Gaps between Buildings for earthquake related movement
When a building experiences earthquake vibrations its foundation will move back and forth with the ground. These vibrations can be quite intense, creating stresses and deformation throughout the structure making the upper edges of the building swing from a few mm to many inches dependent on their height size and mass. This is uniformly applicable for buildings of all heights, whether single storeyed or multi-storeyed in high-risk earthquake zones.
A building needs to be slightly flexible and also have components, which can withstand or counter the stresses caused in various parts of the building due to horizontal movements caused by earthquakes.
In Mexico earthquake it was observed that buildings of different sizes and heights vibrated with different frequencies. Where these were made next to each other they created stresses in both the structures and thus weakened each other and in many cases caused the failure of both the structures.
Bureau Of Indian Standards clearly gives in its code IS 4326 that a Separation Section is to be provided between buildings. Separation Section is defined as `A gap of specified width between adjacent buildings or parts of the same building, either left uncovered or covered suitably to permit movement in order to avoid hammering due to earthquake `. Further it states that ` For buildings of height greater than 40 meters, it will be desirable to carry out model or dynamic analysis of the structures in order to compute the drift at each storey, and the gap width between the adjoining structures shall not be less than the sum of their dynamic deflections at any level.`
Thus it is advised to provide adequate gap between two buildings greater than the sum of the expected bending of both the buildings at their top, so that they have enough space to vibrate.
Unfortunately in India building byelaws permit construction of adjoining houses without gap. The buildings constructed on them are in varying heights and levels of floors. At the time of an earthquake these buildings would vibrate with different frequencies. Since there is no gap between these buildings they would press into each other creating additional stresses at the points of contact. Normally the buildings are designed as an individual, thus no provision is provided for the additional forces coming from the neighboring house causing initial weakening and then complete failure of structural components at contact point in earthquakes of high intensity and where it lasts for a longer duration.
This situation is further compounded when the slab level of one building is near the mid level of the walls and columns of the neighboring building, the walls and columns are normally not designed for taking this additional shear force caused by the horizontal force coming from the neighboring slab. This causes buckling of the columns and walls at times of excessive stresses at the mid points (courtesy your neighbouring building) and thus the collapse of the buildings onto each other starting a chain reaction. Since one cannot predict how one`s neighbour is going to build his house at the time of design it is better to take other precautions such as maintaining gap.
In the case of high-rise multi storeyed residential and commercial complexes expansion joints are provided when the length of the building exceeds a length specified by code. This expansion joint is provided for relieving stresses caused due to expansion or contraction of construction material owing to temperature changes. At this point the buildings are totally separated and a gap of 1 to 2" is provided which is filled with a flexible material. However this is causing a major problem i.e. the deflection of these independent buildings during earthquake is much more in high rise buildings than the expansion joint and since at this point these buildings are separate and of varying size they would swing and hammer with each other and weaken the buildings. Structural components around the expansion joint would be severely damaged and there shall be a chain reaction of forces in the total structure for which the structure has not been designed.
These cracks can be easily observed by just going around Delhi and Gurgaon and various cities of Gujarat. The damage caused in the recent earthquake by the seismic forces in the high rise residential and commercial buildings at the position of expansion joints is clearly visible and indicates to the gravity of the situation.
A major problem as seen in the states adjoining Delhi is that, common wall construction is allowed and long rows of houses or commercial buildings are made with common walls without gaps. Here again engineers designing one`s house cannot be aware of the forces due to earthquake which will come from the neighbouring house, who will be sharing your external wall or structural frame with his. This system is totally against the principle of earthquake proof construction and design since this puts in an unknown factor regarding moments and horizontal forces for which the walls or building frames are to be designed. Further this is also against the provision of the I.S Code where a building or structure is to be separated every 30m for allowing expansion caused due to properties of building materials.
Thus it is advised that in areas where high intensity earthquakes are expected the following precautions be taken:
- Municipal and other housing bodies should insure that a suitable gap is provided between two adjoining buildings at the time of sanction.
- Town Planners while cutting plots should also keep this provision of separation of buildings while deciding their size.
- Instead of expansion joints proper gap as required due to bending/movement of the two parts of the building due to earthquake be provided in all buildings.
- It should be made compulsory to submit structural design, drawings along with certificate from structural engineer at the time of sanction of building plans. These should give details of
a) Soil condition and bearing capacity.
b) Earthquake zone for which the building has been designed.
c) I.S. Codes used for design.
- All relaxations in building byelaws generally given at the time of completion should be incorporated at the time of sanction only in the new byelaws and no relaxation should be permitted afterwards to ensure that no changes are made in structural design after sanction.
- Submission of Structural certificates from the designer to the governing municipal body after casting of foundations and at each floor level should be made compulsory. This should state that the reinforcement and R.C.C casted have been verified and are as per his structural design submitted to the body at time of sanction.
- Common wall system between adjoining buildings should be totally abolished.
To summarize, Building Bye - Laws should be amended keeping in consideration all the recommendations in I.S. Codes. Also start appreciating the importance of the role of the Structural Engineer, and hire them separately independent of the Architect to ensure proper structural design. It`s your life at stake and not any one else`s.