Experimental Study on Retrofit with External Reinforced Concrete Frames and Its Applications on Public Buildings

The Chi-Chi earthquake let people in Taiwan know that the seismic capacities of old buildings are not enough. There are about 25,000 school buildings in Taiwan. They belong to public facility and provide people shelters when their homes are seriously damaged by earthquake. Therefore, school buildings need to raise their seismic ability as soon as possible. However, rebuilding or retrofitting all school buildings will cost a lot and is inefficient. In order to raise the efficiency, National Center for Research on Earthquake Engineering (NCREE) proposed complete procedures for seismic evaluation and seismic retrofit of school buildings. The procedures of seismic evaluation include: simply survey, preliminary evaluation and detailed evaluation (pushover analysis). After screening out school buildings which have less seismic capacity than seismic demand, we need to do seismic retrofit for them. The typical retrofit methods of school buildings are: RC jacketing, adding wing wall and shear wall. All these retrofit methods will affect the function of existing building under construction so they cannot be used for the public buildings such as hospital which the function cannot be affected. In this research, retrofit method with adding external frames, seldom seen in Taiwan, is proposed. Adding external frames is a retrofit method by which two external frames are added along the weak direction of existing building to raise the seismic ability. In this research, external reinforced concrete frames is connected to existing building through reinforced concrete slab. One end of the slab rebar is post-installed to the beam of existing building and the other end is anchored to the beam of external frame. The reinforced concrete slab to transfers the seismic force from existing building to external frames. In order to evaluate the effectiveness of adding external frames, preliminary evaluation method for the retrofitted building is established. Accordingly, external frames can be design preliminarily. In addition, simple detailed evaluation method is extended from the Ultimate Moment Balanced Method. After pushover analysis, shear demand can be acquired for the design of reinforced concrete slab. Moreover, plastic hinge parameters of the slab are developed and incorporated in the pushover analysis so that the design of slab can be checked. From nonlinear static pushover analysis, the capacity curve is achieved and it is turned into performance curve to find out the seismic ability of building after adding external reinforced concrete frames. In this research, we take one school building and two hospitals as example. We hope the method can not only be used for school buildings, but also take the advantage of the method to use for hospitals or other public buildings. The rebars of the slab in the transverse direction are post-installed into existing beam to transfer the seismic force from existing building to external frame so the shear capacity of the slab will influence whether the retrofit is successful. If the slab cannot transfer the seismic force to external frame successfully, shear failure happens at the slab faster than the external beam and column, the retrofit will fail. Since there are few tests about shear force capacity of slab in Taiwan, we design 4 specimens (2 retrofit specimens and 2 newly built specimens) in this research. We use the specimens to discuss how the shear force is delivered and observe how the design of steel bar influences the shear strength of the slab. In addition to comparing the shear strength of specimens with design method (code) to prove the design rule of code is conservative enough, we also turn the curve of lateral force and displacement of specimens into elasto-plastic curve. And we also refer to Hand Book about Seismic Evaluation and Seismic Retrofit of School Building (III) and modify the evaluation curve of lateral curve and displacement to prove that the evaluation curve is more conservative than test result.