While I was on medical 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 Chief Vincent Dunn's Collapse of Burning Buildings, and when I was finished with that, I decided to tackle Francis Brannigan's Collapse 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 a tower 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."

In addition to having a heavy fire condition, the 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 9? 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 fromNational 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.