Time-Dependent Compressive Arch Action Capacity of Reinforced Concrete Frame Beams Resisting Progressive Collapse

Beam compressive arch action (CAA) is an appealing mechanism for reinforced concrete (RC) frames to resist progressive collapse after losing a column during an abnormal event. Past studies have focused predominantly on the short-term collapse resistance of building structures. However, after the column loss in an RC frame, the remaining structural components need to continuously carry the redistributed gravity loads and therefore the long-term load-carrying capacity of the beams bridging over the lost column is of concern. The research presented in this paper experimentally examined the sustained loading capacity of axially restrained frame beams. Four beam-column substructures were tested. Major test variables included loading age ranging from 50 to 267 days and volumetric transverse reinforcement ratios of 0.47% and 0.94%. Three specimens failed after 2.9 h to 25 days of applying the sustained loads. The failed specimens showed accelerated increase in deflection, axial force, and local strains near failure. Poisson’s ratio of concrete in compression was proven a suitable indicator of impending failure. The sustained load did not pose a harmful effect on sectional flexural strength; instead, it was the secondary bending moment demand due to creep that eventually caused the time-dependent failure. Depending on the loading age and transverse reinforcement ratio, the capacity due to compressive arch action reduced by more than 20% when under sustained loads.