# Structural Analysis Procedures to Calculate Equivalent Static Force Acting on a Structure due to Earthquake Loading

May 15, 2017 Abdul Siraj, P.E.

The basic premise behind seismic analysis of structures is to convert the earthquake dynamic forces acting on the structure to equivalent static forces which are then used as inputs in a static structural analysis to obtain the internal forces, stresses, and deformations in the structure. There are two analysis procedures described in the International Building Code that can be used to compute the equivalent  static forces:

The equivalent lateral force procedure – used for seismic design categories B, C, D, E, and F

The simplified analysis – used for seismic design categories B, C, D, E, and F. It can also be used for constructions that are not higher than two stories as well as light frame construction that are not higher than three stories.

For structures in seismic design category A, neither the simplified analysis nor the equivalent lateral force procedure can be used. Rather, these structures should be designed such that the lateral resisting-floor system is capable of resisting the minimum design lateral force, Fx, applied at each floor level per ASCE 7-10, Sections 11.7 and 1.4.3. The minimum design lateral force per Equation 1.4-1 of ASCE 7-10 is equal .01 times the portion of the dead load of the structure located or assigned to level x, wx.

Fx = 0.01wx