CHAPTER 4 RETROFITTING OF REINFORCED CONCRETE BUILDING WITH SOFT STORY AT DIFFERENT LEVEL 4

CHAPTER 4
RETROFITTING OF REINFORCED CONCRETE BUILDING WITH SOFT STORY AT DIFFERENT LEVEL
4.1 INTRODUCTION
In order to well behave under an earthquake, a building should possess adequate strength, redundancy and ductility. The response of a RC-MRFs building subjected to severe seismic forces depends primarily on those properties of its members and en detailing of its individual component and of connections between the components. Many RC-MRFs constructed and designed not according to seismic modem design code or before the development of rigorous seismic design codes and of detailing provisions may not have enough conditions for resisting the seismic actions. It has been clearly observed that many non-ductile RC-MRFs structures are severely damaged or collapsed during small earthquakes with undesirable failure modes such as soft-story mechanisms.
In the 1990s, the Federal Emergency Management Agency (FEMA) and the State of California grew concerned about seismic evaluation and rehabilitation guidelines because of the need of a unified standard of seismic retrofitting. Before and after this time, both experimental and analytical studies were conducted to investigate various retrofit strategies. In addition, many individual investigators and coordinated research programs have provided useful information on seismic retrofitting procedures. The results of this active research have led to significant developments in seismic retrofitting and rehabilitation procedures.

4.2 STRENGTHENING OF SOFT STOREY OF RC BUILDING
Thus the soft storey which is provide at ground level or which is provided at different level of multistory building are more vulnerable to earthquake damage. But in developing nation like India this soft storey are constructed at GL and also at upper floor. A large number of RC frame buildings with open ground soft storey are still being used or constructed in several countries. Since these building are more vulnerable under earthquakes, strengthening of such existing buildings are needed to avoid their severe damage or complete collapse, Simple and yet efficient strengthening technique need to be developed in order to improve the seismic performance of such soft storey buildings in future earthquakes, A strengthening techniques involving masonry infill wall, steel bracings and reinforced concrete shear wall and have to suggest most efficient system out of them.
This chapter presents an overview of some of the retrofitting systems in multistory RCC building with soft storey at different level along with soft storey at GL. In this soft storey at 1) GL & 4th floor 2) GL &8th floor 3) GL &12th floor are analyzed with various retrofitting Strategy. To overcome hinge formation in columns and beams and to improve the performance of building various retrofitting strategies are provided. Here three retrofitting methods are applied such as,
1) Infill with brick masonry.
2) Shear wall.
3) Steel X bracing.
4.3 METHODS OF STRENGTHENING
Generally, there are two ways to enhance the seismic capacity of existing structures. The first approach is a structure-level retrofit, which involves global modifications to the structural system (see Figure 4.1). Common global modifications include the addition of structural walls, steel braces, or base isolators. The second approach is a member-level retrofit (see Figure 4.2). In this approach, the ductility of components with inadequate capacities is increased to satisfy their specific limit states. The member-level retrofit includes methods such as the addition of concrete, steel, or fiber reinforced polymer (FRP) jackets to columns for confinement.

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Fig.4.1 Global modification of the structural system

Fig. 4.2 Local modification of structural components
4.3.1 Structure-level retrofit
Structure-level retrofits are commonly used to enhance the lateral resistance of existing structures. Such retrofits for RC buildings include steel braces, post-tensioned cables, infill walls, shear walls, masonry infills and base isolators. The methods described below are commonly used when implementing a structure-level retrofit technique.
1. Addition of RC structural walls
Adding structural walls is one of the most common structure-level retrofitting methods to strengthening existing structures. This approach is effective for controlling global lateral drifts and for reducing damage in frame members. Generally, repair of an existing shear wall or infilling one of the bays in the frame structure is used. In order to reduce time and cost, shot-Crete or precast panels can be used. The research shows that with the infilling process, details play an important role in the response of panels and the overall structure. The infilling process tends to stiffen the structure such that the base shear can increase. The overturning effects and base shear are concentrated at the stiffer infill locations. Therefore, strengthening of the foundation is typically required at these locations.
2. Use of steel X bracing
The addition of steel bracing can be effective for the global strengthening and stiffening of existing buildings. Concentric or eccentric bracing schemes can be used in the selected bays of an RC frame to increase the lateral resistance of the structure. The advantage of this method is that an intervention of the foundation may not be required because steel bracings are usually installed between existing members. Increased loading on the existing foundation is possible at the bracing locations and so the foundation still be evaluated. In addition, the connection between the existing concrete frame and bracing elements should be carefully treated because it is vulnerable during earthquakes.
3. Seismic isolation
Recently, many researchers have studied seismic isolation as a possible retrofit method. The objective of this type of retrofit is to isolate the structure from the ground motion during earthquake events. The hearings are installed between the superstructure and its foundations. Because most bearings have excellent energy dissipation characteristics, this technique is most effective for relatively stiff buildings with low-rises and heavy loads.
4.3.2 Member-level retrofit
The member-level retrofit approach can provide a more cost-effective strategy than structure-level retrofit because only those components needed to enhance the seismic performance of the existing structure are selected and upgraded. The member level retrofit approaches include the addition of concrete, steel, or fiber reinforced polymer (FRP) jackets for use in confining RC columns and joints.
1. Column jacketing
Column retrofitting is often critical to the seismic performance of a structure. To prevent the story mechanism during earthquakes, columns should never be the weakest components in the building structure. The response of a column in a building structure is controlled by its combined axial load, flexure, and shear. Therefore, column jacketing may be used to increase column shear and flexural strength so that columns are not damaged.
2. Slab-column connection retrofits
In slab-column connections, punching shear failure due to the transfer of unbalanced moments is the most critical type of structural damage. The retrofitting of slab-column connections is beneficial for the prevention of punching shear failures and much research into retrofitting slab-column connections has been conducted reported that adding concrete capitals or steel plates on both sides of the slab can prevent punching shear failures. Both solutions showed improvement in strength along the perimeter.

4.4 DESIGN AND MODELING OF RETROFITTING SCHEMES
To decide the most efficient retrofitting strategy nonlinear static procedure is adopted because it provides a more realistic estimate of the seismic demands under a large seismic event. After going through the extensive literature study and in order to investigate effect of steel X bracing, infill walls and shear walls as a retrofitting scheme on seismic performance of buildings it is very imperative to design and model them very precisely. In this section the methods of modeling of retrofitting scheme are discussed.

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