Question

What is a key difficulty to be overcome when selecting observed seismograms for the design of a structure? Briefly summarize two alternatives.

Answer 1

Main parameter to be identified from any earthquake is intensity of earthquakes,

Two alternatives to be noted down from seismogram are Magnitude and Intensity of Earthquake.

The intensity of earthquake is measured qualitatively in terms of damages caused by it at a particular location. In general, the larger the earthquake, the greater is the intensity. For the same earthquake, the intensity decreases as the epicentral distance of the location increases. Thus the maximum intensity of an earthquake is near its epicentre.

Several intensity scales have been proposed by different investigators. Generally modified Mercalli Intensity (MMI) scale is used in practice. IS 1893:2002 also recommends the use of comprehensive Intensity Scale (MSK 64). Both MMI and MSK 64 scales are similar. However, MSK 64 scale gives more detailed and specific descriptions of the damages caused during and post earthquake.

Both the scales categoriges the damage in twelve classes or degrees. The intensity is usually expressed in roman numerals, I to XII. Some times intensity is alos expressed in grades 1,2......12. In general greater the intensity at a location, the greater would be the damage caused by the earthquake.

Difference between magnitude and intensity should be carefully noted. While the magnitude of an earthquake has a unique value, the intensity decreases as the distance from the epicentre increases. In other words, for a certain earthquake, the intensity varies with the distance. Sometimes, a sound source (or an electric bulb) analogy is used to demonstrate the difference between magnitude and intensity. While the strength of the sound source (or electric bulb) is constant, the intensity of sound (of light) decreases as the observer moves away from the source.

The Gutenberg-Richter relation is commonly used in practice; according to which, the magnitude and intensity are approximately related as,

M = 1.3+0.6 I0

Magnitude M and Intensity on MMI

M = 2, MMI = I, II

M = 3, MMI = III,

M = 4, MMI = IV, V

M = 5, MMI = VI, VII,

M = 6, MMI = VII, VIII,

M = 7, MMI = IX, X

M = 8, MMI = XI

Above mentioned are very important points to be noted from Sesmogram and further analysis of structure.

General principles of earthquake resistant design are as follows,

1. Design Basis Earthquake

2. Pseudostatic analysis

3. Components of acceleration

4. Increase in permissible stresses

5. Increase in allowable bearing pressure.

6. Horizontal and vertical inertia forces

7. Resonsnce

8. Base shear

All the components of the structure and foundations are designed to resist these forces in aadition to the normal forces.