Performance of Cellular Composite Floor Beams at Elevated Temperatures

Analysis Approach of Cellular Composite Floor Beams

at Ambient and Elevated Temperatures

A. Nadjai1, N. Goodfellow1, F. Ali1, O. Vassart2

1University of Ulster, School of the Built Environment, FireSERT Block 27,BelfastBT37 OQB, UK

2ArcelorMittal, Esch/Alzette, G.-D. of Luxemburg

Abstract

In the UK and Europe, in particular, the development of a wide variety of innovative composite floor systems has been notable. Investigation of the behaviour of composite beams with isolated web openings in otherwise solid webs has shown that the slab significantly increases the shear-carrying capacity beyond that of the steel beam alone. This is due to the enhanced flexural and shear capacity of the upper part of the beam across an opening, although an unsupported web-post is more susceptible to buckling. In fire, the temperature distribution across a composite member is non-uniform, since the web and bottom flange have thin cross-sections and a greater exposed perimeter than the top flange. The deterioration of the material properties of the web will therefore become an important effect on the overall performance of the member in the event of fire (Fig.1).

The model shown in Fig. 2 shows the forces and moments acting on the web post between adjacent openings. Horizontal shear forces are developed in the web post in order to transfer the incremental tension force to the bottom tee. Web posts in asymmetric beams will also be subjected to in-plane moments in order to maintain equilibrium between the top and bottom tees.Due to the shear forces transferred across the web posts between openings failure can occur due to out of plane buckling, as illustrated in Fig. 2. The tendency for the web post to buckle will depends on the width of the web post the height of the opening and the d/t ratio of the web.

Six specimens tested at the University of Ulster with two models of different steel geometries and loading conditions were tested under monotonic loading and at elevated temperatures. The aim of this paperis to presents a simple model of analysis based on the interaction formula that relates the shear forces, moments and section capacities. The model predicts the ultimate failure loads at ambient temperatures and temperature failure time at fire conditions. Good agreement with experimental and Finite Element Diana Software results has been obtained.