For all those buildings which are made with old specifications or design comes under red mark. So, to safe the buildings from such damages methods of retrofitting is introduced.
One of those is shear wall method which is a type of retrofitting method. Modelling and design of single and multi-storey structures now-a-days is based on this method so to safe the buildings from vibrations generated by the earthquake.
Actions taken to upgrade the seismic resistance of an existing building so that it becomes safer under future earthquakes is done by retrofitting method. Retrofitting is of various types using different methods as well as different material on the basis of cost and conditions. This can be in the form of providing seismic bands, eliminating sources of weakness or concentrations of large mass and openings in walls, adding shear walls or strong column points in walls, bracing roofs and floors to be able to act as horizontal diaphragms, adequately connecting roofs to walls and columns and also connecting between walls and foundations.
2.1 CONCEPT OF RETROFITTING:-
Retrofitting is technical interventions in structural system of a building that improve the resistance to earthquake by optimizing the strength, ductility and earthquake loads. Strength of the building is generated from the structural dimensions, materials, shape, and number of structural elements, etc. Ductility of the building is generated from good detailing, materials used, degree of seismic resistant, etc. Earthquake load is generated from the site seismicity, mass of the structures, important of buildings, degree of seismic resistant, etc.
Due to the variety of structural condition of building, it is hard to develop typical rules for retrofitting. Each building has different approaches depending on the structural deficiencies. Hence, engineers are needed to prepare and design the retrofitting approaches. In the design of retrofitting approach, the engineer must comply with the building codes. The results generated by the adopted retrofitting techniques must fulfill the minimum requirements on the buildings codes, such as deformation, detailing, strength, etc.
2.2 METHODS OF RETROFITTING:-
Increase the capacity/strength of the system (Seismic Resistance Based Design)
1. Concrete Jacketing.
2. Retrofitting using steel.
1. Concrete Jacketing:-
In this technique, the wall dimensions are increased by adding new concrete to the original web. Additional reinforcement could be used to increase the strength and ductility of the wall. The new reinforcement can be vertical and horizontal bars that form the reinforcement mesh or it can be diagonal bars. The new reinforcement should be anchored to the wall foundation. One way of anchoring is by placing the reinforcement in holes that are drilled in the foundation, and then it is grouted with epoxy. The new concrete is casted with the new dimensions and cured after solidification. Fiorato et al. (1983) tested two RC walls, one rehabilitated using diagonal bars after removal of the damaged web concrete in the plastic hinge region and the other one is rehabilitated by increasing the web thickness (jacketing). The tests showed that the strength and deformation capacities of the rehabilitated walls had increased, while their initial stiffness was almost half that of the original walls. It should be noted that, in some cases when the wall foundation is not over-designed, it will be needed to strengthen the foundation as well in order to be able to carry the additional weight of the wall and the increased lateral load expected to be carried by the wall.
2. Retrofitting using steel:-
Steel is the most common material that was used for retrofitting of RC structures. Steel sections were used to retrofit RC shear walls with different schemes to enhance different response parameters. The lower added weight to them structure (compared to concrete jacketing) and the minimum disruption to the building occupants are advantages of using steel retrofitting systems (Ghobarah and Abou Elfath 2001). On the other hand, steel vulnerability to corrosion, the need for scaffolding, the difficulty of handling the heavy steel plates at the site are problems that arise when retrofitting using steel (Bakis et al. 2002). The following sections discuss the main techniques that were used for retrofit of RC shear walls using steel and the corresponding experimental tests.
1. Using steel sections
In this technique, steel plates are attached to the wall to increase the wall strength, stiffness, ductility or a combination of them. The steel plates can be attached vertically or horizontally according to the enhanced property. Elnashai and Pinho (1997) studied the effect of rehabilitation scheme used for retrofitting shear walls using steel plates on the enhancement of a certain property (e.g. wall stiffness, strength or ductility) without altering the other properties. Figure 5 shows different rehabilitation schemes of the walls studied by Elnashai and Pinho (1997). They concluded that enhancing the wall stiffness without altering the strength can be achieved by using external steel plates bonded along the wall length near the edges as shown in Figure (a), the plates can be bonded along the whole height or along the expected plastic hinge height, and a gap should exist between the plates and the foundation or the top slab in order not to affect the wall strength as the critical section will remain as before. Increasing the wall strength without altering the stiffness can be achieved by using external unbonded steel plates or bars connected with an Interaction Delay Mechanism (IDM) as shown in Figure (b). The IDM allows the added plates or bars to work only after a certain displacement is exceeded. The plates or bars can be attached to the slabs between the wall height, and then enclosed by a ductile material that provide corrosion and fire resistance to the steel. This retrofitting scheme can be used provided that the concrete will be able to carry the additional shear and compression forces applied on it due to strengthening without crushing. Increasing the wall ductility with a minor increase of the stiffness and strength can be achieved by using U-shaped external confining steel plates that are bonded to the wall using epoxy, and bolted using prestressed bolts as shown in Figure (c). Increasing the wall ductility will increase the energy dissipation capacity of the wall which will enhance the seismic behaviour of the retrofitted wall. To increase the shear strength of RC walls, Elnashai and Pinho (1997) used staggered horizontal steel plates that ar epoxy-bonded to the wall and bent at both wall extremities. The increase of the wall shear strength would lead to ensure ductile flexural behaviour of the wall under dynamic loading. This would increase the wall deformation capacity (ductility) and hence increasing the energy that could be dissipated by the wall during an earthquake. Cho et al. (2004) studied RC wall strengthened with channel steel sections as boundary elements, the channels were connected with the concrete wall using headed studs. The tests showed that the boundary elements improved the performance of the wall significantly, and lead to a higher energy dissipation capacity and ductility. The tests showed also that local buckling of the added steel sections is an important issue that should be considered.
(Fig-2:-using steel section)
2. Using steel bracing:-
Steel bracings are mostly used for rehabilitation of nominally-ductile moment resisting frame structures. They can provide the adequate strength, stiffness and ductility required for the structure, provided that a special attention should be directed to their connections with the existing structure. Steel bracings can be also used to enhance the seismic performance of RC shear walls. In that case, the steel bracing can be anchored to the RC wall at small intervals to minimize the buckling length, which will increase the capacity of the bracing member compared to the case of retrofitting the moment resisting frames that is governed mainly by buckling of the compressed bracing member. It is usually recommended to add vertical steel strips at the wall edges when using diagonal bracings, due to the fact that the diagonal forces in bracing members will have a vertical (compression/tension) components that will add higher forces on the wall, in that case it is better to provide vertical strips at the wall ends to resist a part of these forces with the concrete. Taghdi et al. (2000) tested a RC wall that is retrofitted using this technique. Figure 6 shows the retrofitted wall at 1.0 % drift. The tests showed that the retrofitted wall reached an ultimate lateral load capacity up to 2.8 times its original capacity, and an energy dissipation capacity up to 4 times the original one, which indicates the efficiency of this technique in retrofitting RC walls.