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Tension Chord Model and Flexural Stiffness for Circular CFST in Bending

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Abstract

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.

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Abbreviations

A c :

Concrete area

A ct,eff :

Effective concrete area in tension

A s :

Steel area

D :

Diameter

E c :

Concrete modulus of deformation

E s :

Steel modulus of elasticity

EI :

Flexural stiffness

EI cr :

Cracked stiffness

E c I c :

Flexural stiffness of the uncracked concrete infill

E s I s :

Flexural stiffness of the hollow steel tube

M :

Bending moment

T :

Tensile force

T s , T c :

Tensile force carried by steel and concrete, respectively

U s :

Perimeter of the steel–concrete contact

f c :

Concrete compressive strength

f ct :

Concrete tensile strength

f y :

Steel yielding strength

s r :

Crack spacing

s rm :

Average crack spacing

t :

Thickness

x :

Abscissa

x cr :

Neutral axis depth

y :

Vertical coordinate, from the center of the tube

z :

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

αx :

Semi-angle to define the effective concrete area in tension

εs, εc :

Steel and concrete strain, respectively

εsm :

Average strain at the centroid of the tension chord

κ:

Curvature

κcr :

Curvature of the cracked section

κm :

Average curvature

θ:

Angular coordinate

θcr :

Angular coordinate of the neutral axis

σs, σc :

Steel and concrete stress, respectively

τ:

Bond stress

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Authors and Affiliations

  1. Department of Continuum Mechanics and Structures, E.T.S. Ingenieros de Caminos, Canales y Puertos, Universidad Politécnica de Madrid, c/Profesor Aranguren 3., 28040, Madrid, Spain

    Carlos Zanuy

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  1. Carlos Zanuy
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Correspondence to Carlos Zanuy.

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Zanuy, C. Tension Chord Model and Flexural Stiffness for Circular CFST in Bending. Int J Steel Struct 19, 147–156 (2019). https://doi.org/10.1007/s13296-018-0096-9

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  • DOI: https://doi.org/10.1007/s13296-018-0096-9

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