STRUCTURAL TIMBER TO BS 5268
Timber may be used for temporary timbering to trenches, scaffolding, etc.
It may also be used permanently to trusses, joists, Posts, etc.
Timber may be used in various forms e.g. solid build-up, laminated, etc.
Stress grading of timber
Two methods are generally available;
a) Visual grading
b) Mechanical grading/Machine grading
Visual grading
This is based on the number of defects, their size and the position and
associated grain disturbances.
Before the technique of visual stress grain is applied, the strength of the
timber must be determined.
The strength tests are made on a small clear specimen 20 x 20mm taken
from a wide range of species.
The code requires for six strength tests and two physical tests on each
specimen mainly:
i. Static bending) a test that measures the maximum load a material
can withstand before breaking when bent in a fixed position.
ii. Impact bending*a procedure that uses a sudden impact to determine
a
iii. Compression parallel to the grains
iv. Hardness
v. shear parallel to the grains
vi. Cleavage
The physical tests are usually done for the, moisture content and specific
gravity (gs) or Ss.
Once the strength values are known the first stage is to do the basis tree
from these tests, which is in effect as safe stress for an ideal structural
member free from all strength reducing characteristics.
The second stages the assessment of the influence of the defects in
strength.
Natural defects
The main defects are;
a) Knots: The weakening defects of knots are brought about by local
disturbances in the grain direction it produces and it is not due to any
inferiority in the materials of the knot.
i. Spray knot:
X is the dimension to be measured.
ii. Arris knot:
X and Z are the dimensions to be measured
The size of the knot is taken as x +z/3
iii. Edge and face knot:
Average of A and B for face nots.
X is measured dimension in edge knots.
iv. Margin knot
X is the dimension to be measured.
b). Wane
Is a reduction in cross sectional area of rectangular timber section across
the corners due to the section being taken from a location close to the
outer circumference of the tree.
c). Slope of the grains
It is the measure of direction of the fibres from the longitudinal axis of the
pieces.
If the fibres occur at an angle, then any forces applied between any
longitudinal axes will create components of force on those fibres thus
reducing strength.
Timber is much weaker across the grains than along the grains and so
excessively cross grained timber is undesirable.
d). Rate of growth
Although this is not as important as other features a lamination is imposed
indicated by the average number of growth rings per 25mm.
e). Fissures (resin pockets)
A fissure is any separation of fibres in a longitudinal plane and includes:
checks, shakes and splits.
Their existence reduces the cross section area resisting shear and bending
stresses.
f). Bowing, springing, twisting and cupping.
These defects do not affect the strength of timber and grading rules are
generally for obvious visual and practical reasons.
g).Sap stain (blue stain)
This is not a structural defect and is limited only by visual acceptance.
h). Warm holes
Permitted to slight extent provided that there is no active infestation.
h). Decay
The decayed timber should never be accepted.
Mechanical stress grading
Commonly referred to as machine grading.
Visual stress grading has the disadvantage of not being capable to see the
main body content of timber and therefore it cannot separate the naturally
weak from the naturally strong.
It is therefore possible for a timber with many apparent defects to be
stronger than a similar piece with no defects.
On the other hand machine grading makes allowance for the combined
effect of visual defects and hidden density giving a more accurate strength
grade and hence lead to a more economical usage.
Mechanical stress grading examines the relationship which exists between
the deflection of a piece of timber under a small concentrated load and its
breaking strength.
BS 4978 allows for machine stress grading of GS and SS and they carry their
prefix m to give MGS and MSS as the recognized approved markings. Other
grades are designated as M50 and M75.
CP 112 gives basic grading rules for visual assessment. They are formulated
by fixing various strength ratios and specifying limitations for the natural
defects such that they give the required ratios.
Four ratios are fixed at 40%, 50%, 65%, 75% and the design stresses are
applied accordingly. These stresses are commonly known as ‘’numerical
grades’’ and are now recognized to be accurate in comparison to the ratio
and machine grading rules.
BS 4978; therefore, summarizing the grade stresses currently available to
the designers we have
Visual: 40, 50, 65, 75, GS and SS
Machine: MGS, MSS, M50 and M75
Definitions
i. Basic stress
Stress which can be safely permanently sustained by timber containing no
strength reducing characteristics.
ii. Dry stress
Stress applicable to timber having moisture content not exceeding 18%.
iii. Grade stress
Stress which can safely be permanently sustained by timber of a particular
grade.
iv. Green stress
Stress applicable to timber having moisture content exceeding 18%.
v. Connector
A device generally consisting of a plate disc or ring which when partly
embedded in each or one of the contact face of two members held
together by a connecting bolt, is capable of transmitting a load from one
member to the other.
vi. Horizontally connected beam
One which the laminations are parallel to the neutral axis (N.A).
vii. Vertically laminated beams
One which the laminations run at right angles to the neutral axis (N.A).
viii. Knot ratio
The area of the cross sectional areas of a knot(s) to the full cross sectional
area of the lamination or the ratio of the total width of a face knot to the
total width of the lamination.
ix. Strength ratio
Ratio of grade stress to basic stress.
x. Glue laminated m embers
Members obtained by gluing together of laminates having their grains
essentially parallel.
Joints
1. End joints
Permitted in laminations if their presence is adequately allowed for in their
design. The more common joints are finger joints and scarf joints
i. Finger joint
Used extensively to obtain mass utilization of materials
Fingers are of longer, short portion efficient rating derived by tests.
ii. Scarf joint
Slopes vary from 1 in 6 to 1 in 12 Cp112 gives efficient rating for plan scarf
joint used in glue laminated construction. If joints are clamped by nailing,
the nail head must be recessed to avoid damage if surface preparation
follows.
iii. Framing anchors
Cut and punched from thin pieces of galvanized metal
Ordered by length and nailed directly.
iv. Flat straps
Proprietary galvanized with the pre-punched holes at round 25mm centre
to centre and 2mm thickness of 25mm and 5mm can be pre-bent to order.
Used for holding down jointing length and lateral restrained.
v. Pre-punched plates
Flat galvanized plates with pre-punched holes used for end to end
splicing ,jointing, trusses, etc.
vi. Truss clips
As the name implies, it is used for holding down the ends of trusses over
wall plates
Thin galvanized metal strips bent to shape are used.
2. Nailed joint
Usually basic loads are given for certain nails. Nails can be driven into holes
i.e. pre-holed holes or without preparation of a pre-hole.
Nails can be subjected to single/double shear. Sizes of nails vary from 20 –
150mm in length.
3. Screwed joint
As for nailed joint but there are several types of screws available but the
most common is the slotted countersunk head which is generally driven
into a countersunk hole or finished flush with the surface of the timber.
Size vary from 1.52 – 6.30mm diameter with length from 6.4 – 63.5mm
4. Bolts
Bolts and nuts in conjunction with timber connectors are the two types of
fasteners commonly used in structural timber connection that must sustain
quite sizeable loads.
Spacing and edge distances must be in accordance with the code
Sizes used range from 9 – 38mm in diameter.
All bolts specified in timber are assumed to be black bolts in accordance
with BS916.
CP112 recommends that bolt holes should not be more than
1.6mmdiameter larger than the bolts.
5. Connector joints
Connectors are the most efficient of mechanical fasteners.
They should be installed only on the high stress areas.
Types of connectors
i. Split-ring
This is the most popular and economic wood-to-wood connection device
They are available in 62, 64, 100 and 104mm internal diameter and require
bolts.
Because of the joint in split ring it is susceptible to moisture movement in
the joint and so is good where shrinkage or swelling may occur.
ii. Shear plates
Available in 50 – 100mmdiameters.
Corresponds in capacity to split ring of the same sizes.
Used for steel-to-wood connection or for demontable wood-to-wood
connections
They are installed like the split rings but it is flush with the timber face
Once installed are used , back to back in timber connection or singly while
connected to a steel plate
Used for flushing wood columns to footings.
iii. Toothed plates
Light but effective shear developers. Mostly used for soft wood as they are
easily embedded.
They consist of a thin gauge plate with the edge cut at regular intervals
and pieces of edge twisted to produce jointed tooth ready for timber
surface.
Two types are available; 2-sided and single sided in a range of 50, 62 and
75mmeither square or circular in shape.
Double toothed plates are for woo-to-wood connection and are generally
used for permanent joints on or off site.
Single sided plates are for wood-to metal connection or wood-to-wood
joints in case of demountable joints or where fabrication is involved.
iv. Toothed rings
Similar in performance and installation to the double sided toothed plates.
Available in 50- 75mm in diameters.
Wood to wood connection and installed by pressure hence no grooves are
required.
v. Spike grids
May be circular, flat and single styled. Embedded in the wood surface by
pressure.
Wood to wood connection in heavy frame: e.g. highway and railway truss
roof formwork .
Sizes ranges from 105mm square and 250mmdiameters.
Modification factors
These depend on condition of loadings and are applied to basic stresses to
obtain permissible stresses.
i. Duration of loading factor (K3)
This modification factor relates to the load duration e.g. long term loading,
medium, short term and very short term loading and re shown in table 17
of BS 5268.
Table 17 BS5268
Duration of loading Value of K3
1. Long term ( e.g. dead, permanently 1.00
imposed)
2. Medium term (e.g. dead + snow, dead + 1.25
temporary imposed)
3. Short term (e.g. dead + imposed +wind) 1.50
4. Very short term (e.g. dead + imposed 1.75
+wind)
ii. Depth factor (K7)
The grading bending stresses given in table 8of BS5268 only apply to timber
sections having a depth (h) of 300mm .
Fot other depth of beams the grade bending stresses are multiplied by the
depth factor K7 defined in clause 14.6 of BS5268as follows:
K7 = 1.17 for solid beams having a depth > 72mm
K7 = [300/h]0.11 for solid timber with depth 72mm < h 300mm.
K7 = 0.81(h2 + 92300)/(h2 + 56800)for solid beams with h > 300mm where h
= depth
iii. Loading shearing factor (K8)
The load shearing factors given in table 9 apply to individual members e.g.
isolated beams and columns.
When four or more members e.g. rafters, joists or wall studs are spaced a
maximum of 610mm centre to centre act together to resist a common
load ,the great spaces will be multiplied by a load shearing factor K8 , i.e. a
value of 1.1.
iv. Form factor (shapes)
Square sections are placed so that the diagonal is vertical, i.e. the load is
placed in the direction of the diagonal.Type equation here.
K = 1.18√ 2
v. Deep bearing factor (d = 300mm)
d 2+143
K = d 2+88
vi. Notches factor
Notches at the end of beam plus effective depth.
K = de/d
Other calculations of z x x based on effective size of notch can be
determined
vii. Length of bearing factor
For bearings shorter than 150 mm from the end of wall , one should apply
the following factors:
Bearing 12 25 38 50 78 100 150 or
length more
K values 1.70 1.53 1.36 1.19 1.14 1.10 1.00
viii. Slenderness ratio factor
Slenderness ratio Factor (K)
<5 1.25
5 1.25
10 1.23
20 1.20
30 1.17
40 1.13
50 1.08
60 1.00
70 0.90
80 0.707
90 0.68
100 0.58
120 0.42
140 0.32
160 0.25
180 0.20
200 0.16
220 0.14
240 0.12
250 0.11
TIMBER BEAM DESIGN
Joists are normally employed for spacing up to 500 mm and usually carry
floor and roof sheeting, whereas beams and girders are spaced apart and
support heavy floor /roof.
Joists may be supported by walls, trusses or girders.
Structural beams are usually rectangular in section and of uniform depth
throughout the span.
In heavier loadings shaped sections may be adopted.
CONSIDERATION IN DESIGN OF TIMBER STRUCTURES
i. Condition of exposure
It should never be exposed to adverse weather conditions unless surfaces
are protected.
ii. Direction of grains
Timber is usually stronger when loads are perpendicular to the grains unlike
when grains are parallel to load.
The scope of grain should be given consideration to bending, shock, and
compressive stress.
iii. Strength
Consideration should be given to the direction of grains to give maximum
strength.
iv. Deteriorating defects
Includes effects from dry rot, wet rot, termites etc.
TIMBER BEAM DESIGN PROCEDURE
i. Determine the dimensions of the cross section.
If the extreme fiber stress does not exceed the allowable stress for material
used, i.e. design for strength in bending.
ii. Check the section to see that the allowable horizontal shear stress is not
exceeded.
iii. Check the section for deflection.
iv. Determine the dimensions of the end bearing provided that the stresses in
compression perpendicular to the grain do not exceed the allowable given
in the code.
DESIGN CONSIDERATION
The following design consideration will be included:
1. Bending
It is based on the theory of simple bending
Mr f E
I
= y
= R
Where Mr = moment of resistance of the section
I = second moment of area of the section.
f = permissible bending stress parallel to the grains
E = young modulus (Emean)
R = radius of curvature
From the above equation it can be re-arranged as follows:
Mr I
f
= y
I
But y = Z (section modulus)
Mr
Therefore Z = f or Mr = Zf
The moment of resistance should be greater than the maximum bending
moment, i.e. Mr > B.M.
Alternatively;
Z (section modulus) required should be less than Z (section modulus)
provided.
2
bd
For rectangular sections; Z =
6
For usual conditions Mr = Mmax.
bd 2 f
Or 6
= Mmax
Hence dimensions of the beam can be obtained if the ratio of b:d is given.
2. Shear
The maximum shear in a rectangular section is given by:
1.5Q
qmax. = b . d
Where Q = maximum shear force
b = breath of section
d = depth of section
This value must not be greater than the permissible shear stress in timber
parallel to grains i.e. fq < Pq
3. Bearing
There should be adequate bearing area at the support so that the
permissible stresses in compression perpendicular to the grains is not
exceeded by the bearing stress at the supports.
Q
Bearing stress = b . x
Where Q = maximum shear
b = breadth of section
x = length of bearing
This value must not exceed the permissible stress perpendicular to the
grains in timber.
4. Deflection
The permissible deflection = 0.003 x span.
The maximum actual deflection is obtained from the appropriate standard
formula for various loading conditions e.g.
i. Simply supported uniformly distributed loads.
4
5 wL
δ =
384 EI
ii. Point load simply supported
3
WL
δ =
48 EI
iii. Point load cantilever
3
WL
δ =
3 EI
iv. Cantilever carrying uniformly distributed loads
4
wL
δ =
8 EI
The maximum actual deflection must not exceed the allowable permissible
deflection.
Example 1
A joist in a suspended timber floor had 75 x 225 mm and 4m effective length
spaced at 450 mm c/c. Check the joist adequacy with respect to bending and
hence check for shear, bearing and deflection.
Given the following data:
Live loads = 2.0 KN/m2
Dead loads and finishes = 1.0 KN/m2
Allowable deflection = 0.003 x span
Bending parallel to grain = 7.5 N/mm2
Shear parallel to grains = 0.71 N/mm2
Compression perpendicular to grains = 1.9 N/mm2
Other data
Emean = 9.9Kn/m2
Length of bearing at support = 100 mm
