5.1 INTRODUCTION
The strengthening and enhancement of the performance of deficient structural elements in a structure or the structure as a whole is referred to as retrofitting. Retrofitting of a building is not same as repair or rehabilitation. Repair refers to partial improvement of the degraded strength of a building after an earthquake. In effect, it is only a cosmetic enhancement. Rehabilitation is a functional improvement, wherein the aim is to achieve the original strength of a building after an earthquake. Retrofitting means structural strengthening of a building to a pre-defined performance level, whether or not an earthquake has occurred. The seismic performance of a retrofitted building is aimed higher than that of the original building. The present report does not cover the repair techniques for a damaged building or distressed elements.
A survey of existing residential buildings reveals that many buildings are not adequately designed to resist earthquakes. In the recent revision of the Indian earthquake code (IS 1893: 2002), many regions of the country were placed in higher seismic zones. As a result many buildings designed prior to the revision of the code may fail to perform adequately as per the new code. It is therefore
recommended that the existing deficient buildings be retrofitted to improve their performance in the event of an earthquake and to avoid large-scale damage to life and property.
5.2 GOALS OF RETROFIT
The goals of seismic retrofitting of a building can be summarized as follows (IS 13935: 1993; White, 1995).
1. Giving unity to the structure.
2. Eliminating sources of weakness or features that produce concentration of stresses in members.
3. Enhancing the redundancy of the lateral load resisting systems, thereby eliminating the possibility of progressive collapse.
4. Increasing the lateral strength and stiffness of the building.
5. Increasing the ductility (energy absorption) and damping (energy dissipation). Avoiding the possibility of brittle modes of failure.
6. The retrofit scheme should be cost effective, should consistently and reliably achieve the intended performance objective.
5.3 DEFINITIONS
i) Retrofit strategy
The options available for retrofitting individual elements or the building as a whole is termed as retrofit strategies.
ii) Retrofit scheme
A combination of several retrofit strategies is termed as a retrofit scheme for a building.
iii) Retrofit programme
The complete process involved in retrofit of a building is termed as a retrofit programme.
5.4 STEPS OF RETROFIT
A retrofit programme consists of the following steps (Basu, 2002).
i) Seismic evaluation
The evaluation of a building involves data collection, visual inspection, in-situ testing, examination of as-built information and structural analysis. The structural analysis can be linear static (equivalent static method), linear dynamic (response spectrum analysis or time-history analysis), nonlinear static (pushover analysis) and nonlinear dynamic (nonlinear time-history analysis). If the demand-to- capacity ratios of the components are greater than one or if the building fails to achieve the target performance level, then retrofit becomes necessary.
ii) Decision to retrofit
Based on the extent of deficiency of the building, the economic viability, the expected durability of the upgraded building and the availability of the materials, a decision is taken whether to repair, retrofit or demolish the building.
iii) Selection and design of the retrofit scheme
The selection of the retrofit strategies from the options available and their design, influence the decision to retrofit. Hence, knowledge of the retrofit strategies is essential. The design and the detailing should address the transfer of load and the compatibility of deformation between the existing elements, modified elements and the new elements as per the assumptions in the analysis.
iv) Verification of the retrofit scheme
Structural analysis is necessary to justify the selected retrofit scheme. Alteration of the load path, redistribution of the member forces and the changes in the failure
modes after retrofitting, need to be studied. The increase in strength at the cost of a ductile failure mode changing to brittle is not desirable. The selection and design of the retrofit scheme may need to be revised accordingly.
v) Construction
The effectiveness of the retrofit scheme greatly depends on the quality of execution. Hence, the proper execution as per the suggested detailing and specifications is imperative.
vi) Monitoring
Monitoring the performance of the retrofitted building is necessary to detect any defect or remaining deficiency. This will lead to a refinement of the design guidelines and the specifications for future retrofit projects.
5.5 PERFORMANCE OBJECTIVES
For seismic retrofit of buildings, a performance-based analysis is preferred whenever the necessary tools for the analysis are available. The decision to retrofit and the choice of retrofit strategies are open-ended tasks, as compared to seismic design of a new building. The performance-based analysis is a rational method that aids the decision-making and selection of retrofit strategies in a retrofit programme of a building. The definitions of performance levels are explained in Chapter 4.
A performance objective in a performance-based analysis is the selection of a building performance level under a selected earthquake level. The selection of a performance objective in a retrofit programme is guided by the benefit from improved safety, economic decisions, available technical expertise, inconvenience during the intervention and other considerations. Depending upon the importance of the structure, the building performance levels and earthquake levels are chosen.
stations etc. should be functional immediately after an earthquake. For retrofit of multi-storeyed buildings in India, Collapse Prevention (CP) under maximum credible earthquake (MCE) can be selected, as explained in Chapter 4.
A structure with a trial retrofit scheme needs to be re-analysed to check its performance. If a performance point is achieved satisfying the above objective, then the retrofit scheme is satisfactory. But for severely deficient structure a performance point may not be achieved with an acceptable retrofit scheme. There may be partial increase in strength and ductility. This can be accepted as a reduced performance objective as compared to the basic safety objective.
5.6 RETROFIT STRATEGIES
Retrofit strategy refers to options of increasing the strength, stiffness and/or ductility of the elements or of the whole building. For a building, a combination of retrofit strategies may be selected under a retrofit scheme. Retrofit strategies may be broadly classified as local strategies and global strategies. Retrofit of individual members or elements is referred to as local retrofit, whereas the retrofit of the building as a whole is termed as global retrofit. This classification need not be watertight and strategies falling in either group are expected. It may be necessary to combine both local and global retrofit strategies for an effective retrofit scheme.
5.6.1 Global Strategies
The global retrofit strategies are applied to improve the overall behaviour of a building. If a building has inadequate strength to resist lateral forces, it exhibits inelastic behaviour at very low levels of ground shaking. Analysis of such a building indicates large demand-to-capacity ratios in the components throughout the structure. By providing supplemental elements to the building’s lateral force resisting system, it is possible to raise the threshold of ground motion at which the
onset of damage occurs. Addition of shear walls and braced frames, for example, is effective for this purpose. Reduction of plan and vertical irregularities, reduction of mass and improving the connections between the elements are other global retrofit strategies.
In buildings with a large number of deficiencies, it is usually more economical to try a global retrofit strategy first and then if further strengthening becomes necessary, local retrofit strategies can be adopted.
5.6.2 Local Strategies
Local strengthening allows the under-capacity elements or connections to resist the demands predicted by the analysis, without significantly affecting the overall response of the structure. This scheme tends to be economical when only a few of the building’s elements are deficient. The local retrofit strategies discussed here include strengthening of beams, columns, joints, walls and footings.
5.6.3 Energy Dissipation and Base Isolation
A number of technologies are available to allow the energy imparted to a structure to be dissipated through the action of special devices such as viscous fluid dampers, yielding plates or friction pads. These are called energy dissipation devices.
Base isolation produces a system with a fundamental response that consists of nearly a rigid body translation of the structure above the bearings. Most of the displacement induced in the isolated system by the ground motion occurs within the compliant bearings, which are specifically designed for the large displacements. Most bearings also have excellent energy dissipation characteristics.
The cost of energy dissipation and base isolation systems is high and at present their use is limited to important structures like hospitals and monumental structures in India. These devices are not covered in this manual.
5.6.4 Mitigating Geological Hazards
Some of the geological hazards are fault rupture, liquefaction, differential compaction, landslide and earthquake induced tsunamis or flood. Mitigation of geological hazards generally is expensive. Some schemes for the mitigation of these hazards are described in FEMA 356 (2000).