04-25-2000, 08:27 AM #1phyrngnFirehouse.com Guest
Lightweight Trusses and Vertical Ventilation
What are your thoughts on vertically ventilating a lightweight truss roof? I have been told NOT to do it, because you'll draw heat up to the gusset plates and cause collapse. I've also been told that vertical ventilation is essential in many cases, including lightweight truss roofs to get the heat out of the structure and prevent heat buildup underneath a greater surface area of the roof/ceiling. Which is right?
04-26-2000, 10:37 AM #2benson911Firehouse.com Guest
Every scenario is different. If the fire needs to be vented vertically, do it. If the lightweight trusses are involved in the fire, do not put anybody on them or under them.
Venting hot smoke, gases and fire through an attic with truss roof construction shouldn't cause a collapse, unless the fire is so intense the trusses become involved in the fire. If your venting causes the attic structure to become involved in fire, then you have a much larger fire problem than just the trusses.
Remember, the gusset plates aren't failing; they're transferring heat to the wood to which they are attached and causing it to pyrolize, burn and take away the gusset plates anchor. Then, the truss fails. This doesn't take long (possibly 5 minutes.)
If you are venting the attic to release the fire, smoke and gasses from an attic fire, then vent from an aerial ladder and attack the fire from the side (gable end) to avoid being caught in a collapse.
Many other scenarios are possible. Size up every fire like it will be your last and worst fire ever. Do not go blindly into a fire unless you DO want it to be your LAST and WORST. Be careful, but be aggressive.
04-26-2000, 12:47 PM #3fireseekerFirehouse.com Guest
you have asked the 6 million dollar question. This is the situation every officer hates to see. You know when you arrive on the fireground that ventilation is paramount to your crew's and any victims safety. You also know that lightweight construction is built for economic reasons, not safety. There use to be the thought that with ordinary construction that 30 minutes was a good rule of thumb to use before you needed to worry about the integrity of a structure. Today, that 30 minutes has been reduce to 5-10. So, there you have the dilema. Hopefully, the officer will be able to make a sound judgement upon the information that is available to him/her. The bottom line is that the first consideration is for the safety of the crew, after that, everything is secondary. If the decision was easy, nobody would get hurt, lost, or die. It is a situation where you do what you can with what you have. take care...
04-28-2000, 05:55 PM #4FFD#60Firehouse.com Guest
I would tend to agree that the situation is the determining factor. Our Dept. has recently had the opportunity to test this theory. Arrived at a multi-family dwelling with 16 apartments. Only fire visible was rolling out of a 2nd story balcony doorway. Started interior attack with 2-1 3/4 lines. We started to pull the ceiling to see if the fire had gone above and past us. Unfortunately the fire had passes us (thanks to 14mph winds). At that time command decided to cut a trench in front of the fire wall to take some of the heat and fire off of it. It did take a lot of work off of the fire wall. However, we did have collapse of the lightweight trusses over the area where the fire originated. The choice to do a trench next to the fire wall probably saved the rest of the building. Also, if available it is nice to have an aerial to do vertical vent. from the bucket or ladder. Preplanning helped a lot in setting up the aerial for the trench cut by the fire wall and knowing the type of building construction, exposures, etc...
Keep safe brothers and sisters
[This message has been edited by FFD#60 (edited April 28, 2000).]
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