Experimental study on the flexural performance of concrete-encased square steel tube truss beams

Flexural performance experiments on five concrete-encased square steel tube truss (SSTT) beams were conducted in this paper. The crack patterns, failure modes, vertical load-midspan deflection curves and strain distribution of this type of composite beams were studied. The cracking, yield and ultimate loads were then obtained. The results indicated that the five composite beams had a bending failure, i.e, the lower chords in tension first yielded and a subsequent crushing of concrete followed in the compression zone. Under the vertical loads, the composite beams experienced four stages: the integral, cracking, yielding of steel tubes of lower chords, and the failure stage. Moreover, the steel tubes of the lower chords had a gradual yield during the loading process, whereas as the composite beams reached the ultimate bearing capacity, the steel tubes of the upper chords were still in the elastic state, which showed a linear change with the increasing loads. In addition, the strains across the mid-span section of the composite beams conformed to the plane hypothesis. Based on the test results, the formulae of normal sectional flexural capacity, short-term flexural rigidity and crack width for concrete-encased SSTT beams were proposed, which could be used as a reference in practical engineering.