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Steel Designers' Manual - 6th Edition (2003)
In the analysis the member forces and moments due to joint fixity should be
calculated and superimposed on the global member forces. For trusses in buildings
the secondary effects due to joint fixity may normally be ignored provided the
slenderness, in the plane of the truss, is greater than 50 for the chord elements
and 100 for most of the web members. If this condition is satisfied the
members are assumed to be pin jointed in the analysis. Secondary effects due to
axial deformations are usually ignored in building trusses. Local effects due to joint
eccentricities and where loads are not applied at nodes should be taken into
account.
For bridge trusses to BS 5400: Part 3: 2000, the effects of joint rigidity are required
to be taken into account. Secondary stresses due to axial deformations may be
ignored at the ultimate limit state but should be considered at the serviceability limit
state and for fatigue checks. As for building trusses, the local effects due to joint
eccentricities and cases where loads are not applied at nodes must be considered in
bridge trusses.
19.6.2 Effective length of compression members
Detailed design considerations for elements 553
For building trusses the fixity of the joints and the rigidity of adjacent members
may be taken into account for the purpose of calculating the effective length of
compression members. The designer should be careful to ensure that the critical
slenderness is identified. For chords, out-of-plane unrestrained lengths do not necessarily
relate to the truss nodes, and effective length factors are usually unity; in-plane
effective length factors may be demonstrated to be less than unity if the restraining
actions of tension members and non-critical compression members are mobilized
at the ends of the member. Single angle elements, for both the webs and chord,
have minimum radii of gyration that do not lie either in, or normal to, the plane of
the truss.
For compression members in bridge trusses the effective lengths may either be
obtained from Table 11 of BS 5400: Part 3: 1982 or be determined by an elastic
critical buckling analysis of the truss.
In the case of simply supported underslung trusses the top compression chord
will be effectively restrained laterally throughout its length provided the connection
between the chord and the deck is capable of resisting a uniformly distributed
lateral force of 2.5% of the maximum force in the chord. The effective length in
such a case is taken as zero where friction provides the restraint, or as equal to the
spacing of discrete connections where these are provided.
The economic advantages of underslung trusses over through or semi-through
trusses is obvious in this respect, due to the dual function of the deck structure.
In the case of unbraced compression chords, that is, chords with no lateral
restraints, the provision of U-frames is necessary. The effective length of the compression
chord is a function of the stiffness of the chord and the spacing and