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Steel Designers' Manual - 6th Edition (2003)
526 Members with compression and moments
resist the applied moments and thrust; rather for members required to participate
in plastic hinge action, the ability to sustain the required moment in the presence
of compression during the large rotations necessary for the development of the
frame’s collapse mechanism is essential. This requirement is essentially the same as
that for a ‘plastic’ cross-section discussed in Chapter 13. The performance requirement
for those members in a plastically designed frame actually required to take
part in plastic hinge action is therefore equivalent to the most onerous type of
response shown in Fig. 13.4. If they cannot achieve this level of performance, for
example because of premature unloading caused by local buckling, then they will
prevent the formation of the plastic collapse mechanism assumed as the basis for
the design, with the result that the desired load factor will not be attained. Put
simply, the requirement for member stability in plastically-designed structures is to
impose limits on slenderness and axial load level, for example, that ensure stable
behaviour while the member is carrying a moment equal to its plastic moment
capacity suitably reduced so as to allow for the presence of axial load. For portal
frames, advantage may be taken of the special forms of restraint inherent in that
form of construction by, for example, purlins and sheeting rails attached to the
outside flanges of the rafters and columns respectively.
Figure 18.12 illustrates a typical collapse moment diagram for a single-bay pinbase
portal subject to gravity load only (dead load + imposed load), this being the
usual governing load case in the UK.The frame is assumed to be typical of UK practice
with columns of somewhat heavier section than the rafters and haunches of
approximately 10% of the clear span and twice the rafter depth at the eaves. It is
further assumed that the purlins and siderails which support the cladding and are
attached to the outer flanges of the columns and rafters provide positional restraint
to the frame, i.e. prevent lateral movement of the flange, at these points. Four regions
in which member stability must be ensured may be identified:
(1) full column height AB
(2) haunch, which should remain elastic throughout its length
25'A symmetrical E
about
4-
Fig. 18.12 Moment distribution for dead plus imposed load condition
C