NCCI: Design of simple column bases with shear nibs

SN021a-EN-EU
NCCI: Design of simple column bases with shear nibs
This NCCI provides the rules for the design of shear nibs for column bases. The rules given
are complementary to those given in NCCIs SN037 and SN043 for the design of simple and
fixed base plate joints respectively.

Contents
1. Introduction 2
2. Types of shear nib 3
3. Parameters 5
4. Design model 6
5. Design situation 1: Dimension a base plate with a shear nib to resist the shear force 8
6. Design situation 2: Determine the shear resistance of a column base joint with a
shear nib 12
7. References 13

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NCCI: Design of simple column bases with shear nibs
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NCCI: Design of simple column bases with shear nibs
SN021a-EN-EU
1. Introduction
The types of column bases concerned by the present NCCI are the simple column bases
described in SN037, and the fixed column bases described in SN043.
The shear resistance developed by friction between the column base plate in compression and
the joint material (grout), as calculated in SN037, is often adequate for most typical simple
base plate joints and fixed base plate joints.
For simple base plate joints, if there is axial tension acting shear resistance by friction cannot
be developed. For fixed base plates, shear resistance by friction alone may not suffice when
high shear is combined with a low moment and either low axial compression or axial tension.
In the latter situations other means are required to transfer the shear force.
Means other than friction for transferring shear force to the foundation are as follows:
Shear / bearing of the anchor bolts (see 6.2.2(7) of EN 1993-1-8).
Setting the column end with its base plate within a pocket in the foundation pad. The
pocket depth is usually 300 mm or more and is filled with non-shrink concrete once the
column is in place. This type is suitable for fixed column base plate joints. The shear
force is transferred by lateral bearing of the embedded column part on the pocket infill
concrete. The concrete surround of the pocket may require reinforcement in accordance
with EN 1992-1 to transfer the column end forces and moments.
Setting the column end with its base plate in a shallow pocket, usually not more than 100
mm. The behaviour of the joint can be assimilated to that of a shear nib mentioned below.
The shallow pocket is not usually recommended for simple base plate joints because the
column end rotations are likely to produce local damage to the concrete above and around
the base plate.
Providing a tie from the column end into an adjacent ground floor slab. This may require
ensuring that there is appropriate reinforcement in the slab to anchor the horizontal tie
force.
Providing a shear nib (key) welded to the underside of the base plate which is
accommodated in a foundation pocket of sufficient depth and size. The pocket is filled
with non-shrink concrete after the column and the anchor bolts are positioned.
It is not common practice to use anchor bolts in shear. To do so, one must take precautions to
ensure that the shear force transfer to the foundation through the anchor bolts is possible
without causing excessive lateral movement at the column base (see 6.2.2(5) of
EN 1993-1-8). If anchor bolts are grouted in sleeves they may not be dependable in
shear/bearing. Oversized holes are often used in base plates in order to account for the usual
tolerances in the positioning of anchor bolts set in concrete. In the latter case the plate-
washers used under the anchor bolt nuts would need to be welded to the base plates so as to
allow transferring the shear force to the anchor bolts. It is recommended that hole sizes in
these plate washers may be reduced to a minimum, for instance d +1,5 mm (where d is the
nominal anchor bolt diameter). With these precautions, the design resistance of anchor bolts
in shear/bearing, which is given in 6.2.2(7) of EN 1993-1-8, can be added to the friction
resistance when relevant.
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NCCI: Design of simple column bases with shear nibs
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NCCI: Design of simple column bases with shear nibs
SN021a-EN-EU
Neither the design of foundation pockets (but see remark below for the shallow pocket
type) for fixed base plate joints nor that of ties to the floor slab is considered in this NCCI.
The subject of the present NCCI is the design of a shear nib under the base plate for
transferring shear forces to the foundation.
A shear nib (or shear key) typically consists of a short length of steel section welded to the
underside of the base plate. Once the concrete is poured into the reserve hole for the anchor
bolts and the column grouted in its final position, the nib is embedded in the foundation. The
shear force acting on the column base can be transmitted to the foundation by the nib acting
horizontally leading to compression over the vertical surface of the nib against the concrete
foundation.
In practice, the following two design situations are encountered:
1. The column section and the design forces are known. The dimensions of the required
base plate and shear nib are to be determined.
2. The column section, base plate, shear nib and foundation dimensions are known. The
design compressive resistance of the column base is required to be determined, including
that of the shear nib.
The usual procedure is to begin with the design of the base plate using the design procedures
given in sections 4 or 5 of SN037or SN043 as relevant. The design of the nib is then
undertaken using procedures given in Sections 5 and 6 respectively of the present NCCI.
2. Types of shear nib
Figure 2.1 shows two types of shear nib in common use, one being a short length of angle
capable of resisting relatively modest shear forces and the other a short length of I section
used if the shear forces to be transmitted are relatively high.
Note: Figure 2.1 shows typical simple base plates details with nibs. For fixed base plates (see
figure 1.1 of SN043) the anchor bolt rows are not on the column major axis as shown here,
but usually beyond the column flanges on projected parts of the base plates.

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NCCI: Design of simple column bases with shear nibs
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NCCI: Design of simple column bases with shear nibs
SN021a-EN-EU

8
4
d
n
7
1
2
5
9
10
3
3
4
d
n
6
1
2
5
9


Key :
1. I section column
2. Base plate
3. J oint space to be filled with grout
4. Anchor bolt
5. Concrete foundation
6. Angle section shear nib
7. I section shear nib
8. Steel positioning/levelling plate
9. Pocket reservation to be filled with non shrink concrete
or grout after column positioning
10. Foundation reinforcing bar

Figure 2.1 Typical column bases with shear nibs
Other types of shear nibs than those shown in Figure 2.1 are :
a vertical plate welded to the base plate, which plays the role described below for the
vertical leg of the angle nib.
A horizontal plate of sufficient dimensions (thickness embedded in the concrete, welded
perimeter to the base plate) to develop the necessary resistances of the concrete in bearing
and of the welds.
While the design rules given below specifically cover the nib types shown in Figure 2.1, they
may easily be adapted to the design of the latter types as well as to the shallow pocket type
mentioned above in Section 1.
Ideally, shear nibs are welded to the base plate in a central position relative to the column
axes. In the case of an angle nib on a simple base plate, while the angle length (nib width) can
be is centred about the column minor axis, the angle leg protruding down into the foundation
must be slightly off the major column axis in order to avoid the anchor bolts. If the angle
length is greater than that of the anchor bolt spacing, the horizontal leg of the angle section
requires holes to allow the anchor bolts on the minor axis to pass through. For a nib of an
unequal angle it is usual to weld the smaller angle leg to the base plate.
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NCCI: Design of simple column bases with shear nibs
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NCCI: Design of simple column bases with shear nibs
SN021a-EN-EU
3. Parameters
The following table provides the parameters referred to in this NCCI:
Table 3.1 Parameters (includes those for SN037)
Parameter Definition
Ratio of the base plate width or depth
of the design distribution area within
the foundation to the width or height
of the base plate.

cc
Coefficient taking account of long
term effects and unfavourable effects
due to the manner of loading on the
compressive strength of concrete
(see EN 1992-1-1)

j
Foundation joint material coefficient.

c
Partial factor on the concrete
compressive strength (see EN 1992-
1-1).

M0
Partial factor on the bending
resistance of the base plate.
b
a
Angle nib leg plan height (leg length
welded to the base plate).
b
p
Width of the base plate.
b
f
Width of the foundation
(corresponding to the column width).
b
fc
Width of the column section (width of
the I section column flange).
b
eff
Effective width of a base plate T-stub
in compression.
b
n
Plan width of a shear nib.
c Additional bearing width (outside the
column section perimeter).
d
f
Depth of the foundation.
f
yb
Yield strength of the anchor bolt.
f
yp
Yield strength of the base plate.
f
jd
Design bearing strength of the
foundation joint.
f
cd
Design compressive strength of the
concrete according to EN 1992-1-1.
f
un
Tensile strength of the nib steel.
Parameter Definition
h
f
Angle nib leg length embedded in
the foundation
h
c
Depth (height) of the column
section.
h
n
Plan height of an I section shear
nib.
h
p
Depth of the base plate.
t
fc
Column flange thickness.
l
eff
Effective length of a base plate T-
stub in compression.
d
eff,n
Effective depth of a shear nib.
d
n
Total depth of a shear nib.
t
wc
Column web thickness.
t
an
Leg thickness of an angle shear
nib.
t
fn
Flange thickness of an I section
shear nib
t
p
Base plate thickness.
A
c0
Compression area under the base
plate of dimensions b
p
and h
p
.
F
f,Rd
Design friction shear resistance.
F
v,Rd
Design shear resistance of the
column base plate joint.
N
sec,Ed
Secondary axial force in the nib
foundation.
N
j,Rd
Design compressive resistance of
the column base.
Design shear force at the column
base.
V
Ed


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NCCI: Design of simple column bases with shear nibs
SN021a-EN-EU
4. Design model
The mechanical model adopted for the nib is shown schematically in Figure 4.1. The column
base shear force is resisted by pressure developed over the vertical face (or faces) of the nib
embedded in sound foundation concrete. The eccentricity between the horizontal reaction on
the nib and the applied column base shear causes a secondary moment creating a couple of
additional vertical forces (N
sec,Ed
) at the base plate joint, a compressive force and a tensile
force. The tensile force may be resisted either by the anchor bolts or by the nib itself. In the
present NCCI, it is conservatively assumed that the tensile force is resisted by the nib. The
additional compression force between the base plate and the joint material (grout) is often
neglected in design, although it could be added to that in the column flange compressive
T-stub when doing the final check on the design of the base plate joint.

d
eff,n

max

f
c,d
V
E,d
N
secE,d
N
secE,d
V
E,d
4
1
2
5
3
h
c
/2

max

f
c,d
6


Key :
4. Nib 1. I section column
2. Base plate 5. Concrete foundation
6 Triangular distribution of pressure on the nib 3. J oint material ( grout )

Figure 4.1 Shear nib model showing the forces and stresses induced: distribution of compressive
stresses over shear nib and secondary forces
The following simplifying assumptions are made in the design model [1]:
Both embedded flanges of an I section nib provide equal horizontal resistance to the
applied column base shear force.
For the full width of an angle leg or flange within the concrete foundation, there is a
triangular distribution of compressive stresses over the effective depth of the nib (see
Figures 4.1 and 4.2).
The effective nib depth, d
eff,n
, is taken as equal to the full height of the nib , d
n
, below the
base plate minus a thickness at the top surface to allow for the possible inadequacy of the
packing of the joint material (grout) beneath the base plate. It is usual to assume that the
latter thickness is equal to that of the grout layer, which is typically 30 mm and rarely
over 50 mm thick. In the following it is taken as 30 mm thick.
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NCCI: Design of simple column bases with shear nibs
SN021a-EN-EU
The secondary moment is considered to be resisted by a couple of forces acting on the
column base, one a normal tension force in the base plate over the shear nib and one a
compressive force between the base plate and the grout which is centred under one of the
column flanges. Assuming the shear nib to be centred at the column centroid and a grout
layer thickness of 30 mm, one obtains the following axial tension design forces:
o I section nib : Axial tension in a nib flange:

)
1 1
( 30
3 2
1
)
2
( 30
3
)
1
( 30
3
c fn n
n eff,
Ed
c
n eff,
Ed
fn n
n eff,
Ed Ed
h t h
d
V
h
d
V
t h
d
V N +

+ =

+ +

+ =
o Angle nib: Axial tension in the vertical leg:
c
n eff,
Ed Ed
2
30
3 h
d
V N

+ =
In order to ensure against pull-out of the nib from the concrete foundation and to have an
efficient shear nib, the following limits are placed on the nib dimensions:
o Height of an I section nib section: h
n
0,4 h
c

o Effective depth in the foundation of an I section nib: 60 mm d
eff,n
1,5 h
n

o Effective depth in the foundation of an angle nib: 60 mm d
eff,n
1,5 b
n

In the case if a simple base plate, the respect of the latter limits on the nib dimensions
is recommended so as to avoid creating a fixed column base condition.
Being embedded in the concrete, angle legs or I section flanges are considered to be
subjected to negligible local bending. To support this assumption, the following
maximum slenderness criteria are imposed:
o I section nib: Maximum flange slenderness: ( b
fn
/ t
fn
) 20
(a criterion which all IPE and HE sections meet except HEA 260, 280 and 300)
o Angle nib: Maximum leg slenderness: ( d
,n
/ t
an
) 10
(not all standard hot rolled angle sections meet the latter requirement)
For an I section shear nib, the shear force is transferred from the base through the web.
The moment at the underside of the base plate level is resisted by a force couple in the
flanges. Rather than assume the anchor bolts to be active, the secondary normal tensile
force is considered to be shared by the flange sections. The flange in tension the most
loaded. The column web opposite the flange also resists the total force thus obtained.
For the leg of an angle section shear nib, both the shear force and the secondary normal
force are taken by the vertical leg section. Bending at the top of the vertical angle leg is
neglected.
The basic design approach is to ensure that the compressive stresses over the vertical surface
of the nib in contact with the foundation neither exceed the design compressive strength of the
concrete nor lead to excessive stresses in the nib member (leg, flange or web).
The supplementary design checks required are as follows:
The column web is checked for the concentrated force corresponding to the secondary
tensile force in a nib angle leg or nib flange,
The base plate to nib fillet welds resistances are checked for both the horizontal shear and
for the secondary tensile forces.
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NCCI: Design of simple column bases with shear nibs
SN021a-EN-EU

d
eff,n

max

f
c,d
V
E,d
N
secE,d
N
secE,d
V
E,d
t
a
b
n
N
secE,d
d
eff,n

max
f
c,d
h
n
N
secE,d
V
E,d
M
secE,d
/ (h
n
- t
fn
)
h
c
/

2
t
fn
b
n
h
n
V
E,d


Figure 4.2 Dimensions of shear nibs, distribution of compressive stresses and secondary forces
5. Design situation 1: Dimension a base plate with
a shear nib to resist the shear force
If the column forces are given, the following procedure can be followed to dimension the base
plate and the shear nib. It is conservatively assumed that the shear nib provides all of the shear
resistance required, i.e. both the friction resistance when the column is in compression is
ignored as well as the resistance of the anchor bolts to shear.
While it is usual to have a shear nib of the same steel (f
yn
) as that of the base plate, they may
be of different steel grades.
The rules given cover the case of a column base shear force acting in the plane of the column
web i.e. a shear force parallel to the column section minor axis. The design method can be
adapted for cases of when the shear force is parallel to the principal column axis or for when
there are components of shear force along both axes.
Step 1: Dimension the base plate by referring to SN037 or to SN043
The values of the base plate dimensions (h
p
, b
p
, t
p
) are established for the column section (h
c
,
b
c
, t
wc
, t
fc
) load and the concrete (f
cd
) to be used in the foundation is identified.
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NCCI: Design of simple column bases with shear nibs
SN021a-EN-EU
Step 2: Dimension the shear nib if required
Note: It is not usual to have to make a choice between the two types of shear nib.
Assume the joint material (grout) layer to be 30 mm thick
Adopt a practical shear nib width, b
n
, within the following limits, min b
n
b
n
. max b
n
:
Angle nib : mm )
30
: 90 max( min
cd
Ed
n
f
V
b and max b
n
b
p
2t
fc
I section nib : mm )
15
: 90 max( min
cd
Ed
n
f
V
b and max b
n
b
p
2t
fc

Angle shear nib:
The suitable and available angle sections are identified (h
a
, b
a
, t
a
). Noting that it is usual to use
unequal angles, equal angle legs can be used also. The suitability of a given angle section
requires that:
t
a
h
a
/10
where h
a
is the length of the longer leg, the leg to be embedded in the concrete foundation.
a) Estimate the minimum required depth of angle nib:
mm )
2
: 60 max( min
cd n
Ed
n eff,
f b
V
d
b) Check the maximum practical limits on the nib depth:
) : 8 , 0 min( mm 30 min
a f n efff,
h d d + .
If the latter condition is not met, restart using a greater nib width b
n
(length of angle
section).
c) Choose an angle size such that:
mm 30 ) (min
n eff, a
+ d h ; 8 , 0
f a
d h ; 6 , 0
c a
h h 6 , 0
c a
h b and t
a
h
a
/10
Take mm 30
a n eff,
= h d
Estimate the secondary tensile force in the vertical angle leg:
c
n eff,
Ed Ed sec
2
30
3 h
d
V N

+ =
Check the leg thickness under combined shear and tension using the Von Mises
criteria:
3
) 30 3 / ( 2
3
2
c
n eff,
n yn
Ed
2
yn n
Ed
2
yn n
sec
+

+
=

h
d
b f
V
f b
V
f b
N
t
Ed
a

Page 9
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NCCI: Design of simple column bases with shear nibs
SN021a-EN-EU
If it is not possible to complete the checks by modifications of the shear nib width
and/or depth, change to an I section shear nib.
I section shear nib:
Take the following steps in order.
a) Choose an I section: the nib width, b
n
=b
f, nib
,within the maximum and minimum
limits given above.
b) Check that the nib section height h
nib
0,4 h
c,
.
If satisfied, the nib height becomes h
n
=h
nib
.
If the condition is not met restart the procedure with a shallower I section for the nib.
c) Check the flange slenderness of the nib section:
(h
n
/t
f,
)
nib
20
d) Make an estimate of the required minimum nib depth:
mm ) : 60 max( min
cd n
Ed
n eff,
f b
V
d

e) Check the maximum recommended limits on the effective nib depth (in the
concrete):
) 5 , 1 : 8 , 0 min( mm 30 min
n f n eff,
h d d + .
If the latter conditions cannot be met, restart using a different I section of greater
width (b
f
, h
c
)
nib section
.
f) Confirm the suitability of the section choice: h
n
0,4 h
c
; t
fn
b
fn
/10 ;
g) Check the nib section web shear resistance:
V
pl,Rd
=A
vn
f
yn
/(
M0
3 ) V
Ed

If necessary, restart the process with another section providing adequate web shear
resistance.
h) Adopt the value for the nib depth: mm ) : 60 max(
cd n
Ed
n eff,
f b
V
d
For shear nib depth chosen, estimate the secondary normal force in nib flange:
)
1 1
)( 30
3
(
c fn n
n eff,
Ed secEd
h t h
d
V N +

+ =
i) Check the nib flange resistance in tension: A
fn
f
yn
/
M0
N
sec Ed

If all the all checks above are satisfied, the nib section chosen is adequate.
Page 10
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NCCI: Design of simple column bases with shear nibs
SN021a-EN-EU
Step 3: Determine the nib to base plate fillet weld sizes
Fillet welds are usually adopted. The minimum throat size is 3mm.
Angle shear nib:
An all round perimeter fillet weld is adopted. The shear force is assumed to be taken by the
two side welds and the toe weld, all of equal throat size a
V
. The normal force is assumed to be
taken by the weld at the angle heel of weld size a
N
. The weld steel strength is taken f
u
=min
(f
up
: f
un
)
The minimum required weld sizes are then:

) 2 (
3
n n u
Ed M2 w
V
b h f
V
a
+

single fillet around the angle leg perimeter

n u
Ed sec M2 w
N
2
b f
N
a

single fillet at the end of the vertical leg
I section shear nib:
The nib web is assumed to take the column base shear force and the nib flange is assumed to
take the secondary normal force. Double fillet welds are usually used.
Web double fillet welds :
) 2 (
3
nib f, nib c, u
Ed M2 w
V
t h f
V
a

Flange double fillet welds :
) 2 (
2
wn fn u
Ed sec M2 w
N
t b f
N
a

Step 4 : Check of the local resistance of the column web
The column web is subjected to the concentrated secondary tensile force N
secEd
. The following
local resistance check is made:
N
sec Ed
(t
wc
b
eff
)f
yc
/
M0
The force is assumed to be distributed over the following effective width in the column web:

Angle shear nib: b
eff
=t
a
+2t
p
+5 (2 a
wc
)
I section shear nib: b
eff
=t
fn
+2t
p
+5 (2 a
wc
).
where a
wc
is the throat size of the column web to base plate double fillet weld.
If the local column web resistance is not adequate the web should be reinforced locally, either
by a vertical stiffener or by a doubler plate.
Page 11
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NCCI: Design of simple column bases with shear nibs
SN021a-EN-EU
6. Design situation 2: Determine the shear
resistance of a column base joint with a shear nib
Step 1: Determine the shear resistance of the nib based on the concrete
Angle shear nib:
2
cd n eff, n
Rd
f d b
V =
I section shear nib:
cd n eff, n Rd
f d b V =
Step 2: Determine the shear resistance of the nib based on the welds
The weld steel strength is taken f
u
=min (f
up
: f
un
).
Angle shear nib:
w M2
n n V u
Rd
3
) 2 (

b h a f
V
+
=

) 90 ( 2 2
3
n eff, w M2
c n N u
Rd
+
=
d
h b a f
V


I section shear nib:
w M2
nib f, nib c, V u
Rd
3
) 2 2 (

t h a f
V

=

) (
) (
) 90 (
) 2 (
2
3
fn n c
fn n c
n eff,
wn n
w M2
V u
Rd
t h h
t h h
d
t h a f
V
+

+

=


Step 3: Determine the shear resistance of the nib based on the angle leg or flange
and web resistances
Angle shear nib:
Resistance of the leg section under shear and axial forces:


3
3
) 90 ( 2
2
c
n eff,
a n
M0
yn
Rd
+

+
=
h
d
t b
f
V

I section shear nib:

) 90 )( (
) ( 3
n eff, fn n c
fn n c
M0
yn fn
Rd
+ +

=
d t h h
t h h
f A
V

(nib flange in tension)

f A
V = (nib web in shear)
Page 12
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NCCI: Design of simple column bases with shear nibs
SN021a-EN-EU
Step 4: Determine the shear resistance of the nib based on column web resistance
Angle shear nib:
) 90 (
) 2 5 2 (
2
3
n eff,
wc p a c a
M0
yn
Rd
+
+ +
=
d
a t t h t f
V

I section shear nib:
) 90 )( (
) 2 5 2 )( ( 3
n eff, fn n c
wc p a fn n c wc
M0
yn fn
Rd
+ +
+ +
=
d t h h
a t t t h h t f A
V

Step 5: The design resistance is taken as least value for the shear resistance V
R,d

given by steps 1 to 4
7. References
1 Lescouarch, Y.
Pinned column bases, CTICM collection, 1982 (in French).
Page 13
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NCCI: Design of simple column bases with shear nibs
SN021a-EN-EU
Quality Record
RESOURCE TITLE NCCI: Design of simple column bases with shear nibs
Reference(s)
ORIGINAL DOCUMENT
Name Company Date
Created by Ivor Ryan CTICM 20/12/2005
Technical content checked by Alain Bureau CTICM 20/12/2005
Editorial content checked by
Technical content endorsed by the
following STEEL Partners:

1. UK G W Owens SCI 07/04/06
2. France A Bureau CTICM 07/04/06
3. Sweden B Uppfeldt SBI 07/04/06
4. Germany C Mller RWTH 07/04/06
5. Spain J Chica Labein 07/04/06
Resource approved by Technical
Coordinator
G W Owens SCI 31/07/06
TRANSLATED DOCUMENT
This Translation made and checked by:
Translated resource approved by:

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