Recommendations for earthquake resistant design of structures

Earthquakes, Tsunamis, Landslides, Floods and Fires are natural calamities causing severe damage and sufferings to persons by collapsing the structures, cutting off transport systems, killing or trapping persons, animals etc. Such natural disasters are challenges to the progress of development. 

However, civil engineers as designers have a major role to play in minimizing the damages by proper designing the structures or taking other useful decisions. 

Because of the vastness of the topic, “Disaster management and mitigation”, this article includes understanding the earthquakes, behavior of the materials of construction and structures and the extent to which structural engineers make use of the knowledge in taking proper decisions in designing the structures made of reinforced concrete.

Earthquakes have many other effects besides vibrating the structures in response to ground shaking at its foundation. These other effects may even exceed that due to vibration. 

Unfortunately, the procedure of their estimation and the required steps for the design are considered outside the scope of structural engineering. Different seismic resistant design codes have provisions to take into account the vibration of structures. 

Terminology for Earthquake Engineering

(a) Epicenter of an earthquake

Epicenter is the geographical point on the surface of earth vertically above the focus of the earthquake. (cl.3.10 of IS 1893 (Part 1): 2002).

(b) Focus of earthquake

Focus is the source of the elastic waves of the originating earthquake inside the earth which cause shaking of ground (cl. 3.13 of IS 1893 (Part 1): 2002).

(c) Intensity of earthquake

The intensity of an earthquake indicates the strength of shaking during the earthquake and is expressed by a number according to the modified Mercalli Scale or M.S.K Scale of seismic intensities (cl. 3.15 of IS 1893 (Part 1):2002).

(d) Magnitude of earthquake (Richter’s Magnitude)

The magnitude of an earthquake is expressed by a number, which is a measure of the energy released in an earthquake. The magnitude of an earthquake is defined as logarithm to the base 10 of the maximum trace amplitude, expressed in micron, which the standard short-period torsion

seismometer (with a period of 0.8 second, magnification of 2800 and damping nearly critical) would register due to the earthquake at an epicentral distance of 100 km (cl. 3.18 of IS 1893 (Part 1): 2002).

(e) Critical damping 

Critical damping is the damping beyond which the free vibration motion will not be oscillatory (cl. 3.3 of IS 1893 (Part 1): 2002).

(f) Maximum Considered Earthquake (MCE)

   Maximum Considered Earthquake is the most severe earthquake whose effects are considered by IS 1893 (Part 1): 2002, as given in cl. 3.19 of this standard.

(g) Liquefaction

Liquefaction is a state in saturated cohesionless soil wherein the effective shear strength is reduced to negligible value for all engineering purposes due to pore pressure caused by vibrations during an earthquake when they approach the total confining pressure. In this condition the soil tends to behave like a fluid mass (cl. 3.16 of IS 1893 (Part 1): 2002).

Bureau of Indian Standards for Earthquake Design

In our country, several major earthquakes have occurred in the Himalayan-Nagalushai region, Indo-Gangetic Plain, Western India, Kutch and Kathiawar regions. Taking into account seismic data from studies of these Indian earthquakes, Bureau of Indian Standard first published IS 1893 “Recommendations for earthquake resistant design of structures” in 1962 and revised in 1966. Considering the local seismology, accepted level of seismic risk, building topologies and materials and methods used in construction, presently the Bureau of Indian Standards has the following seismic codes:

Indian Standard Codes for Earthquake Design

1. IS 1893 (Part 1), 2002; Indian Standard Criteria for Earthquake Resistant Design of Structures (5th Revision),

2. IS 1893 has other four parts:

(a) Part 2 for liquid retaining tanks–elevated and ground supported,

(b) Part 3 for bridges and retaining walls,

(c) Part 4 for industrial structures including stack like structures and

(d) Part 5 for dams and embankments.

However, they are yet to be finalized. Hence, provisions of  Part 1 will be read along with relevant clauses of  IS 1893: 1984 for structures other than buildings.

3. IS4326: 1993, Indian Standard Code of Practice for Earthquake Resistant Design and Construction of Buildings, (2nd Revision),

4. IS 13827: 1993, Indian Standard Guidelines for Improving Earthquake Resistance of Earthen Buildings,

5. IS 13828: 1993, Indian Standard Guidelines for Improving Earthquake Resistance of Low Strength Masonary Buildings,

6. IS13920: 1993, Indian Standard Code of Practice for Ductile Detailing of Reinforced Concrete Structures Subjected to Seismic Forces, and

7. IS 13935: 1993, Indian Standard Guidelines for Repair and Seismic Strengthening of Buildings.

To download more civil engineering codes go to this link

The regulations of these standards will not result in structures having no damage during earthquake of all magnitudes. However, the regulations shall ensure that, as far as possible, structures will be able to respond without structural damage to shocks of moderate intensities and without total collapse to shocks of heavy intensities.