An earthquake is a sudden, rapid shaking of the earth surface caused by the breaking and shifting of rocks beneath. During earthquake, ground motion occurs in a random fashion in all directions radiating from a point within earth crust, called epicentre. It causes vibrations of structures and induces inertia forces on them.
As a result structure may collapse resulting into loss of property and lives. Earthquakes do not kill people, vulnerable buildings do so. Hence there is need of designing earthquake resistant buildings, especially in the earthquake prone areas.
Improving Earthquake Resistance of Small Buildings
The earthquake resistance of small buildings may be increased by taking some precautions and measures in site selections, building planning and constructions as explained below:
1. Site Selection: The building constructions should be avoided on
- Near unstable embankments
- On sloping ground with columns of different heights
- Flood affected areas
- On subsoil with marked discontinuity like rock in some portion and soil in some portion.
2. Building Planning: Symmetrical plans are safer compared to unsymmetrical. Hence go for square or rectangular plans rather than L, E, H, T shaped. Rectangular plans should not have length more than twice the width.
3. Foundations: Width of foundation should not be less than 750 mm for single storey building and not less than 900 mm for storied buildings. Depth of foundation should not be less than 1.0 m for soft soil and 0.45 m for rocky ground. Before foundation is laid remove all loose materials including water from the trench and compact the bottom. After foundation is laid back, fill the foundation properly and compact.
4. Masonry: In case of stone masonry:
- Place each stone flat on its broadest face.
- Place length of stones into the thickness of wall to ensure interlocking inside and outside faces of the wall.
- Fill the voids using small chips of the stones with minimum possible mortar.
- Break the stone to make it angular so that it has no rounded face.
- At every 600 to 750 mm distance use through stones.
In case of brick masonry:
- Use properly burnt bricks only.
- Place bricks with its groove mark facing up to ensure better bond with next course.
In case of concrete blocks:
- Place rough faces towards top and bottom to get good bond.
- Blocks should be strong.
- Brush the top and bottom faces before laying.
In general walls of more than 450 mm should be avoided. Length of wall should be restricted to 6 m. Cross walls make the masonry stronger. It is better to build partition walls along main walls interlinking the two.
5. Doors and Window Openings:
- Walls with too many doors and windows close to each other collapse early.
- Windows should be kept at same level.
- The total width of all openings in wall should not exceed 1/3rd the length of wall.
- Doors should not be placed at the end of the wall. They should be at least at 500 mm from the cross wall.
- Clear width between two openings should not be less than 600 mm.
- In sloping roofs with span greater than 6 m use trusses instead of rafters.
- Building with 4 sided sloping roof is stronger than the one with two sided sloping, since gable walls collapse early.
7. Chejjas: Restrict chejja or balcony projections to 0.9 m. For larger projections, use beams and columns.
8. Parapet: Masonry parapet wall can collapse easily. It is better to build parapet with bricks up to 300 mm followed by iron railings.
9. Concrete and Mortar: Use river sand for making mortar and concrete. It should be sieved to remove pebbles. Silt should be removed by holding it against wind. Coarse aggregates of size more than 30 mm should not be used. Aggregates should be well graded and angular. Before adding water, cement and aggregates should be dry mixed thoroughly.
10. Bands: The following R.C. bands should be provided
(a) Plinth band (b) Lintel band (c) Roof band (d) Gable band.
For making R.C. bands minimum thickness is 75 mm and at least two bars of 8 mm diameters are required. They should be tied with steel limbs of 6 mm diameter at 150 mm spacing. If wall size is large, diagonal and vertical bands also may be provided.
11. Retrofitting: Retrofitting means preparing a structure in a scientific manner so that all elements of a building act as an integral unit. It is generally the most economical and fastest way to achieve safety of the building. The following are some of the methods in retrofitting:
- Anchor roof truss to walls with brackets.
- Provide bracings at the level of purlins and bottom chord members of trusses.
- Strengthen gable wall by inserting sloping belt on gable wall.
- Strengthen corners with seismic belts.
- Anchor floor joists to walls with brackets.
- Improve storey connections by providing vertical reinforcement.
- Induce tensile strength against vertical bending of walls by providing vertical reinforcement at all inside and outside corners.
- Encase wall openings with reinforcements.
Improving Earthquake Resistance of Tall Buildings
Tall buildings are subjected to heavy horizontal forces due to inertia during earthquake. Hence they need shear walls. A shear wall is a R.C.C. enclosure within the building built to take shear forces. It is usually built around lift room.
These shear walls must be provided evenly throughout the buildings in both directions as well as from bottom to top. Apart from providing shear walls, the following techniques are also used for making tall buildings earthquake resistant:
- Base Isolation
- Using Seismic Dampers.
Base Isolation: The idea behind base isolation is to detach (isolate) the building from the ground in such a way that earthquake motions are not transmitted up through the building, or at least greatly reduced. The concept of base isolation is explained through an example of building resting on roller [Fig. 1].
When the ground shakes, the roller freely roll but the building above does not move. If the gap between the building and the vertical wall of foundation pit is small, the vertical wall of the pit may hit the wall.
Hence 100% frictionless rollers are not provided in practice. The building is rested on flexible pads that offer resistance against lateral movements [Fig. 2]. This reduces some effect of ground shaking to the building. The flexible pads are called base-isolators, whereas the structures protected by means of these devices are called base-isolated buildings.
Seismic Dampers: Another approach for controlling seismic damage in buildings is by installing seismic dampers in place of structural elements, such as diagonal braces. When seismic energy is transmitted through them, dampers absorb part of it, and thus damp the motion of the building. Figure 3 shows the following types of seismic isolation bearings:
(a) High density rubber bearings (b) Laminated rubber bearings and (c) Friction pendulum bearings.
I.S: Codes on Earthquake Resistant Building Design
After observing Indian earthquakes for several years Bureau of Indian Standard has divided the country into five zones depending upon the severity of earthquake. IS 1893-1984 shows the various zones. The following IS codes will be of great importance for the design engineers:
IS 1893–2002: Criteria for Earthquake Resistant Design of Structures (5th revision).
IS 4928–1993: Code of practice for Earthquake Resistant Design and Construction of Buildings. (2nd revision).
IS 13827–1992: Guidelines for Improving Earthquake Resistance of Low Strength Masonary Building.
IS: 13920–1997: Code of practice for Ductile Detailing of Reinforced Concrete Structures Subjected to Seismic Forces.
IS: 13935–1993: Guidelines for Repair and Seismic Strengthening of Buildings. 20.7
Cyclone Resistant Buildings
A cyclone is a storm accompanied by high speed whistling and howling winds. It brings torrential rains. A cyclone storm develops over tropical ocean and blows at speed as high as 200–240 km/hour. It is usually accompanied by lightning, thunder and continuous downpour of rain.
Cyclones extend from 150 km to 1200 km in lateral directions with forced winds spiraling around a central low pressure area. The central region of light winds and low pressure, known as the ‘eye’ of cyclone has an average diameter of 20 to 30 km.
This central eye is surrounded by a ring of very strong winds extending up to 40 to 50 km beyond centre. This region is called ‘wall cloud’. In this region strongest winds and torrential rains occur. Beyond this region winds spiralling extend outwards to large distances, which goes on reducing with the distance from the centre of the cyclone. The following care should be taken in designing buildings in cyclone prone areas:
- Foundations should be deeper
- R.C.C. framed structures are to be preferred over load bearing structures 3. Sloping roofs should be avoided.
- Cantilever projections should be avoided.
- Roof and parapet wall should be properly anchored to the columns and walls.
- Height of the buildings should be restricted.
- Suitable wind load should be considered in the building design.
- Openings in the wall should be less.
- Structure should not rest on loose soil.