Fire Resistance of Innovative and Slender Concrete Filled Tubular Composite Columns

The behaviour of composite compressive members at room temperature was widely studied during the last decades. Due to these investigations, simplified design methods have been developed, which are provided by Eurocode 4 [1]. In this way composite columns became more and more popular and are used in numerous high rise and multi-storey buildings.

In addition to their load bearing capacity and architectural attractiveness cross sectional shape, circular and elliptical cross sections provide a number of advantages. They are able to reach a fire resistance (FR) up to R90 and higher without additional fire protection. Furthermore, additional steel cores inside the columns may increase the FR and lead to higher load bearing capacity both at room temperature and in the fire situation. This is caused by the slower temperature increase in the steel core due to protection by the concrete filling. Especially the performance of these columns with integrated steel core is not fully clarified yet considering the exceptional scenario event of fire. There are no recommendations for the structural fire design available yet.

In particular the load introduction is not explored in detail. Some investigations to this critical scope can be found in literature, but prior for conditions at room temperature. In case of fire there are only a few investigations yet. The special fact in this case is that the load distribution is not uniform at the columns end. For example in a hollow section column with concrete filling and a solid head plate (as shown in Figure 2), the outer tube warms up faster due to thermal expansion and carries the load initially. Subsequently, its maximum load bearing capacity will be reached and the tube begins to yield, so that the concrete will carry the load. Regarding hollow section columns with solid steel core inside, this phenomenon appears in the same manner. It is still unclear to which magnitude the load distributes between concrete filling and steel core.  Moreover, there are many opportunities to introduce the load into the column. This manner of load introduction implies a good composite action between steel and concrete, so that the loads distribute to the whole cross section. Eurocode 4 and DIN 18800-5 provide recommendations regarding this special scope for room temperature conditions. For elevated temperatures there is no information available.

To get a deeper understanding of this behaviour, experimental and numerical investigations are planned within the proposed research project. Three different kinds of load introduction shall be performed in small scale fire tests to study the scope of load introduction. The composite action will be ensured by studs. The specimens will be loaded by a constant axial force and will be exposed to ISO-standard fire for 90min. The distribution of stress in the cross-section will be measured to get a deeper understanding of the load distribution during a fire. 10 small scale fire tests shall be performed in this context. Numerical simulations will be benchmarked by test results and will allow a wider study of parameters.

Partner

• Universidad Politécnica de ValenciaCentre Technique Industriel de la Construction Métallique (CTICM)
• Institut für Stahlbau, Leibniz Universität Hannover
• Imperial College London, Structures Section, Department of Civil Engineering
• University of Coimbra
• AIDICO, Technological Institute of Construction
• CONDESA, Conducciones y Derivados