5 Tricks in Indian Building Codes to Design Earthquake Resistant Buildings That Can Save Upto 50 Percent in Building Costs

Online, October 2, 2010 (Newswire.com) - In the ever increasing competitive environment that exists in the Indian Real Estate market, developers are trying to find novel ways of reducing costs and increasing profit margins. Costs and affordability still are the two governing criteria that influence if a real-estate project will be a success or not. There exist 5 tricks that can be used by architects and structural designers while designing earthquake resistant buildings as per the Indian Building Codes that can save upto 50 percent in the structural cost of a building.

India is divided into 4 seismic zones numbered 2, 3, 4 and 5 with 5 depicting the highest seismic vulnerability. During building design the architects and structural engineers follow the design guidelines as given in the building codes published by the Bureau of Indian Standards. Though the designers refer to dozens of codes the most important of these are IS-1893, the seismic code, IS-456, the concrete design code and IS-13920, the ductile detailing code.
The basic aim of the design codes is to ensure stable earthquake resistant designs that can effectively withstand natural disasters like an earthquake. However there exist many tricks in the above named codes that can save upto 50 percent in the building structure costs without having to explicitly break any laid down clause. These tricks are possible as the above codes are of old vintage and each one has been written without correlating it to the other and have poor explanations. Designers can make effective use of these design tricks to save a lot of money.

Trick 1: All buildings in India have to be designed for earthquake forces. The seismic design philosophy is to calculate an empirical force and apply this as the horizontal/lateral seismic force also known as Base Shear in technical parlance. This force is calculated as a percentage of the weight of the building and in 99 percent of the cases this becomes the governing load case for design. Higher the value of this force, higher is the reinforcement in the beams, columns and shear walls and higher is the construction costs. When using a response reduction factor of 5 the maximum value of this horizontal force applied at the base of the building is 2.5 percent of the building weight for Zone-2, 4 percent of the building weight for Zone-3, 6 percent of the building weight for Zone-4 and 9 percent of the building weight for Zone-5. IS-1893 gives two formulae to calculate the empirical time-period of the building, the code does not describe this clause in detail nor does it lay emphasis its implication. If designers use the moment frame empirical time-period for buildings that contain shear walls or/and brick infill walls then the design base shear can be reduced by upto 100 percent and thereby giving huge cost benefits.

Trick 2: For design of shear walls use the wall design section given in IS-456, the quantity of shear reinforcement steel can be reduced by upto 75 percent.

Trick 3: Most buildings that have shear walls will not be able to enforce 25 percent of the base shear to be resisted by the columns. This would imply that a response reduction factor of 5 cannot be used and a response reduction factor of 4 will have to be used. If this can be ignored then a 20 percent benefit can be achieved in reducing forces and therefore the quantity of steel.

Trick 4: All buildings higher than 25 stories have to be classified as special structure and special design guidelines have to be listed out by any one of the Indian Institutes of Technology (IIT). This is not explicitly mentioned in the code and when the designers choose to ignore this clause it can result in huge savings. The main reason for this is that if the code parameters are used the base shear will come to hardly 0.4 or 0.5 percent of the building weight for highrise buildings. If special guidelines are listed by Government authorized organizations this force can go as high as 1 to 3 percent of the building weight.

Trick 5: Ignore the weight of basement floors and basement walls in calculating the building weight and this can reduce the building weight by upto 25 percent and therefore result in huge savings in quantity of steel.