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The 2005 edition of American Society for Civil Engineers’ Minimum Design Loads for Buildings and Other Structures (ASCE 7-05) Section clearly states that the prescribed seismic design forces of Section 12.8 need to be used for the determination of drift when using the equivalent lateral force procedure. This includes the following minimum base shear equations set forth in ASCE 7-05 Section 12.8, including Supplement No. 2 (see June 2008 Code Simple for more information on Supplement No. 2):

1. Cs shall not be less than Cs = 0.044SDSI ≥ 0.01
(Equation 12.8-5)

2. Cs shall not be less than (0.5S1)/(R/I) when S1 is equal to or greater than 0.6g
(Equation 12.8-6)

If one of these minimum values governs, then ASCE 7-05 requires it to be used to determine drift. This has historically been a controversial topic. However, there is an important change coming in ASCE 7-10 Section that will not require the Cs = 0.044SDSI minimum base shear to be used in drift determination.

In order to better understand why this change has taken place in ASCE 7-10, let’s discuss why they exist.

Cs ≥ 0.044SDSI — Following the 1933 Long Beach earthquake, the California Riley Act required every building to be designed for a minimum service-level seismic force equal to 3 percent of its weight. This minimum requirement eventually became seismic zone-dependent in the Uniform Building Code (UBC). In the 1997 UBC, this minimum base shear was brought up to strength level and was made soil-dependent; it became 0.11CaIW — where Ca was a function of Z, the Seismic Zone Factor, and of the soil characteristics at the site of the structure; I was the importance factor; and W was the effective seismic weight. In the 2000 NEHRP Provisions, this minimum base shear became 0.044SDSIW, based on SDS = 2.5Ca.

Cs ≥ 0.01 — This minimum is intended to provide a nominal level of structural integrity that will improve the performance of buildings in the event of a possible earthquake.

Cs ≥ (0.5S1)/(R/I) when S1 is equal to or greater than 0.6g — Following the 1994 Northridge Earthquake, a lower-bound on the design base shear, applicable in Seismic Zone 4 only, was added to the 1997 UBC. This minimum was specifically intended to account for the large displacement and velocity pulses that were observed in near-fault ground motion in the Northridge earthquake. A corresponding minimum was added to the 1997 NEHRP Provisions. This minimum was originally applicable to structures assigned to SDC E and F only. In the 2000 IBC, this equation was also applicable to all structures located where the mapped MCE spectral response acceleration at 1-second period, S1, equaled or exceeded 0.6g, which is roughly equivalent to Zone 4 of the 1997 UBC. The applicability of this minimum value is the same in ASCE 7-05 as it is in the 2000 IBC.

Now let’s take a look at each of the minimum base shears and discuss what ASCE 7 requires and why.

Cs ≥ 0.044SDSI — The 1997 UBC exempted the minimum base shear of 0.044SDSIW from drift computation, but not without a lot of controversy surrounding it. This exemption was not adopted by ASCE 7-02, ASCE 7-05, or the first four editions of the IBC. Now the exemption has been brought back in ASCE 7-10. This change is significant when it comes to the design of tall buildings. Tall buildings are drift-controlled, rather than strength-controlled. The design of many tall buildings, irrespective of seismic design category, has likely been governed, in the absence of this exemption, by drift computed under the 0.044SDSIW minimum. Many consider this to be unreasonable, since this minimum design base shear is essentially a minimum strength requirement, with its magnitude arbitrarily set.

Cs ≥ 0.01 — ASCE 7-05 and ASCE 7-10 require that the 0.01 minimum be used for drift determination if it governs. However, this seldom controls because it is such a low value.

Cs ≥ (0.5S1)/(R/I) when S1 is equal to or greater than 0.6g — ASCE 7-05 and ASCE 7-10 require that the 0.5S1/(R/I)W minimum be used for drift determination if it governs. This minimum is not exempt and should not be exempt from use in drift determination because it has a physical basis.

S.K. Ghosh Associates, Inc., is a structural seismic and code consulting firm located in Palatine, Ill., and Aliso Viejo, Calif. Presidents S.K. Ghosh, Ph.D., and Susan Dowty, S.E., are active in the development and interpretation of national structural code provisions. They can be contacted at skghosh@aol.com and susandowty@gmail.com, respectively, or at www.skghoshassociates.com.