IMPROVING EARTHQUAKE RESISTANCE OF SMALL BUILDINGS

By taking some precautions the earthquake resistance of small buildings can be increased and measures in building planning, site selections and constructions as described below:

1. Site Selection: The building constructions should be ignored on

i.            with columns of different heights On sloping ground

ii.            Near unstable embankments

iii.            On subsoil with marked discontinuity like rock in some portion and soil in some portion.

iv.            Flood affected areas

2. Building Planning: Symmetric plans are safer compared to unsymmetrical. Therefore go for square or rectangular plans expect E, L, H, T shaped. Rectangular plans should not have length more than twice the width.

3. Foundations: for single storey building Width of foundation should not be less than 750 mm and for storied buildings not less than 900 mm. Depth of foundation should not be less than 1.0 m for soft soil and for rocky ground 0.45 m. remove all loose materials including water from the trench and compact the bottom before foundation is laid. Back-fill the foundation correctly and compact after foundation is laid.

4. Masonry: In case of stone masonry:

1.   Into the thickness of wall place length of stones to ensure interlocking outside and inside both faces of the wall.
2.   Place flat each stone on its broadest face.
3.   To make it angular Break the stone so that it has no rounded face.
4.   Fill the voids by using small chips of the stones with minimum possible mortar.
5.   At every 600 to 750 mm distance utilize through stones.

In case of brick masonry:

1.   Properly use burnt bricks only.
2.   To ensure better bond having next course Place bricks with its groove mark facing up.

In case of concrete blocks:

1.   Place rough faces towards top and bottom to obtain good bond.
2.   Blocks should be strong.
3.   Before lying Brush the top and bottom faces.

Generally walls of more than 450 mm should be ignored. Length of wall should be limited to 6 m. Cross walls make the masonry stronger. It is better to build partition walls along with main walls interlinking the 2.

5. Doors and Window Openings:

1.   Walls which contain too many doors and windows close to each other early collapse.
2.   Windows should be kept at same level.
3.   In wall, Total width of all openings should not exceed from 1/3 rd the length of wall.
4.   Doors must not be placed at the end of the wall. They must be at least at 500 mm from the cross wall.
5.   Clear width between two openings must not be less than 600 mm.

6. Roof:

1.   Instead of rafters in sloping roofs with span greater than 6 m use trusses.
2.   Building with four sided sloping roof is stronger than the one with 2 sided sloping, as gable walls collapse early.

7. Chejjas:

Limit chejja or balcony projections to 0.9 m. Use beams and columns for larger projections.

8. Parapet: Masonry parapet wall can easily collapse. It is more better to build parapet with bricks up to 300 mm followed by iron railings.

9. Concrete and Mortar: for making mortar and concrete Use river sand. It should be sieved to remove pebbles. By holding it against wind Silt should be removed. Coarse aggregates of size more than 30 mm must not be used. Aggregates should be well angular and graded. Before adding water cement, aggregates should be dry mixed thoroughly.

10. Bands: The following R.C. bands should be provided

(1) Plinth band (2) Lintel band (3) Roof band (4) Gable band.

For making R.C. bands minimum thickness is 75 mm and at least two bars of 8 mm diameters are needed. With steel limbs they should be tied of 6 mm diameter at 150 mm spacing.

Also vertical and diagonal bands may be provided if wall size is large.

11. Retrofitting: Retrofitting means preparing a structure in a scientific manner so that all elements of a building act as an integral unit.

Generally it is the most economical and fastest way to achieve safety of the building. some of the methods in retrofitting are following:

a.      Provide bracings at the level of purlins and bottom chord members of trusses.

b.      To walls with brackets anchor roof truss.

c.      Strengthen corners with seismic belts.

d.      By inserting sloping belt Strengthen gable wall on gable wall.

e.      By providing vertical reinforcement Improve storey connections.

f.       Anchor floor joists to walls with brackets.

g.     By providing vertical reinforcement Induce tensile strength against vertical bending of walls at all inside and outside corners.

h.       Encase wall openings with reinforcements.