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Steel Designers' Manual - 6th Edition (2003)
552 Trusses
simple trusses. For simple, statically-determinate trusses, methods of analysis include
joint resolution, graphical analysis (Bow’s notation or Maxwell diagram) and the
method of sections. The last method is particularly useful as the designer can limit
the analysis to the critical sections.
Statically-indeterminate trusses are more laborious to analyse manually; methods
available include virtual work, least work and the reciprocal theorem with influence
lines. For a full discussion on these methods of analysis the reader should refer to
textbooks on structural analysis.
Computers are nowadays readily available to designers and provide a useful
means of analysing the most complex of trusses. In addition, joint and member
rigidities can easily be incorporated in the modelling thus avoiding laborious hand
calculations in determining out-of-balance moments caused by joint deformations.
Local stresses caused by loads not applied at the panel points, joint eccentricities
and axial deformation should generally be calculated and superimposed on the
direct stresses. However, stresses due to axial deformation are normally neglected
except for bridge trusses and trusses of major importance.
Careful consideration must be given to the out-of-plane stability of a truss and
resistance to lateral loads such as wind loads or eccentric loads causing torsion about
their longitudinal axis.An individual truss is very inefficient, and generally sufficient
bracing must be provided between trusses to prevent instability. In bridges, plan
bracing is normally provided between trusses at the chord levels in addition to stiff
end portals to prevent lateral instability.
19.6 Detailed design considerations for elements
19.6.1 Design loads
The current British Standards for steel structures in buildings and bridges are both
limit-state codes. The magnitude of the partial load factors to be applied is dependent
on the load type, the load combination and the limit state (ultimate or serviceability)
under consideration.
The following approach may be adopted in deriving the critical load combinations
for each truss member:
(1) The member forces and moments are calculated for each, unfactored, load type
(dead, superimposed dead, imposed, wind, etc.) using an appropriate method of
analysis.
(2) Load combinations are identified and the appropriate load factors for each
combination applied for both serviceability and ultimate limit states.
(3) The critical loads in each element and joint are extracted for both limit states.
The above process is long-winded but with experience the designer can often take
short cuts in determining the critical load combinations for each element.