EARTHQUAKE RESISTANT DESIGN

by SP WANGDI
1. All buildings are basically cantilevers taking off from the ground and are structurally weaker in the lateral direction as compared to the vertical direction. During an Earthquake, the seismic forces are acting on the building in the horizontal direction.
2. Try to avoid irregular shaped buildings in plan like L,C,Y etc without expansion joints. Buildings with this plan configuration in one block are most vulnerable as there is a variation in the lateral stiffness within the building and are most vulnerable to seismic forces. The simple box structure (Machis ko Khol) is best for Earthquake resistant design and half the battle against Earthquakes is already won by following a box configuration.
3. Make your columns as strong as possible within your budget especially the ones from the foundation to the first floor through the ground floor. During an Earthquake the ground floor columns are usually subjected to maximum shear forces and most vulnerable to fail. In case of a failure, the structural repairs of the ground floor columns to the foundation are tedious and expensive whereas the repairs of the columns in the upper storeys are relatively easier.
4. Be careful of “Soft Storeys” in your building. Soft storeys are created when the partition walls are absent to create halls/basement parking and large rooms. In the absence of partition walls which help to resist lateral forces on the building, the columns will be subjected to greater lateral loading and these have to be designed accordingly or braced.

5. As the partition walls help to resist seismic lateral forces, do not make windows and other openings in partition walls larger than necessary as you are losing lateral stiffness of the building when you do so.
6. The most detrimental factor on a building during an earthquake is its own weight. Make the structure as light as possible but satisfying the strength requirements of the structural members. The general tendency observed in Sikkim is that Mason’s tend to overdesign the RCC beams and slabs which actually creates unnecessary extra loading on the column and the ground on which the building is resting. For example, for most common buildings 10 mm and 8mm steel rebars are more than sufficient to reinforce RCC slabs but many Masons still use 12mm diameter rebars in slabs. Modern Technology has developed steel rebars with much higher yield strength and we have to take advantage of this.
7. Place the stirrups in beams and columns more closely towards all column-beam junctions. Stirrups resist shear forces in a structural member and shear failure is more dangerous than a bending failure.
Stirrups (Rings) are perhaps the most important in Earthquake resistant design as it helps to confine the concrete within the core of the column and the beams. It also provides ductility (property of the building to bend and sway) which helps to dissipate the seismic forces acting on the building. For this reason, the Bureau of Indian Standards Codes have specified that the minimum dia of stirrups should not be less than 8mm as it contributes so much to the overall structural strength of the building. However, most of the masons in Sikkim seem to economise on the use of stirrups by using 6mm dia mild steel bars which is not proper.
8. During an Earthquake, the joints ie column beam junctions are the most vulnerable to damage and one must be careful while concreting the joints. The column beam junction contain the most steel with rebars coming from all directions which results in a dense rebar cage. Ensure that the stone chips are of appropriate size which can go through the cage for proper concreting.
9. If you are facing budget constraints and compromising on the quality of stone chips use the best quality in this order, firstly the column beam junctions, followed by the columns, beams and then the slab. I should not be saying this but people do compromise on the stone aggregates due to budget constraints.
10. The strength of the concrete depends primarily on the water cement ratio for the same quality of materials used. Most of the masons and labourers tend to add more water than necessary for easy workability which actually reduces the strength of the concrete. Also always mix the water by batching instead of using a water hose to maintain the consistency in the concrete. Invest in a mechanical vibrator as it is difficult for the labourers to do manual compaction.
11. Plan your construction joints in advance. A construction joint is formed when you stop the concreting for the day and continue it the next day. Ideally, construction joints should not be made at points subjected to maximum shearing forces.
12. Geo Technical Report--- No matter how well a building is designed and constructed, all the loads encountered by the building is ultimately transferred to the ground. Therefore, one must have a fair idea about the geology of the soil , land mass on which the building is to rest. The Mines & Geology Department can help you in this regard. During an Earthquake the seismic waves travel through the earth with different characteristics depending on the type of soil through which it is travelling and the building has to be designed for seismic safety keeping this in mind. During the Sikkim Earthquake on 18/09/2011, i feel buildings with foundation on rock suffered more damages than buildings with foundation on soil due to the kind of vibrations induced by the Earthquake on the buildings.
[The writer is a B.Tech and Member, Indian Buildings Congress]