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Earthquake causes ground motions in a random fashion, both horizontally and vertically in all directions from the epicenter. As a result, the structures found in the ground vibrate, inducing inertial forces on them. It is therefore essential to ensure stability, strength, and serviceability with acceptable levels of safety by suitable design and detailing.
Due to horizontal shaking of the ground, horizontal inertia forces are generated at the level of the mass of the structure. The lateral inertia forces are transferred by the floor slab to the walls or columns, to the foundations, and finally to the soil. So each of the structural elements like floor slabs, walls, columns, foundations, and connections between them must be designed to safely transfer these inertia forces through them.
In transferring the inertial forces, the most critical elements of a building are walls or columns. In traditional construction, floor slabs and beams receive more attention and care during design and construction as compared to walls and columns. Walls are relatively thin and often made of brittle materials like masonry. They are poor in carrying horizontal earthquake inertia forces along the direction of their thickness. There are many examples in the past of the failure of masonry walls during an earthquake. Similarly, poorly designed and constructed reinforced concrete columns can be disastrous.
The design approach of Earthquake Resistant Building:
The design and construction of an earthquake-proof building that will not get damaged during a strong earthquake are rare and too expensive. Therefore the engineering intention in this sense is to make buildings earthquake resistant which will resist the effects of ground shaking. Although they may get damaged severely but would not collapse during strong earthquake. The safety of people and contents is assured in earthquake-resistant buildings, and thereby a disaster is avoided.
According to IS1893 (Part 1):2002 adopts the following criteria or fixing the level of design seismic loading.
Criterion 1: Structure should possess at least minimum strength to withstand minor earthquakes (<DBE) which occur frequently without damage. Here DBE is Design Basis Earthquake.DBE is expected to occur once during the life of the structure. This means that under minor but frequent shaking, the main members of the building that carry vertical, as well as horizontal forces, should not be damaged.
Criterion 2: The structure should be able to resist moderate earthquakes (DBE) without any significant structural damage. Some non-structural damage may also occur. It means that under moderate but occasional shaking, the main members may sustain repairable damage while other parts of the building may be damaged such that they may even have to be replaced after the earthquake.
Criterion 3: Structure should withstand a major earthquake or Maximum Considered Earthquake (MCE) without collapse. This means under strong but rare shaking, the main members may sustain severe damage, but the building should not collapse.
After every minor shaking, the building should be fully operational within a short time, and the repair cost will be small. And after moderate shaking, the building should be operational once the repair and strengthening of the damaged main member are completed. But after a strong earthquake, the building may become dysfunctional for further use but will stand so that people can be evacuated and property recovered.
Architectural features and structural shapes of Earthquake Resistant Building:
The behavior of earthquake-resistant buildings critically depends on the overall shapes, size, and geometry. The structural system must be required to carry the earthquake forces to the ground. The choices of shape and structural system have significant behavior on the performance of the building during a strong earthquake.
1. Size of Building:
The horizontal movement of the floors during ground shaking is large in all the buildings having a large height-to-base ratio. In short but very long buildings, the damaging effects during an earthquake are very large. In buildings with large plan areas like warehouses, the horizontal seismic forces can be excessive to be carried. So, buildings with much larger or much smaller overall sizes do not perform well during earthquakes.
2. Horizontal Layout of Building:
A building having simple geometry like rectangular, circular, oval shape,etc. performs very well during strong earthquakes.Buildings with re-entrant corners like those U, V, H, and star-shaped plan have sustained significant damages. The interior corners in the plan of a building are avoided by making the buildings in two parts. An L-shaped building can be separated into two rectangular parts by using a separation joint at the junction. Buildings having a simple plan but the columns/walls are not equally distributed in the plan are more prone to twist during earthquake shaking.
3. Adjacency of Buildings:
When two buildings are too close to each other, they may pound on each other on strong shaking. With the increase in building height, this collision can be a greater problem. When building height does not match, the shorter building may pound at the mid-height of the column of the taller one. This can be a very big problem.
4. Vertical Layout of Building:
The earthquake forces developed during earthquakes at different floors of buildings need to be brought down along the height of the building by the shortest path. Any deviation or discontinuity in this path leads to poor performance during an earthquake. Buildings with vertical setbacks especially in hotels cause a sudden jump in earthquake forces at the level of discontinuity. Buildings having fewer columns or walls in a particular storey tend to damage or collapse which is initiated at that storey. Many buildings with an open ground storey are more vulnerable to earthquake damages.
Buildings on slopy ground have unequal height columns along the slope causes ill effects like twisting and damage in shorter columns. Buildings with columns that hang or float on beams at an intermediate storey cause discontinuity in the load transfer path. In some buildings, reinforced concrete walls are there to carry the earthquake loads to the foundation.
5. Regular and Irregular Configurations:
To perform well in an earthquake, a building should possess four main attributes.
I) It should have a simple and regular configuration.
II) Adequate lateral strength.
III) Good ductility.
IV) Adequate stiffness.
Buildings having simple regular geometry and uniformly distributed mass and stiffness in the plan as well as elevation, suffer much lesser damages than the buildings with irregular configuration.