D. AJITHA SIMHA details the matrix of occupancy, type of construction and the resulting FAR values
The National Building Code (NBC) 1970 version introduced the concept of interlinking occupancy and type of construction with the Floor Area Ratio (FAR). The FAR is the quotient of total area of all the floors of a building divided by the plot area. In Mumbai (Maharashtra) it is called Floor Space Index (FSI). Generally, for all buildings, the density of dwelling units, road width and services to be provided govern the determination of FAR. However the importance of different occupancies and fire resistance construction has now been highlighted. This concept has been based on the approach of BOCA code (Building Officials Conference of America), widely accepted in the U.S.
Different occupancy classifications have already been listed; this classification is based on the fire load (amount of combustible materials) due to the occupancy. The types of construction have also been listed, namely Type 1, Type 2, Type 3 and Type 4. These are in the descending order of fire resistance; they can be taken as 4 hr, 3hr, 2hr and 1 hr fire resistance constructions, in that order.
The concept is in preparing matrix of occupancy, type of construction and the resulting values of FAR in a relative manner. To clarify, if a residential occupancy (of 1 hour fire load) is constructed with 4 hr (Type 1) structure, theoretically, there is no limit to the total area to be constructed. Alternatively, a hazardous occupancy (4 hr fire load) constructed with 1 hr (Type 4) structure is not permitted. For other occupancies and varying types of construction, FAR values are indicated. This concept is very much in vogue, for example, in New York and Chicago. It is for the local body, like the Municipal Corporation, to decide on the actual values based on the following considerations;
1. Density of dwelling units per hectare
2 Traffic emanating from the complex and the road width
3 Parking space available
4 Local fire-fighting capabilities
5 Availability of services such as water supply, drainage, sewerage system and electric supply.
Work it out
An example of application of FAR is as below:
Plot area = 3,000 sq. m; ground coverage permitted is 1/3 of plot area. Therefore the ground coverage works out to 1,000 sq. m. FAR permitted in that locality is 4. This means that the permitted total area of all floors of the building is 3000 x 4 = 12,000 sq. m. Since the ground coverage is only 1,000 sq.m., the number of floors shall be 12,000/1000=12.
While rewriting the building bylaws based on NBC, in most of the municipal corporations and municipalities in India, this concept has been brought to their notice.
Corporations including those in Mumbai and Delhi have seriously considered this and tried to incorporate it in a suitable manner in their Development Control Rules.
(The author is Retd. Dy. Director-General, Bureau of Indian Standards, New Delhi; former U.N. Consultant for Basic Approach to Writing of Building Code, Habitat Centre, Nairobi, Kenya; and ex-Secretary, Guiding Committee, NBC)