Effect of Soil Condition in Earthquake Response
Muhammad Shydul Alam
The effect of soil condition on earthquake
response has been studied by considering the effects of soil-structure
interaction and soil amplification. In this study, idealized Reinforced Concrete
buildings (1, 2, 5 and 10-storied) are subjected to ground vibrations recorded
during the Northridge earthquake in 1994.
Surface foundations on soils of shear wave velocity cs = 300
ft/sec, 1000ft/sec and 10000ft/sec (representing soft, medium and hard soil
respectively) are used as the substructure. The maximum forces (shear force and
bending moment) are compared for different conditions of the foundation and the
The results from soil-structure interaction show that the maximum
bending moment occurs for the 5-storied building. The soil-structure interaction
does not significantly modify the bending moments for any of the structures.
This is because the considered foundations are surface foundations and are quite
stiff compared to the structures themselves.
However, the maximum forces are significantly affected if the effect of
soil amplification is also taken into account. For structures built on soft
soils the maximum bending moment occurs for the 2-storied building instead of
the 5-storied building. This is because the natural frequency of the soil layer
nearly coincides with the natural frequency of the 2-storied building, thereby
amplifying the energy content of the earthquake at those frequencies.
in assessing the possible impact of an earthquake on a particular structure, it
is important to know the main energy content of the vibration and the natural
frequencies of the structure, and avoiding resonance.