1617.4 - Equivalent Lateral Force Procedure forSeismic Design of Buildings

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October 2005 » Columns » CODE SIMPLE

The International Code Council’s 2003 International Building Code (IBC) Section 1617.4 refers to theAmerican Society of Civil Engineers’ Minimum Design Loads for Buildings and Other Structures.

S. K. Ghosh, Ph.D.

1617.4—Equivalent Lateral Force Procedure for Seismic Design of Buildings

The International Code Council’s 2003 International Building Code (IBC) Section 1617.4 refers to theAmerican Society of Civil Engineers’ Minimum Design Loads for Buildings and Other Structures (ASCE 7-02) Section 9.5.5 for the commonly used equivalent lateral force procedure. At the end of ASCE 7-02Section 9.5.5, there is a very important, and often overlooked provision, which reads as follows:

For regular structures 5 stories or less in height and having a period, T, of 0.5 sec or less, the seismicresponse coefficient, Cs, shall be permitted to be calculated using values of 1.5g and 0.6g, respectively, forthe mapped maximum considered earthquake spectral response accelerations Ss and S1.

This section allows lesser mapped spectral response acceleration values to be used in the case of regularstructures five stories or less in height having a period of 0.5 seconds or less. The darkened areas on themaps in Figure 1 and Figure 2 illustrate those areas of the 48 contiguous states where the reduced valuesapply.

Fig. 1

Fig. 2

As most readers would recall, the UniformBuilding Code (UBC), which was publishedby the International Council of BuildingOfficials, traditionally truncated the value ofthe seismic zone factor, Z, to 0.4, therebytruncating the effective peak acceleration to0.4g. The performance record of structuresin UBC Seismic Zone 4, which satisfied therestrictions in the paragraph quoted fromASCE 7-02, has by and large been quitesatisfactory. The provision underdiscussion essentially continues this truncation for structures that fall under this restriction. Effective peakacceleration of 0.4g corresponding to the design earthquake of the UBC (10 percent probability ofnonexceedance in 50 years) is being taken equivalent to short- and long-period spectral responseaccelerations corresponding to the Maximum Considered Earthquake of the IBC and ASCE 7 of 0.4g x 1.5x 2.5 = 1.5g and 0.4g x 1.5 = 0.6g, respectively.

Answers to FAQs:

Q: My project is a two-story office buildingof masonry (Seismic Use Group I) and islocated in a very seismically active citywhere SS = 2.05g and S1 = 0.911g. Thesite class is B and Fa = 1.0 and Fv = 1.0.Because my building is less than fivestories and has a period less than 0.5second, can I use SS = 1.5g and S1 = 0.6gfor the purpose of assigning the seismicdesign category?

A: Yes, provided your structure isregular.Table 1 provides a comparison ofseismic design category assignments for the different cases.