A modern lightweight metal gusset truss roof with a ceiling. This photo depicts significant volume and velocity of smoke during initial portions of the test.
A modern lightweight metal gusset truss roof with a ceiling. This photo depicts the collapse of the continuous plastic ridge vent.
A lightweight 14-inch metal gusset truss floor with cord splices and framed stair opening without ceiling.
Modern lightweight 14-inch metal gusset truss floor with cord splices and framed stair opening without ceiling.
A modern lightweight 12-inch wood I-joist floor without a ceiling. The collapse time was 6:03.
A lightweight 12-inch wood I-joist floor without a ceiling.
Quadrant view of recorded video images taken during the research experiments available within the web-based outreach program.
Mannequins represent standing and crawling firefighters for testing. This photo depicts the typical loading for the floor assembly tests.
Mannequins represent standing and crawling firefighters for testing. This photo depicts the typical loading for the roof assembly tests.
A standing mannequin is engulfed in flames as it falls through a floor behind a crawling mannequin.
Multiple firefighter fatalities at large commercial fires attract the attention and scrutiny they warrant. However, the fire service must also recognize that most line-of-duty injuries and deaths occur in single- and multiple-family dwellings during routine, "bread-and-butter" fires.
A review of 22 investigations conducted by the National Institute for Occupational Safety and Health (NIOSH) Firefighter Fatality Investigation and Prevention Program between 1997 and 2009 illustrates this issue: These incidents involved fires in residential buildings that resulted in 28 fatalities and numerous injuries. These incidents can be linked to rapidly spreading fire in areas of unprotected wood construction, the collapse of unprotected dimensional lumber or the collapse of lightweight engineered wood components.
In addition, fire departments nationwide have experienced numerous near-miss incidents involving lightweight structural components and truss construction; the National Fire Fighter Near-Miss Reporting System has documented more than 80 reports of such incidents. Near-miss incidents involving buildings with weakened or collapsed structural components are not accurately recorded in the National Fire Incident Reporting System (NFIRS) data-collection system. Without modifications to NFIRS, we will continue to lose the ability to diagnosis how serious this problem has become for the American fire service.
The fire service has been aware of the hazards associated with lightweight construction for decades. Numerous fire service representatives have conducted small-scale burn tests to demonstrate their concerns regarding the structural stability of engineered lumber in fire conditions. Literally dozens of articles have been written by some of the most notable authors in the fire service, but until recently, we lacked the scientific data and statistical evidence to quantify the anecdotal evidence from numerous fireground events. Engineered lumber manufacturers understandably submit for testing only those assemblies for which they expect to achieve the desired passive fire-resistive ratings. Current model building codes allow unrated, or unprotected, assemblies (i.e., wood covered with non-rated sheathing materials or exposed wood) to be used in residential construction, particularly in single-family homes. Unprotected wood assemblies, assemblies without active or passive fire protection measures, are proving to be inherently unsafe.
Because the use of lightweight and engineered assemblies has become virtually the exclusive means of constructing floor and roof systems in residential buildings, the fire service must develop a better understanding of how these assemblies behave under fire conditions. It is also imperative that we test these built assemblies in a scientifically valid and quantifiable way so that the results can be used to effect changes to building codes and construction practices.
Underwriters Laboratories Inc. (UL), in partnership with the Chicago, IL, Fire Department (CFD) and the International Association of Fire Chiefs (IAFC), was awarded funding from the Department of Homeland Security (DHS)/Federal Emergency Management Agency's (FEMA) Assistance to Firefighters Grant (AFG) program to investigate the hazards faced by firefighters when confronted with lightweight and wood-constructed residential buildings. The results of tests comparing the fire performance of conventional and modern construction will improve the understanding of the hazards of lightweight construction and assist incident commanders, company officers and firefighters in evaluating fire hazards and allow a more informed risk-benefit analysis when assessing life-safety risks to building occupants and firefighters.
The findings of this research have been developed into a web-based training program for the fire service. This free interactive program is at www.ul.com/fireservice, access online fire service training and take the interactive training program titled, "Structural Stability of Engineered Lumber in Fire Conditions." Given the changes in how buildings are now constructed and how modern materials burn, survival may depend on how firefighters pay attention and respond to the results of this work.
The Research Evidence
UL and its partners were awarded the AFG funding to subject a representative group of floor and roof assemblies to the industry-standard fire-resistance testing method, ASTM E119: Fire Tests of Building and Construction Materials. The experimental series consisted of 12 furnace fire tests of assemblies representative of typical residential "legacy" and "modern" floor and roof construction. The tests included six structural elements, three ceiling-finish configurations, four floor or roof finishes and three floor-penetration configurations. All of the test assemblies conformed to the dimensions and span of the available test furnace (14 by 17 feet). Measurements taken during each experiment include observation of the conditions of the ceiling and floor or roof surfaces, temperatures in the concealed space above the ceiling membrane, deflections of the floor and roof surfaces, and failure times of the tested assemblies.
This research conformed to the standard requirements of the ASTM E119 testing method with one exception — the applied floor and roof loading was less than the ASTM E119 loading requirements. The ASTM E119 testing method is normally used to certify fire-resistive construction. The standard set by ASTM E119 describes a fire-test method that establishes a benchmark fire-resistance performance between different types of building assemblies. This test relies on a standardized fire and time temperature curve intended to represent a fully developed contents fire within a residential or commercial structure with temperatures reaching 1,000 degrees Fahrenheit at five minutes and 1,700°F at 60 minutes. The ASTM E119 fire-endurance test is designed to express fire resistance ratings in terms of hours: half-hour, one-hour, two-hour, three-hour or four-hour rated assemblies. These hourly time ratings are not intended to convey the actual time a specific component or assembly will withstand a real fire event. Variations result from room size, combustible content and ventilation conditions. The ASTM E119 test method is designed to provide a useful benchmark for use by building code officials and fire protection engineers, enabling a comparison of fire performance between test samples within the laboratory environment.
Following are some findings from the research:
- Residential fires may actually pose commercial fire risks. Many of today's "typical" house fires are in buildings that, based on size and interior volume, can and should be categorized as commercial structures with commercial fuel loads. Combined with modern synthetic fuel loads, fires in large, unprotected and un-firestopped voids made of lightweight engineered building materials can be catastrophic.
- Thermal imagers (TIs) do not provide an adequate indication of a weakened floor or pending collapse. There is a potentially dangerous misconception in the fire service that TIs can detect fire on the floors below or above a firefighter. TIs detect variations in surface temperatures for objects in the camera's field of vision. They cannot detect temperatures in areas that are thermally shielded from the camera's view by the finish materials of a floor or ceiling. During these tests, average temperatures below the assembly were in excess of 1,200°F, while average temperatures on top of the carpet were less than 100°F. The application of water during suppression operations will further mask or eliminate the thermal signatures available to the TI's sensor.
- Floor collapse can occur in six minutes. Engineered wood floor assemblies have the potential to collapse very quickly under well-ventilated fire conditions. When it comes to lightweight construction, there is no margin of safety. There is less wood to burn, and therefore less time before the assembly fails. During this research, the shortest failure time was noted during the unfinished/unprotected engineered wooden I-joist test. This assembly experienced a total structural collapse at six minutes.
Tested assemblies weaken significantly well before they completely collapse. The ASTM E119 definition of collapse requires that the floor totally collapse. Several fire-test standards recognized outside the United States, such as ISO 834:1: Fire-Resistance Tests, Elements of Building Construction, Part 1, look instead at how long a test sample is able to maintain its ability to support the applied load during the fire test, taking into account when a floor is progressively deflecting or failing prior to a complete structural collapse — clearly a critical piece of information for the fire service. The bottom line: If the ISO standard was applied to the unprotected engineered wooden I-joist assembly, the accepted failure time would change from 6:03 to four minutes. The engineered I-joist assembly clearly demonstrated the significance of the difference between the ASTM E119 and the ISO 834:1 standards. At 3:30, the engineered I-joist floor assembly began to vibrate vertically approximately eight inches for 90 seconds.
All lightweight assemblies studied in the tests exhibited similar differences in time-to-failure based on the standard applied, but none exhibited as severe a vibration period prior to collapse. Fire service instructors must reinforce that the dramatic vibrations exhibited by the engineered I-joist assembly were a very late indicator of a pending structural collapse and that this behavior will not be exhibited by all floor systems prior to structural collapse.
- Plastic ridge vents can mask fire intensity. In the early stages of the modern roof-assembly test, a significant amount of smoke was emitting from the continuous plastic ridge vent. As the temperatures increased, the ridge vent melted and collapsed upon itself, sealing the natural opening. The heavy smoke emitting from the continuous ridge vent diminished to a light smoke trail, although the fire was still raging below. Temperatures in the attic space went from approximately 200°F to 1,400°F in less than 60 seconds. These are flashover conditions and can cause global failure of the ceiling. Firefighters conducting size-up and attempting to read smoke conditions from the exterior may be deceived by this change in the volume and velocity of the smoke venting from the roof, and roof teams may conclude that the roof is safe to operate on, when in fact it may be rapidly approaching the point of collapse.
- Collapse times of the tested assemblies. The collapse times for all of the assemblies are shown in the chart above.
In addition to the collapse times, a large amount of significant useful data for the fire service was obtained during these fire tests to include: the observation of the conditions of the ceiling and floor or roof from both sides of the assembly, temperatures in the concealed spaces, deflections of the floor and roof surfaces prior to collapse, thermal imager video from the top of the assembly, and video and audio recordings from the simulated firefighters' perspectives.
Over the past few decades, millions of single-family homes have been built with truss-constructed roofs that create large, undivided attic spaces and unfinished basements that have unprotected lightweight wood floor systems above them. This study established that while all unprotected wood floor assemblies are susceptible to early failure when exposed to fire, modern lightweight assemblies fail significantly sooner and the failures are more global. Unprotected combustible wood construction also poses the threat of accelerated fire development.
Following are some tactical considerations for firefighting operations in lightweight constructed structures.
Use existing awareness literature, multimedia training aids and after-action reports to conduct department-wide awareness and building construction training, using the following resources:
- Visit www.ul.com/fireservice to access online fire service training and take the interactive training program "Structural Stability of Engineered Lumber in Fire Conditions." This program explains the UL study in detail, including the motivation, methodology, testing and lessons learned.
- Visit www.woodaware.info, the awareness-level informational website directed to the fire service. This program describes both traditional and modern wood products used in residential construction. These publications were developed under a cooperative agreement between the U.S. Fire Administration (USFA) and American Forest & Paper Association.
- NIOSH publication 2009-114, Preventing Deaths and Injuries of Firefighters Working Above Fire-Damaged Floors.
- NIOSH publication 2005-132, NIOSH Alert Preventing Injuries and Deaths of Fire Fighters Due to Truss System Failures (www.cdc.gov/niosh/docs/2005-132/).
- Review NIOSH fire fatality investigations so your department may benefit from the lessons learned at these incidents (www.cdc.gov/niosh/docs/wp-solutions/2009–114/).
- Develop standard operating guidelines (SOGs) for lightweight construction. Francis Brannigan spent his life teaching the fire service the importance of "knowing your enemy." Residential structures, and particularly "starter castle"-sized single-family homes made from lightweight engineered wood assemblies, are a very different enemy than legacy-constructed 1,500-square-foot homes. SOGs not specifically developed for lightweight construction are inadequate.
- Collect and refer to standardized pre-fire planning information. Thorough pre-fire planning highlights potential tactical considerations for responding companies approaching an unfamiliar structure. When possible, provide pre-plans on mobile data terminals or via radio transmissions for responding units and incident commanders.
- Consider fire department and mutual aid resources. Review your department's dispatch protocols to ensure that the assignment and staffing complement are sufficient for not only the occupancy, but for the size and construction of the subject structure itself. Upgrade alarm responses early and often. If you do not have adequate resources to conduct an interior operation while protecting firefighters' lives, consider a defensive operation.
- Adjust your initial size-up considerations. Assume this is a lightweight-constructed building unless/until you know otherwise. Complete a 360-degree survey of the structure before committing to an overall strategy and tactical assignments. Start laying 2½-inch hoseline for your fire attack. Continually monitor the exterior of the building using a TI, looking for signs of fire in the truss voids between floors. Consider the age of the structure, construction features, occupancy and visual indicators of the fire's progress, behavior and location. Follow Brannigan's dictum and distinguish between a contents fire and a structure fire. Once fire is attacking the structural components of a lightweight constructed building, you're out of time. Get out.
- Conduct a risk-benefit analysis. Continually assess potential victims' survival profile. Aggressive interior attack should cease if and when the occupants are accounted for. Aggressive interior attack should also cease if and when fire conditions preclude victim survival. This does not mean we must always abandon the building and "surround and drown." It does mean that we should identify when we are the only viable life hazard within the building.
- When multiple "immediate" tasks must be accomplished sequentially, make fire control your first priority. Rescuing trapped occupants is the first strategic priority, but not necessarily the first tactical priority. More people are saved by a well-placed and advanced hoseline than by any other tactic. Controlling the fire removes the hazard from the victim, which is much more efficient than trying to locate and remove the victim from the hazard. Do not conduct unsupported or unprotected search and rescue operations.
- Check below your area of operation. Always check below the apparent fire location before committing to interior operations. Do not advance until conditions below the area of operation are verified.
- Open void spaces upon entry. Lightweight construction is balloon framing with lots of extra holes punched in the joists. Once fire enters the voids, it will travel anywhere and everywhere. Upon entering the fire floor, make an inspection hole in the ceiling above and the floor below. Inspection holes can help verify the conditions of structural framing members and uncover areas of hidden fire within the void spaces.
- Consider a transitional fire attack. When possible, attempt to operate from protected positions. An interior attack can begin with a stream operated from the exterior. A correctly applied solid stream from the exterior of a fire area with flames visible and venting from one or more openings will slow the fire's growth and buy the time needed to allow for a more considered and deliberate interior operation. Bring the nozzle as close as possible to the exterior opening and play the solid stream off the ceiling and walls. Shut down and reposition as soon as knockdown is achieved. Provide an adequate fire stream for the fire conditions encountered. This tactic works best with an adult-sized (i.e., 2½-inch) hoseline.
- Upon extinguishment, aggressively ventilate all fire areas. Verify the integrity of the lightweight structural components as soon as possible. To do this, we need to see; to see, we need to ventilate. This is especially true for basement fires. Once the fire is controlled, ventilation and opening of the ceiling voids has to happen now, not later.
- Vent attic fire areas from the exterior. With truss roofs, ventilate and attempt knockdown of attic fires from the exterior (aerial platforms, fire-protected areas or adjacent buildings) before committing personnel to the floor below the attic. Exterior attacks can also be accomplished from above with a penetrating nozzle or from-the-gable-end attack. Interior extinguishment and search teams should remain below the landing of the stairs leading to the top floor. If a lightweight attic or cockloft area is so charged with smoke or fire that it is truly in need of ventilation, it is already at the point of collapse. No personnel should be allowed to operate on or below this structure.
- Vent, enter and search (VES). Monitoring crew locations is critical. This search and rescue method has the advantage of always letting you know exactly where the search team is operating. Search team members can also be sure of their means of egress; they brought it with them. VES can also be conducted on the ground floor with the appropriate-sized ladders or in a frame dwelling by making doors out of all the first-floor windows.
- Maintain operational flexibility. You don't know when the "countdown to collapse" clock started. Conduct a continuous risk assessment and be prepared to revise the operational plan when necessary. Maintain awareness of operational progress and consistently monitor fireground communications. If little or no progress toward fire control is being made, interior occupants have been accounted for, interior occupants are no longer viable and/or primary search and rescue has been completed, consider moving to a defensive operation.
- Always use an adequately sized rapid intervention team (RIT). Deploy a dedicated handline with the RIT. Many RIT scenarios, especially those dealing with rapid flashover or structural collapse, can benefit greatly from the ability to control fire.
A Final Word
Fires in today's "modern" residential buildings pose greater risks than their "legacy" predecessors. These structures are subject to rapid fire spread through areas of unprotected wood construction, the collapse of unprotected dimensional lumber, and the collapse of lightweight engineered wood components. Understanding the testing methods employed and the results of this study, even on a basic level, will assist firefighters in conducting a safer fireground operation the next time the alarm bell sounds. A working knowledge of these results is also a critical step for all members of the fire service who are actively engaged in the growing movement to enhance firefighter safety by modifying the current code requirements for residential construction.
For More Information
• For a complete interactive training program that explains the UL study in detail, including the motivation, methodology, testing and lessons learned, visit www.ul.com/fireservice, access online fire service training and take the interactive training program titled, "Structural Stability of Engineered Lumber in Fire Conditions."
• For further information, please e-mail Robert.G.Backstrom@us.ul.com, Stephen.Kerber@us.ul.com or James.Dalton@cityofchicago.org. To submit additional information and/or photos on local fire incidents within your area that may inform the issues discussed within this article, please contact James.Dalton@cityofchicago.org.
JAMES DALTON is the coordinator of research and development for the Chicago, IL, Fire Department. PETER VAN DORPE is a battalion chief for the Chicago Fire Department. ROBERT G. BACKSTROM is a senior staff engineer with Underwriters Laboratories. STEVE KERBER is a research engineer with Underwriters Laboratories and has 12 years of firefighting experience.
|Structural Element — Ceiling Finish||Type of Construction||Ceiling Materials||Floor/Roof Subfloor/Finish||Collapse Time (min:sec)|
|2 x 10 Joist Floor — Without Ceiling||Legacy||None||1 x 6 and Hardwood||18:45|
|2 x 10 Joist Floor — With Ceiling||Legacy||Gypsum Board||OSB and Carpet||44:45|
|2 x 10 Joist Floor — With Ceiling||Legacy||Lath and Plaster||1 x 6 and Hardwood||79:45|
|12-inch Wood I-Joist Floor — Without Ceiling||Modern Lightweight||None||OSB and Carpet||6:03|
|12-inch Wood I-Joist Floor — With Ceiling||Modern Lightweight||Gypsum Board||OSB and Carpet||26:45|
|14-inch Finger Joint Truss Floor — Without Ceiling||Modern Lightweight||None||OSB and Carpet||13:06|
|14-inch Finger Joint Truss Floor — With Ceiling||Modern Lightweight||Gypsum Board||OSB and Carpet||26:45|
|14-inch Metal Gusset Truss Floor with Cord Splices and Framed Stair Opening — Without Ceiling||Modern Lightweight||None||OSB and Carpet||13:20|
|14-inch Metal Gusset Truss Floor — With Ceiling||Modern Lightweight||Gypsum Board||OSB and Carpet||29:15|
|14-inch Metal Gusset Truss Floor with Cord Splices, Recessed Lights and Ducts With Ceiling||Modern Lightweight||Gypsum Board||OSB and Carpet||30:08|
|Metal Gusset Truss Roof — With Ceiling||Modern Lightweight||Gypsum Board||OSB and Shingles||13:06|
|2 x 6 Joist and Rafter Roof — With Ceiling||Legacy||Gypsum Board||1 x 6 and Shingles||40:00|