What is Seismic Retrofitting

3. What is Seismic Retrofitting Seismic retrofitting is the modification of existing structures to make them more resistant to seismic activity, ground motion or soil failure due to earthquakes. There are several techniques which have come forward nowadays by which an existing structures can be modified and make it less prone to earthquakes. There is no such technique to make the structure fully earthquake proof but seismic performance can be greatly enhanced through proper initial design or subsequent modifications.

4. Objectives: • Public safety To protect human life, ensuring that the structure will not collapse. 2. Structure survivability The structure remaining safe may require extensive repair but not replacement 3. Structure functionality Primary structure undamaged and the structure is undiminished in utility for its primary application 4. Structure unaffected A high level of retrofit is preferred for historic structure of cultural significance

5. Need for Seismic Retrofitting of Buildings • Buildings not designed according to the codes of practice • Buildings designed to meet the modern seismic codes, but deficiencies exist in the design and/or construction • Essential buildings must be strengthened like hospitals, historic monuments and architectural buildings • Buildings that are expanded, renovated or rebuilt • Deterioration of strength of the buildings • Not considering the safety of buildings while construction Common seismic retrofitting techniques • Base Isolators • Supplementary dampers • Tuned mass dampers • Slosh tank • Active control system

6. Base isolators: Base isolation is one of the most powerful means of protecting a structure against earthquake forces. It is meant to enable a building to survive a potentially devastating seismic impact through proper initial design or subsequent modifications. Base isolation system consists of • Isolation units 2. Isolation components Isolation units consists of shear or sliding units. They are intended to provide the decoupling effect to a building. Isolation components are the connections between isolation units and their parts having no decoupling effect of their own.

7. Working of base isolators: Base isolators is a technique developed to prevent or minimize damage to building during an earthquake. When a building is built away (isolated) from the ground, resting on flexible bearings or pads known as base isolators, it will move little or not at all during an earthquake. They consists of basic components – a lead plug, rubber and steel, which are generally placed in layers.

8. Rubber – The rubber provides flexibility. At the end of an earthquake, the rubber bearing will slowly bring the building back to its original position. Lead – lead has plastic property. During an earthquake, the kinetic energy of the earthquake is absorbed into heat energy as the lead is deformed. Steel – If layers of steel are used with rubber, the bearing can move in the horizontal direction but is stiff in the vertical direction.

9. Structures with modern isolation base

10. Supplementary Dampers: A supplementary Damping System is essentially an energy dissipation system that is incorporated into the design of a structure to absorb vibration energy, thereby reducing motion. Supplementary damping is the most efficient and cost effective way to achieve energy dissipation in building. This would inadvertently mean decreasing the energy dissipation demand on the structural components i.e. beams/columns/slabs thereby increasing the survivability of the building structure.

12. Dampers are mechanical devices that look somewhat like huge shock absorbers and their function is to absorb and dissipate the energy supplied by the ground movement during an earthquake so that the building remains unhamered. Whenever the building is in motion during an earthquake tremor or excessive winds, dampers help in restricting the building from swaying excessively and thereby preventing structural damage. The energy absorbed by dampers gets converted into heat which is then dissipated harmlessly into the atmosphere. Dampers thus work to absorb earthquake shocks ensuring that the structural members remain unharmed.

13. Tuned Mass Dampers: is a device mounted in structure to reduce the amplitude of mechanical vibrations. Their application can prevent discomfort, damage or outright structural failure. They are frequently used in power transmission, automobiles and buildings. Tuned mass dampers stabilize against violent motion caused by harmonic vibration. It reduces the vibration of a system with a comparatively lightweight component so that the worst case vibrations are less intense. Practical systems are tuned to either move the main mode away from a troubling excitation frequency or to add damping to a resonance that is difficult to damp directly

15. Slosh Tank: In fluid dynamics, slosh refers to the movement of liquid inside another object undergoing motion. To reduce structure motion due to external loadings in high rise building, slosh tank is one of the inventions that can be installed in different locations and levels into a structure in order to increase damping and decrease vibrations. It can either be installed on the top floor of a structure or in some certain floors or even at each floor of a building.

18. Active control System: Active control systems are force delivery devices integrated with real-time processing evaluators/controllers and sensors within the structure. They act simultaneously with the hazardous excitation to provide enhance structural behaviour for improved service and safety. An active structural control system consists of: • Sensors located around the structure to measure either external excitations, or structural response variables, or both. • Devices to process the measured information and to compute necessary control force needed based on a given control algorithm. • Actuators, usually powered by external sources, to produce the required forces.

20. HOW BUILDINGS RESPOND TO EARTHQUAKES The earthquake as well as wind load acting on the buildings are termed as ‘lateral loads’ since their effect is felt mainly in the horizontal direction. This is in contrast to the weights of the building, which act vertically down due to gravity. Forces due to earthquake, called seismic forces, are induced in a building because of the heavy masses present at various floor levels. Such forces are called inertial forces, is calculated by the products of the masses and their respective accelerations. If there is no mass, there is no inertial force. Accelerations generated by the seismic waves in the ground get transmitted through the vibrating structure to the masses at various levels, thereby generating the so-called horizontal seismic forces. The building behaves like a vertical cantilever, and swings horizontally almost like an inverted pendulum, with masses at higher levels swinging more. Hence, the generated seismic forces are higher at the higher floor levels. Because of the cantilever action of the building (fixed to the ground and free at the top), the forces accumulate from top to bottom. The total horizontal force acting on the ground storey columns is a sum of the forces (seismic loads) acting at all the levels above. This is termed as the base shear and it leads to highest stresses in the lowermost columns.

22. Heavier buildings attract larger seismic forces. On the other hand, lighter buildings are affected less. This was an important lesson learned from the great Assam earthquake of 1897 (magnitude M8.1), which destroyed almost all buildings (up to 3 storeys) built in British India. Following this disaster, constructions were limited to single and double storeyed “Assam type” dwellings with light roofing as ideal earthquake-resistant construction in North-East India, which falls under the highest seismic zone (zone V) in the country. Over the years, these basic lessons have been forgotten, and numerous high-rise buildings have mushroomed, especially in recent times in the urban centres of the country. Many of these buildings are seismically deficient. Figure shows an old building in Guwahati, originally 4- storeyed, to which three additional storeys were added recently ⎯ an example of a potential man- made disaster, waiting to happen, in a highly congested area. To prevent such disasters, local building authorities must strictly ensure that all new constructions should comply with design standards. Existing buildings that are highly unsafe must be declared unfit for occupation (and, if located in congested areas, must be demolished), unless they are retrofitted appropriately.

24. 1. Effect of Stiffness  Buildings are expected to behave elastically under service loads. Elasticity is that property by which a body or a structure, displaced by a load, regains its original shape upon unloading. It is by virtue of this property, that buildings that are pushed horizontally by wind or mild earthquake loads, return to the original vertical configuration after the wind or the tremor has passed. How much the building deflects under a given load is measured by a property called stiffness, which may be defined as “the force required to cause unit deflection”. The stiffness required to resist lateral forces is termed as lateral stiffness. The stiffer the building, the less it will deflect (‘drift’)

26. Should our buildings be relatively stiff or flexible? Certainly, it is desirable for the building to behave elastically under lateral loads, including forces under low earthquake levels that are likely to occur occasionally during the life of the building. But, it would be highly uneconomical to design ordinary buildings to behave elastically under high level earthquakes, which are rarely expected to occur during the design lifetime of the building. Because of the limitation of resources, the design standards allow us to take some risk of damage in the event of a rare severe earthquake. What we need to ensure is that the building, although likely to be severely damaged in the event of the rare earthquake, does not collapse, so that lives are not lost. How do structural engineers achieve this? They allow the building to behave inelastically (that is, the building does not regain its original shape after the earthquake) at such high load levels and thus dissipate energy. The building’s original stiffness gets degraded, and it becomes flexible.

27. Of course, there must be adequate stiffness in buildings. This can be achieved by providing adequate lateral load resisting systems (such as masonry walls with bands in small buildings, frames, braces or shear walls in large buildings). Otherwise, they will get severely damaged and may even collapse under low level earthquakes. In the case of exceptionally important buildings (such as nuclear reactors), it is even desirable to have sufficient stiffness to ensure elastic behaviour even under rare earthquakes. They must survive at all cost, and that too without damage.  The mass and lateral stiffness of the building contribute to another important structural property, called the natural period of vibration. It is the time taken by the building to undergo a cycle of to-and-fro movement (like a pendulum). Buildings with high stiffness and low mass have low time period, whereas buildings with low stiffness and high mass have high natural period. The value of this natural period also governs the magnitude of seismic force that the building will attract. This is similar to the effect of resonance in a vibrating system. The conventional buildings of a few storeys, common in urban areas, have low natural period and hence attract higher seismic forces as a fraction of their weight.

28. 2. Effect of Ductility The ability of a structure to deform with damage, without breaking suddenly (without warning), is termed as ductility. With ductility, a building can continue to resist seismic forces without collapsing. There is a story of a proud tree teasing a blade of grass for not being able to stand erect in the wind. But, when a very severe storm came, it was the tree which fell down and the blade of grass survived. This is because the blade of grass was able to undergo very large deformation without breaking down, unlike the big tree which snapped suddenly at its base, when its strength was exceeded.  In a similar way, buildings too can exhibit either ductile or brittle (non-ductile) behaviour, depending on the structural material, design and detailing. Generally, conventional masonry buildings exhibit brittle behaviour, when provided with earthquake resistant features. On the other hand, well-designed buildings made with reinforced concrete or structural steel can exhibit ductile behaviour.

31. Materials like brick, stone and plain concrete are relatively brittle. When bricks or stones are used in masonry wall construction without adequate bond, they can fall apart suddenly, even if the walls are relatively thick. This indeed is how the failure of many buildings occurred during the Laturearthquake. Many such buildings have weak mud mortar and absence of bond stones. Ordinary buildings are commonly classified as either load bearing or framed structures. Most low-rise buildings are load bearing buildings made of masonry walls, which resist both gravity (vertical) loads and lateral loads due to wind and earthquake. As the building height increases, and in high seismic zones, lateral loads tend to govern the design. In such cases, it is structurally efficient and economical to adopt framed buildings. In such buildings, it is the framework (skeleton) of the building, comprising beams, columns and footings, made usually of reinforced concrete, which mainly resist both vertical and lateral loads. In high-rise buildings, reinforced concrete shear walls are often introduced to enhance the lateral load resisting capacity.

33. Unlike brick and stone, materials like steel and reinforced concrete (if properly designed and detailed) possess considerable ductility. Ductility is required at locations of very high stress, such as the beam-column joint. The horizontal bars in the beam should be anchored well in the joint and the vertical ties spaced closely near the joint. The vertical bars and the closely spaced horizontal ties in the column should be continuous throughout the joint. These important details are often overlooked in reinforced concrete construction, whereby the desired ductility is not achieved.

35. If the structural components (walls, beams, columns etc.) in a building can “hang on” through ductile behaviour, without breaking down during the brief period of the major earthquake, the building will not collapse, even though it may get damaged. Such ductile buildings attract lesser load with increasing deformation (since stiffness gets reduced) than the buildings which remain stiff (like the tree in our story). Also, a significant amount of the input energy due to the earthquake in the building gets dissipated through the yielding (deformation without increase in stress) of the ductile materials. Otherwise, the entire input energy needs to be stored as elastic strain energy. The strain energy and the dissipated energy are shown by the shaded areas in the base shear versus drift graphs. However, as the drift is generally much less in the latter type of buildings, the total seismic force to be resisted by such a building is very high to handle the same input energy. This is not practical for ordinary buildings, because it calls for very large member sizes and will be prohibitively expensive. If the desired ductility can be provided in the building, the design seismic force Vinelasticcan be much lower (up to 20%) than the corresponding force in an elastic building Velastic .

37. 3. Effect of Strength and Integrity Every structural component has strength, which is the magnitude of the maximum internal force (such as axial force or bending moment) it can resist under a certain type of loading. When this strength is exceeded by the applied load, the material fails (or collapses). The strength depends not only on the type of material, but also on other factors, such as the size of the cross-section. However, the thicker wall will attract higher earthquake force.  As mentioned earlier, the load attracted by a structural component during an earthquake depends on the mass and lateral stiffness of the building. If the building is not designed to “yield”, and behave in a ductile manner, it will be required to resist higher load during an earthquake to prevent a sudden failure. A structural component should be designed to have a strength that is not less than the maximum internal force, associated with the overall seismic load on the building, and the associated ductility.

38. If the strength is not adequate, the building component will fail. The failure of the vertical components in a building (such as load bearing walls, columns or footings) is more critical than that of the horizontal components (such as beams and slabs), because the former type of failure is likely to trigger a possible collapse of the entire building. The failure of a beam may cause only a local distress; but the failure of a ground storey column or a footing can trigger overall building (global) collapse. In the recent Gujarat earthquake, many multi-storeyed buildings collapsed because of the failure of the columns in the ground storey.

40. Thus, an important principle adopted in the seismic resistant design of framed buildings is this: Soil must be stronger than foundations; foundations must be stronger than columns; columns must be stronger than beams. To ensure that forces are safely transmitted from beams to columns, from columns to foundations, and from foundation to soil, the connections at the beam-column joints, column- foundation joints and foundation-soil interface should have the required strength (and ductility). This requirement is part of ensuring the integrity of the building. Retaining the stiffness in the building is the other aspect of integrity. Consider, for example, a single room, bounded by four masonry walls and covered by a roof slab. If the connections between the walls and between the slab and the walls are not effective, the building has limited strength and stiffness to resist seismic forces, and can collapse even under a minor earthquake shaking. Indeed, this is precisely what happened to a large number of dwelling units during the Latur earthquake .

42. In a masonry building, if reinforced concrete bands are provided in walls at plinth, lintel and roof levels and the wall-to-wall connections provide good bond, the walls will act together like a box. This will enable the building to resist even a major earthquake effectively. The more the number of lateral load-resisting systems in a building, the less is the potential for global collapse. This is because when one of them fails, the loads get redistributed, and other systems take an increasing share of the load. This potential for having multiple load paths in a building is called redundancy and it makes the building as a whole stronger.

44. 4. Effect of Layout and Configuration Providing earthquake resistance to buildings is primarily the responsibility of civil engineers. But architects also have a major role to play. Some architectural features, relating to overall size and shape, are unfavorable and invite potential seismic disaster. It is desirable that the client also knows something of these features, if the building is located in a high seismic zone. Prevention is always better than cure. In such situations, safety is preferable to fancy looks. Otherwise, structural design has to be done carefully and competently. In general, if the basic architectural features favoring good seismic resistance are adopted, the cost of making the building earthquake proof is less.

45. The, building should have a simple geometrical shape in plan, such as rectangular or circular. All rectangular shapes are not uniformly good. If the building is too long (in one direction) or too large in plan, it is likely to be damaged during earthquakes. Buildings with large cut-outs in the walls, floors and roofs are undesirable, as this affects their integrity. Similarly, buildings which have ‘L’, ‘U’, ‘V’, ‘Y’ or ‘H’ shapes in plan are also undesirable, inviting severe stresses at the interior corners called re-entrant corners. Each wing of the building tends to vibrate separately in the event of an earthquake, causing serious problems at the common core region, leading to potential collapse.

48. Buildings with asymmetry in plan are bound to twist under an earthquake, inviting further damaging effects. A general rule of thumb is to make the building as simple, solid and symmetric in plan as possible. If complex geometries are absolutely required, then it is desirable to break up the building plan into separate simple rectangular segments with proper separation joints, so that they behave as individual units under an earthquake.

49. Buildings should not only be simple in plan but also in elevation. The walls and columns should continue uninterrupted from top to bottom, to ensure transmission of forces to the supporting ground through the shortest and simplest path. If there is any discontinuity in this path of load transmission, there is a danger of potential damage to the building in the event of an earthquake. Hence, hanging or floating columns (columns which begin in an upper storey from a beam) and discontinuity of walls in the ground or other storey (open ground storey) should be avoided.