What is Earthquake Loading?

Earthquake loading comprises the forces from inertia of mass of structure. These forces are generated due to shaking of foundationof a building by seismic disturbance. The effect of translational forces from inertia of a structure is more significant than rotational or vertical component of shaking and seismic resistant design pay concentration to this force.

Dynamic response of a structure against shaking of ground produces the magnitude of loading due to earthquake. On the basis of structural properties and past seismic record of a region, seismic loading are determined. 

Now we will know other form of earthquake forces like forces due to–

a. Landsliding

b. Subsidence

c. Active faulting beneath foundation

d. Vibration generated liquefaction 

These sources of forces are local. But they sometimes become too massive to challenge any economic seismic resistant design. During site selection for a structure a earthquake engineer must consider these geological and geotechnical considerations and alternate location should be chosen to avoid earthquake forces of these form.

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How is Earthquake Loading Determined by Equivalent Lateral Force Method?

Equivalent lateral force method: 

At first we will know the application and feasibility of this method, later we will know how it determines earthquake loads. 


It is very simple and quick method. This method is not applicable to exceptionally high buildings having unusual structural arrangement i.e. irregular structure and tall buildings cannot be analyzed for earthquake in this method. Now question is what irregular structure is. To have idea about structural irregularity please read previous posts as linked here. 


In this method base shear is first determined by simple calculation and then horizontal forces equivalent to base shear is distributed throughout height of structure in some manner prescribed in local codes or UBC, NBC etc. This distributing from base through height of structure permits static analysis for the structure. The 

base shear is determined by estimating fundamental period of structure and expected maximum ground velocity or acceleration and including some relevant factors. But, for a design earthquake less force are used in the structure above to count damping properties provided by components of buildings, to count the force reduction as members yield beyond elastic limit and to count greater strength expected from structure in working stress level.

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