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Learning

Earthquake Design
30 and
Earthquake Tip
Construction

What is Important in Foundations of Earthquake-Resistant Buildings?

Selecting Site for Effect on Ground Shaking on Building Site
Earthquake-Resistant Buildings Even if local soil stratum underneath a proposed
The site of a building should be free from any structure is stable, ground shaking may be modified
collateral damage due to earthquake-related effects. when earthquake waves propagate through the soil
Ideal sites are: overlying rock layers; this phenomenon is referred to
(a) Away from a potential fault rupture zone; as Site Effect. Even when shaking at the base rock is
(b) Above the level of inundation under tsunami waves moderate, the motion at a site may be amplified by soil
generated in the adjoining ocean by earthquakes above rock, and this needs to be accounted for in
(Figure 1); design. Site effect was noticed first in the 1819 Kutch
(c) Beyond the forest or wooded areas with potential earthquake in India. It was very prominent in 1985
fire hazard arising from earthquakes; and earthquake that affected Mexico City; ground response
(d) Free from detrimental earthquake actions in the was amplified by up to 7-8 times at building sites
ground, like liquefaction, settlement and lateral located on lake bed (which was akin to a bowl of jelly)
spreading (See IITK-BMTPC Earthquake Tip 31). in contrast to those located on hard rock (Figure 3) in
Mexico City. Peak Ground Acceleration (PGA) is a
Tsunami measure of severity of shaking of ground. During the
Wave 1985 earthquake, PGA at soft soil site (SCT) was
significantly larger than at rocky site (UNAM) (Figure
3b).
Soil
Amplification of ground motion depends on soil
Hard Rock properties (e.g., shear modulus, damping, soil layers
and their properties, saturated versus dry soil, and
Figure 1: Tsunami Waves – Buildings on coasts loose versus dense soil), and ground motion
can inundate, if tsunami wave is high enough characteristics. In general, stiff soils have lower
amplification, while soft soils higher. Seismic design
Even if the site is devoid of the above, steep slopes codes provide design spectra for underlying soil strata
or vertical cuts in natural hills (otherwise safe under of different soil types.
other loads acting on them) can slide during
earthquakes (Figure 2). Vulnerable soil embankments
can slide or spread laterally due to liquefaction. Other Mexico City
earthquake hazards at hill slope sites include rolling Soft soils of lake

stones and debris. When the ground shakes underneath SCT UNAM
buildings with elongated plan or long span structures (e.g., Site Site
suspension bridges), the motion at different supports
may not be synchronous. Differential shaking of such
structures at their supports induces additional effects, 3-5 m Basaltic rock,
underlain by softer soil
and should be accounted for in their design.
(a)
Slope
Stability
Concern UNAM Site
Acceleration Time t

SCT Site
Acceleration Time t
Slope
Stability
Concern Soil
(b)

Hard Rock
Figure 3: 1985 Mexico City Earthquake – a classic

example of local site response (a) layout of
Figure 2: Unstable hill slopes – landslides are Mexico City, and (b) acceleration response at
major concerns in hilly areas two sites on distinctly different soil conditions