Report - PEER - University of California, Berkeley

mild steel reinforcement are filled with non-shrink cementituous fiber grout. Thegrout at the interfaces ensures continuity between precast members while grouting theducts enables the reinforcement to contribute to the stiffness and strength of thehybrid frames. The mild steel reinforcing bars are debonded over a short length nearthe interfaces to control the inelastic strain accumulation and avoid premature fractureof the reinforcement. A friction mechanism is relied upon for shear transfer across theprecast connection interface.The hybrid frame concept has been studied over the past decade, which includedexperimental verification using component (Stone et al. 1995; Stanton et al. 1997) andstructure level testing (Priestley et al. 1999; Sritharan 2002). More recently, thehybrid frame has been implemented in a few buildings including a 38-story apartmentcomplex in San Francisco, California (Englekirk 2002).h c(Area = Apt)h(Total area =2*As)1.2 BenefitsFigure 1. The connection details of a precast hybrid frame.The use of unbonded steel reinforcement at the precast connections causes flexuralcracks to concentrate at the beam ends. Consequently, the beams will undergominimal structural damage when the hybrid frame is subjected to inelastic lateraldeformations. An added benefit of reducing the beam damage is that the frameelongation resulting from the formation of plastic hinges at the beam ends will besmaller than that expected in a monolithic concrete frame. Nonlinear elastic responsefrom the unbonded post-tensioning tendons and hysteretic behavior from the mildsteel reinforcement will enable the hybrid frames to dissipate energy and minimizeresidual displacements. The reduced residual displacements will also make theseframes less sensitive to P-∆ effects. The post-tensioning tendons that run across thecolumn width reduce the principal tensile stresses in the beam-to-column joints. Thereduction to the principal tensile stress suggests that the amount of joint shearreinforcement could be reduced when compared to the joints in equivalentconventional concrete frames.446