Due precautions are required as regards choice of building materials to ensure the least damage to structures in case of an earthquake.
Major M D Apte, Chartered Engineer
It is now an accepted fact that all the countries on the earth are liable to earthquakes. Lands can only be grouped into 5 zones (Zone I where the event is active at its highest to Zone V where it is the least active) depending upon the frequency and the severity of the earthquakes. The severity has an indeterminate character and the entire planet needs be considered as prone to earthquakes. The world has accordingly been mapped in five seismic zones. A map of India indicating various seismic zones is shown for ready reference.
Many South American and European countries (especially those experiencing severe and frequent earthquakes) have devised some practices and regulations for construction of buildings to minimize damage to the structures during an earthquake. In India, the Disaster Management Cell at the Central Government level has initiated certain guidelines. The guidelines provided by some European and South American countries have been compiled in a small booklet titled ‘Earthquake-Resistant Confined Masonry Construction’ by National Information Center for Earthquake Engineering (NICEE) located in IIT Kanpur. At the actual implementation stage some modifications to these guidelines may have to be incorporated to suit local specifications as approved (if any) by the local authorities.
As touted, cement is the best building material and lasts long (if constructed properly). That is why mainly RCC (columns and beams framed) structures are erected everywhere even as a quake-resistant structure. The cement (as against lime) is considered to be the best for multi-storied structures. Buildings of up to 100 stories high have been constructed with cement all over the world. Currently, billions of tons of cement is being manufactured and used annually for construction throughout the world. Rather, its quantity of use is being considered as an important parameter for measuring the ‘Development Index’ of nations.
Cement appears to be a substance which is ‘unfriendly’ to Nature. Problems with use of Cement for construction are serious ones regarding environmental pollution and Green House Gas (GHG) emission. During manufacturing (even in dry process), GHG emission is at the rate of 11 tons of CO2 emitted per 30 tons of cement manufactured. It is a huge amount by any standard. It has been indicated by laboratories that currently the atmosphere contains CO2 to the extent of over 380 PPM. Cement appears to be one of the main abettors to the GHG emission which is required to be reduced to sustain safety of environment to life on Earth. Therefore, use of cement in construction needs be discouraged. In addition, while using one ton of cement per month in a year, for creating or maintaining construction assets including roads, heat is generated and passed on to the environment at the rate of more than 3000 calories each day for the whole year.
Cemented surfaces during daytime absorb solar heat and release it into the atmosphere during the cool night hours. The heat released adds to the earth’s warming effect. When tall buildings are constructed, people need lifts for vertical movement and electric power; this use of energy adds to the GHG emission. Being away from Nature (soil), certain health benefits due to contact with soil (to improve immunity against pathogenic germs, smooth functioning of metabolism process in human body due to constant environmental pressure etc) are not available for people.
Construction of tall structures using cement concrete results in a very heavy structure and in case of earthquakes becomes very risky for the people living in them. Under enhanced seismic threat, use of cement on a large scale is not recommended. Any natural local material like slaked lime, instead of cement, used in masonry of buildings will surely result in greener construction.
As an effective measure against severe damage, it is suggested that the plan of a structure is rectangular in shape with length to width ratio not exceeding 4.0. This will give equal resistance to seismic forces on any directions.
The buildings in earthquake prone areas therefore should not be very tall, rarely more than 3-4 storey high which do not require lifts. Since India is earthquake prone with some variation in frequency and severity, this rule must apply all over the country. Another advantage is that RC columns need not be very large and 110x110 mm cross section may be adequate. It also becomes clear that the walls between columns can be of half-brick thickness, making the construction lightweight.
This method has been devised in countries facing frequent and severe earthquakes. In this, rows of columns held between plinth and floor or between floors or between floor and terrace tie-beams (continuous from plinth to parapet wall top) standing at a distance of 4 to 5 meters apart as well as at the junction of walls and also on both sides of wall openings like doors or windows, must be all along the walls of the building. The ground floor columns will be erected without (individual) footings but on plinth tie-beams. All tie-beams will be of 110x110 mm size. The walling within the confines of tie-columns and tie-beams will be of half brick masonry (110 mm thick).
Confined masonry is suitable for low-to-medium rise buildings. A site with ground acceleration up to 0.2g (corresponding to Zone III) can have a building up to 4-storey tall. A location (falling in Zone IV) can have a building up to 3-storey high since it has the design ground acceleration up to 0.3g. The plot of land, (in Zone V) where design ground acceleration is in excess of 0.3g, should have a building of up to 2-storey high only.
A new concept of wall density has been included in this construction. It is the ratio of cross sectional area of the walls in one direction on a floor in the numerator and the total floor area of the building since that level up as denominator. At every storey, wall density must not be less than 2% for design ground acceleration below 0.2g; less than 4% where design ground acceleration exceeds 0.2g but is below 0.3g; and less than 5% where design ground acceleration exceeds 0.3g. Cross sectional area of walls in a floor in a direction is the total length of the wall in that direction in one floor (less all the openings for doors and windows in that floor) multiplied by the thickness of the wall/s. The denominator in the wall density ratio consideration is the floor area of the building of all upper floors including that on which the walls being considered are standing.
In any wall on ground floor, the total length of openings (in that wall) must not exceed 50% of the length of that wall. This percentage will not exceed 42% in second floor while on subsequent upper floors it will not exceed 33 % of the length of the wall.
NICEE has provided broad guidelines to Architects for designing the building and guidelines for Engineers for constructing the structure. These have been attached as Appendix A and Appendix B to this article. It must however be noted that all the details provided in appendices are for guidance only. Engineers may have to modify them suitably keeping in mind special needs of the locations where local authorities might have already approved certain way of construction and details of specifications.
Reinforcement for tie-columns as suggested is for (top) two floors of the building. In case taller buildings are proposed, the main steel reinforcement may be changed to bigger size bars to strengthen the lower columns. The size of columns, however, needs no increase. With floor panels of about 25 sqm supported by at least four tie-columns, more than 12 mm longitudinal bars may not be needed as main reinforcement. This will ensure saving of cement and steel.
The foundation for walls need be taken to hard murum base in the ground since the structure is not very tall and therefore lighter. In the foundation trenches over a lean concrete layer of about 15 cms depth, the rubble walls can come up to plinth level ending in with plinth beam around the room floors. Tie-columns need to be founded in the rubble masonry at the same time.
In the buildings steel as well as cement is minimum and hence its mass is restricted. The construction will be greener since cement used is very small in quantity. Lime being the primary material used for masonry purposes, use of cement mortar will be also dispensed with. Thin walls, columns and beams with not very tall construction, the structure will be (comparatively) very light and therefore in case of earthquakes, the damage is likely to be limited, making the building less risky. By using the guidelines offered by NICEE as well as the Disaster Management Cell of the Government of India we can make all our buildings more environment-friendly and safer.
In view of the increase in earthquake frequency in and around India (as experienced in Mar-Apr 2012), it is suggested that construction of buildings must be restricted to maximum 4 storeys. Such buildings will not need elevators which will make them energy efficient as well. Staircases are suggested to be separate from the building (rectangular) plan.
Reference: Earthquake-Resistant Confined Masonry Construction Booklet by NICEE Kanpur