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How is Earthquake Loading Determined by Equivalent Lateral Force Method?

In the previous post we discuss earthquake loading and seismic design principles as well. In this post we will try to get an overall idea about a method of determining of magnitude of earthquake loading. As discussed previous this method uses record of past seismic activity and their behavior and properties of structures like fundamental periods etc.
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


Earthquake base shear distribution throughout the height of building in staad pro analysis


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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