Glulam Beams
7.5 JOINTS IN LAMINATIONS .1 Joints in horizontal laminations
With structural glulam the end jointing of individual laminations is carried out almost certainly by finger jointing or scarf joints. If a butt joint is used, then the laminate in which it occurs has to be disregarded in the stress calculations, and this usually makes the member uneconomical. Finger joints and scarf joints may be considered not to reduce the EIvalue of the member. Finger joints can be used on either axis of laminates and have equal strength whichever way they are cut.
There are two ways of establishing if the strength of an end joint in a lamina- tion is adequate for its function. The simple way is to provide a joint with an effi- ciency in bending which compares with the efficiency in bending of the laminate.
The second way is to calculate the strength required of the joint in bending, tension or compression (or combinations of bending and tension, or bending and com- pression), and to provide a joint of adequate strength.
When a horizontally laminated member is made from C24 or C16 laminates, providing the end joint has an efficiency in bending of at least:
70% for C24 laminates, or 55% for C16 laminates,
t =F S bI
v x x
Fig. 7.9
Fig. 7.10
then BS 5268-2 (clause 3.4 and Table 96) permits the joint to be used without a further design check.
Guidance on the efficiency in bending, tension and compression of various finger joints is given in BS 5268-2 Annex C, reproduced here as Table 7.3 with length l, pitch pand tip width t, as illustrated in Fig. 7.11.
Note that, although the efficiency in tension is the same as in bending, the effi- ciency in compression is higher. The efficiencies in bending and tension have been established by test. Tests in compression tend to indicate an efficiency of 100%
but, rather than quote 100%, the following formula is used:
Efficiency rating in compression =[(p-t)/p] ¥100%
End joints should be staggered in adjacent laminations as illustrated in Fig. 7.12.
This will ensure that the actual joint efficiency is larger than the quoted value because, even in the outer lamination, as well as the strength of the joint in isola- tion, the adjacent lamination will act as a splice plate which will increase the Glulam Beams 133 Table 7.3 Finger joint profile efficiency ratings
Finger profile Efficiency ratings (%)
Length l Pitch p Tip width t Bending and tension Compression
(mm) (mm) (mm) parallel to grain parallel to grain
55 12.5 1.5 75 88
50 12 2 75 83
40 9 1 65 89
32 6.2 0.5 75 92
30 6.5 1.5 55 77
30 11 2.7 50 75
20 6.2 1 65 84
15 3.8 0.5 75 87
12.5 4 0.7 65 82
12.5 3 0.5 65 83
10 3.7 0.6 65 84
10 3.8 0.6 65 84
7.5 2.5 0.2 65 92
Fig. 7.11 Types of structural finger joint.
strength of the joint. Finger joints are usually randomly spaced but BS 4169 requires that finger joints in adjacent laminations shall be offset by a distance not less than the width of the lamination and calls for excessive grouping of finger joints in critical areas (i.e. tension zones) to be avoided.
When designing with C24 or C16 grade laminates and unable to provide an end joint efficiency rating of 70% or 55% respectively, the designer is required to check the strength of the joint that is to be used against the actual strength required. To assist with this, BS 5268-2 gives coefficients which, for softwood, are:
K30 =1.63 for bending parallel to grain K31 =1.63 for tension parallel to grain K32=1.43 for compression parallel to grain.
Example
Consider a horizontally laminated beam 540 mm deep of 12 laminates and calcu- late the permissible extreme fibre stress in bending for the outer lamination under medium-term loading. The beam is C24 grade throughout and the finger jointing to be used has an efficiency rating in bending of 55% (i.e. less than the 70%
required for ‘blanket approval’ with C24 grade).
The permissible bending stress in the lamination, as limited by the joint being used, is calculated as:
The C24 grade stress for the species (7.50 N/mm2)
¥the relevant moisture content factor K2(1.00 for dry exposure)
¥the relevant load–duration factor K3(1.25)
¥the modification factor for depth of member K7(0.893 for 540 mm)
¥the ratio for efficiency in bending of the joint (0.55 in this case)
¥factor K30(1.63) Fig. 7.12
Permissible bending stress in laminate = 7.50 ¥1.00 ¥1.25 ¥0.893 ¥0.55 ¥1.63
= 7.50 N/mm2
If the laminates were free of end joints, the permissible bending stress for this member would be:
The C24 grade stress for the species (7.50 N/mm2)
¥the relevant moisture content factor K2(1.00 for dry exposure)
¥the relevant load–duration factor K3(1.25)
¥the modification factor for depth of member K7(0.893 for 540 mm)
¥the laminating factor K15(1.45 in this case)
Permissible bending stress =7.50 ¥1.00 ¥1.25 ¥0.893 ¥1.45 =12.1 N/mm2 The design check is extended in the case of finger joints in a member taking axial tension as well as bending.
Example
Consider a horizontally laminated beam 540 mm deep of 12 laminates and calcu- late the permissible extreme fibre stress in bending for the outer lamination under medium-term loading. The beam is C24 grade throughout and the finger jointing to be used has an efficiency rating in bending of 55% (i.e. less than the 70%
required for ‘blanket approval’ with C24 grade). All as the previous example.
As before, the permissible bending stress is 7.5 N/mm2.
The permissible tension stress in the lamination, as limited by the joint being used, is calculated as:
The C24 grade stress for the species (4.50 N/mm2)
¥the relevant moisture content factor K2(1.00 for dry exposure)
¥the relevant load–duration factor K3(1.25)
¥the modification factor for width of member K14, see section 4.9.1 (0.937 for 540 mm width)
¥the ratio for efficiency in bending of the joint (0.55 in this case)
¥factor K31(1.63)
Permissible tension stress in laminate =4.5 ¥1.00 ¥1.25 ¥0.937 ¥0.55 ¥1.63
=4.72 N/mm2
For the purpose of this example assume applied bending stress is 4.0 N/mm2and applied tension stress is 1.5 N/mm2.
Section is adequate despite the 55% efficiency rated finger joints. Combined bending and compression would be assessed in a similar fashion.
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Glulam Beams 135
7.5.2 Joints in vertical laminations
With a member made up of vertical laminates the stress grades which apply are those for solid timber (e.g. C24, C16, etc.). As with joints in horizontal laminates the designer has the option of providing a joint of a certain efficiency for a par- ticular stress grade or of comparing the strength of a joint of a certain efficiency with the strength required for the actual stresses in the member.
When a horizontally laminated member is made from C24 or C16 laminates, providing the end joint has an efficiency in bending of at least:
70% for C24 laminates, or 55% for C16 laminates,
then BS 5268-2 (clause 3.4 and Table 96) permits the joint to be used without a further design check.
Efficiency ratings for a range of commercial finger joint profiles is given in Annex C of BS 5268-2 and in Table 7.3.
When designing with C24 or C16 grade laminates and unable to provide an end joint efficiency rating of 70% or 55% efficiency respectively, clause 3.4 of BS 5268-2 requires the permissible stresses to be reduced accordingly.