Waved Joint for Seismic-Resistant Precast Floor Diaphragms

Floors made of precast concrete hollow-core slabs may be constructed without a structural concrete topping and with transverse tie reinforcement placed only out of the slabs. Longitudinal joints between adjacent precast units are filled with mortar on site. To ensure good diaphragm performance under seismic action, a special joint has been devised, in which the slab sides are profiled as continuous sinusoidal waved shear keys. The research involved experimental investigations and analytical modeling. The features of the joint and the means of production were first worked out. Then, a series of destructive tests were performed, on both short joint samples and full-scale floors, made of extruded prestressed hollow-core slabs without topping, all subjected to large in-plane loading reversals. They showed that, after the adhesion between the grout and precast concrete is overcome, a stable cyclic shear transfer mechanism develops, based on wedge action and friction raised at the wavy slab–mortar interface. The analytical studies led to a specific interface finite element, condensing the behavior of a segment of joint into two degrees of freedom. The model considers also the possibility of the joint being partially open, which may happen when the floor undergoes in-plane bending. This interface element, when implemented in a global model with “shell” finite element representing the slabs, allows for numerical tests on the ultimate behavior of floors of various configurations.