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Steel Designers' Manual - 6th Edition (2003)
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Design of steel bridges to BS 5400: Part 3 487
Fig. 17.5 Section geometry and classification – compact sections
The Code further defines cross-sections as compact or non-compact, and a different
design approach is required for each classificatiod. 7
The classification of a section depends on the width to thickness ratio of the elements
of the cross-section considered, as shown in Fig. 17.5 for a compact plate
girder.
A compact cross-section can develop the full plastic moment resistance of the
section, i.e. a rectangular stress block, and local buckling of the individual elements
of the cross section will not occur before this stage is reached (see Fig. 17.6 (b)).
However, in a non-compact section local buckling of elements of the cross section
may occur before the full moment resistance is reached, and hence the design of
such sections is limited to first yield in the extreme fibre i.e. a triangular stress block
(see Fig. 17.6 (c)).
These classifications can best be illustrated by considering a beam in fully
restrained bending. This gives the idealized moment resistance of the cross sections
as follows.
Compact sections
Non-compact sections
M1 y
1
Z
M
Z
s
=
g g
s
=
g g
pe y
m f3
m f3
where sy is the yield stress, Z pe is the plastic section modulus, Z is the elastic section
modulus, g m is the partial factor on strength, and g f3 is the partial factor on loads,
reflecting the uncertainty of loads.
The use of the plastic section modulus for compact sections does not imply that
plastic analysis can be employed; in fact it is specifically excluded by BS 5400: Part
3. The achievement of a rectangular stress block does not necessarily mean that
there has been redistribution of moments along the member.