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Steel Designers' Manual - 6th Edition (2003)
girders, that have to be checked for fatigue. The details of the connection may have
a significant effect on the fatigue life, and by careful detailing fatigue may be
‘designed out’ of the bridge.
17.6.5 Design format
One of the significant features of BS 5400: Part 3 is that plastic redistribution across
the section is now permitted, in certain circumstances, prior to the attainment of
the ultimate limit state. Such redistribution is much less than permitted in simple
plastic design of buildings, but can give a considerable enhancement in strength. For
example, the design bending strength of compact sections is based on the plastic
section modulus, and the basic shear strength is based on rules which take account
of tension field action, which only develops in the presence of considerable plasticity.
The interaction between shear strength and bending strength of a cross-section
is empirically based and implicitly assumes that plastic strains may occur prior to
the attainment of the design strength. Finally, the design strength of compact sections
that are built in stages assumes that a redistribution of stresses may take place
within the cross-section. The ultimate limit state check for the completed structure
is simply to ensure that the bending resistance (given by the limiting stress times
the plastic section modulus divided by partial factors) is greater than the maximum
design moment (obtained by summing all the moments due to the various design
loads).
This recognition of the reserve of strength beyond first yield is limited to situations
where plasticity may be permitted safely. The principal limitations are as follows.
(a) Plastic section moduli may only be used for compact sections, i.e. those that can
sustain local compressive yielding without any local buckling.
(b) Where the structure is built in stages and is non-compact, plasticity is permitted
implicitly only in considering the interaction between bending and shear
strength.
(c) Where longitudinal stiffeners are present, neither the plastic section modulus
nor the plastic bending / shear interaction may be used.
(d) If the flanges are curved in elevation, similar restrictions to (c) apply.
References to Chapter 17
References 495
1. British Standards Institution (2000) BS 5950: Structural use of steelwork in
building: Part 1: Code of practice for design: rolled and welded sections. BSI,
London.
2. British Standards Institution (1982) BS 5400: Steel concrete and composite
bridges: Part 3: Code of practice for the design of steel bridges. BSI, London.