Retrofitting of Soft Storey Building by using Different Bracing System due to Seismic Load
Abstract
In the present study an attempt has been made to evaluate an existing building located in seismic zone V using equivalent static analysis. Indian Standard IS-1893:2002 (Part-1) is followed for the equivalent static analysis procedure. Building is modeled in commercial software STAAD Pro. Seismic force demand for each individual member is calculated for the design base shear as required by IS-1893:2002. Corresponding member capacity is calculated as per Indian Standard IS456:2000. Deficient members are identified through demand-to-capacity ratio. A number of beams and column elements in the first floor of the present building are found to be deficient that needs retrofitting. A local retrofitting strategy is adopted to upgrade the capacity of the deficient members. This study shows that steel jacketing is an efficient way to retrofit RC members to improve flexure as well as shear capacity.
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Introduction
The increase in urbanization for the past few years has made the vehicle parking a major concern. In order to overcome this usually we provide the first storey of the building for parking. The open ground storied structure is a structure in which the infill wall is absent at the ground storey for the purpose of parking or social gathering. According to earthquake reports the structure having open ground storey leads to complete collapse due to absence of infill wall. Distribution of strength, mass, stiffness should be consistent throughout the building both vertically and horizontally as per design philosophy of structures. Improper orientation of walls results in soft storey, weak storey and torsion effect. The severe damage can be seen on the structure due to irregularity of structures. Due to modern era of construction the buildings without open ground storey is unavoidable because there is shortage of area for parking so we have to provide some special measures on the structure to mitigate the effect of soft storey on the structure . It is very necessary to conduct an in-depth study on the nonlinear behavior of the structure so that it gives the proper response of the structure during earthquakes.
Conclusion
The nonlinear behavior of the structure taken as case study of Zone V (Guwahati) had been analyzed for Time History as well as Pushover Analysis. It was subjected to a suite of six different earthquakes which were scaled as per the target spectrum of Zone-V and the performance of the structure was evaluated. The storey drift, storey displacement, base shear, time period, performance points and capacity spectrum have been observed and evaluated for base model and retrofitted with shear wall and cross bracings (both in different models). The behavior of retrofitted structure with shear wall may be significantly different from what has been observed for base model and cross bracing retrofitted structure. Synthesis of the observed seismic response has led down to the following conclusions-
1. Storey drift is reduced in base model due to introduction of shear wall as compared to cross bracing in great extent.
2. Storey displacement is reduced by 75 to 80% in model II and 23 to 26% in model III as compared to model I.
3. The time period of vibrations for all the three models was analyzed. It is considerably reduced for models II and model III as compared to model I.
4. Base Shear of models were analyzed and it is clearly shown that the base shear of the structure heavily increases and makes the structure more stable against seismic action by using shear wall for retrofitting of the structure.
5. The pushover analysis highlights the performance points in different models. It is shown that the performance of base model is poor as compare to other two models. After retrofitting the base model with shear wall the hinges are not formed beyond immediate occupancy level which makes structure safer.
6. In capacity spectrum curve model II shows less ductility demand under higher acceleration.
A financial feasibility study was also carried out, taking in to consideration the cost-benefit ratio, and it can be concluded that shear wall is an effective technique of retrofitting the structure against lateral loadings.