**The most happening principles to be considered for the design of

earthquake-resistant structures are discussed as follows:**Design basis

earthquake:

In the earthquake-resistant design, it can�t be possible to make the structure

absolutely earthquake proof that will not suffer any damage during the rarest of

the earthquakes. A fully earthquake-proof structure will be very huge and highly

expensive. Instead an attempt shall be made that the structure should be able to

withstand the minor earthquakes that take place frequently in that region.

Moreover, the structure should be able to resist the moderate earthquakes called

design basis earthquakes (DBE), without significant structural damages. Such

earthquakes occur once during the life time of structure. Even a major

earthquake called maximum considered earthquake (MCE) with intensity greater

than that of the design basis earthquake would not be able to cause collapse of

the properly designed and constructed structure and losses would be limited.

**Pseudo-static earthquake:**Earthquakes cause dynamic loading on the

structures. However, for the design of earthquake-resistant structures, the

dynamic analysis is usually not carried out. Instead a pseudo-static analysis

shall be employed in which the earthquake forces are replaced by equivalent

static forces. These forces are considered in addition to the normal loads on

the structure for its design. It is assumed that the forces due to earthquake

are not likely to occur simultaneously with other occasional forces such as wind

loads, maximum flood forces or maximum sea wave forces.Components of

acceleration:

Earthquakes can cause acceleration in any direction. It is the usual practice to

consider the components of acceleration in the vertical direction and in two

perpendicular horizontal directions. Moreover, the acceleration components can

be either positive or negative in these three directions. Since the three

components of earthquake acceleration may not act at the same time with their

maximum magnitude, the code recommends that when maximum response from one

component occurs, the response from the other two components can be 30 percent

of their maximum values. All possible combinations, including plus or minus

signs should be considered in the design. Principally the horizontal

acceleration is the most predominant.

Increase in permissible stresses:

The vertical component of acceleration can increase the normal vertical loads on

the structure. Because of the provision of adequate factors of safety used in

the normal design of structures, most of the structures are able to resist the

additional momentary vertical loads due to earthquakes.

According to the code when earthquake are considered along with the normal

design forces, the permissible stresses in materials in the elastic method of

design can be increased by one-third. However, for steels having a definite

yield stress the increased stress may be limited to the yield stress and for

steels without a definite yield point, the stress may be limited to 80 percent

of the ultimate strength or 0.2 percent proof strain, whichever is smaller.

Increase in allowable bearing pressure:

The allowable bearing pressure in the soils can be increased by 25 to 50 percent

depending upon the type of foundation as per details given in the code.

Horizontal and vertical inertia forces:

The predominant direction of ground motion is usually horizontal. Therefore, the

horizontal seismic forces are most important for the earthquake-resistant

design. However, as per the code the vertical inertia forces are to be

considered in the design unless checked and proven that they are significant.

When effects due to vertical earthquake loads are to be considered, the design

vertical acceleration spectrum is taken as two-thirds of the design horizontal

acceleration spectrum.

Resonance:

Based on code the resonance of the type as visualized under steady-state

conditions will not occur because the earthquake have irregular motion of short

duration in which there is not adequate time to build up the required

amplitudes. However, if the structure�s fundamental period is close to that of

site, resonance may not occur. Such conditions have been observed for some tall

buildings on deep soft soils.

Base shear:

Inertia forces generated in the structure due to an earthquake are assumed to be

transferred to the base structure as the base shear. The base transfers these

forces tot eh foundation, which in turn transfers to the ground.