By Dr. N. Subramanian
Ph.D., FNAE, F.ASCE (Life), M.ACI, FIE (Life), F.ICI (life), F.ACCE (I) (Life)
Award winning author, consultant and Mentor (Former AvH Fellow, Germany)
Gaithersburg, Maryland, USA
According to www.usgs.gov, more than 500,000 earthquakes occur on Planet Earth each year. Out of which people will feel about 100,000 of them and about 100 will cause damage. Most earthquakes are moderate in size and still can be destructive. A severe earthquake, which may strike occasionally, will have severe consequences resulting in loss of thousands of lives and economic investment worth billions of dollars. For example the March 2011Tohoku, Japan earthquake resulted in the loss of about 15,900 lives(in addition 6152 injured and 2611 missing), 127,290 buildings collapsed, 272,788 half-collapsed and 747, p89 partially collapsed. Additionally, the Fukushima-Daiichi nuclear reactors suffered extensive damage. The economic losses of all these damage amounting to US$ 235 billion.
Even after extensive research activities, engineers still do not have any reliable method of predicting these earthquakes beforehand, unlike other natural disasters like cyclones or floods. It is generally accepted that earthquakes and volcanic eruptions occur due to the movement 13 larger and several smaller plates, ranging in thickness from 32-240 km, that form the crest of our earth. These crest plates were found to be in constant motion at the rate of a few millimeters per year (theory of platetectonics) . According to www.usgs.gov, the Arctic Ridge has the slowest rate (less than 2.5 cm/yr), and the East Pacific Rise near Easter Island, in the South Pacific about 3,400 km west of Chile, has the fastest rate (more than 15 cm/yr). Researchers have found that the majority of Earth’s volcanoes and earthquakes take place along the 40,000 km long and 500 km wide horseshoe-shaped belt, called the Ring of Fire, around much of the rim of the Pacific Ocean. The Pacific coasts of South America, North America, and some islands in the western Pacific Ocean form the ring of fire. About 76% of the Earth’s seismic energy is released as earthquakes in the Ring of Fire. However, earthquakes can also occur in regions other than the Ring of Fire. For example, the Latur earthquake of 1993 occurred in a place which was considered a stable region on earth. Earthquakes are recorded on a strong-motion accelerograph or seismograph.
The earthquake’s magnitude (a measure of the amount of energy released) and its intensity (measure of the apparent effect and damage caused) are of interest to the structural engineer. The magnitude of an earthquake is measured by the Richter scale which is a logarithmic scale. Thus, an earthquake of magnitude R6 is 31.6 times more powerful than an R5 earthquake. Earthquakes of Richter magnitude 6, 7 and 8 are categorized as moderate, major and great earthquakes, respectively. Two commonly used intensity scales are the Modified Mercalli Intensity (MMI) scale and the MSK scale.
Earthquakes cause the ground to shake violently in all directions, lasting for a few seconds in a moderate earthquake or for a few minutes in very large earthquakes (For example, the 9.1 magnitude December 26, 2004 Sumatra, Indonesian earthquake had a rupture length of 1200 km and lasted 500 seconds, whereas the 6.7 magnitude January 17, 1994, Northridge, California earthquake had only 14 km rupture length and lasted 7 seconds. An earthquake begins at a hypocenter, and from there the rupture front travels along the fault, producing waves while it is moving. As the intensity of these waves diminishes when they travel through the ground, their effect is less away from the fault. There can be P (primary), S (secondary), R (Rayleigh) and L (Love) waves. P waves are the fastest (6-13 km/s) and S waves are slower than P waves (3.5-7.5 km/s). S waves are found to be responsible for most of the earthquake damage.
In general, smaller buildings such as houses are damaged more by higher frequencies (hence they need to be closer to the hypocenter to be severely damaged), whereas larger structures such as tall buildings and bridges are damaged more by lower frequencies (affected by the larger earthquakes, even at considerable distances from the epicenter). Though the ground motion is horizontal and vertical, the predominant direction is usually horizontal. However, vertical acceleration should be considered in structures with long spans and in those structures in which stability is a criterion in design. Special attention should be paid to the effect of vertical component of ground motion on cantilever beam, girders and slabs. The long-period components that occur at the tail-end of earthquakes (with periods closer to the fundamental period of the building) will have a profound influence on the behaviour of structures.
It has to be noted that soils can greatly amplify the shaking in an earthquake. Hence a soft, loose soil may shake more intensely than hard rock at the same distance from the same earthquake. For example, the extensive damage of the 1985 M8.1 and the 2017 M7.1 Mexico City earthquakes were due to the amplification of seismic waves by the soft sediments of ancient lake bed, over which the Mexico City was built.
In India the design of structures to resist earthquake forces has to follow the provision of IS 1893 and IS 13920 codes. It is important to note that as per the earthquake design philosophy adopted in IS 1893
- Minor and frequent earthquakes should not cause any damage to the structure
- Moderate earthquakes should not cause significant structural damage but could have some non-structural damage, (structure will become operational once the repair and strengthening of the damaged main members are completed).
- Major and infrequent earthquakes should not cause collapse,(the structure will become dysfunctional for further use, but will stand so that people can be evacuated and property recovered).
Hence the structures are designed for much smaller forces than actual seismic loads during strong ground shaking. Note that this approach is different than that adopted in the case of wind, dead, live and other loads, where the structure is designed for the anticipated loads.
Buildings having simple regular geometry and uniformly distributed mass and stiffness in plan and elevation have been found to suffer less damage in earthquakes than buildings with irregular structures. Hence, columns and walls should be arranged in grid fashion and should not be staggered in plan. The effect of asymmetry will induce torsional oscillations of structures and stress concentrations at re-entrant corners. These irregularities are grouped as Plan irregularities and Vertical irregularities in IS 1893, which gives very strict stipulations for these irregularities. For the purpose of determining seismic forces, the country is classified into four seismic zones.
Several systems can be adopted to provide adequate resistance to seismic lateral forces. The most common systems are: moment resisting frame (MRF), combined system of moment frames and shear walls, braced frames with horizontal diaphragms and a combination of the above systems. In moment resistant frames, strong-columns- weak-beams concept is emphasized. To perform well in an earthquake, a building should posses the following four main attributes: (a) simple and regular configuration, (b) adequate lateral strength, (c) adequate stiffness and (d) adequate ductility. It is preferable that the structure has clear load path from top to bottom and also has alternate load bath. Hence a structure with MRF and shear walls is better than only a MRF. Long cantilevers and floating columns should be avoided. Appendages like sunshades (chajjas) and water tanks should be designed for higher safety levels.
The structural engineer now has the option of using a variety of devices to ensure safety or serviceability of any structure under severe earthquakes. These devices can be either in the form of
- Base isolators which isolate the structure from ground vibration (they are special laminated rubber-bearing pads, made of alternate layers of steel and rubber and have a low lateral stiffness, and are placed between the ground and the foundation of the structure). Base isolators are found useful for short-period structures.
- Energy absorbing devices like dampers (seismic energy dissipating devices) mounted on structures (especially on diagonal braces). They act like the hydraulic shock absorbers provided in automobiles, absorbing the vibration of sudden jerks and transmitting only a part of the vibration above the chassis of the vehicles.