Tension Chord Model and Flexural Stiffness for Circular CFST in Bending

The flexural behavior of circular concrete-filled steel tubes (CFST) is addressed in the present paper, with the aim of understanding the contribution of the concrete infill and the steel tube to the flexural stiffness under in-service loads. The provisions given by current codes of practice are oversimplified and they result in a very different contribution of the concrete infill from one code to another. In the present paper, a mechanical approach is proposed by taking into account the stress transfer mechanism from the steel to the concrete through bond stresses. The paper firstly addresses the response of the cracked section in pure bending. Secondly, a tension chord model for CFST is proposed, so that the contribution of concrete in tension between cracks (tension stiffening) can be evaluated. The model capabilities are compared with experimental results from the scientific literature and with own experimental results of the author, showing that the proposed approach can predict the flexural stiffness of CFST very satisfactorily. For the first time, a model with sound mechanical background is introduced to calculate the bending stiffness of CFST.