Experimental study on the structural behavior of double z-shaped steel–encased concrete composite beams

This paper presents an experimental investigation into the structural behavior of a novel deck configuration utilizing double Z-shaped steel–concrete composite beams (2ZSCCBs). It is proposed as an alternative to conventional U-shaped steel–concrete composite beams (USCCBs). The 2ZSCCB consists of a plain concrete beam partially encased by double cold-formed Z-shaped steel sections, connected using threaded bolts without welding—addressing the challenges of welding thin-walled sections. The experimental program involved five full-scale beam specimens tested under a four-point bending setup. One specimen was a conventional reinforced concrete beam (BRC), used as the reference, whereas the other four were 2ZSCCBs without traditional shear or flexural reinforcement. Four key parameters were investigated: (i) diameter of bottom flange bolts, (ii) length of bottom flange bolts, (iii) presence of side bolts in the web, and (iv) use of angle connectors. Results showed significant improvements in load-carrying capacity and stiffness for the 2ZSCCB specimens compared to the reference beam. Bolt connectors in the tension zone were especially effective. The ultimate load capacity of the 2ZSCCBs was approximately three times higher than that of the BRC beam with the same cross-section. The use of 12 mm diameter, 80 mm long bolts (B2) at the bottom flange notably enhanced ultimate strength, increased strain energy absorption, developed a tiny relative slip between the bottom flange of 2Z-shaped steel and encased concrete, and changed the failure mode from flexural to shear-compression failure. Additionally, the inclusion of side bolts (B3) further enhanced performance by preventing bolt shear failure and improving energy absorption.