REINFORCED CONCRETE STRUCTURES
Chief Scientist & Head,
Advanced Seismic Testing and Research laboratory
CSIR-Structural Engineering Researcg Centre
Earthquake Engineering as an independent field of science has developed in the last 50 years. The dense network of accelerograms in seismic areas, the developments in analyzing complicated structures in elastic and inelastic ranges using powerful computers, the experimental methods of testing of structures and elements using servo controlled actuators and seismic simulators, refinement and extensive use of in-situ measuring techniques and the broadening of knowledge in soil-structure interaction effects have helped the development of this relatively new field of engineering. The philosophy of seismic resistant design has changed its focus from a strength-based design to an energy-based design during the last two decades and the designed structures should be in a position to absorb and dissipate the kinetic energy imparted to it during an earthquake. The latest seismic resistant design follows three directions 1. Design of structures with members able to dissipate significant amount of energy through stable cycles of inelastic deformation while sustaining a limited amount of damage. 2. Seismic isolation of structures, in tandem with dampers with a view to deflect the major frequency components of the input. 3. Use of special energy dissipating devices, for limiting the degree of damage sustained by the structure.
India has a long and painful history of catastrophic seismic events. More than 60% of the country is vulnerable to strong earthquakes and the major cause of the seismic history of India is due to the movement of the Indian plate against the Eurasian plate and the rate is around 5cm/year, as claimed by geo-physicists. There are also major faults and existences of new faults are realized after each major seismic event.
Table –1 shows the major seismic events in India and their magnitude, year of occurrence and damage.
Table 1. Famous Indian Sub-Continent Earthquakes
Place| Year| Magnitude in Richter’s scale| MM Intensity| Human loss| Kutch | 1819| | | 1500|
Assam| 1897| 8.7| XII| 10000|
Kangra(Punjab-Kashmir)| 1905| 8.6| X| 19000| Bihar-Nepal| 1934| 8.4| X| 10700|
Quetta(Baluchistan)| 1935| 7.6| | 30000| Assam-Tibet| 1950| 8.7| XII| |
Koyna| 1967| 6.5| VIII| 200|
Bihar-Nepal (Dharbanga)| 1988| 6.6| | 1000| Uttarkashi| 1991| 6.6| IX| 770|
Latur| 1993| 6.4| IX| 10000|
Jabalpur| 1997| | | |
Chamoli- Uttarkashi| 1999| | | |
Bhuj-Kutch| 2001| | | 20000|
2.0 METHOD OF SEISMIC ANALYSIS
The major earthquakes have predominant lateral components and the vertical components are half or even less than these lateral components. Also, most of the structures are flexible and critical under lateral loads and horizontal components assume paramount importance than the vertical ones excepting in the case of bridges. As per the linear structural dynamics theory, a continuum system with small damping and subjected to random loadings predominantly vibrates in its fundamental lateral mode. For example, total response of a 2% damped system under an earthquake is 90-95 % in its fundamental mode and 5-10% in all the other higher modes.
Table 2. Typical comparison of Wind and Seismic Loadings
Characteristics| Wind| Earthquake|
Frequency Range| 0.05 to 1.0 Hz| 1.0 to 10.0 Hz| Loading Definition| Wind speed Fourier amplitude or power spectrum (like Davenport / Kaimol etc)| Response spectrum of physical systems.| Response...