Steel Buildings in Europe

Part 5: Detailed Design of Trusses 5 - 22 It is therefore essential, where truss structures are concerned, to control the effect of connection slack on the displacements. In order to do this, it is often necessary:  either to limit slack in category A connections: drilling at +1 mm, even +0,5 mm and using shear bolts on a smooth bolt shank (to limit the increase in slack by deformation) of the threads and pieces; or  to use ‘fit bolts’; or  to use preloaded bolts (category C connections); or  to use welded connections instead of bolted connections. In cases where loading in the members does not result in reversal of axial force, it is possible to calculate a value for the effect of slack in all the connections. The following calculation illustrates this phenomenon for the worked example. Each of the chords, upper and lower, has a continuous connection with bolted splice plates around the mid-span. In addition, the diagonals are connected by bolting on gusset plates welded to the chords. Holes are 2 mm larger than the bolt diameter. Figure 3.6 Worked example – Position of the chord connections using splice plates In a spliced connection of a chord, the effect of slack on the deflection can be evaluated by assuming that the bolts are initially centred on their holes. If the diameter of the holes is d + 2 mm (where d is the bolt diameter), a chord in tension is extended by 4 mm, as shown in Figure 3.7. 1 mm 1 mm d 1 mm 1 mm d g g + 4 mm Figure 3.7 The effect of slack under load In order for a diagonal to be loaded, 2 mm has to be recovered at each end: the length of a diagonal in tension is increased by 4 mm; a diagonal under compression is reduced by a further 4 mm.