Membrane roof systems installed on steel roof decks traditionally result in a uniform transfer of wind (uplift) loads from the roof membrane to the steel roof deck and underlying supporting structure (e.g., steel joists). For example, in a built-up membrane roof system—which has been used commonly in the U.S. roofing industry for more than 125 years—the built-up membrane is continuously adhered to rigid board insulation. The rigid board insulation, which is used to span the steel deck’s flutes, is mechanically attached to the steel roof deck in a closely spaced pattern (e.g., one fastener per every 3 sq. ft.), resulting in a near-uniform uplift load path. Polymer-modified bitumen roof systems and adhered single-ply membrane roof systems are installed in similar configurations and result in a similar uniform uplift load path. In the 1960s, single-ply membrane roof systems were first introduced into the U.S. roofing market. By the late 1970s, the seam-fastened, mechanically attached method of installation was first introduced. With this installation method, the single-ply membrane sheet is mechanically attached along its outer edges into the roof deck, which results in a larger tributary uplift load per fastener and placement of fasteners in linear, non-uniform loading configurations of the roof deck and underlying supporting structure. When first introduced, membrane sheet widths in seam-fastened single-ply membrane roof systems typically were five feet wide, resulting in rows of mechanical fasteners spaced at five feet on-center. Since the early 2000s, single-ply membrane sheet widths have become wider, with 10-ft.-wide sheets now commonplace, resulting in rows of mechanical fasteners spaced at 10 feet on-center. Currently, single-ply membrane roof systems have clearly overtaken conventional built-up and polymer-modified bitumen membrane systems in market share. The seam-fastened, mechanically attached method of installation also has overtaken traditionally adhered methods of application. The National Roofing Contractors Association’s (NRCA’s) annual market survey shows seam-fastened, mechanically attached single-ply membrane roof systems make up the majority of all membrane roof systems currently installed. With the present emphasis on wind resistance in design, a closer look at how seam-fastened mechanically attached singleply membrane roof systems interact with steel roof deck and joist construction is in order. A common method of single-ply membrane sheet layout is shown in Figure 1. A common placement of mechanical fasteners is shown in Figure 2. These concentrated J u l y 2 0 1 8 RC I I n t e r f a c e • 2 3 Figure 1 – Typical membrane layout by roofers. line loads can severely overstress the steel deck and may also cause the steel joist below the deck to be overstressed under uplift loading. The behavior of such fastening systems, when the roof system is subjected to uplift loadings, is shown in Figure 3. The current trend in securement is for the membrane installer to mechanically fasten the membrane to the deck only along the edge of the sheet rolls to speed up the roof installation, thereby lowering installation costs. Unfortunately, the structural engineer of record and the steel deck and joist suppliers are usually unaware of the concentrated load pattern of the roof membrane attachment. In fact, the architect of record may not be aware of the ramifications of such attachments. The architectural roofing specifications may simply state that the roof membrane shall be installed per manufacturer’s recommendations. The roofing installer’s foreman is the one who generally decides on the exact layout of the membrane sheets on the roof. That decision is made based on what layout can be installed in the fastest and least expensive manner. Roofing suppliers and FM Global recommend the fastener line loads not be installed parallel to the deck ribs, but rather perpendicular to the deck flutes. Placing the lines of attachment parallel to the deck ribs will only load a one-foot width of the steel deck. This recommendation helps but may not eliminate potential severe overstress of the deck. Currently, the Steel Deck Institute’s (SDI’s) position paper, “Attachment of Roofing Membranes to Steel Deck,” states: “SDI does not recommend the use of roofing membranes attached to the steel deck using line patterns with large spacing unless a structural engineer has reviewed the adequacy of the steel deck and the structural supports to resist the wind uplift loads transmitted along the lines of attachment. Those lines of attachment shall only be perpendicular to the flutes of the deck.” DECK STRENGTH EXAMPLE To illustrate the potential effect of the attachment pattern, determine the deck strength for the following conditions illustrated by Figures 1 and 2. Use load and resistance factor design (LRFD) load combinations and the American Society of Civil Engineers’ ASCE 7-10. Thus, the controlling ASCE load combination is 0.9D + 1.0W. (Wind calculations are not shown for brevity.) Given: A roof system located in Kansas City, Missouri. Category II Building. Exposure C per ASCE 26.7.3. This is an enclosed building with a flat roof (¼ in./ft.). The building is 100 by 100 ft. in plan and has an eave height equal to 30 ft. The metal deck is 1.5-in., 22-gage, wide rib (WR) deck on joists 6 ft. on-center. Fy = 33 ksi. The roof dead load on the metal deck = 5 psf. The membrane is 10 ft. wide in the interior zones and 5 ft. wide in the perimeter zones. From the SDI Roof Deck Design Manual (RDDM), φMn (negative moment capacity) = 5.358 kip-inches, and φMp (positive moment capacity) = 5.088 kip-inches. Interior Zone (Field of Roof) Uplift line loads are determined using component and cladding ASCE requirements (ASCE Chapter 30). The fasteners are placed perpendicular to the deck span and are spaced 1 ft. on-center. Therefore, the membrane area is 10 sq. ft. (1- x 10-ft.-wide sheet). The uplift pressure is 33.3 psf. Assume that at some location in the field of the roof, the fastener line will be positioned at the center of a deck end span. From a structural analysis of a three-span deck, the maximum moment occurs in the end span (positive 2 4 • RC I I n t e r f a c e J u l y 2 0 1 8 Figure 2 – Typical fastener layout at corner zones. Table 1 – Summary table of required deck strength to actual deck strength. Figure 3 – Line-attached membrane under uplift. Courtesy of the Steel Deck Institute. moment); Mr = 4.85 kip-inches. For a uniformly loaded deck, the maximum moment occurs over the two supports (negative moment); Mr = 1.24 kip-inches. East-West Perimeters (Attachments Perpendicular to Deck Span) The first line load is 5 ft. from the building edge, and the second is 10 ft. from the edge. The third is 20 ft. from the edge. First Line Load: The membrane area for the first line load = (5 ft. x 1 ft.) = 5 sq. ft. Second Line Load: The membrane area for the second line load = (2.5 ft. + 5 ft.)(1 ft.) = 7.5 sq. ft. Third Line Load: The membrane area for the third line load = (5 ft. + 5 ft.)(1 ft.) = 10 sq. ft. The uplift pressure is 55.8 psf for the first 10 ft. from the building edge and 33.3 psf for the remainder of the three-span deck. The maximum moment is 4.32 kipinches and occurs in the second span as a positive moment. For a uniformly loaded deck, the maximum moment is 2.31 kip-inches (negative) and is located over the first support from the building edge. North-South Perimeters (Attachments Parallel to Deck Span) Note: Fasteners running parallel to the deck flutes is a severe condition and not recommended. If used, the following loading conditions occur. Line Load: Parallel to Deck Flutes The uplift pressure is 55.8 psf. The membrane area for the line load = (5 ft. x 1 ft.) = 5 sq. ft. Line load on 1 ft. of deck width = (5 ft.) (55.8 psf)- 4.5 plf = 275 pounds per ft. Positive moment = Mw = (0.08)wL2 = (0.08)(0.275 kips per ft.)(6 ft.)2 = 0.79 kipfeet = 9.50 kip-inches. Negative moment: Mw = (0.10)wL2 = (0.10)(.275 kips per ft.)(6 ft.)2 = 0.99 kip-feet = 11.9 kip-inches. For a uniformly loaded deck, the maximum negative moment is 2.22 kip-inches. Corner Condition Zone The uplift pressure is 84.0 psf for the first 10 ft. from the building edge, and 55.8 for the remainder of the three-span deck. Line load on 1 ft. of deck width = [(5 ft.) (84 psf)]/2-4.5 plf = 206 pounds per ft. for first 10 ft. The division by 2 is to account for load distribution between fastener lines. Then (5 ft.)(55.8 psf)-4.5 plf = 275 pounds per ft. beyond. Based on a continuous beam analysis, the maximum negative moment equals 11.55 kip-inches and occurs over the second interior support from the corner. The maximum positive moment occurs within the third span and equals 9.64 kip-inches. For a uniform load, the maximum negative moment equals 3.48 kip-inches and occurs over the first interior support. The maximum positive moment occurs in the first span and equals 2.73 kip-inches. See Table 1 for a summary of the conditions. Conclusions North-south perimeters and corner zone failures occurred. It is interesting to note that with the amounts of overload shown in these calculations, there are not more reported deck and joist failures. There may be a number of reasons for fewer reported failures. For example: 1. The design uplift anchorage of the deck to the joists, while increased for mechanical attachment as compared to adhered membranes, does not exceed the factors of safety in J u l y 2 0 1 8 RC I I n t e r f a c e • 2 5 ® Innovation based. Employee owned. Expect more. For information, call 241-515-5000 or go to www.PolyguardBarriers.com by Exclude termites and pests over the life of the structure by incorporating non-chemical TERM® Waterproofing Termite Barrier System in the envelope. Enhanced waterproofing to stop leaks of: • Moisture • Energy • Termites • Sealant upgrade, tested since 1999 by Texas A&M entomologists • 100% ground level horizontal coverage • 6 new details • Reduces pesticide needs • Increases occupant comfort • Excludes termites and other pests Simple upgrade includes: Non-chemical physical barrier: Sustainability upgrade … the design of the deck attachment fasteners. 2. The majority of roofs have not seen roof uplift loads of those predicted by ASCE 7. 3. The decks may have higher yield strengths than those used in the design example. The SDI RDDM tabulates roof deck capacity based on a lower bound yield stress of 33 ksi. Many manufacturers provide decks with yield stresses of 40, 50, or 80 ksi (limited to a 60-ksi design stress by the AISI S100 standard). A design stress of 60 ksi versus 33 ksi will increase the deck flexural strength by about 70%. APPLICATION WHEN REROOFING An important point to note is that, per NRCA, approximately two-thirds of the roofing installed every year in the U.S. is reroofing of existing buildings. Buildings that are 20 to 30 years old are unlikely to have higher-yield-strength steel decks. Therefore, caution is required when evaluating the roof deck when reroofing. HIGHER WIND REGIONS The analysis described above was performed on a building located within the basic wind velocity zone of 115 mph per ASCE 7-10. Particular attention must be paid to the design of the deck for regions where the wind velocity is higher. At higher design wind speeds, a deck that is adequate to support an adhered membrane roof 2 6 • RC I I n t e r f a c e J u l y 2 0 1 8 ISSUE SUBJECT SUBMISSION DEADLINE October 2018 Walls July 13, 2018 November 2018 The changing workforce August 15, 2018 December 2018 Historical restoration September 14, 2018 January 2019 Mid-rise construction October 15, 2018 February 2019 Building envelope (misc.) November 15, 2018 March 2019 Fenestration December 14, 2018 Publish in RCI Interface RCI Interface journal is seeking submissions for the following issues. Optimum article size is 2000 to 3000 words, containing five to ten graphics. Articles may serve commercial interests but should not promote specific products. Articles on subjects that do not fit any given theme may be submitted at any time. Submit articles or questions to Executive Editor Kristen Ammerman at 800-828-1902 or kammerman@rci-online.org. This article refers to FM Approval Standard 4470 regarding steel decks and a factor of safety of 2. In fact, the FM Global requirements for steel decks are covered in FM Approval Standard 4451, Approval Standard for Profiled Steel Panels for Use as Decking in Class 1 Insulated Roof Construction, and FM Global Property Loss Prevention Data Sheet 1-29. FM Approval Standard 4451, Section 4.3.1.6, states, “Stresses induced to steel roof decking shall be determined by rational analysis using Allowable Strength Design (ASD) principles and shall not exceed the allowable stresses per the latest edition of the North American Specification for the Design of Cold-Formed Steel Structural Members, AISI S100-2007.” FM Global Property Loss Prevention Data Sheet 1-29, Section 2.2.3.2, includes tables that are based on ASD principles and the allowable stresses per AISI S100-2012. This section also allows for “a performance-based approach” with the required assumption (Subsection C), “Assume maximum allowable stresses are determined using allowable strength design (ASD) in accordance with AISI S100-2012, or comparable standard outside the United States.” Therefore, the FM Global requirements are no more stringent than AISI S100 for steel deck spans and thicknesses. This confusion may have been the result of interpretation of the definition of Service Wind Load within FM Approval Standard 4470, which states, “The uplift load resulting from a windstorm that a roof assembly must resist. The service load is equal to one-half of the rated load in psf (kPa).” As part of the FM Approval process covered by this standard, simulated wind uplift pressure testing is performed on roof assemblies. The rated load achieved by a particular roof assembly in uplift pressure testing is equivalent to two times the uplift pressure from a windstorm the roof assembly is required to resist. Therefore, the factor of safety of two applies only to the performance of a roof assembly during uplift pressure testing. It does not apply to deck span calculations. Anthony Longabard Staff Engineering Specialist FM Global CLARIFICATION with uniform uplift deck loading may not be structurally adequate to support widely spaced line loads from a mechanically attached membrane roof. FM GLOBAL REQUIREMENTS Often, FM Approval Standard 4470, Single-Ply, Polymer-Modified Bitumen Sheet, Built-Up Roof (BUR), and Liquid-Applied Roof Assemblies for Use in Class 1 and Noncombustible Roof Deck Construction, is required for roof systems. These requirements are more stringent with respect to deck spans and thicknesses because the FM standard uses a factor of safety of two (see sidebar clarification for updated information), whereas the AISI S100 standard mandates an ASD factor of safety of 1.67 for flexure. These requirements can be found on FM’s RoofNav website. Go to www. roofnav.com and select Reference Materials, followed by Approval Standards. The FM Standard provides wind ratings based on fastener row spacing, deck spans, deck thicknesses, and deck yield strengths. For the above example, for a 33-ksi, 1.5- in. WR deck spanning 6 ft., a 20-gage deck is required to obtain a 60 rating (30 psf ASD). RECOMMENDATIONS Design recommendations for single-ply roofing when concentrated line securements are used to connect the membrane to steel roof deck instead of uniformly distributed securements include: 1. When FM Global requirements are to be followed, maximum deck spans, deck thicknesses, and deck yield strengths as required by FM Global must be used. a) Based on the maximum concentrated line loads determined from FM Global, specify the required joist net uplift. b) Coordinate design requirements with the general contractor and the architect. 2. When FM Global requirements are not required: a) Determine the uniform net uplift forces based on the building code in force. b) Using the spacing between the lines of fasteners for the membrane, perform a structural analysis of the deck as a three-span beam, placing the first concentrated load at the mid-span of the first deck span. The subsequent loads are placed according to their spacing. This analysis will produce a moment diagram that is close to the maximum that would be achieved from an influence line analysis. c) From this analysis, specify a deck that has a flexural capacity that exceeds the maximum positive and negative design moments. d) If no deck is found that will work, change the spacing of the supports (joists) or alter the spacing of membrane fastening. Changing the spacing of the membrane fastening is something that requires coordination with the entire roofing team (specifier, manufacturer, and installer). e) Determine the net uplift requirement for all joists based on the final selected line securement spacing and forces. f) Specify these requirements to J u l y 2 0 1 8 RC I I n t e r f a c e • 2 7 The Original & Best Performing Liquid Flashing www.apoc.com • (800)562-5669 Ideal for Roofing, Waterproofing & Building Envelope Applications Fast Install with up to 50% Labor Savings Solid Monolithic & Waterproof Configuration Use on Vertical or Horizontal Applications Available in Multiple Sizes & Containers the joist manufacturer. g) Coordinate design requirements with the general contractor and the architect. ECONOMICS Based on experience, using the wide attachment spacing may not be economical when one considers the increase in deck costs and joist costs as compared to the labor savings using the wide securement spacing. For any given project, these cost comparisons should be made. EDITOR’S NOTE: This article is republished with permission from STRUCTURE, March 2017. 2 8 • RC I I n t e r f a c e J u l y 2 0 1 8 Thomas Sputo is president of Sputo and Lammert Engineering, LLC, Gainesville, FL, and Technical Director of the Steel Deck Institute. He is also an emeritus senior lecturer in the Department of Civil and Coastal Engineering at the University of Florida. He holds a PE license in twelve states plus an SE license in Illinois. He may be reached at tsputo50@gmail.com. Thomas Sputo, PhD, PE SE, FASCE James M. Fisher is vice president emeritus of Computerized Structural Design, Milwaukee, WI, and a consulting engineer to the Steel Joist Institute. He has authored and coauthored several books and numerous technical publications. Fisher is active on several AISC specification task committees, as well as AISI Cold-Formed Specification Committee and ASCE 7. He may be reached at jfisher. florida@gmail.com. James M. Fisher, PhD, PE, Dist. MASCE Hemp is making a comeback in the U.S. in more ways than through the legalized use of marijuana for recreational and medicinal purposes. It is undergoing a revival as a building material. While both George Washington and Thomas Jefferson grew hemp on their plantations in the 1700s, it was outlawed in the U.S. starting in the 1930s (but never in Europe). But recently, The Industrial Hemp Farming Act of 2018 was introduced in the Senate by Majority Leader Mitch McConnell (R-KY), Senator Rand Paul (R-KY), Senator Ron Wyden (D-OR), and Senator Jeff Merkley (D-OR). The bill would allow hemp to be regulated as an agricultural crop, as well as encourage federal research funding for hemp’s industrial uses. Congress must pass a farm bill by the end of September, when the current law expires. Canada has been exporting hemp to the U.S. for several years. Today, 34 states have enacted hemp bills, and 19 states grew hemp in 2017. Mixing hemp’s woody core with lime and water results in a natural, light concrete that retains thermal mass and is highly insulating. Its proponents say it has good acoustics, provides low humidity, and requires no pesticides. It also grows from seed to harvest in about four months. It can be made into fiberboard, roofing, flooring, wallboard, caulking, cement, paint, paneling, particleboard, plaster, plywood, reinforced concrete, insulation, insulation panels, spray-on insulation, concrete pipes, bricks, and biodegradable plastic composites. Both marijuana and hemp are of the cannabis sativa species, but hemp is cultivated for its fiber, hurd, and seeds, and contains just 0.3 percent of tetrahydrocannabinol (THC), the compound that leads to a high. At least 25,000 products are currently made of hemp, including apparel, foods, plastics, skin products, and dietary supplements. And now it is gaining attention as a nature-friendly alternative to fiberglass insulation. It is nontoxic, fireproof (no, it won’t go up in smoke, as some joke), carboncapturing, and mold- and pest-resistant. Hempcrete or hemplime, developed in France in the 1980s, is made from the inner woody core of the hemp plant that is mixed with a lime-based binder and water. This produces a lightweight insulating material weighing an eighth of the weight of concrete. Currently, there are said to be only 50 homes in the U.S. that have used hemp insulation and/or exterior or interior walls, but it has been used in construction around the world for hundreds of years. Hemp Technologies Global was involved in what has been claimed to be the first modern hemp home in the U.S., built in 2010 for Russ Martin in Asheville, NC. Just BioFiber, founded in Calgary, Alberta, in 2014, has patented prefabricated structural building blocks made from hemp. ASTM International formed a committee in 2017, D37, to create technical standards and guidance regulating cannabis and its products and processes. RCI member Ralph Paroli, past chairman of the board of ASTM and director of research and development in measurements science and standards at the National Research Council of Canada, is chair of the committee. — realhemp.com, cnbc.com, nytimes.com, cannabiz.media Houses Hewn from Hemp? Hempcrete. Photo courtesy of GreenBuilt.
