General principles of earthquake-resistant design


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.


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