Thursday, February 09, 2017

IS: 1893: Design of Earthquake Resistant Structures

This standard is intended for earthquake resistant design of normal structures. It is applicable to buildings, elevated structures, bridges, concrete, embankments and retaining wall. It has been endeavored to ensure that, as far as possible, structures are able to respond, without structural damage to shocks of moderate intensities and without total collapse to shocks of heavy intensities. It takes into account:
a) Seismic zone factor. (Fo)
b) Importance factor, to account for the varying degrees of importance for various structures. (I)
c) The coefficient of Flexibility for design of multi- storeyed building is given in form of a curve with respect to period of buildings.
d) Performance factor depending on the structural framing system and brittleness or ductility of construction.
e) Intensity of shock due to earthquake.
f) The seismic zone map, the object of this map is to classify area of the country into five seismic zones.

Earthquake cause random motion of ground, which can be resolved in any three mutually perpendicular directions. This motion causes the structure to vibrate. The vibration intensity of ground expected at any location depends upon the magnitude of earthquake, the depth of focus, distance from epicenter and the strata on which the structure stands. There might be cases in which structure have less importance factor and relatively small structure for which no analysis need be made. There is Clause in code which gives permissible increase in allowable bearing pressure or resistance of soils. 

  • Design Seismic Coefficient for different zones:

Following two methods are used to calculate horizontal seismic coefficient:
a) Seismic Coefficient Method
b) Response Spectrum Method

a) Seismic coefficient Method takes into account only static characteristic of structure.

b) Response Spectrum Method takes into account structure dynamic characteristics and ground motion caused due to earthquake force. It is recommended for use in structures where it is desired to take both effects.

  • Design Live loads:

Clause in code gives the percentage of live load, which should be considered while calculating horizontal earthquake force. This percentage of live load shall also be used for calculating stresses due to vertical loads. Under the earthquake condition, the whole frame except the roof may be assumed loaded with live load proportions. Where the probable loads at the time of earthquake are more accurately assessed, the designer may alter the proportions or even replace the entire live load proportions by the actual assessed load.

  • Design criteria for multi- storeyed Buildings:

The criteria for design of multi- storeyed buildings are:
- In case of building with floors capable of providing rigid horizontal diaphragm action, a separate building or any block of a building between two separation sections shall be analyzed as a whole for seismic forces. 
- In case of buildings where floors are not able to provide the diaphragm action, building frame may be analyzed frame by frame with tributory masses for seismic forces.

Modal analysis:

This is applied to buildings having irregular shape and / or irregular distribution of mass and stiffness in horizontal and/ or vertical plan.

The Lateral forces at roof of floor i depend on:

-Base pressure
-Load of the roof or any floor i = dead load+ appropriate amount of live load 
- Height measured from the base of building to the roof or any floor i
-Number of storey, including the basement floors.