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Winds of Change

Mcn  Bonus  Feature  Jun17 3 Low Rez

New codes and requirements affect metal construction

Metal building systems and their metal roofs provide value, long-term durability and protection. The performance of metal roofing, along with other roofing materials, is highly dependent on our understanding of wind loading. Proper design and construction of metal roofing that meet or exceed industry requirements and satisfy applicable code requirements stand a better chance of both building and occupant survival. New wind requirements are being introduced this year.

American Society of Civil Engineers' (ASCE) 2016 edition of ASCE Minimum Design Loads and Associated Criteria for Buildings and Other Structures will be available in June 2017. This edition provides the most up-to-date and coordinated loading standard for structural building design. Incorporating improved coordination and routine updates, ASCE 7-16 includes many significant changes for the metal construction industry. It will have a userfriendly, online version for the first time.

 

A Six-Year Cycle

ASCE presently develops and publishes the standard on a six-year cycle. The ASCE 7 committee works cooperatively with industry associations and government agencies. ASCE 7-16 will be included as a reference standard in the 2018 International Building Code, which is adopted nationally as state and local jurisdiction building code.

Donald R. Scott, SE, F.SEI, F.ASCE, is senior principal with PCS Structural Solutions Inc., Tacoma, Wash., and is chair of the ASCE 7 Wind Load Subcommittee, and of the National Council of Structural Engineers Associations Wind Engineering Committee. He says there has been continual refinement of the ASCE 7 wind load provisions over the past several cycles resulting in the current ASCE 7-16 provisions that to date, best match correct wind loading on buildings. He believes much of this past cycle concentrated on the production of new basic wind speed maps to better reflect the regional variation of wind speed across the country.

Over the past several cycles of ASCE 7, the wind loads task group has been conducting a thorough review of the basis and requirements for computing wind loads. A number of major changes have been introduced over the years. A new windhazard study for the entire United States has updated data from hundreds of weather stations, rationalization of the return period for design winds for structures assigned to different risk categories and review of wind tunnel data on model buildings.

"The wind hazard study on which these new maps are based include data from over 1,000 weather stations across the country and are adjusted for the roughness characteristics (i.e., trees, water, grass, etc.) at each of these sites," Scott says. "It also, for the first time, differentiates the analysis of the wind speeds by storm type. This is because each type of storm has different wind characteristics."

 

Wind Load Changes

According to Ronald Hamburger, structural engineer and senior principal, Simpson Gumpertz & Heger, San Francisco, and chair of the ASCE-7 committee, the three main changes to the wind load requirements in ASCE 7-16 that most affect metal buildings are:

• The design wind speed maps have been revised
• Air density variation with altitude has been recognized
• The pressure coefficients for cladding and components have been revised

The revision of the design wind speed maps results in lower design wind pressures for main wind force resisting systems (moment frames and braced frames) throughout most of the United States, the exception being along the Atlantic and Gulf coastlines, where hurricane hazards control the maps. "Typically the reduction in wind speed is approximately 10 mph which equates to approximately a 20 percent reduction in required wind design forces for frames," Hamburger says. "At high-altitude sites, such as Carson City, Nev., Denver, Lake Tahoe, Nev., Los Alamos, N.M, and Santa Fe, N.M., further reduction, on the order of up to an additional 20 percent is possible. However, the pressure coefficients for cladding and components have been substantially revised both as to the number and size of zones that must be considered and also as to the amplitude of the coefficients."

These effects have been known in the industry for approximately 10 years now, but Hamburger says a thorough investigation was required to justify the revisions within the standard. Even after this investigation, the revisions were peer reviewed by additional experts before they were included in the standard.

"The resulting pressure coefficients used to design purlins, girts and cladding, and their attachments, are substantially larger at roof edges and eaves than was the case under ASCE 7-10," Hamburger says. "However, when taken together with the other changes previously discussed, the net wind loads on these elements remains the same throughout most of the U.S. The primary exception is a relatively narrow band along the Atlantic and Gulf coastlines where design loads are controlled by hurricane considerations and do increase significantly."

W. Lee Shoemaker, Ph.D., PE, director of research and engineering, Metal Building Manufacturers Association, Cleveland, says the ASCE 7-16 wind speed maps updates will result in reductions in most of the interior regions of the U.S. For example, he says for much of the interior region, the wind speed for a Risk Category II building (standard building) will change from 115 mph to approximately 105 mph. This update would reduce wind loads in these areas by about 15 percent.

"This reduction would apply to the main wind force resisting system, and wall component and cladding, but other changes in ASCE 7-16 that increase roof component and cladding design pressures will more than offset this reduction in most cases," he adds. "In regions with the highest wind speeds, i.e., the hurricane-affected areas, only minor adjustments were made in the ASCE 7-16 wind speed maps. Additionally, ASCE 7-10 had one map that applied to both Risk Categories III and IV, and ASCE 7-16 has a map for each. This is favorable to Risk Category III buildings since the wind speeds will not be as conservative."

 

Metal Roof Impacts

The most significant change in ASCE 7-16 affects all roofing products, including metal. This is because the roof component and cladding wind pressure coefficients that account for the aerodynamic flow of wind over different-shaped roof surfaces increased substantially due to the evaluation of newer wind tunnel studies. Shoemaker says some roof shapes will not be affected because they were not included in the newer studies, such as monoslope (single-sloped roofs), stepped roofs, multi-span gable roofs and sawtooth roofs. "Gable and hip roofs will see a significant increase in wind pressure coefficients," he adds.

Bill Coulbourne, PE, owner of Coulbourne Consulting, Rehoboth Beach, Del., says the revised component and cladding wind zones and external pressure coefficients will clearly impact building shapes frequently used by the metal building industry. These metal building shapes include, "Those with heights less than 60 feet and roof slopes that extend from 0 degrees to 45 degrees," he says. "The external pressure coefficients are generally greater than those in previous versions, and thus wind pressures for components and cladding would be higher than previously determined, all other things being equal including wind speeds and exposure/terrain coefficients."

Mark Detwiler, lead research and develop- ment engineer, NCI Building Systems, Houston, and incoming chairman of the MBMA technical committee, sees ASCE 7-16 impacting metal roof designs for all low-rise structures. "The layout and width of the roof pressure zones that group locations of similar load magnitudes has changed, particularly so for roof slopes less than or equal to 7 degrees," he says. "This change will likely add some complexity to roof designs, with potentially a greater number of different structural support and attachment schemes employed to meet the demands in the multiple roof pressure zones. In addition to the change in pressure zones, there are design wind suction increases for the different roof areas that in some cases are pretty dramatic. For the smaller effective wind areas historically used to design metal roofs and their connections, there are some roof areas that may see as much as a 100 percent increase in design-pressure requirements."

Because ASCE 7-16 revisions may potentially require a greater number of roof subsections with different support and attachment schemes, Detwiler believes it will be critical for metal building system providers to clearly communicate these changing requirements to construction contractors in a manner that is easy to understand, implement and inspect. "In addition, there may be some locations where a particular metal roof style that has shown historically good performance will no longer be an appropriate choice for the increased ASCE 7-16 demands," he adds.

 

Impact on the Contractor

The impact of the new wind codes on contractors will certainly be seen in the components and cladding needed to resist wind pressures. The number, size and spacing of fasteners required to attach roof components such as roof panels are based on the wind pressures determined from ASCE 7-16. "When pressures go up, more fasteners are required when the size of spacing stays the same; likewise when the wind pressures are reduced, the number of fasteners is reduced when the size remains the same," says Coulbourne. "The areas where fasteners are required and the number, size and spacing required should be provided to contractors from building designers."

Scott believes for contractors the changes to the ASCE 7-16 wind provisions will have little impact on their construction. "The buildings constructed along the hurricane coastlines (Atlantic and Gulf Coast shorelines) will be designed for somewhat larger roof pressures that may require closer spacing on the roof purlins and heavier cladding material," he says. "Elsewhere in the United States, particularly in the western states, the loading will be less on the overall building framing, resulting in the use of smaller members overall."

Zach Priest, PE, director of PRI Construction Materials Technologies LLC, Tampa, Fla., predicts the biggest change for contractors on the lowslope side is how complicated it will become to determine the size of the roof zones. "From what I have seen, some buildings will only have Zone 2 and 3, and some will have Zone 1', 1, 2, and 3," he says. "This is a radical departure from where things are today, so imagine this will be an adjustment. Obviously, the wind loads will dramatically increase in some cases, so this will put pressure on manufacturers to ensure they have designed systems to meet the new wind loads."

"These new wind speed maps reduce design wind speeds over 95 percent of the continental United States, and this reduction can result in as much as a 20 percent reduction in wind speeds in the western United States in comparison to previous editions of ASCE 7. These new wind speed maps will result in much lower loads on the overall building wind resisting systems, reducing construction costs."

Donald R. Scott, SE, F.SEI, F.ASCE

Mike Momb, technical director of Hansen Pole Buildings LLC, Browns Valley, Minn., says the biggest impact to designers and installers of metal roofs from ASCE 7-16 will come in the attachments of cladding and flashings. "ASCE 7-16 comes with upward revisions in the design pressures for components and cladding, in some cases more than doubling the pressures," Momb says. "Generally the attachments of trims such as rakes and fascias tend to not be spelled out on building plans, but are more often left to the discretion of the installer. Contractors should make certain the RDP (registered design professional: architect or engineer) who has prepared the structural plans has called out specifics for the connections of all steel trims. Failure to do so could result in some unhealthy consequences to the builder in the event of a catastrophic event."

Shoemaker agrees that because of roof design pressure increases, contractors will probably notice purlins and joists that are spaced more closely together as well as the need for additional roof cladding fasteners. However he predicts the biggest change that contractors will need to be aware of affects roofs with a slope less than 7 degrees. "This is because of the change in how the roof wind zones are determined," he says. "We are all familiar with 'corner zones' and 'edge zones' where pressures are higher, and prior to ASCE 7-16, all buildings had the same basic zone layout. However, the roof zones in ASCE 7-16 for low-slope roofs can change significantly depending on the building height and plan size." He also believes that as the industry makes the transition to ASCE 7-16 in the coming years, there will be challenges, but attention to detail in the field is imperative.

 

Sidebar: Other Industry Wind Changes

The metal roofing industry (Metal Construction Association (MCA) and Metal Building Manufacturers Association (MBMA)) is involved in developing a test method for the capacity of edge metal on a roof. Depending on the metal roofing detailing, the edge metal could be the initiation of failure from wind loads.
W. Lee Shoemaker, Ph.D., PE, director of research and engineering, Metal Building Manufacturers Association, Cleveland

The American Iron and Steel Institute has been co-funding research into more accurately determining standing seam roof capacity with the Insurance Institute for Business and Home Safety in South Carolina and Western University in Canada. The expectation is that our current testing methods for determining capacity may prove to have some significant overconservatism that can be utilized safely.
Mark Detwiler, lead research and development engineer, NCI Building Systems, Houston