Bowstring truss.

[johnny46]

Is there a way to tell from the outside whether a curved roof is bowsting truss or not?

[DeputyChiefGonzo]

Is there a way to tell from the outside whether a curved roof is bowsting truss or not?

Whenever I come across a curved roof.. the first thing that does come to mind is "bowstring truss".

The second thought is potential failure of the truss under fire conditions.

Treat every curved roof as a bowstring truss until you can verify it isn't.

[johnny46]

this is what I've been taught, but I just got done watching a video with some guys on a curved roof and wondered. The only way I've been aware of knowing the difference is pre-planning

[GFDLT1]

I don't know of any way from just looking at it, but there are some quick ways to find out. You can either make an inspection cut from the bucket/stick or if you inside truck crew carries a camera with them have them point it up toward the ceiling and report back what they find. I have been told that with steel trusses you can use the TIC to identify what type of roof construction you are dealing with. You would have more exposure to this working in Houston then I would.

[FireCapt1951retired]

In the 33 years I spent on the department, I never saw a curved roof that wasn't a "bowstring truss" and we had a lot of them, except fot just a few almost all have burned down now.

[ChiefChip]

In the 33 years I spent on the department, I never saw a curved roof that wasn't a "bowstring truss" and we had a lot of them, except fot just a few almost all have burned down now.

Lt1951, You are right. We only have four or five in my city today. I have been on the job 21 years and have never seen a curved roof that wasn't bowstring. I have seen a couple that had a pitched roof added over the bowstring. The only way we knew about it was by preplanning.

In the absence of preplanning, consider all curved roof structures bowstring and take the appropriate precautions. Also, watch out for the parapet that can be found on the front of a building with bowstring construction.

Finally, to whomever started this thread, read Brannigan's book and whatever you can find regarding trusses by Vincent Dunn.

[johnny46]

Thanks all.

[kieranhope]

One way to tell if an arched roof is bowstring is the tie rods may pass through the exterior walls to outside plates; however, many times these plates are stucco'd over, which covers these identifiers. In Los Angeles, we don't have as many bowstring arches as ribbed arches. In the area I work, most of our arch trusses are ribbed, not bowstring. Due to the large 2x10s or 2x12s used as the bottom cord of the arch, these ribbed arches allow significant ventilation time before failing in sections. If unsure what type of arched roof, we cut an inspection hole where the truss is tied into the walls. This lets us visualize whether the bottom cord is tie rod w/turbuckles (bowstring) or 2x12s (ribbed). We also have a couple of tiltups with arched roofs that are panelized roof construction. If it is bowstring, it depends where the fire is, i.e. in the attic versus below a drop ceiling, if and how we will ventilate.

[johnny46]

One way to tell if an arched roof is bowstring is the tie rods may pass through the exterior walls to outside plates; however, many times these plates are stucco'd over, which covers these identifiers. In Los Angeles, we don't have as many bowstring arches as ribbed arches. In the area I work, most of our arch trusses are ribbed, not bowstring. Due to the large 2x10s or 2x12s used as the bottom cord of the arch, these ribbed arches allow significant ventilation time before failing in sections. If unsure what type of arched roof, we cut an inspection hole where the truss is tied into the walls. This lets us visualize whether the bottom cord is tie rod w/turbuckles (bowstring) or 2x12s (ribbed). We also have a couple of tiltups with arched roofs that are panelized roof construction. If it is bowstring, it depends where the fire is, i.e. in the attic versus below a drop ceiling, if and how we will ventilate.

That's some good information. Now go and defend your boys on the cool truckie video thread.

I reckon knowing your buildings is the best way to deal properly with fires, but if there's a trick for those few times we're working out of our territory, then I want to know it.

[fdofficer4]

Yes you can tell from the outside, it's called a pre-plan

[johnny46]

Yes you can tell from the outside, it's called a pre-plan

So if you preplan, you can tell from the outside, or do you need to go inside on the preplan?

[kayakking]

Just FYI...we have 2 structures in our city with the characteristic arch from the outside, but they are actually large glulams and not a truss at all.

Both of these structures were once Safeway grocery stores, but now one is a church that left the glulams visible from the floor and the other is a boxing gym that has a dropped, suspended ceiling...you have to get on a ladder and move ceiling tiles to know what the roof structure is.

So, as has been said before...you've gotta preplan to KNOW what you have.