Column core response during progressive collapse of RC flat slabs

Reinforced Concrete (RC) flat slab structures are sensitive to punching shear failure. Numerous static loading tests have shown that a sudden shear failure surface is developed in the slab surrounding a supporting column, while the column is not damaged. When the detached slab impacts the slab underneath, an impact punching shear failure occurs, where the entire slab volume around the column is severely damaged and the connection failure occurs along the column core circumference. This paper investigates this event, and identifies that severe damage occurs within the column core domain as well. The column core is the key part of the slab-column connection, and its completeness is crucial to carry the building gravitational loads and its damage may end with a catastrophic collapse of the entire building. The paper focuses on the damage development in the column core during a representative flat slabs impact event. This is carried out employing an advanced numerical analysis capable of following the progressive damage evolution in any unit cell of the connection and mapping the increasing damage shortly after impact. Different parameters are examined such as the column width and cross-section sides aspect ratio. The analysis clearly shows that a larger cross-section area reduces the damage to the column core and a square cross-section shape is preferable than an elongated rectangular cross-section. Attention is given to the extent of damage and its effects. Opposed to the shear surface that is formed in the case of static punching shear, the impact punching shear causes complete failure of the slab surrounding the column core. The damage invades into the column core domain eroding the concrete beyond the column stirrups and vertical rebars. A major inner part of the column is damaged as well. This affects reduction of the column axial load capacity, loss of the stirrups' confinement, exposure of vertical rebars that may buckle as a result and reduced bending resistance of the column cross section. This major deterioration of the column core has not been identified nor reported in the literature. The obtained results are compared with the column core damage observed at different failed connections of a progressively collapsed parking garage building, and the observed damages correspond very well with the above analysis results.