Flexural behaviour of RC beams reinforced with compressive steel bars and two-piece enclosed stirrups

- Two-piece stirrups can provide sufficient tie stiffness and restraining effect.
- Compressive reinforcement buckles before peak load.
- Reinforcement buckling almost not affects the load carrying capacities of RC beams.
- Flexural behaviour of beams reinforced with one- and two-piece stirrups is identical.

Compressive reinforcement buckling accounts for the flexural behaviour of reinforced concrete (RC) beams with two-piece enclosed stirrups featured with relatively weak transverse restraining. Specimens of six beams and ten hooks were tested to gain experimental evidence on the flexural behaviour of beams and tie stiffnesses of stirrups, respectively. The reinforcement buckling possibility was investigated using an analytical approach in literature for an isolated elastic bar axially loaded under compression and laterally supported by springs. A finite element method based model was developed, verified and used for parametric studies to clarify the mechanics mechanism of flexural behaviour of RC beams. Results revealed that, compared to the case of traditional one-piece stirrups, premature reinforcement buckling was prevented due to sufficient restraining effect provided by two-piece enclosed stirrups. Reinforcement buckling could occur prior to peak load which differed from the assumption in the classical analytical method. The load carrying capacities of the RC beams were insignificantly affected by the reinforcement buckling. Consequently, identical flexural behaviour was confirmed for RC beams reinforced with one- and two-piece stirrups.