Abstract Each study of a catastrophic fire or wind blow-off could begin with one question: What did the loss preven¬ tion agency (insurer) require in terms of construction? The same would be an appropriate starting point for investigation of collapse events; however, the governing building code may have as much to say about those requirements as does the loss prevention agency. Delayed stormwater runoff may well be the event which precipitates a collapse. More insidiously, a struc¬ tural framing system may have been overstressed and permanently deformed well in advance of actual failure. This study will review some common scenarios which can result in roof collapse. The review is offered to bring a heightened awareness to the matter. Lyle D. Hogan, FRCI Roofing assemblies rarely make headlines until an event of significant non¬ performance. Structural failure is foremost among these (photos 1 and 2). The occurrence leaves owners panicked, occupants inconvenienced (if not injured), and reporters clamoring for more rolls of film. As opposed to floor decks which are customarily sturdy, roof deck design does not anticipate such load¬ ing. Roof decks and framing mem¬ bers are comparatively light. Available load-carrying capacity is usually the sum of dead load (all permanent construction) and live load (anticipated temporary imposi¬ tions), factored according to the method of design. At the point of overloading, vari¬ ous structures behave in different manners. Concrete members tend to break abruptly, quickly releasing pent up energy. Fractions of broken concrete may separate violently. Similarly, wood structures may splinter and fail suddenly. On the other hand, metal build¬ ings and conventional steel struc¬ tures may deform slowly. The shift¬ ing weights of drifted snow and stockpiled rooftop loads (Photo 3) may take several minutes to fail. It is not uncommon for steel structures to experience a “partial collapse”, marked by extreme deformation of members yet falling short of com¬ plete failure. One potentially perilous combina¬ tion is a ballasted roof assembly on a poorly-drained surface. As shown in Photo 4, the 1-1/2 inch stone ballast disappears under water merely a few feet from the perimeter. At midspan, a depth of 4-1/2 inches of water was measured. The structural framing members below had gone into nega¬ tive camber. At such a time, the deformed surface promotes ponding water which induces further concavi¬ ty. This combination has resulted in a yielded structure on too many occasions. A structural analysis should be performed prior to adding dead load to an existing assembly. For ballast¬ ed assemblies, the structural analyst may have based his or her calcula¬ tions on 10 pounds per square foot of newly-imposed load. He or she may be thunderstruck to find how much stone may actually be placed from ordinary dispensing equipment. This writer has measured 23-28 pounds per square foot of stone ballast deposited on a roof. It was an over¬ roof assembly on a deck exhibiting marginal drainage. (We carefully exited the roof.) Site-specific drainage features (high scuppers, high drains, deformable roof decks, insufficient total drainage orifice, constricted drains, etc.) may combine to render the structural analysis dubious at best, irresponsible at worst. The pru¬ dent structural analyst will search for these features, evaluating their con¬ tribution, if any, to a potential col¬ lapse. Photo 5 depicts a sure contrib¬ utor to ponding and roof loading which could go unnoticed both before and after collapse. A simple review of structural framing plans is an inadequate struc¬ tural analysis. The review must be matched against features now char¬ acterizing the project. The cumula¬ tive weight of internal process pip¬ ing, sprinklers, light fixtures, and electrical conduits may sufficiently combine with other dead load com¬ ponents to result in overload and col¬ lapse. This is represented in Photo 6, where the collapse took upwards of 32 Interface Photo 1 Photo 2 20 minutes, according to jolted onlookers. Note that the weight of water in a pipe is significant. The water in vir¬ tually any size of PVC pipe flowing full weighs more than the pipe. With cast iron (schedule 40), the water is a lesser component of total weight per foot but figures prominently at greater pipe diameters. More obscure perhaps is the diam¬ eter of the opening afforded at drain flashing. 1 Constriction there will vio¬ late the intent of the stormwater run¬ off rate when less than the diameter of the piping. 2 Factory Mutual Corporation is fully aware of loss potential in this manner. Document 1-54 is dedicated to the subject of collapse and sets out requirements for considering drifting snow load potential among varying roof levels. Note that two buildings do not have to be connect¬ ed for a drifting potential to exist.3 ASCE-7 (formerly ANSI A 58.1) addresses a “rain-on-snow” scenario. This recognizes the tendency of water to remain in snow much longer on relatively flat roofs than on those of steeper slope. A surcharge loading consideration is proposed, additive with ground snow loads for a given area. In summary, several practices can prevent a partial or full collapse. The importance of regular roof mainte¬ nance is academic (Photo 7). Retrofitting a roof with supplemen¬ tary scuppers may be wise in certain instances; it may be required in oth¬ ers. Reroofing a building or signifi¬ cant portion may prompt structural enhancement. This prompting may be from the specific loss prevention agency involved or from the building code prevailing (latest revisions). The embellishments could proceed along many paths. Modern standing seam metal roofs are carried on a system of clips. This arrangement will not provide a shear diaphragm as would directly attached (panel rib¬ type) profiles. As with any other joist or beam, rotation of these coldrolled purlins (Z’s or C’s) sharply Photo 3 Photo 4 March 1996 33 Photo 5 Photo 6 Photo 7 reduces load carrying capacity. One major insurer has mandated strength¬ ening this type of construction with braces between outlying purlins.4 Linking up the purlins in this manner induces better distribution of loading in a manner similar to the bridging used between floor joists of framed wood construction. Wood structures might be stiff¬ ened by means of steel flitch plates sistered into place and bolted through existing joists. Steel joists may receive additional diagonal bracing; this improves load distribu¬ tion among neighboring joists and reduces rotation of the members. As always, replacement roof types which would result in a net loss in dead load are a worthwhile consider¬ ation. References 1. Smith, Thomas L., “Tips for Avoiding Ponding-induced Roof Col¬ lapse”, Professional Roofing, Janu¬ ary 1995, pg. 42. 2. Hogan, Lyle D., “Providing Posi¬ tive Roof Drainage”, Plant Engineer¬ ing, June 4, 1992, pg. 58. 3. Factory Mutual Research Corp., Loss Prevention Data Sheet 1-54, pp. 5, 6. 4. Allendale Insurance, letter dated September 6, 1994, and “Loss Pre¬ vention Alert” for metal roof sys¬ tems. “THE NEW BIBLE OF ROOFING” The Science and Technology of Traditional and Modern Roofing Systems THE MOST COMPREHENSIVE BOOK EVER WRITTEN ON ALL ASPECTS OF ROOFING materials & systems containing over 3,150 pages, with 80 chapters in 2 hardcover 8Vz”xH” volumes 1,100 illustrated figures and photos and over 500 tables with index. Includes manufacturing, selection, performance, testing, design, economic, statistics, application, management and maintenance. The latest ref¬ erence and further reading guide for each chapter. Chapters on materials include all major foreign and domestic roofing material manu¬ facturers and suppliers toll free telephone numbers and addresses. Truly a self-contained reader-friendly encyclopedia of roofing. This “New Bible of Roofing” is internationally recognized and accepted worldwide as a major publication and wealth of knowledge written for architects, engineers, spec writers, roofing consultants, contractors, building superintendents and managers. Suitable for college and university roofing course curricula. 1992 ABI World Lifetime Achievement Award winner. Hardcover (set) $350.00 (USD) ISBN: 0-9629669-0-8. 15 lbs overnight shipping to US add $16.00, Canada add $40.00 (USD). For this rare bargain delivered overnight, send your check payable to Dr. H. Laaiy, 9037 Monte Mar Dr., Los Angeles, CA 90035-4235, or call (800) 559-6090 CODE 45 for brochure and order form. 34 Interface
