Technical
Figure 2 Truss arrangement
26 NSC
Nov/Dec 19
Example 2
To illustrate the effect of bolt slip consider a pin jointed Pratt
truss (N frame), shown in Figure 2. Member areas are based on a
tensile stress of 350 MPa and 150 MPa in compression, with the
area limited to a minimum value.
The deflection of the truss centre under the total design load
is estimated to be 175 mm (span divided by 230), calculated
by virtual work. An FE model gives a deflection of 179 mm.
The deflection can be apportioned to 110 mm of bending
deflection (deformation of the truss booms) and 65 mm of shear
deflection, from the bracing members. Making this distinction
is useful if the deflection is to be reduced because the elements
making the greatest contribution to the total deflection can be
identified.
In estimating the effect of bolt slip, it is assumed that with
automated saw and drill lines, the accuracy of holing is such that
slip can occur in all holes simultaneously. If all the members are
bolted with 2 mm oversize holes and 1 mm of slip is assumed
at each end of a member, a total of 2 mm per member, the
deflection increases by 43% to about 250 mm. The effect on
the mid-span deflection of other assumptions about which
members experience slip is shown in Table 2. Possible scenarios
are 1) that pipe-flange type bearing splices are effected in
compression booms with no slip; 2) that both booms are
effectively continuous with the bracing members bolted to them
and 3) that the truss is shop-welded with bolted splices.
Condition Deflection (mm) % increase
No slip 175 -
All members bolted,
250 43
1 mm slip in each joint
No slip in compression boom,
1 mm slip in other joints
220 26
No slip in booms,
1 mm slip in bracing joints
200 14
1 mm slip at 2 bolted splices in
booms and diagonals
190 9
If the most unfavourable assumptions are made about the
position of the bolts in their holes a slip of 4 mm at each end of a
member is theoretically possible as shown in Figure 3.
The corresponding deflections are set out in Table 3. It can be
seen that the theoretical increase in the mid-span deflection is
very large. This is not surprising when the elastic deformations
in the compression members are about 3 mm and an average of
about 5 mm in the tension members.
A truss designed with joints made with preloaded bolts of
Category C where the friction coefficient assumed in design
is not achieved may well experience increased deflection in
service. However, the magnitude of the increased deflection is
uncertain. The potential percentage increases indicated in Tables
1 and 2 are unlikely to be realized for several reasons.
24
Table 2 Effect of bolt slip on deflection – 2 mm slip per member
/Trusses#Pratt_truss_.28.27N.27_truss.29
/Trusses#Pratt_truss_.28.27N.27_truss.29
/Trusses
/Preloaded_bolting