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AD 361: Mono pitched portal frames in fire boundary conditions

In SCI publication P313, Single Storey Steel Framed Buildings in Fire Boundary Conditions, Section 2.5.4 gives guidance on the calculation of overturning moments for mono pitched frames. However, it has come to SCI’s attention that the expressions in that Section can give negative values of overturning moment for certain configurations of mono pitched frames.

The purpose of this AD Note is to provide clarification on the use of Section 2.5.4.

The problem of negative values of overturning moment given by expressions in Section 2.5.4 occurs when the frame being checked has short rafter spans or lightly loaded rafters, as this tends to cause low rafter utilisation. In such cases, the value of the horizontal force associated with the plastic collapse mechanism, H, has a small or even negative value, according to the expression on page 14 of P313. With a negative value of H, the values of OTM1 and OTM2 given by the expressions on page 13 can also become negative.

A negative value of H has no physical significance as far as the fire performance of the frame is concerned; it simply means that at the rafter temperature considered (890°C) the frame will not develop a mechanism under the applied loading.

So, as a first step, H should not be taken as less than zero. However, with H = 0 and light loading on the rafter, the values of OTM1 and OTM2 may be small. Noting that the design model for duo pitched rafters includes a minimum value of overturning moment, it would be prudent to adopt a similar approach for mono pitched frames. The column base should be designed to resist a minimum value of overturning moment equal to 10% of the column moment resistance.

Contact: W I Simms
Tel: 01344 636525

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