Standard engineering practice or junk science?, Part 2
By Richard Schulte
Schulte & Associates, Evanston, Ill.
Good judgment and the ability to have a "feel" for the answer to engineering problems are traits that it is essential to develop in the practice of engineering. One of the problems with those involved with fire modeling is that many modelers have a tendency to believe the results of their modeling efforts, regardless of whether or not the results are logical.
On March 21, 1996, two fires, one occurring in the early morning and the second occurring in the middle of the afternoon, destroyed the McFrugal's Warehouse, a one-million-square-foot warehouse in New Orleans. An expert for the plaintiffs in the subsequent litigation, Dr. Craig Beyler, utilized the Large Eddy Simulation (LES) fire model to support his contention that the installation of in-rack sprinklers in multiple-row racks (Class IV commodity, 21-foot-high storage) where ignition of the first fire took place would have had little or no effect upon the sequence of events leading to the destruction of the warehouse. Beyler's rationale for this bold statement was that the fire would have spread to combustibles located above the in-rack sprinklers, and that the ceiling sprinklers located 50 feet above the top of the storage were inadequate to control a fire burning at the top of the racks.
Ordinarily, a fire that occurred more than 12 years ago, where the parties to the litigation have settled out of court, would be of little interest in 2008. However, Dr. Beyler is now utilizing the Fire Dynamics Simulator (FDS) model to support Hughes Associates Inc.'s (HAI) research into the concept of "ganged" roof vent operation in buildings protected by standard spray sprinklers. Part 1 of this article (briefly) addressed Beyler's use of the LES model in the McFrugal's Warehouse litigation. This article will address Dr. Beyler's research into the "ganged" roof vent concept funded by the American Architectural Manufacturers Association (AAMA) Smoke Vent Task Group, a trade group of manufacturers of smoke/heat vents.
The AAMA Smoke Vent Task Group first announced new research into the use of roof vents and draft curtains in buildings protected by sprinklers in a memorandum dated September 10, 1999. It appears that the reason for the AAMA announcement of new research was the release of adverse findings (from the vent manufacturer's standpoint) on the interaction of sprinklers, roof vents and draft curtains from research sponsored by the National Fire Protection Research Foundation (NFPRF) in September 1998. (The findings of this research are published in a document referred to as NISTIR 6196-1. This report can be found on the National Institute of Standards and Technology (NIST) Building and Fire Research Laboratory (BFRL) Web site.)
The NFPRF-sponsored research confirmed Factory Mutual Research Corporation's 1994 finding that draft curtains have an adverse impact on the operation of standard spray sprinklers when a fire occurs near or directly under a draft curtain. The NFPRF-sponsored research also confirmed Factory Mutual's research from 1974 that automatic roof vents are unlikely to operate where the temperature rating of the operating mechanism of the vents is equal to or higher than the temperature rating of the sprinklers, and the sprinkler system successfully controls the fire.
After a code change proposal to delete the provisions for roof vents in the sprinklered buildings contained in the drafts of the International Building Code (IBC) and International Fire Code (IFC) was defeated in the code development hearings held at the St. Louis Convention Center in October 1999, the Smoke Vent Task Group's research project appears to have been abandoned. Subsequently, when several other code change proposals to delete the IBC/IFC code provisions for roof vents in sprinklered buildings were introduced into code change cycles, another new research project was announced by the Task Group in the Spring 2006 issue of the AAMA newsletter, AAMAnet.work. The following is an excerpt from that announcement:
"To concretely demonstrate the value of S&HV in terms of property protection, occupant safety, firefighter safety, and firefighter effectiveness, AAMA's Smoke Vent Task Group (SVTG) has inked a contract with Hughes and Associates to conduct Large Eddy Simulation (LES) and Computational Fluid Dynamics (CFD) studies in cooperation with the University of Maryland and the National Institute of Standards and Technology. Test results will provide data for an optimized S&HV design approach suitable for inclusion in Chapter 8 [Chapter 11] of NFPA 204, 'Guide for Smoke and Heat Venting,' or in model building codes."
Rather than addressing the interaction between standard spray sprinklers and automatic roof vents as was expected, the Hughes Associates Inc. (HAI) study addresses a new concept, the "ganged" operation of roof vents controlled by the operation of the sprinkler system water flow alarm. The reason for proposing a new method for operating roof vents in sprinklered buildings is that even the smoke/heat vent manufacturers (or, at least, the consultant employed by the manufacturers, HAI) now acknowledges that vents do not operate as intended in buildings protected by standard spray sprinklers (although they "flip-flop" on this point when testifying at the ICC code development hearings). The HAI report on their study of the "ganged" operation of roof vents is titled "Analysis of the Performance of Ganged Operation of Smoke and Heat Vents with Sprinklers and Draft Curtains" and is dated February 18, 2008. The timing of the release of this report is rather interesting, given that the ICC code development hearings in Palm Springs, California, took place between February 18 and 29, 2008. Releasing the report on February 18 allowed HAI to talk about the report in the hearing as if it were in the public domain, while limiting the time for a review of the HAI work by the Fire Code committee and others testifying on code change proposals involving roof vents.
Note: The HAI study on the "ganged" roof venting concept can be found at the following Web address: www.iccsafe.org/cs/cc/ctc/balanced/Thornberry_AAMA_Modeling_Study.pdf.
Excerpts from the HAI report include the following:
The objective of this study was to evaluate the performance of gang operated smoke and heat vent systems in sprinklered facilities. The gang operation concept involves opening all the vents within the coverage area of the sprinkler system in which the fire originates one minute after the first sprinkler has operated." (Executive Summary, page 11)
"A numerical study was undertaken to explore the performance of gang operated smoke and heat vents, working with and without draft curtains, in an 7,430 m2 (80,000 sq. ft.) sprinklered warehouse." (Executive Summary, page 11)
"Comparison of sprinkler operations between vented and unvented cases clearly shows that the operation of sprinklers was not affected by smoke and heat vents or by smoke and heat vents with draft curtains. The time to first sprinkler operation, the number of sprinkler operations and the pattern of operation were not impacted by the venting system. The use of a one minute delay in vent operation allowed all sprinklers capable of applying water to the fire to operate before the vents operated, thus assuring that the sprinkler system performance would be unimpeded by the venting." (Executive Summary, page 11)
"The impact of smoke and heat venting upon the visibility and general environment for firefighting was significantly enhanced. For challenging fires without smoke and heat venting, loss of visibility was nearly complete. With smoke and heat venting, excellent visibility was maintained throughout the facility, including in the area of actual sprinkler operation." (Executive Summary, page 11)
"While only limited data for smoke production from controlled fires is available in the literature, the modeling results show that smoke and heat venting is very effective in removing smoke even for heat release rates associated with controlled fires. This facilitates fire department operations to extinguish the controlled fire." (Executive Summary, page 12)
"The operation of the smoke and heat vent system had no effect on the operation of sprinklers and as such maintained the operational effectiveness of the sprinkler system while improving the conditions within the building in support of fire department operations." (Executive Summary, page 12)
"Smoke and heat vents, working in conjunction with draft curtains, have long been used as an effective fire protection measure. They can typically be found installed in industrial buildings, warehouses, stores, and malls." (page 13)
“Current U.S. design of smoke and heat vents typically utilizes thermal activation of individual vents. In Europe, it is typical to have smoke and heat vents operate in a ganged fashion. Ganged operation can provide more efficient venting in sprinklered facilities where the activation of individual vents is limited by the thermal management provided by the sprinklers. To eliminate any potential effect of venting upon the operation of sprinklers that provide water to the burning area, vent operation one minute after the first sprinkler operates will allow the sprinklers to operate without potential interference, yet provide effective venting of smoke and heat. For ganged operation based upon water flow, vents located within the area covered by the sprinkler system in which a sprinkler activates will open one minute after sprinkler water flow is detected." (page 13)
"The modeled building was an 7,430 m2 (80,000 sq. ft.) warehouse that was 8.23 m (27 feet) tall. The space was divided into two sprinkler systems each having a coverage area of 3,720 m2 (40,000 ft2). Thirty industry standard 2.44 m?1.22 m (8'?4') vents were placed in each coverage area in a manner such that they were not coincident with sprinklers and were not obstructed in any way. Ganged operation occurred over the entire sprinkler system coverage area for a single sprinkler system." (page 13)
"The Fire Dynamics Simulator, version 4 (FDS4), was used to perform the field calculations. FDS4 is a three-dimensional large eddy simulation CFD program developed at the National Institute of Standards and Technology's (NIST's) Building and Fire Research Laboratory (BFRL) [McGrattan & Forney, 2004; McGrattan, 2005]." (page 14)
"For the standard arrangement, pallet loads are positioned so that 15.2 cm (6") flues result. As was mentioned earlier, this small gap allows good radiative feedback between pallet loads, resulting in a rapid growth rate. However, due to resolution limitations, the burning rack flue space was set to 0.610 m (2'), which means two cells separating each pallet load in the x - and y -directions. The reason is that two cells by two cells allow more dynamic flue flows than would be the case with one cell flues. And, because of the flame spread methodology presented above, the desired growth rate is ensured in spite of the larger flues." (page 24)
"The normal inlet vent requirement for makeup air area is twice that of the roof vents in order to avoid inlet flow restriction. Garage door style inlet vents were used for makeup air in the simulations that included smoke and heat vents. The inlets were designed to be adequate and to minimally impact the simulation results." (page 25)
"FDS4 has two ways to account for radiative transport. One technique uses the finite volume method to approximate the solution of the equation of transfer. This feature was turned off for the following reasons. The main concern of this investigation is the activation of sprinklers. Sprinkler activation typically occurs after the ceiling jet engulfs a sprinkler head. Because of the smoke, radiation transfer within the upper layer is not a contributing factor. It is possible to have heat from the upper layer reach a sprinkler outside of the ceiling jet by means of radiative transfer. As is usually the case, the higher the temperature difference, the more intense the radiative heat transfer. However, activating sprinklers will cool the ceiling jet. The ten-foot sprinkler spacing is standard because it has been found not to lead to activations ahead of the ceiling jet. If this were not the case then a runaway activation sequence could occur that could exceed the sprinkler design area. Hence, based on the cooling effects of the sprinkler sprays and the spacing of the heads, it is not expected that radiative heat transfer will be a contributing factor in sprinkler activation. Furthermore, because the heat release rate is specified (i.e., not driven by flame spread) and because the heating of targets was not part of the scope of the present investigation (again, the cooling of the sprinklers minimizes this effect), there was no motivation to perform radiative heat transfer calculations that would extend the overall run time of each simulation. Therefore radiation effects were taken into account by using FDS4's constant loss feature. The specified radiative fraction was 35%." (pages 25 and 26)
"The fact that the beams could act as small draft curtains motivated moving them off the ceiling by two feet. The connection to the ceiling would be by purlins." (page 27)
"For a variety of reasons, comparisons with the commodity tests on a project are not appropriate. For example, in the current set of simulations, the heat release rate is well defined. It was not measured for the commodity tests [in McGrattan, Hamins, & Stroup, 1998]. Without this information, it is difficult to determine which experiments best correspond with the sixteen simulations available in this report. Some of the NFPARF [NFPRF] tests, such as P-3, are not applicable at all because fuel was placed directly underneath the draft curtain. Nonetheless, the averages of the first sprinkler activations will be compared. For tests P-1 - P-5 on a project [in McGrattan, Hamins, & Stroup, 1998], the average is 70.6 s. For Runs 1 - 9, the average is 71 s. The difference is 14%. The comparisons presented in the section are of suitably low percent differences to conclude that the validation exercise is a success." (page 31)
"Figure 9 shows that in the time span of less than six seconds four sprinklers activated." (pages 32 and 33)
"With smoke and heat vents installed, Figure 22 shows that eighteen sprinklers operated as compared to the 19 sprinklers that operated in Run #4 (i.e. no dramatic change). As before, most sprinklers operated before 135 s. The final two sprinkler operations lagged their unvented counterparts in Figure 20 by no more than 40 s. The sprinkler activation map in Figure 23 shows that the ring-by ring activation pattern was not affected by the presence of the vents." (page 44)
"The simulations performed in this study were based upon the experimental work of McGrattan et al [1998]. The results with respect to sprinkler operation are consistent with that work both in terms of the time to first sprinkler operation and the total number of sprinklers operating. The simulations clearly span the range of performance that can be expected in practice with sprinkler operations numbering from four up to the number of sprinklers in the design area." (page 91)
"This investigation has shown that ganged operation of smoke and heat vents is highly effective in removing heat and smoke from the building. The action of the smoke and heat vent system markedly improved the visibility throughout the building and significantly reduced the exposure of the building and contents to smoke. Draft curtains, although not vital to the performance of the smoke and heat vents, did limit lateral spread of smoke to other zones and enhanced the extraction of smoke from the building. The operation of the smoke and heat vent system had no effect on the operation of sprinklers and as such maintained the operational effectiveness of the sprinkler system while improving the conditions within the building in support of fire department operations." (page 92)
Discussion
In the litigation over who was "at fault" in the fire at the McFrugal's Warehouse, Dr. Beyler asserted that the installation of automatic roof vents and draft curtains would have reduced the number of ceiling sprinklers that operated in the first fire. Beyler further stated that if fewer ceiling sprinklers had operated in the first fire, it would have been unnecessary to shut down all of the sprinkler systems in the building, and this would have prevented the second fire from destroying the entire building.
While Dr. Beyler touted the benefits of roof vents and draft curtains in his expert's report and deposition in the case, he also opined that providing two levels of in-rack sprinklers in the mulitple-row racks as required by NFPA 231C would not have substantially contributed to the control of the first fire. In other words, at least for the McFrugal's Warehouse fire, it was Beyler's opinion that roof vents and draft curtains would have performed better than in-rack sprinklers.
In his deposition in the case, Beyler stated that the two most important features in a roof vent/draft curtain installation are the depth of the curtains and the number of roof vents that operated. It seems rather obvious that the number of roof vents that operate in a fire is important if the vent/curtain system is going to perform as intended. It also seems logical that, if automatic roof vents do not open in a fire (for whatever reason), or that a limited number of vents open, then the vent system will likely not perform its intended function.
The HAI report on the "ganged" operation of roof vents confirms that this is the case with the statement that "for challenging fires without smoke and heat venting, loss of visibility [in the building modeled] was nearly complete." Given that even Beyler now admits that sprinkler operation interferes with, and often prevents, the operation of automatic roof vents, this, in essence, is an admission that automatic roof vents do not work in buildings protected by sprinklers (assuming that the sprinkler system is operative and controls the fire). This statement effectively rebuts Beyler's assertion in the McFrugal's Warehouse fire litigation that roof vents and draft curtains should have been included in the design of the building. After all, why include a fire protection feature that doesn't work in the building design?
Aside from rebutting Beyler's testimony in the McFrugal's Warehouse litigation, the HAI report does a good job of illustrating the ability for "ganged" roof vent operation to vent heat and smoke, but there is little doubt that the "ganged" vent concept would be effective. In fact, it would probably be even more effective than shown in the HAI report if additional vents were provided (or, even better, if the roof of the building simply retracted when a fire was detected). The key issue with "ganged" venting is not whether smoke and heat will be effectively vented, but whether or not "ganged" venting will have an adverse impact on the operation of the sprinkler protection. Unfortunately, the HAI report falls far short of fully addressing this issue.
While the NFPRF-sponsored tests in 1997/1998 were able to demonstrate that the activation of standard spray sprinklers interfered with the opening of automatic roof vents utilizing a single 0.64 inch orifice ordinary temperature sprinkler, using this same type of sprinkler and only conducting 16 computer simulations is hardly enough work to draw a conclusion that the "ganged" operation of roof vents will not have an impact on the operation of sprinklers. In order to draw this conclusion and state with any degree of confidence that the "ganged" operation of vents will not impact the operation of sprinklers, it will be necessary to "test" a number of variables, including sprinkler type (upright or pendant, metal link or glass bulb, quick response or standard response); orifice size; temperature rating; pressure; varying pressures during sprinkler operation; sprinkler spacing; various densities; sprinkler location with respect to the ceiling; fire location with respect to sprinklers; varying ceiling heights; various storage configurations and the use of in-rack sprinklers with this concept. This, of course, presumes that the use of the FDS is appropriate for predicting the operating time of as many as 20 to 50 sprinklers.
The HAI study indicates that "the ten-foot sprinkler spacing is standard because it has been found not to lead to activations ahead of the ceiling jet." This statement is erroneous. NFPA 13 limits the spacing of standard spray sprinklers to a maximum of 100 square feet when the required density exceeds 0.25 gpm/SF. Sprinkler spacing of less than 100 square feet is permitted by NFPA 13 and spacings of 12 feet by 8 feet are not unusual. For example, in the fire at the bulk merchandising facility in Tempe on March 19, 1998 (where visibility at the floor of the building was zero when fire fighters arrived at the building, even though roof vents were provided and three vents and a skylight opened prior to their arrival), the typical sprinkler spacing was 9 feet, 7 inches by 8 feet (77 square feet).
With respect to the variables of sprinkler orifice size and operating pressures on operating times, consider the following excerpts from a paper titled "An Investigation of the Causative Mechanism of Sprinkler Skipping" by Paul A. Croce, John P. Hill and Yibing Xin of FM Global Research. This paper appeared in the May 2005 issue of the Journal of Fire Protection Engineering.
". . . (3) skipping propensity increases with water discharge density; (4) skipping beyond the second ring was not observed. Possible conclusions are that skipping tends to happen with a strong fire plume and a higher water discharge density, and is affected by the fire plume position and the sprinkler type and spacing."
"The six most severe fires shared the characteristic that four sprinklers directly over the pile were actuated first and that most or all of the twelve sprinklers in the next surrounding ring were skipped. In these tests, the skipping phenomenon is common and occurs with both 12.7 and 13.5 mm (1/2 and 17/32 in.) sprinklers."
"Both the tests were conducted using 3.05 x 3.05 m2 (10 x 10 ft2) spacing, 74oC (165oF) sprinklers and a 0.305 mm/s (0.45 gpm/ft2) water discharge density. But in one test (No. 68), 12.7 mm (1/2 in.) sprinklers were used under 431 kPa (62.5 psi) and seven heads [sprinklers] opened while in the other test (No. 88), 13.5 mm (17/32 in.) sprinklers were used under 221 kPa (32 psi) and 33 heads [sprinklers] opened. The analysis shows that the different testing results were caused by sprinkler skipping instead of sprinkler type. A series of calculations were also performed to identify the parametric effects on skipping and the conditions under which skipping would occur for 13.5mm (17/32 in.) sprinklers with a 74oC (165oF) temperature rating."
"It was identified that the only consistent prerequisite for skipping thus far was a persistent, high-challenging fire, and both residual and permanent skipping might occur. . . . The impact of reducing and eliminating skipping might include control of a fire, minimization of water demand, and increase of area demand. Due to skipping, sprinkler spray pattern, close spacing, drop size distribution, and deflector have to be considered in the design of sprinkler systems."
"The cooling of the ceiling produced by a sprinkler is an important part of its function. Of equal importance is its ability to deliver water through a buoyant fire plume to the seat of the fire. ... An increase in ceiling cooling is generally accompanied by a decrease in plume penetration."
Conclusion
From the above, it appears obvious that HAI's study to demonstrate that the "ganged" operation of roof vents will not have any adverse impact on the capability of standard spray sprinklers to control and extinguish a fire has a long way to go before it can be considered to be complete and authoritative. Until it is conclusively demonstrated and proven that the "ganged" opening of roof vents will not have an adverse impact on a sprinkler system, the wisest course of action for engineers to take would be to avoid the use of this concept.
The one issue that the HAI study appears to have overlooked is the capability of the FDS model to be able to predict sprinkler-activating times. As with Beyler's expert report on the McFrugal's Warehouse fire, the study does not discuss the validation of the model used to predict sprinkler operating times in detail (and the limited discussion provided on validation is in error). With respect to this issue, Dr. J. Floyd wrote the following in an e-mail note posted on the FDS and Smokeview Discussions bulletin board at 9:37 a.m. on April 3, 2008:
"It should also be noted that there is currently little to no validation basis for the suppression effects of water in FDS. Sprinkler activation has some validation, large drop movement (i.e. bucket test like simulations) has some validation provided one has measured that flow pattern for the specific nozzle being simulated."
Note: Dr. J. Floyd's post can be found at the following address:
http://groups.google.com/group/fds-smv/browse_thread/thread/d11a1b14d7d09953
Who is correct, Dr. J. Floyd or HAI? Perhaps it's time for HAI to do a few large-scale experiments (maybe two- to three hundred) with actual storage arrays to determine whether or not the results of the sprinkler operating times generated by the FDS are actually correct.
Certainly, a comparison of sprinkler operating times predicted by the FDS and the activation times determined in Tests P-1 through P-5 in the NFPRF research in 1997/1998 would be of interest. Actually, this writer has already done this comparison. A spreadsheet of the comparison of the sprinkler operating times can be found on the ICC Code Technology Committee (CTC) Web page under the heading "Balanced Fire Protection." The Web address of the spreadsheet and additional critique of the HAI "ganged" roof vent study can be found at the following addresses:
www.iccsafe.org/cs/cc/ctc/balanced/Schulte_HAI-Ganged_Roof_Vents.xls
www.iccsafe.org/cs/cc/ctc/balanced/Schulte_HAI-Ganged_Roof_Venting_Report_Critique.pdf
www.iccsafe.org/cs/cc/ctc/balanced/Schulte_HAI-ganged_roof_Vent_analysis.pdf
More on a comparison of the HAI model results and the large-scale tests that were a part of the NFPRF research next month.
Richard Schulte is a 1976 graduate of the fire protection engineering program at the Illinois Institute of Technology. After working in various positions within the fire protection field, he formed Schulte & Associates in 1988. His consulting experience includes work on the Sears Tower and numerous other notable structures. He has also acted as an expert witness in the litigation involving the fire at the New Orleans Distribution Center. He can be contacted by sending e-mail to rschulte@plumbingengineer.com.