International Journal of Steel Structures

, Volume 19, Issue 1, pp 147–156 | Cite as

Tension Chord Model and Flexural Stiffness for Circular CFST in Bending

  • Carlos ZanuyEmail author


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.


Concrete-filled steel tubes Bending behavior Serviceability Tension stiffening 

List of symbols


Concrete area


Effective concrete area in tension


Steel area




Concrete modulus of deformation


Steel modulus of elasticity


Flexural stiffness


Cracked stiffness


Flexural stiffness of the uncracked concrete infill


Flexural stiffness of the hollow steel tube


Bending moment


Tensile force

Ts, Tc

Tensile force carried by steel and concrete, respectively


Perimeter of the steel–concrete contact


Concrete compressive strength


Concrete tensile strength


Steel yielding strength


Crack spacing


Average crack spacing






Neutral axis depth


Vertical coordinate, from the center of the tube


Distance of a fiber of the cross-section from the neutral axis


Semi-angle to define the effective concrete area in tension

εs, εc

Steel and concrete strain, respectively


Average strain at the centroid of the tension chord




Curvature of the cracked section


Average curvature


Angular coordinate


Angular coordinate of the neutral axis

σs, σc

Steel and concrete stress, respectively


Bond stress


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Copyright information

© Korean Society of Steel Construction 2018

Authors and Affiliations

  1. 1.Department of Continuum Mechanics and Structures, E.T.S. Ingenieros de Caminos, Canales y PuertosUniversidad Politécnica de MadridMadridSpain

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