Are You Able to Take Advantage of This Relaxation?
ASCE 7-10 Section 12.8.1.3 reads as follows:
12.8.1.3 Maximum SS Value in Determination of CS. For regular structures five stories or less above the base as defined in Section 11.2 and with a period, T, of 0.5 s or less, CS is permitted to be calculated using a value of 1.5 for SS.
This section allows a lesser mapped short-period spectral response acceleration value to be used in the case of regular structures five stories or less in height having a period of 0.5 seconds or less. the figure below illustrate those areas of the 48 contiguous states where the reduced value applies (darkened areas on the map).
C12.8.1.3 Maximum SS Value in Determination of CS. The maximum value of SS was created as hazard maps were revised in 1997. The cap on SS reflects engineering judgment about performance of code-complying buildings in past earthquakes, so the structural height, period, and regularity conditions required for use of the limit are important qualifiers.
Here is a little more background information…as many readers may recall, the Uniform Building Code (UBC) traditionally truncated the value of the seismic zone factor, Z, to 0.4, thereby truncating the effective peak acceleration to 0.4g. Performance record of structures in UBC Seismic Zone 4, which satisfied the restrictions of ASCE 7-10 Section 12.8.1.3, has by and large been quite satisfactory. The provision under discussion essentially continues this truncation for structures that fall under this restriction. Effective peak acceleration of 0.4g corresponding to the design earthquake of the UBC (10 percent probability of non-exceedance in 50 years) is being taken equivalent to a short-period spectral response acceleration corresponding to the Maximum Considered Earthquake of ASCE 7-10 of 0.4g x 1.5 x 2.5 = 1.5g.
Q: My project is a two-story office building of masonry (Risk Category I) and is located in a very seismically active city where SS = 2.05g. The site class is B and Fa = 1.0. Because my building is less than five stories and has a period less than 0.5 second, can I use SS = 1.5g for the purpose of assigning the seismic design category?
A: Yes, provided your structure is regular. The following table provides an example where the relaxation applies (Case 1) and where it doesn’t (Case 2).
| SS | SDS | |
| CASE 1: No. of stories ≤ 5 and T ≤ 0.5 second |
1.5g | 1.00 |
| CASE 2: No. of stories > 5 or T > 0.5 second |
2.05g | 1.367 |
Additional Information. For readers seeking additional background information, when the 1997 UBC introduced near-source factors, thereby effectively removing the cap of 0.4g on effective peak accelerations, the value of SDS (in today’s terms) was capped at 1.21 for structures satisfying the conditions given in 1997 UBC Section 1629.4.2. This was a 26.67% reduction over the maximum value of 1.65 considered by the 1997 UBC. The 1997 NEHRP Provisions, the 2000 IBC, and ASCE 7-02 revived the original cap of 0.4g on effective peak accelerations, translated that into upper bound limits of 1.5g on SS (calculated as explained above) and 0.6g on S1 (0.4g x 1.5), and introduced the restriction now found in ASCE 7-10. ASCE 7-05, the 2006 IBC, and subsequent editions of the IBC deleted the cap of 0.6g on S1, to simplify the provision, on the basis that it would simply not apply for the majority of buildings satisfying the restrictions.
The provision will be further modified in important ways in the 2015 NEHRP Provisions. These modifications are expected to be incorporated into ASCE 7-16, which is planned to be referenced in the 2018 IBC.
