Earthquake questions

 1. What is storey drift?

Ans. It is the relative displacement between the floors above and/or below the storey under consideration.

Storey drift ratio:- It is ratio of lateral displacement of storey relative to lower level storey and height of the storey. In short, it is ratio of storey drift and height of the storey.

2. What is soft storey?

Ans. Soft storey is that storey of building which lateral stiffness is lesser than that in the storey above. According to IS code 1893:2002,  when a storey stiffness is lesser than 70% of stiffness of the storey above then the storey is termed as soft storey.

Storey level lateral stiffness:- Storey level lateral stiffness is the total stiffness of all lateral load resisting system in the considered direction. 

3. What is weak stoery?

Ans. It is in in which the lateral strength of storey is lesser than that in the storey above. 

Storey lateral strength : it is the total strength of all seismic force resisting elements sharing the lateral storey shear in the considered direction. 

4. What is diaphragm action?

Ans. Diaphragm is the horizontal or nearly horizontal structural system which transmits lateral forces to vertical elements connected to it. And this action of transferring lateral load to vertical elements is termed as diaphragm action.

5. What is eccentricity?

Ans. It is the distance between centre of mass (CM) and centre of resistance (CR) of floor i. 

Centre of resistance: The point on the roof (single storey building) or the set of points on the horizontal floors (multi-storey buildings) through which, the resultant internal resistance acts, all floors of the building undergoes:

a. pure translation in the horizontal direction

b. no twist about vertical axis passing through the CR

Centre of mass: The point in the floor of a building through which the resultant of the inertia force of the floor is considered to act during earthquake shaking. 

For more read IS code 1893:2016 Terminology section pp 2,3,4,

Important Terminology

Damping: The effect of internal friction, Inelasticity of materials, slipping, sliding, etc, in reducing the amplitude of oscillation.

Ductility : It is the capacity of a structure (or its members) to undergo large inelastic deformations without significant loss of strength or stiffness. 

Epicentre : Geographical point on the surface of earth vertically above the point of origin of the earthquake. It is used in estimating design seismic force depending on the functional use of the structure.   

Importance factor (I): For important building or critical buildings like school, hospitals, community or public buildings, the value of I =1.5 otherwise 1. 

Liquefaction : A condition when uniformly graded, cohesion-less and saturated soil is subjected to earthquake then it loses its effective shear strength to zero then soil starts behaving like a liquid. The situation is termed as Liquefaction. 

Response reduction factor (R): It is the factor by which the base shear induced in a structure, if it were to remain elastic, is reduced to obtain the design base shear. Depends upon ductility of the building. For moment resisting frame (MRF) R=3; for Special moment resisting frame (SMRF) R=5 

Response spectrum:- It's a tool to convert the seismic ground motion into the load experienced by the building. It is a representation of maximum responses of a spectrum for single degree freedom systems of different natural period but having the same damping under the action of same earthquake ground motion at their bases.  

We calculate response acceleration coefficient (Sa/g) from the response spectra for natural period of the building. 

6. How to calculate earthquake load?

Step 01: Find the zone of the building. (II,III,IV or V). (In Bihar zones III, IV and V are present. Patna                 is in zone IV )

Step 02: With the help of building height and type of building calculate the natural period of the                             building as per IS Code 1893:2016

Step 03: Define importance factor (I) as per the functionality of the building.

Step 04: Choose value of response reduction factor depending upon building type from the code.\

Step 05: Find horizontal acceleration coefficient (Ah) using formula

                

Here Z is zone factor that is PGA in the zone which exceedance probability is 2% in 50 Years. It is also termed as maximum considered earthquake (MCE) for that  particular zone. 

Step 06: Calculate seismic weight of the building and multiply it with the Ah, the result is earthquake                    load experienced by the structure.

7. What is time history analysis?

It is an analysis of the dynamic response of the structure at each instant of time,  when its base is subjected to specific ground motion time history.

8. What is Pounding?

When closely spaced (negligible or zero gap) buildings of different dynamic characteristics are subjected to an excitation like earthquake they tend to collide each other. This collision is termed as pounding. 

9. What are some major measures to avoid pounding?

Ans.  For new buildings provide seismic gap between two adjacent buildings as per code. For existing buildings pounding can be controlled using dampers or by attaching both the buildings so that their dynamic responses could be similar. 

9. What is retrofitting?

Ans. Retrofitting is process of strengthening the seismically deficient structures as per code requirement. There are many process for the retrofitting process:

1. Jacketing: Process of strengthening the columns 

2. Providing seismic belts at sill level, lintel level, openings, and rods at corners 

3. Please check your mail/whatsapp, I have attached a file for more information regarding the                     retrofitting techniques. 

4. For any further query you may call anytime.