Condition Deflection (mm) % increase
No slip 175 -
All members bolted, 8 mm slip
in each joint
No slip in compression boom,
8 mm slip in other joint
No slip in booms, 8 mm slip in
Slip at 2 bolted splices in
booms and diagonals
Figure 3: Worst case slip Table3 Effect of bolt slip on deflection – 8 mm slip per member
Discussion and conclusion
It is almost certainly not the case that each joint in each member
will slip by the same amount, because the force carried per bolt
will not be uniform throughout. For example if the number of
bolts required in a joint is 6.2, determined by dividing the design
load by the bolt resistance, 8 bolts will be provided. This suggests
that the possibility of any dynamic effects due to a sudden slip in
all the joints is unlikely.
The absolute worst-case increased deflection set out in
Table 3 will not occur because in practice the bolts will never
be installed in every joint such that the maximum slip can
occur. According to the NSSS, the maximum deviation from the
intended position of a hole in a group of holes is 2 mm so it is
anticipated that there will be some variation in the position of
the bolt holes in a group (meaning some bolts will already be in
bearing) and reduce the potential slip.
Kulak and others1 discuss the behaviour of bolted joints
and state “High strength bolts are usually placed in holes that
are nominally 1/16 in. 1.6 mm larger than the bolt diameter.
Therefore the maximum slip that can occur in a joint is equal
to 1/8 in 3.2 mm. However, field practice has shown that joint
movements are rarely as large as 1/8 in. and average less than
1/32 in 0.8 mm. In many situations the joint will not slip at
all under live loads because the joint is often in bearing by
the time the bolts are tightened. This might be due to small
misalignments inherent to the fabrication process. In addition
slip may have occurred under dead load before bolts in the joint
were tightened. Generally, slips under live loads are so small that
they seldom have a serious effect on the structure”.
In practice therefore, the maximum slip at each joint may well
be no more than 1 mm.
If further reading is desired, a design guide for single storey
steel buildings2 published by Arcelor Mittal and others includes a
section on estimating deflection due to bolt slip.
1. Geoffrey L Kulak, John W Fisher, John H Struik, Guide to design
criteria for bolted and riveted joints, Second Edition, AISC, 2001
2. Steel buildings in Europe, Single storey steel buildings, Part 5
Detailed design of trusses Section 3.6