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Steel Designers' Manual - 6th Edition (2003)
Fig. 16.15 Effective cross-section
be/2 be/2
__ I -1
(strength and stiffness) are then calculated for this effective cross-section as illustrated
in Fig. 16.15. Tabulated information in BS 5950: Part 5 for steel of yield
strength 280 N/mm 2 makes the application of this approach simpler in the sense that
effective widths may readily be determined, although cross-sectional properties
have still to be calculated.The use of manufacturer’s literature removes this requirement.
For beams, Part 5 also covers the design of reinforcing lips on the usual basis
of ensuring that the free edge of a flange supported by a single web behaves as if
both edges were supported; web crushing under local loads, lateral–torsional buckling
and the approximate determination of deflections take into account any loss of
plating effectiveness.
For zed purlins or sheeting rails section 9 of BS 5950: Part 5 provides a set of
simple empirically based design rules. Although easy to use, these are likely to lead
to heavier members for a given loading, span and support arrangement than either
of the other permitted procedures. A particular difference of this material is its use
of unfactored loads, with the design conditions being expressed directly in member
property requirements.
16.8 Beams with web openings
Beams with web openings 457
One solution to the problem of accommodating services within a restricted floor
depth is to run the services through openings in the floor beams. Since the size of
hole necessary in the beam web will then typically represent a significant proportion
of the clear web depth, it may be expected that it will have an effect on structural
performance. The easiest way of visualizing this is to draw an analogy between
a beam with large rectangular web cut-outs and a Vierendeel girder. Figure 16.16
shows how the presence of the web hole enables the beam to deform locally in a
similar manner to the shear type deformation of a Vierendeel panel. These deformations,
superimposed on the overall bending effects, lead to increased deflection
and additional web stresses.
A particular type of web hole is the castellation formed when a UB is cut, turned
and rewelded as illustrated in Fig. 16.17. For the normal UK module geometry this
leads to a 50% increase in section depth with a regular series of hexagonal holes.
Other geometries are possible, including a further increase in depth through the use