Civil Engineering Association

DESIGN DRIFT REQUIREMENTS FOR LONG-PERIOD STRUCTURES

The code provisions for calculating the design seismic drift of buildings have been substantially revised over the past 40 years. While these changes in the code are fairly well documented, the reasons behind these changes and the consequences of the changes are not as well known. This paper presents a brief history of design drift requirements, technical background for the requirements, and the reasoning behind the changes, starting with the 1961 Uniform Building Code (UBC) through present day. Emphasis is given to the discussion of minimum base shears for calculation of drift for long-period structures. Specifically, in Section 1630.10.1 of the 1997 UBC, it is not immediately apparent why Equation 30-6 may be disregarded in the calculation of drift while Equation 30-7 may not, since both equations tend to give very similar minimum base shears for typical buildings. In prior versions of the UBC, the minimum design base shear was determined by only one equation that could be disregarded during determination of drift. This paper discusses the reasoning behind Equation 30-7 in the current UBC and discusses the current controversy and differences of opinion regarding this equation. Also discussed are equivalent requirements in the National Earthquake Hazards Reduction Program (NEHRP) and Minimum Design Loads for Buildings and Other Structures (ASCE 7-02), which require a similar minimum base shear for determining drift of long period structures.