Housing Vulnerability

The census of Houses, 2001 Census of India, gives the following details of houses bases on materials of construction for walls and roofs (BMTPC Vulnerability Atlas):

a) Type of Roof:

  1. Pitched or sloping including Tiles, slate or shingle; Corrugated iron, zinc or other metal sheets; Asbestos cement sheet's thatch, grass, leaves, bamboo etc.

  2. Flat including brick, stone and lime; reinforced brick concrete/reinforced cement concrete.

b) Type of Wall:

  1. Mud, unburnt bricks, stone laid in mud or lime mortar.

  2. Burnt bricks laid in cement, lime or mud mortar.

  3. Cement concrete.

  4. Wood or Ekra walling.

  5. Corrugated iron, zinc or other metal sheets.

  6. Grass, leaves, reeds or bamboo or thatch and others.

c) Type of Flooring:

Various types like mud, stone, concrete etc.

On the basis of building material, the houses have been categorized in four types to assess the vulnerability to earthquake hazard:-

  1. Category A: Mud Wall (all roofs), Unburnt brick or Adobe wall with sloping roof, unburnt brick or Adobe wall with flat roof, stone wall with pitched/sloping roof, stone wall with flat roof.

  2. Category B: Burnt brick wall with sloping roof, burnt brick wall with flat roof.

  3. Category C: Concrete wall with sloping roof, concrete wall with flat roof, wooden wall (all roofs), ekra wall (all roofs)

  4. Category X: Corrugated iron, zinc or other metal sheet walling (all roofs), bamboo, thatch, grass leaves etc. (all roofs).


House types and Risk

The damage risk of various house types is based on their average performance observed during past occurrence of damaging events. In view of numerous variations in the architectural planning, structural, detailing, quality of construction and care taken in maintenance, the performance of each category of houses in a given event could vary substantially from the average observed. For example, under seismic occurrence, the following observations have been made in many cases (BMTPC Vulnerability Atlas).

(a) All Masonry Houses (Categories A and B).

  • Quality of construction comes out as a major factor in the seismic performance particularly under intensities MSK IX and lower. Good quality constructions perform much better than poor quality constructions in any category. Appropriate maintenance increases durability and maintains original strength.

  • Number of storeys in the house and storey height are other factors. Higher the storey and more the number of storeys, greater is the observed damage.

  • Size, location and number of door and window openings in the walls also determine seismic performance, since the openings have weakening effect on the walls. Smaller and fewer openings and located more centrally in the walls are better from seismic performance viewpoint.

  • Architectural layout, particularly in large buildings, that is, shape of building in plan and elevation, presence of offsets and extended wings also play important role in initiation of damage at certain points and its propagation as well. More symmetrical plans and elevations reduce damage and unsymmetrical ones lead to greater damage.

  • Where clay/mud mortar is used in wall construction, its wetness at the time of earthquake is very important factor in the seismic performance since the strength of fully saturated mortar can become as low as 15% of its dry strength.

(b) Wooden Houses:

  • Quality of construction, that is, seasoning of wood and the joinery are important in seismic and cyclone wind performance. Better the quality better the performance.

  • Wood decays with time due to dry rot, insect and rodent attack etc. therefore, the joints tend to become loose and weak. The state of maintenance of the wooden building will determine its performance during earthquake, high wind, as well as flooding.

(c) Reinforced Concrete Houses:

  • In reinforced concrete construction, good structural design and detailing and good quality construction only could ensure excellent performance. Carelessness in any of these can lead to poor behaviour both under earthquake and cyclones.

Now the average risk levels to various categories of houses for various hazards and their intensities are defined here below for use in the house vulnerability tables.The damage risk to various house types is defined under various seismic intensities on MSK scale. The following damage risks are defined based on this Intensity Scale:

Very High Damage Risk (VH)

Total Collapse of building.

High Damage Risk (H)

Gaps in walls; parts of buildings may collapse; separate parts of the building lose their cohesion; and inner walls collapse.

Moderate Damage Risk (M):

Large and deep cracks in walls, fall of chimneys on roofs.

Low Damage Risk (L):

Small cracks in walls; fall of fairly large pieces of plaster, pentiles slip off; cracks in chimneys, part may fall down.

Very Low Damage Risk (VL):

Fine cracks in plaster; fall of small pieces of plaster.



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