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
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
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.
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.