Asce 7-16 Code

[Osman Briseno]

The new ASCE 7-16 Minimum Design Loads and Associated Criteria for Buildings and Other Structures (Standard) is adopted into the 2018 International Building Code (IBC) and is now hitting your desks. The 2018 IBC and the referenced Standard are being adopted by a few jurisdictions and will become more widely used in 2019. Thus starts the time when practicing engineers learn the new provisions of the Standard and how they apply to their practices. To help in this process, changes to the wind load provisions of ASCE 7-16 that will affect much of the profession focusing on building design are highlighted.

An updated study of the wind data from over 1,000 weather recording stations across the country was completed during this last cycle. This study focused on the non-hurricane areas of the country and used a new procedure that separated the available data by windstorm type and accounted for changes in the site exposure characteristics at the recording anemometers. This separation was between thunderstorm and non-thunderstorm events. Also, a small revision was made to the hurricane wind speeds in the Northeast region of the country based upon updated hurricane models. Consequently, wind speeds generally decrease across the country, except along the hurricane coastline from Texas to North Carolina. The wind speeds in the northern Great Plains region remain approximately the same as in ASCE 7-10. The most significant reduction in wind speeds occurs in the Western states, which decreased approximately 15% from ASCE 7-10 (Figures 1 and 2). To meet the requirements of Chapter 1 of the Standard, a new map is added for Risk Category IV buildings and other structures (Figure 3). These new maps better represent the regional variations in the extreme wind climate across the United States.

Not many users of the Standard utilize the Serviceability Wind Speed Maps contained in the Commentary of Appendix C, but these four maps (10, 25, 50 & 100-year MRI) are updated to be consistent with the new wind speed maps in the body of the Standard.

New additions to the Standard are provisions for determining wind loads on solar panels on buildings. These provisions give guidance to the users of ASCE 7 that has been missing in the past. Previously, designers commonly attempted to use a combination of the component and cladding provisions and other provisions in the Standard to determine these loads, often resulting in unconservative designs.

There are two methods provided in the new Standard. One method applies specifically to a low-sloped roof (less than 7 degrees) (Figure 5) and the second method applies to any roof slope where solar panels are installed parallel to the roof. Each of these provisions was developed from wind tunnel testing for enclosed structures. Thus, these provisions are not applicable to open structures because the flow of the wind over the roof of enclosed structures and open structures varies significantly. Further testing is currently underway for open structures, and these results will hopefully be included in future editions of the Standard.

The wind loads for solar panels do not have to be applied simultaneously with the component and cladding wind loads for the roof. However, the roof still needs to be designed appropriately assuming the solar panels are removed or not present.

The component and cladding pressure coefficients, (GCp), for roofs on buildings with an h < 60 feet, have been revised significantly in ASCE 7-16. The new roof pressure coefficients are based on data from recent wind tunnel tests and then correlated with the results from full-scale tests performed at Texas Tech University. The full-scale tests indicated that the turbulence observed in the wind tunnel studies from the 1970s, that many of the current roof pressure coefficients were based on, was too low. Also, the technology available to measure the results of these wind tunnel tests has advanced significantly since the 1970s. Therefore, the new wind tunnel studies used flow simulations that better matched those found in the full-scale tests along with improved data collection devices; these tests yielded increased roof pressures occurring on the roofs. Thus, the roof pressure coefficients have been modified to more accurately depict roof wind pressures.

The roof zoning for sloped roofs kept the same configurations as in previous editions of the Standard; however, many of the zone designations have been revised (Figure 7). This revision in zone designations was required because the values in zones around the roof in previous editions of the Standard were shown as having the same pressure coefficient, i.e., corners at the eave versus corners at the ridge have been found to have varying pressures.

Previously, designers were required to use various provisions of overhangs, free roof structures, and more to determine the wind loads on canopies. Research became available for the wind pressures on low-slope canopies during this last code cycle of the Standard. This research was limited to low-slope canopies and only for those attached to buildings with a mean roof height of h < 60 feet. Research is continuing on sloped canopies, and the Committee hopes to be able to include that research in the next edition of the Standard.

The first section of this article explained the basic science and forces that act upon buildings and roofs. You may now be wondering how to apply that information? How are forces, fastening patterns, and adhesive application rates determined for roof systems? This is a complicated topic, one on which thousands of pages of detailed information have been published by testing agencies and engineering firms. This article is intended as a light introduction to the topic.

As you would expect (with all other factors being equal), the roof of a short wide building, like a warehouse, will experience far less winds and stresses than those imposed on the roof of a tall skyscraper. (Think about the last time you traveled from home on a mildly windy day to a downtown area with tall buildings where the mild winds had morphed into gusts).

The ASCE 7-16 building specifications break a roof into three separate roof zones: corners, perimeters and center. The corners and perimeters experience much greater uplift pressures than the center of the roof and typically require increased fastening rates. Individual uplift pressures and fastening requirements are calculated separately for each of the roof zones.

The ASCE 7-16 specifications include the mathematic formulas necessary to calculate uplift pressures. A component of the formulas are pressure multipliers linked to risk factors. ASCE 7-16 includes a couple charts listing conditions and their associated multipliers.

Buildings deemed to have a lower risk to human life such as storage buildings will have smaller multipliers and thus be required to meet less stringent uplift pressures than buildings considered essential such as hospitals.

In conclusion, many calculations and factors must be considered in order to follow the proper building codes for a commercial roof. The roofs specifications greatly depend on the environment and shape of the building. These factors make it necessary to insure that you have a quality roofing contractor working for you. Read Part 3 here. Part will be coming soon. Subscribe to our emails to get them in your inbox.

An integral part of building codes in the United States, ASCE/SEI 7-22, which supersedes ASCE/SEI 7-16, is your source for the most up-to-date and coordinated loading standard for general structural design. Available formats include print, PDF, and ASCE Amplify platform.

SEI is currently accepting proposals to modify the 2022 edition of ASCE/SEI 7 Minimum Design Loads and Associated Criteria for Buildings and Other Structures, as the committee prepares for the 2028 revision cycle.

Come join the discussion about ASCE Standards! The Peer-to-Peer Standards Exchange is a new ASCE Collaborate forum to discuss technical issues about ASCE standards. Dive into your technical area with questions and issues with your community. Members can ask and answer questions. Nonmembers will have view-only capability.

In early December, ICC posted the preliminary results of the Group B Online Governmental Consensus Vote, which included structural changes to the IBC, IEBC and IRC. ICC reports that there were more than 162,000 votes cast by eligible Voting Members during the three-week online voting period.

One subject of interest to building Designers, builders and some building-material suppliers was the disposition of a group of code changes that adopted ASCE 7-16 as the reference standard on loads for the IBC and IRC, and changed other parts of the IBC and IRC to reflect that.

The most controversial part of adopting the new ASCE 7-16 standard was its increase in roof component and cladding loads. The higher pressure coefficients in some cases raised the concern that the cost of roofing, roofing materials and roof repairs would be increased. Other items that raised some opposition were the new chapter on tsunami loads and the increase in deck and balcony live loads from 40 psf to 60 psf.

Along with that specific change, several other related changes were approved to correlate the IBC with adoption of ASCE 7-16. These included changes to Section 1604, General Design Requirements; adding in a new Section 1615 on Tsunami Design Requirements; modifications to Section 1613 so that seismic design requirements match ASCE 7-16; and deletion of Section 1609.6, Alternate All-Heights Method for wind design. On this last item, the argument was that since ASCE 7 now includes a simplified wind load design method, a competing method is not needed in the IBC.

Interestingly, a change to remove Strength Design and Allowable Stress Design load combinations from the IBC, which was approved by the IBC Structural Committee, was overturned and denied by the ICC Member voters. So those will remain in the IBC.

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