Earthquake-resistant structures: How safe is your home?

    Earthquake-resistant structures: How safe is your home?

Earthquakes can destroy property and life in a matter of minutes. It is sudden and it cannot be predicted. More than 55% of the landmass in India is vulnerable to earthquake. So, it is important for engineers to learn best practices to construct earthquake resistant residential houses and buildings. Following construction practices and design tips are given by experienced experts in the field of construction.

Shape of building

Buildings should be symmetrical in shape, to properly distribute earthquake load.
If the length of the building is long, provide a minimum 25mm gap (1 inch) and provide expansion joints. Minimize the projections in the building. Projections like balconies should not exceed 1/5th of the dimension of the building in the direction of projection.

 Protection of Masonry walls

Quality control is important during brick work. Only use cement mortar or cement-lime mortar of ratio 1:4 and 1:1:6 respectively. Also, soak the bricks/blocks in water before masonry work, hence it won’t absorb water from mortar during construction. Soaking bricks also remove dirt from it.
Joints between walls are very important. Join walls properly with toothed joints or provide L-shaped dowel bars. Also, ensure good interlocking in junctions.
Thickness of masonry joints should not exceed 10mm (4 inches).
To get enhanced strength against earthquake, provide vertical rod inside corners of the masonry walls. Provide 10mm vertical rod for single storey building and 12mm vertical rod for multi storey earthquake resistant building.
Vertical rods should be provided up to the foundation and loop it to the horizontal band reinforcement. Provide Lintel and Sill bands above and below the openings.

Right way to provide Wall Openings for doors and windows.

Openings should be at least 600mm (2 feet) away from the corner and 500mm (1.5 feet) away from nearby openings.
Even though openings are important for natural lighting and air circulation, keep it minimum for even distribution of load in walls. Width of the opening should be less than 40% of the length of the wall. For example, If the length of the wall is 3m then total width of the opening should not exceed 1.2m

Importance of  RCC belts – Bands

RCC bands at different levels help to distribute load properly. Hence provide RCC bands in Plinth level, sill level and lintel level.
RCC bands should cover the entire structures.
Roof bands are important for Load bearing structures.
For single story building, Rcc bands shall be minimum 75mm (3 inches) deep, with 10mm diameter rod and 6mm diameter stirrups of 150mm (6 inches) spacing. For 2 storey building, bands at ground floor shall be minimum 150mm (6 inches) deep, with 12 mm diameter rod and 6mm diameter stirrups of 150mm (6 inches) spacing.

Earthquake resistant RCC beams

Width of the beam should be greater than one-third of the depth of the beam or 200mm.
Ensure proper shear reinforcement near support. Use bendup bars or straight bars to provide reinforcement at the top of the beam near the support.
Spacing of stirrups should be less than half the depth of beam or 300 mm. Provide 135-degree hook for stirrups bends. Stirrups should be closely spaced near the support for the distance of 2 times the depth of the beam. Stirrups should be closely spaced in lap zones.

Earthquake resistant RCC columns

Spacing of ties shall be greater than half the smallest dimension of the column. But, it should be closely spaced for about 450mm near beam column junction.
Do not lap the bars near the junction, and ties in lap zones should be closely spaced.

Foundation requirements

Footings should rest on firm ground. If not, then Proper compaction is necessary for soft soil.
Concrete bed for foundation should be a minimum of 100mm thickness. Provide proper tie beams between columns. Tie beams shall have minimum 200mm x 200mm and shall have 4 rods of minimum 12mm diameter with 6mm dia stirrups at 150mm spacing.

Further Reading
1. Earthquake Retrofitting – Popular methods to strengthen existing buildings against earthquake