Keywords: fire resistance, slim floor, asymmetric beam, intumescent coating, steel structure, fire test.

Introduction

In slim floor construction, the supporting floor beam is contained within the depth of the floor deck. This provides a solid flat-slab appearance similar to reinforced concrete construction. The most recent advent of the asymmetric beam (ASB), developed by Corus UK (formerly British Steel) and The Steel Construction Institute (SCI), does not require welding of an additional plate, as the SLIMFLOR beam does, and achieves optimum properties for design. The floor is constructed of long span composite slabs with deep profiled steel decks (Figure 67.1).

Because the asymmetric beams are almost totally contained within the floor slab, they have inherently good performance in fire and in most cases can achieve 60 minutes fire resistance without applied protection. However, for more than 60 minutes fire resistance, protection must be applied to the bottom flange. Such fire protection can effectively be provided in the form of intumescent coating, which will add a negligible amount to the depth of the section. The required thickness, however, would be much smaller than that needed for normal UC beams, due to the intrinsically good fire resistance of slim floor construction. The purpose of this project is to evaluate the coating thickness required for specified fire resistance, for a commonly used intumescent coating.

Procedure

The project took the following procedure:

i

Examination of testing set-up and data

ii

Calculations of moment resistance and load ratio for the coating thickness used

iii

Evaluation of beam temperature distribution as a function of protective coating thickness

iv

Determination of minimum coating thickness required for up to 120 minutes fire resistance

v

Examination of the effect of service holes

vi

Assessment for S607 on asymmetric beams by comparison with test data on SLIMFLOR beams.

The reduced moment resistance in fire conditions is calculated using plastic theory, as permitted by BS5950 Part 8 and EC4 Part 1.2. The steel section is split into eight elements as shown in Figure 67.2. The reduction of strength of each element is obtained from factors (based on 2 was obtained from measurements during the test. The concrete is split into four elements. The basic methodology adopted is to obtain load ratios for the given temperature distribution which is in turn determined by the coating thickness.

Result Summary

The calculated coating thickness sufficient for asymmetric beam slim floor construction for all loading conditions are summarised in Table 67.1. A minimum practical coating thickness of 500 g/m2 is assumed. For 60 minutes, protection is not normally required.

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