SEISMIC EVALUATION AND RETROFIT OF IIT MAIN BUILDING

Abstract

The present study deals with the Seismic Evaluation and Retrofit of the historical main building of IIT Roorkee, which falls in Seismic Zone IV. The Building is 150 years old, having burnt clay: brick masonry wall in mud mortar and at a few places in lime-surkhi mortar. It has Jack-arch type roof supported on cast iron ¢i d s and at a few places it has flat tiled rf on wooden girders. As the roofing system is flexible in its plane, it cannot transfer earthquake force between difereAn walls; and individual walls resist earthquake force with acting tributary masses. Stresses due to earthquake effects have been calculated using Simplified Static and Linear Dynamic Analysis Methods. For Equivalent Static analysis, individual walls have been considered with seismic load due to slab assumed to act at the top of wall and seismic load due to self inertia of wall assumed to act at 2/3 `d height of the wall. Walls have been assumed to be simply supported on four edges in out-of-plane action. The walls have been checked against gravity, and in-plane and out-of plane bending lateral forces due to earthquake. The strength of masonry has been considered from literature. The permissible values of stresses have been considered according to IS 1905- 1987 and it has been increased by 33% when considering the seismic actions as per IS:1893-2002. The building also has been modelled in ANSYS using solid elements for masonry and mass elements for mass application and dynamic analysis has been performed for the design response spectrum as per IS: 1893. First, individual solid and perforated walls have been modelled to evaluate the seismic capacity of walls and study of stresses. The_ walls have been fixed at bottom, restrained in out-of-plane at junctions and the slab load is applied on the top of the wall uniformly. In the next step, a 3D FEM model of a representative portion of the building has been developed and dynamic analysis has been performed. The safety of walls and the representative portion of building have been evaluated against compression, in-plane shear, in-plane bending, and out of plane bending modes of failure. The analysis shows the walls are unsafe in in-plane shear, in-plane bending and out of plane bending. Four alternative methods have been considered for retrofitting of the building. These consist of (i) Providing 250 mm shear wall camouflaged into brick wall, (ii) Providing RC jacket on one side of walls, whereas the other side is covered with welded wire mesh iii plastered with cement mortar, (iii) Providing RC jacket on both sides of walls, and (iv) Providing Ferro-cement on both sides of the walls. In the first method, cutting of masonry wall is very difficult and providing adequate foundation to the added shear wall is a challenge. Providing RC jacket of 125 mm on one side of the wall is sufficient to resist the lateral force but welded wire mesh is needed to resist the tensile force in out-of-plane action. The third method is comparatively better than the above two methods because of symmetry in out of plane behaviour and box type action providing better resistance against out-of-plane earthquake forces. Finally, Ferro cement Jacket of 40 mm thick has been suggested on both sides of the walls. Detailed drawings of the two suggested retrofit methods have been provided. The roof of the building consists of rogjricj and it is vulnerable against earthquake forces. Bracings have been suggested to avoid the relative movement between cast iron girders and to support the walls in out-of-plane action. The bracings have been designed for maximum compressive and tensile forces. The 3D model of the building has been developed with braced roof which results in decrease in time period and increase in mass participation in the modal vibration. The validation of suggested retrofit techniques has also performed using Finite Element analysis of the composite model. Dynamic Analysis of the 3D FEM model has been performed to estimate the stresses in masonry and Concrete / Ferro-cement. It has been shown that the stresses in masonry and Ferro-cement / Concrete are within the permissible limits. Finally, estimation of material quantity and cost of retrofit has been carried out for the building for different alternatives. iv