Effect of longitudinal reinforcement and prestressing on stiffness of composite beams under hogging moments

• Laboratory tests were conducted on five composite steel and concrete beams.
• Prestressing significantly improved the cracking moment of composite beams.
• Crack spacing was affected by the transverse reinforcement spacing in some cases.
• A model considering the tension stiffening effect was proposed to estimate stiffness.

To investigate the stiffness of steel–concrete composite beams subjected to hogging bending moments and their enhancement through prestressing using external tendons, laboratory experiments were conducted on five steel–concrete composite beams. Three of the beams were two-span continuous composite beams with box steel girders and the others were simply supported. The parameters investigated included the amount of longitudinal reinforcement and the arrangement of prestressing. From the experimental results, it was found that prestressing significantly improved the cracking moment of composite beams, and crack spacing was dominated by the transverse reinforcement spacing. The stiffness enhancement and crack control suggest the advantages to the mechanical performance of composite beams prestressed by external tendons. A theoretical model is proposed to estimate the effective stiffness of a composite beam subjected to hogging bending moments, taking into account the tension stiffening effect of concrete cracking.