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Steel Designers' Manual - 6th Edition (2003)
348 Plane frame analysis
It will remain the engineer’s responsibility to ensure that stability is provided both
locally and in the overall condition. BS 5950: Part 1 2 provides no specific rules
regarding the stability of the frame as a whole; this means that the engineer must
ensure that the stability is checked using the general rules for all frames. He must
also check that the movement of the frame under all loading cases is not sufficient
to cause damage to adjacent construction, i.e. brick walls or cladding, the serviceability
limit state of deflection.
Plastic analysis
The method of calculating the ultimate load of a portal frame is described in many
publications.The main essence of the method is to assume that plastic ‘hinges’ occur
at points in the frame where the value of M/Mp is at its highest value, the load being
considered as increasing proportionally until the failure or ultimate state is reached.
Because of the straining at the hinge points it is essential that the local buckling and
lateral distortion do not occur before failure. Failure is deemed to have taken place
when sufficient hinges have formed to create a mechanism.
The member capacities are calculated using the rules given in Section 4 of the
Code but with the additional restrictions applied to hinge positions. In addition,
positive requirements are put on checking frame stability for both single-bay and
multi-bay frames. Plastic designed frames are lighter than elastic designed frames,
providing deflection is not a governing point; however, additional bracing may well
be required.
11.2.2 Stability
With the use of lighter frames, various aspects of stability take a more prominent
part in the design procedures. As far as portal frames are concerned the following
areas are important:
(1) overall frame stability, in that the strength of the frame should not be affected
by changes in geometry during loading (PD effect)
(2) snap-through stability, in multi-bay frames (three or more), where the effects of
continuity can result in slender rafters
(3) plastic hinge stability, where the member must be prevented from moving out
of plane or rotating at plastic hinges
(4) rafter stability, ensuring that the rafter is stable in bending as an unrestrained
beam
(5) leg stability, where the leg below the plastic hinge must be stable
(6) haunch stability, where the tapered member is checked to ensure that the inner
(compression flange) is stable.