Development and seismic performance evaluation of New high strength reinforced concrete column and steel beam (New-RCS) joint

Reinforced Concrete column and Steel beam (RCS) structural systems have recently become popular in Taiwan for office buildings. High seismic demand makes the size of conventional concrete columns enormous and the reinforcement over-crowded, which can be solved using high-strength materials. However, due to the lacuna of research regarding the usage of high-strength materials, existing design guidelines for the RCS systems have restricted the maximum grade of steel reinforcement to 410 MPa. Also, using high-strength materials in RCS joints necessitates innovative joint detailing and design to develop sufficient joint bearing and shear strength. Thus, a new high-strength reinforced concrete column and steel beam (New-RCS) joint was developed in this study, incorporating the use of Grade 690 MPa steel as reinforcement along with Grade 84 MPa concrete. A combination of wide flange bearing plate and flange doubler plate joint detailing was developed to improve the bearing and shear capacity of the joint. Also, a design methodology based on moment-curvature analysis was proposed to include the contribution of locked longitudinal reinforcement to bearing resistance. Subsequently, two large-scale interior beam-column subassemblies were subjected to quasi-static cyclic loading to verify the seismic behaviour of the proposed joint. One specimen was designed explicitly considering the material hard-ening and overstrength of steel beams. While the other was designed following the Taiwanese practice, which does not explicitly consider material hardening and overstrength but implicitly does it through strength reduction factors. Both the specimens exhibited a drift capacity of 4% with a stable and ductile hysteretic response and eventually failed through beam plastic hinging.

Automated summary

Reinforced Concrete column and Steel beam (RCS) structural systems are popular for office buildings in Taiwan due to the high seismic demand. High-strength materials are used to solve the problem of large size and overcrowded reinforcement in conventional concrete columns. However, the usage of high-strength materials in RCS systems is restricted in existing design guidelines, and innovative joint detailing and design are required for sufficient joint bearing and shear strength. This study developed a new high-strength reinforced concrete column and steel beam joint, using Grade 690 MPa steel reinforcement and Grade 84 MPa concrete, along with a specific joint detailing to improve bearing and shear capacity. Two large-scale subassemblies were tested to verify the seismic behavior of the proposed joint.