by Daniel P. Sheridan
当我在医疗leave recovering from surgery, I decided to start reading some things that I hadn’t looked at in awhile. I reread for the fifth time ChiefVincent Dunn‘sCollapse of Burning Buildings, and when I was finished with that, I decided to tackleFrancis Brannigan‘sCollapse for the Fire Service. I finished them both. My first day back to the firehouse was a quiet Sunday. The day tour was very uneventful, with just a few runs. The night tour continued to be very quiet as well. At 0300 hours, the teleprinter in the battalion jumped to life, and the tones went off for the battalion to fill a 10-75 (working fire) in a commercial section of the borough.
I assumed that the fire was probably small. The sprinklers probably would have had the fire knocked down, I told myself, and we would not have a big problem. When we got into the car, however, I could tell by the tone of the first-due chief’s voice that this was not going to be the case. He asked for an extra engine and truck and wanted to make sure that the extra truck was atower ladder. That is a great call: start getting the help there early. On the way there, my mind was racing because I had just spent the past few weeks reading Dunn and Brannigan.
I know the area where this fire was very well and that there are lots of truss roofs in that part of the city. The units on the scene were all engaged, and there wasn’t much information being put over the air. I was thinking that if the building had a bowstring truss roof, the guys may not have much time to make an interior attack. When we arrived, the fire was already in its advanced stages. The units were making a push in through the front, but they were confronted by an extremely heavy fire condition. We came in to the fire from the Exposure 2 (B) side and could see a wall of fire in the middle of the building. The fire building took up a quarter of a city block, approximately 250 × 250 square feet (about an acre). It was difficult to determine the type of roof system because of the heavy smoke condition, but because of the hips coming down on the front of the fire building, I was thinking it was a bowstring wood truss. In this case, a hip is a feature of a bowstring truss–the part of the roof that meets the wall in front. It is the part that slopes down in front and back. The bow usually spans the two sides (B and D); the hips come down the front and back.
The biggest problem with trusses is that the failure of one element may cause the entire truss to fail. According to Brannigan, bowstring truss construction is a compressive structure. Both the top and bottom chords are under compression. Trusses normally have the top chord in compression and the bottom chord under tension. “The economy of the truss is found in the fact that it separates compression and tension forces. However, this economy can also cause disaster. The bottom chord being under tension (like a rope) will precipitate failure. Conversely, the top chord being under compression responds like a column. The load-carrying capacity decreases as length is increased.”
除了一个沉重的火灾条件下,hydrant on the Exposure 2 side of the building went out of service at this fire, leaving the units on that side of the building without water. At this point, the chief in charge pulled everyone off the roof and conducted a roll call when he received a report from the roof that they had a gypsum roof. It was learned later that the roof was a steel bar joist truss roof (see photos.) Once the members were pulled off the roof, multiple alarms were transmitted, and the incident commander (IC) set up for a defensive operation.
Steel adds a whole other dynamic to the fire. Although steel is noncombustible, it still presents a huge problem to a fire scene, especially if it is exposed. The problems with steel are the following:
- Steel will begin to start elongating at about 1,000º F.
- If the steel is constrained and can’t elongate, it will push out whatever wall it is butting up against.
- Steel may fail when heated to temperatures of about 1,300º F. Cold drawn steel cables will fail at 800º F.
- Steel is a good thermal conductor and transmits heat readily. This would mean that if you have a combustible roof, the roofing material could possibly ignite.
Steel Elongates
Steel will expand from 0.06% to 0.07% in length for each 100º rise in temperature. The expansion rate increases as the temperature rises. Heated to 1,000º F, a steel member will expand 9½ inches for a section of steel 100 feet long. If the structure is able to restrain the beam, the floors will then become compromised. If the structure cannot contain the steel, in the case of one constructed of masonry, the wall will become compromised and will result in a catastrophic collapse.
Steel Fails
Today’s fire loads may be double what was expected in years past. With modern furnishings, it may be possible to achieve temperatures of 1,500º in five minutes. With this in mind, it seems that we may reach the failure rate for steel much faster in today’s fires. Like wood, steel also relies on its mass or weight. According to the standard fire test from National Fire Protection Association Standard 251,Standard Methods of Tests of Fire Endurance of Building Construction and Materials, the bar joists or lightweight steel truss will absorb heat rapidly and could fail in about seven minutes. Another point to consider is that the heavier the steel is loaded, the faster it will fail.
Steel Conducts Heat
An example that Brannigan uses is that a suit of armor is noncombustible, but no one would fight a fire in it. I have been to lots of duct fires in restaurants. Many times there are no major problems, and we are able to get some water into the duct and extinguish the fire. Other times, we have huge problems, such as when the ducts go through the walls of a five-story multiple dwelling or other types of buildings. There was a horrific fire in Paraguay, South America, a few years back. The fire started in the flue above a grill. The grill was used to cook lots of fatty meats, meaning grease had likely accumulated in the inside of the ductwork over time. The building used foam insulation as part of the roofing materials. I surmise that the grease ignited, resulting in the fact that heat from the fire conducted through the steel and ignited the foam. the heat from the fire conducting through the steel and igniting the foam? I believe more than 300 people were killed in that fire.
We see this every day in the United States, especially in fast food places or Chinese restaurants. The ducts don’t get cleaned, the grease builds up, and we get a good duct fire. The problem is that if the steel ductwork is next to anything combustible, we will have extension. I used to worry about the chimney in my old house. I had a wood-burning stove, and every so often I would get a good chimney fire. The creosote would build up and then when I would get a good fire going, the chimney sometimes would take off. I would always have to go up into the attic and give it a good inspection.
According to Brannigan, with whom I am in total agreement, the heat or fire being generated is of secondary importance to the exposed steel. If water is applied to steel that is being exposed to high heat, the cooling effect of the water will draw the steel back to its original dimensions–in other words, at this fire, for example, it may be just as or even more important to get water on the exposed roof supports than the burning materials. In reality, it is possible to do both at the same time. If we aim our nozzles at the ceiling, we will be cooling the ceiling and extinguishing the fire at the same time.
EXPOSURE PROBLEMS
大约25年前,我们已经在佤邦rehouse that stored giant newspaper rolls for some of the New York daily papers. The rolls would be brought in by rail and loaded into a huge warehouse. Part of this warehouse was under the Triborough Bridge, a major bridge in New York City. I was assigned to a tower ladder at the time. I didn’t quite understand it at the time, but, looking back, the chief in charge had a tremendous challenge on his hands. Not knowing much at the time, I was thinking that the biggest concern was to try and save as much of the warehouse as possible. I was not thinking the bridge was the bigger exposure problem. To add to the exposure problem, the building was in a part of the city that didn’t have many hydrants. We have had rubbish fires in that area over the years where we need to strike a second alarm for water relay.
The bridge was the major exposure at this operation. We set up the tower ladder close to the bridge. After the fire, when we had returned to quarters, every light on the truck was melted.
Some years back, there was a gasoline tanker fire under an underpass on a busy part of the New York State Thruway. As a result of the fire, the Thruway was closed down for quite some time because of damage from the fire. The fact that steel is noncombustible is irrelevant–it will still either fail or transmit heat. We need to identify what is burning and what our structures are made of to be successful and safe at every operation.
