Effects of Axial Restraint on Seismic Performance of R/C Frame Beams and Beam-Column Joints

Document Type

Conference Proceeding

Publication Date


Publication Title

11th National Conference on Earthquake Engineering

Publisher Location

Los Angeles, CA

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Reinforced concrete (R/C) beams tend to elongate after flexural cracking and yielding; however, in an R/C moment frame, the beam growth is restrained by the surrounding structural components such as the columns and cast-in-place floor slabs. Accordingly, large compressive force develops. Experiments were conducted on 1/2-scale interior beam-column subassemblies to study the effects of axial restraint on R/C frame beams and beam-column joints, for which limited data exist. Four interior beam-column subassemblies were tested. The equivalent beam top reinforcement ratios normalized by the yield stress of Grade 60 steel were 0.78% and 1.05%. The test setup permitted applying axial restraint at beam ends and measuring the compressive axial force passively generated in the beams. The restraining stiffness simulated that a beam may encounter in typical R/C frames. Each specimen was cyclically loaded up to a lateral drift ratio of 4%. The beams without axial restraint elongated by 18 mm. Under axial restraint, compressive axial force steadily increased in the beams, reaching an axial force ratio of 0.18. For the beams with lower reinforcement ratio, the axial restraint enhanced lateral loading capacity by as much as 60%. Such an effect can reduce the chance of achieving “strong-column-weak-beam” in R/C moment frames subjected to earthquake loading. The tests also revealed the negative impact of beam axial restraint on beam-column joints. For the specimen with higher beam reinforcement ratio, the axial restraint dramatically increased shear demand in the joint, thereby leading to severe damage in the joint.


Geophysics and Seismology