Keywords: fire resistance, slim floor, intumescent coating, steel structure, fire test, computer modelling.

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 Slimflor beam, developed by Corus UK (formerly British Steel) and The Steel Construction Institute (SCI), is based on a Universal Column section, with a single horizontal plate welded to its bottom flange (Figure 65.1). The floor can be constructed of pre-cast concrete units or long span composite slabs with deep profiled steel decks.

Because the Slimflor 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 plate. Thin film intumescent coatings are ideal in this respect because they add a negligible amount to the depth of the section. Typically thickness of about 1mm is used for protecting conventional steelwork. For slim floor beams the required thickness is low would be expected to be lower 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 two commonly used intumescent coatings manufactured by Nullifire Ltd., UK.

Procedure

The project took the following procedure:

(i)

Correction of temperature data obtained from small furnace tests on unloaded beams at Nullifire Ltd.

(ii)

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

(iii)

Calculations of moment resistance and load ratio for any given coating thickness and the determination of minimum coating thickness for different periods of fire resistance

(iv)

Extension of calculation results for different beam sizes

(v)

Examination of the effect of service holes.

The reduced moment resistance in fire conditions is calculated using plastic theory, as permitted by BS 5950 Part 8 and EC4 Part 1.2. The basic methodology adopted is to obtain load ratios for the given temperature distribution (i.e. temperatures in plate, bottom flange and lower web) which is in turn determined by the type of coating (S607 or S605) and its thickness. The load ratio is defined as:

The calculations were conducted if the beam was designed at room temperature as a composite beam using a deep steel deck with concrete cover above the top of the beam. The shear connection percentage was taken as 40 performance of Slimflor beams designed as composite beams is not as good as beams designed as non-composite. Therefore, these conditions represent the most severe fire situation that could be encountered in normal construction, and therefore the results would in fact give additional safety factors for many practical cases.

Result Summary

The calculated coating thickness sufficient for all types of Slimflor construction is summarised in the Table 65.1. As the calculations were for the most severe fire conditions, reduced coating thickness may be possible for many situations.

Note: For 60 minutes without service holes, protection is not normally necessary. For most practical applications 2000 gm/m2 would be adequate.

The assessment carried out is based on the insulating properties of the coatings. An assessment of the stickability of the products has been carried out by Fire Safety Engineering Consultants Ltd. who have concluded that the "stickability" of the coatings should be adequate for the above fire resistances and protection thickness.

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