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Fire Hazards of Trusses: Sample Report
Structures often play the important role when a building is on fire. Upon many reviews, there are a lot of fires claiming for lives of the inhabitants of the building, but there are also some cases, which had taken the firefighters' lives as well. Such cases should not have happened, but limited information of the building's structures and length of fire could have caused it.
Chesapeake automobile warehouse happened to be a fatal case of fire (NFPA, 2002). The 12-year-old building was constructed under lightweight wood trusses. There were two steel frames and another brick construction located at the building. The trusses were built using the combination of wood and metal plates that joined them altogether.
On Monday morning, March 18, 1996, the repair shop operated as usual. The employees were coming along to start their first day of the week, taking up service works along the arrangement of the combustible store array of auto spare parts and liquid products.
There could be not enough time to check up all apparatus available on the store, or simply that the tool was out of order. Suddenly, a little malfunction had ignited a fire on the part of the store. Upon the absence of fire detector or indoor fire sprinklers, the assortment of aerosol, paints, repair products and flammable liquids in spray cans then fed the fire exhilaratingly. Some of them were packaged in cartons and thick cellulose materials, which caused the fire to spread faster than they had thought. In a flash, the fire climbed up to wood trusses, crackling up to the roof. Unfortunately, most was hidden inside the ceiling area that nobody could have notice how bad it had spread.
The firefighter crew came at 11:30 A.M., noticing only little part of the fire taking up the back part of the store. It seemed to be a manageable fire that the incident commander sent only a group of firefighters to handle it.
What they found inside must have been worse than what it looked like. The officer asked for another back up using radio transmission to the incident commander. Eight minutes afterwards, with the hard clatter, the roof collapsed onto the ground, trapping the firefighters. The fire, on the other hand, flamed larger than before. There was another transmission from the entrapped officer, but it did not convey very well. There was nothing they could do, as the fire was too immense to defeat.
It was fifteen minutes after the unit came to the scene. The accident ended up with the deaths of two firefighters. They possibly did not make it in the gulfing smoke, which intoxicated their respiratory system and the excessive burns.
The Risks of Trusses in Fire Cases
Roofing is the whole system that covers a building, and trusses are the good choice to support it. According to Federal Alliance for Safe Homes (2001), truss system includes "the top chord or rafter (where roof sheathing is nailed), a joist or bottom chord (where the interior ceiling is nailed) and angled pieces that form a web and are used to add strength."
People had been using wood trusses. Trusses are usually designated to fit the specific roof system, produced in even shapes and sizes, and then joined together with high accuracy to eliminate gaps. This construction gives the house enough protection to shelter the walls and floors so that they would stay longer in weather exposures throughout the years.
The common materials used for trusses structures usually varied from wood to metals. The metal plates put joints for the wooden trusses. This material is chosen for its reliability holding the roofing system for years.
However, for this combination, trusses also contain the high risk within a fire. A&R Truss Company said that this construction are combined one with another into a web system, supported with the main plates and metal joints. It is considered "separate, independent structures"; they do not depend to each other. The combination structure is only put there to support certain condition (protecting walls and floors, under normal circumstances). In the case of fire, when one truss get burnt and collapse, the other portion would remain stiff. It provides the time to perform the evacuation.
On the other hand, NFPA record on trusses collapse shows that the danger remains high with this type of structure. Trusses are assembled with lightweight materials for esthetic and economical reason. With its designs - might be specifically done for each building - trusses is reliable for its "loading capability over wide spans," that requires less materials as well as the cost. The weight, type of materials, and type of design determine the specific fire pattern and the period of its endurance when fire attacks.
For example, the fires at Chesapeake store, and the other one at a Branford carpet store, were fed rapidly by the construction of lightweight materials (wood trusses and sheathing). Those materials were built in specific location hidden above the ceilings, where firefighters (and probably the workers too) unable to notice. NFPA called the materials in the case as "fuel" to the hidden fire, with a notification that had the trusses made of metals, such ignition would probably not spread. Fire would have reached on merely the roof or gypsum ceilings, where witnesses and firefighters could easily spot.
However, even though metal trusses (and frames) would stand longer for fire attack than wood ones, some fireballs could intensify hundreds times above its normal temperature. When it reaches the melting point, such trusses would lose its buttress against the roof structure.
Parow (2002) gave example on what happened at the World Trade Center tragedy, where jet fuel burn escalated quickly and destroyed nearly everything adjoining within few seconds in engulfing inferno. The temperature was estimated to reach 1000-3000 degrees. He said, "at 1,000°F steel loses up to half of its tensile strength and starts to buckle and deform, and at 1,400°F it retains only 10-20% of its overall strength." And when the building continued to burn several minutes afterwards, "heat induced additional stresses into the damaged structural frames," melting the floor metal trusses. Even steel would not survive this temperature, and then it caused the building to collapse.
From 1997 to 1995, about thirty firefighters were killed in 16 incidents involving wood trusses roofing," said NFPA. One case was reported from the period of 1977 to 1995.
Fire Suppression Under Trusses Structures
Lightweight wood truss apparently is one of the dangerous types of structure that would be easily collapsed. The Memphis fire, as explained by the United States Fire Administration (1992) gave example of its hazard. There was no sign how the collapse happened before without warning, but unfortunately, it had claimed two lives of firefighters.
The Pilgrims Hope Baptist Church was a small, one-story building in Memphis. The telephone call first rang at 1:54 P.M. On December 26, 2001, followed by the alarm three minutes afterwards. The fire was reported had flamed out of the church and burnt it up all over. Batallion 11 eagerly dispatched three engines and a couple of trucks to the location, each headed with one officer and three firefighters. They arrived at 2.02, however the building could have been burnt for more than 20 minutes since there was even a delay from the last call to the station, as the caller failed to provide the name of the street at the first place. There was "working fire through the roof," as the command reported.
Large fire was seen penetrating the roof an the east wall, destroying gas lines, which means that this had come to very high temperature, enough to melt metal construction. Finding a hydrant hook, the engine 27 moved to the south part to put of the fire on this side, through the window, where flames speeding up forward. By this time, fire had reached the attic, and the firefighters thought they could finish them by making holes on the ceiling and going through.
Battalion 11, arriving two minutes afterwards failed to reach the officers inside, through radio communication. The Incident Command commanded to proceed into the structure making
Positive Pressure Ventilation (PPV)" and extinguish through the sanctuary wall.
It was when another truck requested to advance vertical ventilation; they went up and saw how the roof was extremely on fire. It was impossible to create such ventilation, and then they continued to work back down. The radio kept on failing under the noise level. On the other hand, they kept pulling the ceiling though smoke was heavy, which made it difficult to see each other.
The heavy struggle exhausted the firefighter from Engine 31. He transferred the nozzle to a colleague and rested to the north door. Another crew who moved into the south door then noticed falling rubble fell down the floor. He urged the fellow to back off, as well as the other one who rested at the north door. At the same time, the roof fell down into…[continue]
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