Earthquake-resistant structures: How safe is your home?

The Mumbai Metropolitan region and New Delhi-Gurgaon region has seen a spurt in the vertical growth of buildings. With the recent earthquake in Nepal, the discussion on how safe buildings and houses are in India has again gained prominence. The question on most minds is, how safe is my residential building or office building during an earthquake? 

During an earthquake, a wave propagates from the rock to the soil and then into the structure, creating a sway in the structure. The key to designing an earthquake-resistant structure is to build a ductile structure rather than a stiff structure. The extent of damage to a structure during an earthquake depends upon the distance of the epicentre from the structure horizontally as well as vertically below the ground. If the epicentre is closer to the surface, the damage tends to be larger in structures that are not resistant to earthquakes. It also depends on the type of soil. During earthquakes, certain soil such as sandy soil or deposited layers undergo soil liquefaction, causing greater damage to structures. Soil liquefaction is essentially when the soil bubbles, heaves or surges to the top surface under great pressure. Some areas in Delhi are prone to the occurrence of soil liquefaction during an earthquake, specifically those areas which have higher deposits of sandy silt or clayey soils.

Generally during an earthquake, load bearing structures have brittle failure while well-designed reinforced cement concrete (RCC) structures have ductile failure. Earthquake-resistant design is essentially about ensuring that the damage to buildings during earthquakes is of an acceptable variety, with zero human loss and also that they occur at the right places and within acceptable ranges. All legally built structures are either load bearing or RCC structures. In load bearing structures, the brick walls are thick (between 9 inches to 1 foot) and carry the load to the foundation. It may have beams and the slab is typically made of concrete with steel reinforcement. RCC structures, referred to as framed structures, are made of concrete and steel and the load is carried by columns or shear walls to the foundation resting on concrete piles. 

Load bearing structures were typically built prior to the 1970s, and have low resistance to earthquake. The bricks are stiff and have no way to either pull the structure in the direction opposite of the sway or be ductile enough to allow for small movement in the structure. Load bearing structures exhibit instantaneous failure and fall like a pack of cards. In the recent Nepal earthquake, most structures that fell within seconds of the earthquake were load bearing structures such as the Dharahara Tower in Kathmandu. One way to avoid such catastrophic failure in load bearing structures is to create a disconnect between the foundation of the building and the rest of the above ground structure by using the base isolation method or levitating the building during earthquake from its base by having an air compressor fill air between the foundation and upper storeys. Such methods have been recently adopted in USA and Japan, but so far in India, the system is not very prevalent. 

RCC framed buildings are typically about 30 ft to a few 100 ft long. Most two to fifteen storey buildings in India have stilt parking. There are no walls provided on the stilt floor creating larger flexibility in the ground structure. In an earthquake, such buildings sway like a reverse pendulum with rigidity being provided at the bottom and in such a case, the top portion of the building sways more than the bottom portion. Also, often the ground storey is a weak structure due to limited ability to carry horizontal forces. Such stilt parkings greatly reduce the earthquake resistance of a building and can fall during the earthquake. It is essential to have properly constructed in-fill walls so as to strengthen the buildings. Such strengthening should be done in consultation with a registered structural engineer.
  
Another method to reduce failure during earthquakes is to design a strong core shear wall in seven to 20 storey RCC structures. Typically, this is designed in the elevator area. If the shear wall is designed as per relevant code, it can provide necessary stiffness to reduce excessive sway during the earthquake. For 20+ storey buildings, it necessary to provide a combination of vibration controlled systems to avoid catastrophic failures. This is typically done by providing proper core walls as well as by providing base isolation systems. Another way to reduce vibration in tall buildings is to provide tuned mass dampers and shock absorbers. Tuned mass dampers are essentially a pendulum with a specific viscous fluid which moves the building in the opposite direction of the structure’s natural frequency, thereby avoiding catastrophic failures. Other types of energy dissipation devices such as friction dampers and yielding dampers are also adopted to reduce damage during earthquakes. 

Is Your Building Safe?
In a well-designed earthquake resistant building, the basic mantra adopted is - the soil must be stronger than the foundations, the foundations must be stronger than the columns and the columns must be stronger than the beams. To decide if an existing building is safe or not, consult a competent structural engineer. Get the building assessed, and if found deficient, get it suitably retrofitted. (Handbook of Seismic Retrofit of Buildings)

While buying a new house or renting one, there are a few things one should keep in mind. 

1. Determine if the plan and elevation of a building is simple and regular. Typically, excessive architectural features that are not connected to the main frame of the building are potential seismic disasters. The building should have a simple geometrical plan such as rectangular or circular. Even in rectangular, avoid plans with excessively longer lengths in one direction. Buildings which have L, U, V, Y or H shape in plan are avoidable. If such geometries are unavoidable, one should make sure a separation joint at re-entrant corners is provided so that each side behaves as a separate unit during the earthquake. Typically, buildings with vertical setbacks such as plaza type buildings or buildings with excessive overhangs such as cantilever staircases perform poorly in an earthquake. 

2. For a structure more than two storeys, ask if it has been designed as per relevant National Building codes of India and Indian Standards. 

3. Determine if the building has adequate core shear walls. If the building has stilt parking without walls, ask if relevant ductile designing for columns has been done. 

4. Determine if the building has any columns that run within floors only and do not run all the way into the ground. Such columns are called floating columns and can reduce the structural capacity during earthquakes. 

5. The staircase is the ONLY escape route during an earthquake and should be designed so as to not fail during it. Check that the staircase slabs are integrally connected to the frame of the building to prevent collapse.

6. Ask the builder if proper geo-technical investigations have been conducted prior to the start of the design and construction process. Ask if adequate pile lengths in rock have been provided to get proper anchorage in the soil. Inquire about soil liquefaction studies and slope stability studies conducted during investigations. 

The author holds a PhD in Civil Engineering from Purdue University. She has taught at several Universities in the USA and currently is the Principal Consultant at Renuka Consultants.