Crack patterns and bond effects in the progressive collapse of local prestressed concrete substructures

In large buildings, the combined application of long-span prestressed concrete beams and short-span reinforced concrete beams is common.Such structures are designated as local prestressed concrete (LPC) structures.LPC structures could be classified into two types: unbonded local prestressed concrete (UBLPC) structures and bonded local prestressed concrete (BLPC) structures.Nevertheless, the crack patterns of LPC structures under progressive collapse remains unclear.The bond effect between prestressed tendon and concrete on crack development and loadcarrying capacity is yet to be elucidated.Therefore, this paper tested the progressive collapse resistance of BLPC and UBLPC substructures.The results indicated that LPC substructures exhibited the resistance mechanism comprising two distinct stages: beam-beam stage and beam-link stage.A main crack appeared at the junction of the reinforced and non-reinforced regions of the longitudinal rebar in long-span beam, with surrounding clusters of cracks.The cracking at the ends of short-span beam was significant.Mechanistic analyses revealed the bond effect on crack patterns.Theoretical derivation demonstrated that bonded prestressing provided higher resistance than unbonded prestressing.However, bonded prestressed tendon was more prone to fracture.This study could inform the design of LPC structures against progressive collapse.