Design Approaches to Earthquake Resistant Buildings

EARTHQUAKE RESISTANT BUILDINGS: DESIGN APPROACH

The engineers do not attempt to make earthquake proof building that will not get damaged even during the rare but strong earthquake, such building will be too robust and also too expensive. Instead, the engineering intention is to make buildings earthquake resistant; such buildings resist the effects of ground shaking, although they may get damaged severely but would not collapse during the strong earthquake. Thus, safety of people and contents is assured in earthquake-resistant buildings, and thereby a disaster is avoided. This is a major objective of seismic design Codes throughout the World.

IS 1893 (Part 1) 2002 (Criteria for Earthquake Resistant Design of Structures) adopts the following there criteria for fixing the level of design seismic loading

Criterion 1 : Structure should possess at least a minimum strength to with- stand minor earthquakes (<DBE) which occur frequently, without damage. Here DBE stands for 'design basis earthquake, defined as the earthquake which can reasonably be expected to occur at least once during the life of the structure. This means that under minor but frequent shaking, the main members of the building that carry vertical and horizontal forces should not be damaged; however, building parts that do not carry load may sustain repairable damage.

Criterion 2 : Structure should be able to resist moderate earthquake (DBE) without significant structural damage though some non-structural damage may occur. This means that under moderate but occasional shaking, the main members may sustain repairable damage while the other parts of the building may be damaged such that they may even have to be replaced after the earthquake.

Criterion 3 : Structure should withstand a major earthquake (or maximum considered earthquake, MCE) without collapse, but with some structural and non-structural damage.

This means that under strong but rare shaking, the main members may sustain severe (even irreparable) damage, but the building should not collapse.

Thus, after minor shaking, the building will be fully operational within a short time and the repair costs will be small. And, after moderate shaking, the building will be operational once the repair and strengthening of the damaged main members is completed. But, after a strong earthquake, the building may become dysfunctional for further use, but will stand so that people can be evacuated and property recovered.

The consequences of damage have to be kept in view in the design philosophy. For example, important buildings, like hospitals and fire stations, play a critical role in post-earthquake activities and must remain functional immediately after the earthquake. These structures must sustain very little damage and should be designed for a higher level of earthquake protection. Collapse of dams during earthquakes can cause flooding in the downstream reaches, which itself can be a secondary disaster. Therefore, dams (and similarly, nuclear power plants) should be designed for still higher level of earthquake motion. Design of buildings to resist earthquakes involves controlling the damage to acceptable levels at a reasonable cost. Contrary to the common thinking that any crack in the building after an earthquake means the building is unsafe for habitation, engineers designing earthquake-resistant building recognize that some damage is unavoidable. Different types of damage (mainly visualized though cracks, especially so in concrete and masonry buildings occur in buildings during earthquakes. Some of these cracks are acceptable (in terms of both their size and locations) while others are not. For instance, in a reinforced concrete frame building with masonry filler walls between columns, the cracks between vertical columns and masonry filler walls are acceptable, but diagonal cracks running through the columns are not. In general, qualified technical professionals are knowledgeable of the causes and severity of damage in earthquake-resistant buildings. Earthquake-resistant design is therefore concerned about ensuring that the damages in buildings during earthquakes are of the acceptable variety, and also that they occur at the right places and in right amounts.

 

VIRTUES OF EARTHQUAKE RESISTANT BUILDING: INDIAN SEISMIC CODES

An earthquake resistant building has the following four virtues

(1) Good structural Configuration Its size, shape and structural system carrying loads are such that they ensure a direct and smooth flow of inertia forces to the ground.

(2) Lateral strength: The maximum lateral (horizontal) force that it can resist is such that the damage induced in it does not result in collapse.

(3) Adequate Stiffness: Its lateral load resisting system is such that the earth- quake-induced deformations in it do not damage its contents under low-to-moderate shaking

(4) Good Ductility: Its capacity to undergo large deformations under severe earthquake shaking even after yielding is improved by favourable design and detailing strategies.

 

The following Indian Seismic Codes cover all the above aspects.

Indian Seismic Codes: Seismic Codes are unique to a particular region or country. They take into account the local seismology, accepted level of seismic risk, building typologies, and materials and methods used in construction. Further, they are indicative of the level of progress a country has made in the field of earthquake engineering.

 

The first formal seismic Code in India, namely 18 1893, was published in 1962 Today, the Bureau of Indian Standards (BIS) has the following seismic Codes

 

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

2. IS 4326, 1993, Indian Standard Code of Practice for Earthquake Resistant design and Construction of Buildings (2nd Revision)

3. IS 13827, 1993: Indian Standard Guidelines for Improving Earthquake Resistance of Earthen Building

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

5. IS 13920, 1993: Indian Standard Code of Practice for ductile Detailing of Reinforced Concrete Structures subjected to Seismic Forces

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

The regulations in these standards do not ensure that structures suffer no damage during earthquake of all magnitudes. But to the extent possible, they ensure that structures are able to respond to earthquake shakings of moderate intensities without structural damage and of heavy intensities without total collapse.