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Steel Designers' Manual - 6th Edition (2003)
550 Trusses
bolted connections are less favoured than welded connections due to the increased
fabrication costs, and usually bolted connections require cumbersome and obtrusive
gusset plates. However, bolted connections are more widely used in bridge
trusses, particularly medium- to large-span road bridges and railway bridges, due to
their improved performance under fatigue loading. In addition, bolted connections
may sometimes permit site erection of the individual elements without the need for
expensive heavy craneage. Gusset plates are often associated with bolted bridge
trusses, their size being dependent on the size of the incoming members and the
space available for bolting.
Gusset plates also enable the incoming members to be positioned in such a way
that their centroidal axes meet at a single point, thus avoiding load eccentricities.
Ideally for all types of trusses the connections should be arranged so that the centres
of gravity of all incoming members meeting at the joint coincide. If this is not possible
the out-of-balance moments caused by the eccentricities must be taken into
account in the design.
Some typical joint details are illustrated in Fig. 19.6.
19.5 Guidance on methods of analysis
Loads are generally assumed to be applied at the intersection point of the members,
so that they are principally subjected to direct stresses. To simplify the analysis the
weights of the truss members are assumed to be apportioned to the top and bottom
chord panel points and the truss members are assumed to be pinned at their ends,
even though this is usually not the case. Normally chords are continuous and the
connections are either welded or contain multiple bolts; such joints tend to restrict
relative rotations of the members at the nodes and end moments develop.
Generally, in light building trusses secondary stresses are negligible and are often
ignored. Secondary stresses in light building trusses may be neglected provided that:
• the slenderness of the chord members in the plane of the truss is greater than 50,
and
• the slenderness of most of the web members, about the same axis, is greater than
100.
However, in bridge trusses the secondary stresses can be a significant proportion of
the primary stresses and must be taken into account. The British Standard for the
design of steel bridges, BS 5400: Part 3: 2000, requires the fixity of the joints to be
taken into account although axial deformation of the members may be ignored for
the ultimate limit state.
The magnitude of the secondary stresses depends on a number of factors including
member layout, joint rigidity, the relative stiffness of the incoming members at
the joints and lack of fit.
Manual methods of analysis may be used to analyse the stresses, particularly in