of conductors and earth wires
Conductors and earth wires for use in the construction of an overhead line are designed to meet the
relevant mechanical and electrical characteristics as determined by the design parameters for the line.
Additional factors relating to operation, maintenance and the environmental impact may need to be
considered when specifying the requirements for conductors and earth wires for use in the
construction of the line.

N.1.2 Operational factors

Typical factors involved are:

• target system reliability and line restoration time for different categories of forced outage;
• current carrying capabilities (continuous and short-term);
• constraints on electrical losses (I²R and corona);
• internal and external clearances;
• constraints on line electrical characteristics (series reactances, shunt susceptances etc);
• required lifetime.
N.1.3 Maintenance requirements
An important requirement involved is:

• access along conductors to in-span fittings (e.g. spacers and visibility markers).
N.1.4 Environmental parameters
Typical parameters involved are:

• wind and/or ice loadings affecting strength selection, conductor sag, vibration and galloping
performance;
• pollution - affecting corrosion protection;
• lightning - affecting earth wire and conductor specification;
• radio (and other) interference constraints;
• audible noise constraints;
• visibility marking for birds and aircraft
• visual amenity (e.g. surface finish of conductors);
• electric and magnetic fields;
• conductor grease (e.g. drop point and chemical content);
• maximum and minimum ambient temperatures.

N.2 Selection of conductors and earth wires

In addition to the specified characteristics on the basis of the overhead line design parameters, and
the factors detailed in N.1, consideration should also be given to the choice of conductors for particular
applications.

This consideration may include:

• conductor type - round wire, segmental, stranded or other constructions;

• bundle type - single conductor, twin, triple, quad etc.;
• conductor material of which examples are:
EN 50341-1:2012 - 246 -

• all aluminium conductor (AL1);

• aluminium conductor aluminium alloy reinforced (AL1/ALx);
• aluminium conductor steel reinforced (AL1/STyz);
• aluminium conductor aluminium clad steel reinforced (AL1/SAyz);
• aluminium alloy conductor steel reinforced (ALx/STyz);
• aluminium alloy conductor aluminium clad steel reinforced (ALx/SAyz);
• all aluminium alloy conductor (ALx);
• aluminium clad steel conductor (20SA);
• copper/copper alloy;
• steel;
• conductor and bundle dimensions;
• current carrying capabilities;
• grease type and content;
• surface finish (including painting);
• conductivity;
• stress/strain behaviour;
• tensile strength (including reduction with temperature and time);
• creep;
• optical fibre requirements (including protection);
• corrosion protection;
• vibration characteristics (self damping, vertical and rotational stiffness, mass/length etc.);
• maximum operating temperature (continuous, short-duration, and short-circuit);
• permitted overhead line support loadings.

N.3 Packing and delivery of conductors and earth wires

Conductors shall be packed and delivered to site on suitable drums containing lengths previously
agreed between the purchaser and supplier; the durability treatment for wood drums shall be specified
in the Project Specification. The drums shall give adequate protection to the conductors. Suitable
arrangements shall be made for the return or disposal of empty drums.

N.4 Precautions during installation of conductors and earth wires

At all times during installation conductors shall be handled with care to minimise surface damage. In
particular precautions shall be taken to avoid abrasive contact with the ground or other surfaces.
 - 247 - EN 50341-1:2012

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EN 50341-1:2012 - 248 -

Annex P
(informative)

Tests on insulators and insulators sets

Table P.1  Reference list of tests on overhead line insulators and insulators sets in porcelain
and glass insulating materials (1 of 2)

String insulator units

Insulator Line post Pin type Staywire
sets insulators insulators insulators
Long rod Cap and pin
(Type A) (Type B)

Standard type tests:

Verification of dimensions X X X X X X
a a f
Wet power frequency X X X X X X
withstand voltage test
a a
Dry lighting impulse X X X X X -
withstand voltage test
g
Wet switching impulse - - X - - -
Withstand voltage test

Thermal mechanical X X - - - -
performance test

Mechanical or electro-
mechanical failing load test X X - X X X

Optional type test:

RIV test - X X X X -
b
Pollution performance test - - X X X -

Power arc test - - X X X -

e
Impulse voltage puncture - X - X X -
test
c
Zinc sleeve test - X - - - -

Residual strength test - X - - - -

Sample tests:

Verification of dimensions X X - X X X

Verification of locking system X X - - - -

and displacements

Temperature cycle test X X - X X X

(porcelain and annealed
glass insulators only)
 - 249 - EN 50341-1:2012

Table P.1  Reference list of tests on overhead line insulators and insulators sets in porcelain
and glass insulating materials (2 of 2)

String insulator units

Insulator Line post Pin type Staywire
sets insulators insulators insulators
Long rod Cap and pin
(Type A) (Type B)

Sample test (continued):

Mechanical or electro-
mechanical failing load test X X - X X X

Thermal shock test - X - X X -

(Toughened glass insulators
only)
e
Puncture voltage withstand - X - X X -
test

Porosity test (porcelain X X - X X X

insulators only)

Galvanising test X X - X - -

Optional sample tests:

Impulse voltage puncture - X - - X -

test

c
Zinc sleeve test - X - - - -

Routine tests:

Visual inspection X X - X X X

Mechanical test X X - X - -
(h >300 mm)

Electrical test - X
d
- - X
d
-

Optional routine test:

Ultrasonic examination X - - - - -

a
Test carried out on one short standard string or one long rod insulator.
b
Pollution performance tests are generally carried out on insulator strings without fittings.
c
Test not normally required for systems with nominal system voltages < AC 45 kV.
d
Applicable to insulators in ceramic material (see EN 60383-1).
e
Applicable to line post insulators which are not puncture proof.
f
Test on insulator sets for systems with Us ≤ 245 kV.
g
Test on insulator sets for systems with Us > 245 kV.
EN 50341-1:2012 - 250 -

Annex Q
(informative)

Insulators
Q.1 Specification of insulators
Q.1.1 Factors influencing the specification of insulators
Insulators and insulator sets for use in the construction of an overhead line are designed to meet the
relevant electrical and mechanical characteristics as determined by the design parameters for the line.
Additional factors relating to operation, maintenance and the environmental impact may need to be
considered when specifying the requirements for insulators and insulator sets for use in the
construction of the line.

Q.1.2 Operational factors

Typical factors involved are:

• target system reliability and line restoration time for different categories of forced outage;
• required lifetime for each component;
• nominal system voltage;
• temporary over-voltages;
• insulation coordination and line switching policy;
• electrical clearances.
Q.1.3 Maintenance requirements
Typical requirements involved are:

• working practices - live line (hot line) or dead line;

• access to conductors via insulators (not applicable to lines with nominal system voltage
exceeding AC 1 kV but not exceeding AC 45 kV);
• performance of damaged insulators i.e. residual strength;
• provisions for attachment of maintenance equipment on both suspension and tension sets (not
applicable to lines with nominal system voltage exceeding AC 1 kV but not exceeding AC 45
kV).
Q.1.4 Environmental parameters
Typical parameters involved are:

• altitude and its effect on insulator performance;

• pollution level and type of pollution;
• constraints on audible noise level or radio interference voltage (not applicable to lines with
nominal system voltage exceeding AC 1 kV but not exceeding AC 45 kV);
• lightning (lightning flash density [keraunic] level) and the extent of system protection against
its effects;
• maximum and minimum ambient temperatures;
• visual amenity e.g. colour of insulators;
• vandalism.

Q.2 Selection of insulators

In addition to the electrical and mechanical characteristics specified on the basis of the overhead line
design parameters, and the factors detailed in Q.1, consideration should also be given to the choice of
insulators for particular applications. This consideration may include:

• insulators of ceramic material or glass e.g. string insulator units of the cap and pin type, long
rod type, line post insulators, pin type, and stay wire insulators;
 - 251 - EN 50341-1:2012

• composite insulators;
• dimensions, including length of strings or sets, spacing of individual units, diameter, creepage
distance, shed profile and coupling or fixing arrangement;
• withstand voltages;
• corrosion protection e.g. galvanising of metal parts, zinc sleeves on cap and pin units
(normally for lines with nominal system voltage exceeding AC 45 kV), greasing of connections;
• weight of insulator units, strings and sets.

Q.3 Packing and delivery of insulators

Insulators shall be packed in a manner suitable for safe delivery to site. The size and weight of
individual packages shall be consistent with convenient handling on site and during line construction
e.g. to meet the purchaser's requirements.

The size and weight of bulk packages shall be consistent with the requirements of the means of
delivery and the limitations of mechanical handling.

The design of crates shall give suitable protection and support to the insulator unit(s) and shall as far
as possible prevent impact damage or shed damage under conditions normally encountered during
transportation and handling on site.

The packaging shall comply with any requirements regarding disposal of packing materials.

Q.4 Precautions during installation of insulators

During installation, insulators shall be handled with sufficient care to avoid damage. In some cases
the use of mechanical lifting equipment may be appropriate. Whether insulators are manually or
mechanically lifted into position, due regard shall be given to safety considerations for the personnel
concerned.

When lifting longer insulator strings or sets, it is recommended that a cradle or other device is used to
minimise bending loads and eliminate any risk of distortion of the couplings of string insulator units or
damage to composite insulators.

Insulators with semi-rigid couplings (e.g. clevis, tongue or eye) may suffer damage if submitted to high
torsional loads. A suitable system for relieving stresses may therefore be necessary during conductor
stringing operations.
EN 50341-1:2012 - 252 -

Annex R
(informative)

Hardware
R.1 Specification and selection of fittings
R.1.1 Factors influencing specification and selection
Fittings for use in the construction of an overhead line are designed to meet the relevant mechanical
and electrical characteristics as determined by the design parameters for the line. Additional factors
relating to operation, maintenance and the environmental impact may need to be considered when
specifying the requirements for fittings and in selecting particular designs for use in the construction of
the line.

R.1.2 Operational factors

Typical factors involved are:

• target system reliability, security and safety and line restoration time for different categories of
forced outage;
• required life time for each component;
• operating voltage range;
• current carrying capabilities;
• short circuit performance;
• constraints on electrical losses;
• stress limiting by suitable clamp design.
R.1.3 Maintenance requirements
Typical requirements involved are:

• working practices - live line (hot line) or dead line;

• access to conductors via insulators and fittings;
• provisions for attachment of maintenance equipment on both suspension and tension sets;
• access along conductors to in span fittings (e.g. spacers and visibility markers).
R.1.4 Environmental parameters
Typical parameters involved are:

• wind characteristics for vibration performance;

• constraints on audible noise level or radio interference voltage;
• vandalism;
• visibility marking for birds and aircraft;
• ambient temperature range and maximum and minimum temperatures;
• atmospheric pollution influencing corrosion protection;
• wind/ice loadings affecting strength selection.

R.2 Packing and delivery of fittings

Fittings shall be packed in a manner suitable for safe delivery to the site. The size and weight of the
individual packages shall be consistent with convenient handling on site.

The size and weight of the bulk packages shall be consistent with the requirements of the means of
delivery and the limitations of mechanical handling.

The packaging shall comply with any requirements regarding disposal of packing materials.
 - 253 - EN 50341-1:2012

R.3 Precautions during installation of fittings

During installation, fittings shall be handled with sufficient care to avoid damage. In some cases, the
use of mechanical lifting equipment may be appropriate. Whether fittings are manually or mechanically
lifted into position, due regard shall be given to safety considerations for the personnel concerned.

_____________