What you always wanted to ask.
This quarterly article addresses some of the questions received about structural standards developed by the Structural Engineering Institute (SEI) of the American Society of Civil Engineers (ASCE). In addition, questions from engineers, building officials, and other design professionals are often considered in the development of future editions of the Standards. Following are some questions received along with SEI’s responses.
Seismic Site Effects without
Fa or Fv
ASCE 7-22 Chapter 11 is missing Table 11.4.1 for Short Period Site Coefficients, Fa , and Table 11.4.2 for Long Period Site Coefficients, Fv . Is this a huge typo?
No, it is not a typo. These coefficients are intentionally removed because they are no longer needed. In ASCE 7-22, the risk-targeted maximum considered earthquake (MCER) spectral response acceleration parameters SS, S1, SMS, and SM1 are required to be obtained directly from the United States Geological Survey (USGS) Seismic Design Geodatabase for the applicable site class. And, because the values are now included in the database for all site classes directly, the coefficients formerly used to calculate the spectral response acceleration parameters for short periods (SMS) and at 1 second (SM1), adjusted for site class effects, no longer need to be calculated.
The ASCE 7-22 Chapter 11 Commentary provides a more detailed explanation. While the theoretical basis for MCER ground motions has not changed from ASCE 7-16 (and prior editions), ASCE 7-22 now uses multi-period response spectra (MPRS) to improve the accuracy of the frequency content of earthquake design ground motions and enhance the reliability of the seismic design parameters derived from these ground motions. These improvements make better use of the available earth science, which has in general, sufficiently advanced to accurately define spectral response for different site conditions over a broad range of periods and eliminate the need for site-specific hazard analysis required by ASCE 7-16 for certain (soft soil) sites.
The following brief history explains why these were used for one cycle of ASCE 7 in the 2016 edition.
During the closing months of the 2015 cycle of the Provisions Update Committee (PUC) of the Building Seismic Safety Council, a study was undertaken of the compatibility of current Site Class coefficients Fa and Fv, with the ground motion relations used by USGS to produce the design maps. In the course of this study, it was discovered that the standard three-domain spectral shape defined by the short-period response spectral acceleration parameter, SDS, the 1-second response spectral acceleration parameter, SD1, and the long-period transition period, TL, is not appropriate for soft soil sites (Site Class D or softer), in particular, where ground motion hazard is dominated by large magnitude events. Specifically, on such sites, the standard spectral shape substantially understates spectral demand for moderately long period structures. The PUC initiated a proposal to move to specification of spectral acceleration values over a range of periods, abandoning the present three-domain format, as this would provide better definition of likely ground motion demands. However, this proposal was ultimately not adopted due to both the complexity of implementing such a revision in the design procedure and time constraints. Instead, the PUC adopted a proposal prohibiting the general use of the three-parameter spectrum, and instead requiring site-specific hazard determination for longer period structures on soft soil sites.
Subsequently, Project 17 (NIBS 2019) was charged with re-evaluating the use of multiperiod response spectra as a replacement or supplement to the present three-domain spectral definition and to consider how the basic design procedures embedded in ASCE 7 should be modified for compatibility with the multi-period spectra. As a result, Project 17 developed (and unanimously approved) a comprehensive multi-period response spectra (MPRS) proposal that was subsequently adopted (with changes) in the 2020 National Earthquake Hazards Reduction Program (NEHRP) Recommended Seismic Provisions for New Buildings and Other Structures, which form the basis for related changes to ASCE 7-22.
Furthermore, “mapped” values of seismic parameters SS, S1, SMS, and SM1 are archived in the USGS Seismic Design Geodatabase at gridded locations across United States regions of interest and provided online by the USGS Seismic Design Web Service for user-specified site location (latitude and longitude) and site class. The USGS web service spatially interpolates between the gridded values of these parameters based on site location (latitude and longitude). Chapter 22 provides print copies of seismic parameters SMS and SM1 for default site conditions. Seismic parameters SMS and SM1 (and SDS and SD1) incorporate site effects, eliminating the need for the tables of site factors Fa and Fv of ASCE 7-16.
User Note: The USGS Seismic Design Geodatabase is intended to be accessed through a USGS Seismic Design Web Service that allows the user to specify the site location, by latitude and longitude, and the site class to obtain the seismic design data. The USGS web service spatially interpolates between the gridded data of the USGS geodatabase. Both the USGS geodatabase and the USGS web service can be accessed at https://doi.org/10.5066/F7NK3C76. The USGS Seismic Design Geodatabase is available at the ASCE 7 Hazard Tool https://asce7hazardtool.online/ or an approved equivalent.
New Reliability-Targeted Ground Snow
In reviewing the ground snow in ASCE -22, the values look very different than what we have been using in my jurisdiction from past editions of ASCE 7. How do the new ground snow zones compare to the previous zones or Tables in 7-16?
This is correct. The ground snow included in ASCE 7-2 is very different from the values calculated in the past editions, and these are no longer zone maps. The adoption of reliability-targeted design ground snow loads in the ASCE 7-22 standard represents a significant change from ASCE/SEI 7-16 and prior editions, which previously used ground snow loads with a 50-year mean recurrence interval (MRI). Reliability-targeted loads are adopted here to address the nonuniform reliability of roofs designed according to the 50-year snow load in different parts of the country due to climatic differences. In some parts of the country, designing for the 1.6 load factor times the 50-year value does not meet the reliability targets of the standard (and, in some of these places, failures due to an underestimated ground snow load have been observed). In other places, designing for the 1.6 load factor times the 50-year value is unnecessarily conservative. With the move to reliability-targeted values, the load factor on snow loads has also been revised from 1.6 to 1.0 to represent the reliability basis of the values appropriately. Snow importance factors have also been eliminated because values are now provided for each risk category. The 0.7 factor is intended to provide roughly equivalent strength when the design follows Allowable Stress Design (ASD) procedures. For some materials, the ratio between design strength given by LRFD procedures and design strength given by ASD procedures is 1.5. For some other materials, the ratio varies depending on the limit state being checked. The inverse of 1.5 was rounded to 0.7 for this purpose. Also, note that the 2024 International Building Code and International Residential Code have updated provisions to accommodate this change. ■
This article’s information is provided for general informational purposes only and is not intended in any fashion to be a substitute for professional consultation. Information provided does not constitute a formal interpretation of the standard. Under no circumstances does ASCE/SEI, its affiliates, officers, directors, employees, or volunteers warrant the completeness, accuracy, or relevancy of any information or advice provided herein or its usefulness for any particular purpose. ASCE/SEI, its affiliates, officers, directors, employees, and volunteers expressly disclaim any and all responsibility for any liability, loss, or damage that you may cause or incur in reliance on any information or advice provided herein.
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