Experimental study on the stiffness and cracking behavior of steel hollow section reinforced UHPC beams

Ultra-high-performance concrete (UHPC) possesses high strength and excellent durability; however, its relatively high material cost limits its widespread application in conventional flexural members. Embedding a steel hollow section (SHS) in a reinforced UHPC beam can not only reduce UHPC consumption and member self-weight but also improve the seismic performance of the member. To investigate the flexural stiffness and cracking behavior of steel hollow section reinforced ultra-high-performance concrete (SHSRUHPC) composite beams under vertical loading, flexural tests were conducted on five beam specimens. The effects of SHS thickness and longitudinal reinforcement ratio on the flexural behavior were investigated. The failure modes, crack patterns, load-deflection responses and strain distributions were also analyzed in detail. The results showed that all specimens exhibited typical balanced flexural failure with satisfactory ductility. Strain measurements at the mid-span section indicated that some slip occurred between the SHS and UHPC as the load approached the peak value; nevertheless, the average strain distribution generally satisfied the plane-section assumption. Within the investigated parameter range, increasing either the longitudinal reinforcement ratio or the SHS thickness enhanced the stiffness of the specimens, while the increase in the SHS thickness produced a more pronounced improvement in flexural performance. Based on reasonable assumptions, analytical formulas were proposed for predicting the crack width and short-term flexural stiffness of SHSRUHPC composite beams. The theoretical predictions agreed well with the experimental results within the investigated parameter range. The findings provide a useful reference for the design and engineering application of SHSRUHPC composite beams.