Effect of load history on punching shear resistance of flat slabs

• Unloading/reloading cycles increase slab rotations and reduce punching resistance.
• Moment–curvature model including concrete softening and tension stiffening.
• Modeling of unloading/reloading cycles by bilinear envelopes.
• Prestressing of shear strengthening systems compensate punching resistance reduction.

The unloading of reinforced concrete slabs results in residual slab rotations and reloading to the same load results in irreversible rotation increases. Unloading and reloading (UR) cycles applied to non-strengthened and strengthened flat slabs may thus affect the punching resistance, which is rotation-dependent. A quintilinear moment–curvature relationship, which takes concrete softening and tension stiffening into account, combined with UR cycles, modeled as bilinear envelopes, is developed to predict residual slab rotations and irreversible rotation increases. A parametric study shows that the effect of UR cycles on the punching resistance of concrete is normally small, however, it may be significant if the slab is strengthened after unloading, particularly for thin and low-reinforced slabs, which exhibited plastic slab rotations before unloading. Prestressing of the strengthening system may reduce the residual slab rotations and thus limit or compensate the loss of punching resistance.