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CHAPTER 6 CONCLUSIONS 6

March 28, 2019 0 Comment

CHAPTER 6
CONCLUSIONS
6.1 GENERAL
This study highlights the poor seismic performance of RCC buildings with soft storey at different level along with soft storey at ground level. The study is carried out for three buildings i.e. building with soft storey at a) GL &4th floor, b) GL &8th floor, c) GL &12th floor. All buildings are modeled in SAP 2000 and nonlinear static procedure (N SP) or pushover analysis is carried out. Buildings were found to be deficient due to formation of hinges in columns at ground level soft storey as well as in columns of upper soft storey. Hence it is seen that buildings are failed by soft storey mechanism in GL hence retrofitting is suggested. It is also seen that most of the hinges are formed in beams. As a result different strengthening schemes are used to improve performance of deficient buildings. All these different retrofitting strategies are aimed to improving performance level for DBE condition. Based on results following conclusions are drawn.

6.2 PERFORMANCE OF SOFT STOREY KEPT AT DIFFERENT LEVEL
• Soft storey buildings designed as per IS: 456-2000 are seismically deficient. These buildings are unable to produce sufficient lateral load resisting capacity during an earthquake to avoid sever damages. In all the three buildings with soft storey at different level the linear hinges are formed in columns of bottom soft storey at performance point.
• At performance point in all beams B-IO hinges are formed and in brick wall CP-E level hinges are formed.
• It is observed that as the uppers soft storey in multistory building is kept at higher level the base shear of building increases.
• As the soft storey is shifted to higher level the intensity of hinge formation becomes lower and lower.
• As the soft storey is shifted to higher level the displacement of building increases.
• As the soft storey is shifted to higher level the Ta of building decreases.
6.3 EFFECT OF INFILL WALLS
• It is observed that when brick infills are provided as a retrofitting strategy the base shear of building is increased and also displacement gets reduced in both X and Y direction as compared to without brick infill in soft storey.
• The Ta of building is reduced but which is not in permissible limit.
• Hinges formed in beams are reduced to some extent but hinges formed in columns at ground soft storey are not removed completely. Hence it cannot be used as retrofitting strategy. The hinges are also formed at Centre of infill walls.
6.4 EFFECT OF SHEAR WALLS
• The study of hinge formation patterns in case of buildings retrofitted with RC shear walls show that hinges are not developed in columns.
• Hinges formed in beams are at operational level (B) at performance point.
• Hinges crosses collapse (C) level in case of masonry struts.
• After retrofitting with shear walls it is observed that the base shear carried at performance point is increased also Ta is decreases which are in permissible limit.
• Roof displacement of the buildings when they are retrofitted with shear walls is less as compared to steel X braces and infill walls.
• As the shear walls are provided up to the upper soft storey the roof displacement decreases and base shear increase.
6.5 EFFECT OF STEEL X BRACES
• The study of hinge formation patterns in case of buildings retrofitted with steel X braces show that hinges are not developed in columns.
• Hinges formed in beams are at operational level at performance point.
• Hinges formation in bracing are at B-IO level for soft storey at 4th, 8th and 12th floor.
• It is observed that most of the steel braces fail in compression because of buckling at performance point, if stronger braces are used; failure mechanism may be transferred to a column which is not accepted. Hinges crosses collapse(C) level in case of masonry infills at 4th, 8th, 12th. The hinges are formed at center of masonry infill strut.
• After retrofitting with steel X bracing it is observed that the base shear carried at performance point is increased.
• The displacement of building when retrofitted with X bracing is reduced than that of retrofitted with infill walls.
• The Ta of building is reduced but which is not in permissible limit.
6.6 COMPARISON BETWEEN THREE RETROFITTING STRATEGIES
• It is seen that when alternate brick walls are provided as a retrofitting scheme the hinges from columns of bottom storey are not removed hence it is not very efficient as a retrofitting strategy.
• When bracings are provided as a retrofitting strategy the hinges from columns get removed completely. Hinges are formed in bracing.
• When soft storey of building is retrofitted with shear walls the hinges from columns get totally removed at the performance point. The hinges from beams are reduced to linear level and hinges formed in walls are CP range which is acceptable criteria in FEMA356. It is because of shear walls increase stiffness of the building more than other two retrofitting schemes.
• Roof displacement for infill walls and steel X braces are found to be closer.
• The Ta of model retrofitted with shear wall is in permissible limit.
• At performance point, maximum roof displacement is observed in steel X braces as compared to model retrofitted with shear walls because steel itself is more ductile material than concrete; also steel braces contributes negligible mass and stiffness to the original mass and stiffness of the structure.
• At performance point, maximum roof displacement is observed in infill walls as compared to model retrofitted with shear walls because stiffness of shear wall is greater than infill wall.
• A number of different strengthening systems can be adopted to improve the seismic performance of deficient buildings. The performance of particular retrofitting strategies depends upon the structural properties of original deficient building. In this case shear walls placed in outer bay as well as steel X bracings improved the performance to desired level. But performance is better when shear walls are provided in outer bays at comer.
• According to IS-1893 (PART I):2002 there is no any formula for calculation of Fundamental natural time period of Multistoried building containing soft storey at different level. So NSP analysis is best for calculating Ta of such Building.

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