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Steel Designers' Manual - 6th Edition (2003)
494 Plate girders
force component in the flange if the beam soffit is haunched), a force to account
for the destabilizing effect of the web, forces from any local cross bracing (including
forces from restraint systems), and a force due to the tension field. The derivation
of these is far more precise and lengthy than BS 5950: Part 1: 2000 requires,
largely because plate girders in bridges are usually part of a much more complex
structural system than in buildings.
Bearing stiffeners are almost always placed symmetrically about the web centreline.
Stiffeners that are symmetrical about the axis perpendicular to the web, i.e. flats
or tees rather than angles, are preferred.
17.6.2.8 Intermediate stiffeners (clause 9.13 of the Code)
Intermediate stiffeners are usually placed on one side of the web only. Standard flat
sizes should be used. BS 5400: Part 3 does not prescribe stiffness criteria for intermediate
stiffeners (which have an associated length of web), but it does prescribe
design forces for which these stiffeners are designed, similar to those outlined for
bearing stiffeners.
17.6.3 Design of beams at the serviceability limit state
In practice, due to the proportions of partial safety factors at the ultimate and
serviceability limits, the serviceability check on the steel section is automatically
satisfied if designs are satisfactory at the ultimate stage.
The design of beams is carried out using an elastic strain distribution throughout
the cross-section, in a manner generally similar to that used at the ultimate limit but
with different partial safety factors, i.e. different values of gfL, gm and gf3. The determination
of effective section must take account of shear lag, where appropriate.
Stresses should be checked at critical points in the steel member to ensure
that no permanent deformation due to yielding takes place. 8–10 Plastic moment
resistance is not considered at the serviceability limit state.
In composite construction, crack widths in the concrete deck may often govern
the design at the serviceability limit state. For non-compact sections the analysis at
the ultimate limit state is carried out using a triangular (elastic) stress distribution.
Consequently the serviceability limit check for bending is always satisfied, since the
factors are less than those at the ultimate limit state.The Code lists the clauses which
require checking at both limit states.
17.6.4 Fatigue
Fatigue should be checked with reference to BS 5400: Part 10. Generally it is only
bracing connections, stiffeners and shear connectors, and their welding to the