BRE FB5 PDF

Introduction

Current structural fire design methods, which form the basis of BS5950 Part 8"', Eurocode 3 Part 1.2'*' and Eurocode 4 Part l.2'3', are mainly based on the results from standard fire testd4' on isolated members. The underlining assumption is that if a column, beam and floor reach a certain minimum fire resistance in an isolated fire test, then if these elements are used to construct an entire structure, the structure will have an equivalent fire resistance as that observed in the small-scale fire tests.

This assumption ignores any structural interaction between elements, which can be either detrimental or beneficial. For example, restraint to thermal expansion, which occurs in a complete building when a zone of cold structure surrounds a zone of heated members, can cause plasticity or buckling in the steel frame much earlier in a fire compared to members in isolated fire tests. Alternately, changes in geometry of the composite floor as it is heated and deflects, can be extremely beneficial to the survival of the building. The effect of slab deflections is opposite to that on columns (Figure 2). In columns a change in geometry reduces the load carrying capacity due to the P-6 effect, whereas in slabs an increase in displacement can lead to an increase in load carrying capacity. This behaviour is known as membrane action and cannot be identified from the current isolated fire tests on composite floors, since they are limited to one-way spanning flexural behaviour.

Results from the six localised fire tests'5s6' conducted on the full-scale steel-framed building, constructed at the B E Large Building Test Facility, together with observations of actual fires at Broadgate7 and Basingstoke', showed that actual buildings perform far better than current design methods suggest. It is generally agreed that this enhanced performance is due to membrane action in the composite floor slab. It is therefore a logical steep to develop a design method that incorporates membrane action of composite slabs in fire.