By Ron Hamburger, Chair ASCE 7-16 Main Committee
The adoption of ASCE 7-16 into the 2018 I-Codes has encountered a roadblock in the ICC process in the form of a public comment opposing update from the 2010 to the 2016 edition. The purpose of this article is to inform SEAOC members, and to request the assistance of ICC Governmental Members within our membership, who can help to defeat the public comment at the October Public Comment Hearings.
At the April ICC Structural Committee Hearings in Louisville, Ky., a coalition lead by the American Roofing Manufacturers Association (ARMA) and the National Home Builder’s Association (NHBA) opposed the adoption to the 2016 edition of ASCE 7. The coalition put forward a successful assembly motion that will result in an automatic public comment at the ICC Final Action Hearings in October. This public comment will move to modify proposal ADM-94 that, among other actions, administratively adopts ASCE 7-16 in place of ASCE 7-10. The public comment will move to retain ASCE 7-10 instead of ASCE -16, and, if successful, will create a significant problem for structural engineers and building officials as well as the ICC.
The reason that ARMA launched this challenge is over concern that wind pressure coefficients for roofs contained in ASCE 7-16 “substantially” increase wind pressure design requirements for roofing systems. Indeed, ASCE 7-16 does modify and increase the wind pressure coefficients at eaves, edges, and ridge lines, as well as increase the width of these zones for low-slope roofs. The net increase in pressure is about 38% in the worst locations, which is limited to coastal hurricane zones within 600 feet of the shore line (Exposure D). Both research and empirical evidence indicate that an increase is warranted.
However, since ASCE 7-16 also includes new wind maps that reduce design wind speeds significantly in most of the country, except in eastern coastal zones within 600 feet of the shore line the new wind pressure coefficients have no net increase on design wind pressures. In fact, because of the lower wind speeds, the wind design requirements for main wind force resisting systems will decrease significantly over much of the United States. During the ASCE 7-16 balloting process the Metal Building Manufacturers Association (MBMA) conducted an independent peer review of the new pressure coefficients prior to the final vote to ensure the changes were correct. The results of the peer review indicated that the increases were appropriate.
Beyond the wind coefficient issues, we understand that NHBA also opposed the update to ASCE 7-16 because of concern that seismic design requirements in some portions of the country would increase within the new standard. ASCE 7-16 seismic design requirements may increase for some sites and some buildings because of the adoption of new maps and because of a change in site class coefficients. A review of 34 cities in areas of high seismicity indicates that in most cases the changes are typically less than +/-20%. In fact, in two-thirds of these cities the changes are less than +/-10%, and on average the new standard will result in a slight decrease in ground motion relative to the ASCE 7-10 maps, although up to an additional 18% increase in design base shear may be seen in structures requiring response spectra analysis.
As with the wind maps, significant decreases occur in Southern California. An increase does occur in the region surrounding Las Vegas, Nevada, and the basis for the increase was developed and supported by the State of Nevada Geologist’s office. The new site class coefficients have small effect on short period buildings of most interest to home builders but can result in significant increase in base shear coefficients for tall buildings with long periods located on Class D or E sites. ASCE 7-16 requires site specific spectra for such buildings, which has been common practice for these classes of buildings for many years.
While concern over an increase in roof pressures is understandable, it is also important to recognize the major improvements in ASCE 7-16 includi the following:
- New wind speed maps that result in reduced wind speeds for much of the country and clarify the special wind study zones, including new maps for Hawaii;
- New regional snow data generated by state Structural Engineers Associations in Colorado, Oregon, New Hampshire, Washington and other mountainous states, that is now directly referenced and eliminates many, older site-specific Case Study zones;
- Entirely new chapter with tsunami design provisions, which is important to west coast states, Alaska, and Hawaii.
Why help is needed
Most importantly, and the reason for this letter, these updated requirements of ASCE 7-16 — including the seismic, snow and wind hazard maps, and site coefficients discussed above — have been coordinated with the 2018 IBC and approved for adoption under many, separate proposals that are not affected by the public comment to ADM-94. Therefore, if this public comment is successful and ASCE 7 is not updated to the 2016 edition, the 2018 IBC will have an uncoordinated and confusing mixture of requirements: some based on ASCE 7-10 and some on ASCE 7-16. This will create major enforcement problems for building officials and general confusion for anyone attempting to follow and use the code.
How to help
The next step in the code adoption process will occur at the Public Comment Hearings in Kansas City in October when ICC Governmental Members will vote yea or nay on this and other public comments. We urge ICC Governmental Members to vote against the public comment to ADM-94 challenging adoption of ASCE 7-16. We also urge engineers who know building officials and other ICC Governmental Members to contact them and urge them to vote against this public comment.