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truss vs stick built burn test

Very interesting stuff. Now here is my dumb limey question.

What qualifies a roof structure as "stick built". Most all new construction over here is using the kind of prefab trusses shown failing rapidly failing but older "1930's and before) tend to be in-situ constrcted , using bigger timbers but less intermediate bracing - is that what "stick built" means?
 
Stuart Ablett said:
I could be wrong Ian, but I would think that "Stick Build" would be closer to "Timber Framed" :dunno:
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Thanks Stu. The problem is, that over here, timber framed covers any building which is dependent on timber for its structural walls. So it can mean either substantial oak beams or pre-made plywood panels. Interestingly, in our market the former are somewhat at a premium but the latter are definitely seen as being less attractive. With most of our housing stock being brick or brick and block built there was a flurry of developments in the 70's and 80's which used the plywood panel type construction but where the builders were really not familiar with the demands of that kind of work. Lots of problems with quality followed which became even worse when people started making alterations and additions with even less skillfull builders involved. Standard timber construction now tends to be restricted to small scale apartment type buildings where the likelihood of future alterations is limited. It would be quite difficult (in most areas) for a spec developer to sell single family homes in timber framed construction without taking a reduction in his sale price.

I guess that part of the reason as well is that where land makes up a very large part of the cost of housing (comparatively) then the added cost of a more labour intensive building method becomes proportionally less of the total cost. I guess that in Tokyo you have a whole different set of pressures again!:)
 
In my line (architecture), stick built means framed in place with dimensional lumber - usually 2 by's. Trusses are often made with the same types of materials, but are pre-engineered for efficiency (strength vs. amount of material used), factory built, shipped to the site, and lifted into place. Engineered trusses consume less material and make sure you have members where you really need them. Also, the connections are engineered as well. They also install much faster than stick building a roof. My guess is they've found that in the case of a fire, the gusset plate connections fail quickly because they only bite into the first 1/2 inch of material (which burns off quite quickly). Nailed lap joints, on the other hand, use a heavier gage of metal (more fire resistant) than the gusset plates and penetrate much deeper into the material.

Typically, trusses are a better product. My neighborhood is about 100 years old - predominantly brick bearing wall homes with stick built floor plates and roofs. It predates engineered wood framing - a lot of guess work went into it - a lot of bad guess work. I have worked on many homes in my neighborhood and others of similar vintage and have yet to come across one that didn't have significant problems with the stick framing - generally at the stair openings and the roof framing. Renovation typically includes structural modification to address failure.

Timber framing is another animal altogether. This involves large solid wood members. Timber structure performs quite well in a fire if the connections are done right. What happens is the fire chars the outer inch or so of the members, which then insulate the core of the timber from the fire. It will still fail, but lasts much longer than other wood construction and even steel.

Paul Hubbman
 
Thanks Paul - that's kinda what I guessed. I was interested to see in the video that the pint of inital failure appeared to be exactly where your explanation would predict. The diagonal bracing between the horizontal bearer and the angled bearer can be seen making a clean seperation rather than any of the timbers burning through.

We had a fire at home a couple of years ago and the forensics guy from the insurance company timed the fire by how much of the adjacent skirting board was charred. I seem to recall he worked on 1mm of char per minute of burn and his timing and the other known facts about the fire agreed. Interesting stuff this!
 
Tod,
Neat video - does make sense that the smaller framing pieces using light gauge metal fastening would go first in a fire. I noticed the same failure point that Paul did. I wish they had a shot of the plates afterwards to see if the cleats or the wood failed.

Ian,
Back in the late '90's I was at Batamat in Paris (really big building/construction trade show) and trusses were making a big splash. Not to say they hadn't been used previously, but for some reason there were a number of companies featuring them and their benefits. Batamat was a really cool experience - got to go twice. They've got everything related to building construction there - usually in full scale!

Wes
 
Good information, Paul. Do you know why they don't "nail" the gusset plates in more than 1/2"? I expect those are applied by machine. Seems the machine could drive the plates in with fingers longer than 1/2". It would just mean stamping the plates out with longer fingers.

Mike
 
I don't really know why gussets are made as flimsy as they are with relatively short "teeth". My guess would be that they are engineered for efficiency in ideal conditions - not fire. If the fingers were longer, you'd likely need a thicker plate of metal to give the teeth sufficient stability. The more steel used, the lower the efficiency of the connection. It's probably a few bucks per house to make the gusset plates more substantial, but truss builders are focused on economies of hundreds or thousands of homes per year.

A change like that would likely need to be regulated through a change in the code requirements. It's the classic conflict between business (seeking to optimize efficiency) and government (needing to regulate for health, safety, and welfare).

Most houses now are built to burn, as long as the occupants have a reasonable chance of getting out unhurt. Rebuilding those homes that do burn is cheaper than requiring non-combustable construction for all homes. That said, charred roof framing is one thing - structural collapse is something else entirely.
 
If a fire gets to that point, does it really matter?? I would assume in most cases i's going to have to be torn down anyway to repair or replace it. Just as long as no one is inside, and suspect at that point they wouldn't be alive anyway.
 
paul has been dead on in his explanations as far as i know:eek:
the gang-plates used to join truss members are pressed into the members by either passing the entire truss through a large set of pinch rollers,or by manual pressing with a hydraulic press...link.. increasing the length or diameter of the fasteners would require that the pressing equipment be substantially beefed up and might negate any cost savings?
as to the pros-n-cons of engineered lumber, trusses, stick framing, timber framing, ect. it really all depends on the building....what it`s to be used for, who`s constructing it, and what the budget is...
trusses have allowed builders to flourish that couldn`t have years ago, they`ve made large open spans available for little money and have proven their reliability in most applications, but trusses as well as most engineered lumber products cause the oldtimers to wonder about their long term service in the construction field.
the united states is a very young nation in terms of building life and the engineered products are younger yet, so those who are thinking of building or buying property with an existing building would be well served to have a professional look as closely as possible at any questionable construction....
 
Jeff Horton said:
If a fire gets to that point, does it really matter??
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I'm with Jeff, it really doesn't matter at that point. If a fire gets to that stage there is little chance your going to rehab the building, it'll need to be demolished any way. Personally, If it was my house and it's not a minor fire, then I'd prefer it burned right to the ground. Less clean up and disposal costs.

Mike
 
M Toupin said:
I'm with Jeff, it really doesn't matter at that point. If a fire gets to that stage there is little chance your going to rehab the building, it'll need to be demolished any way. Personally, If it was my house and it's not a minor fire, then I'd prefer it burned right to the ground. Less clean up and disposal costs.

Mike
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Mike, Jeff

I think if it matters then it matters to the firemen risking their life. I believe some quoted numbers in the number of firemen killed by truss roofs falling in over a span of a few years.

Garry
 
Paul Hubbman said:
I don't really know why gussets are made as flimsy as they are with relatively short "teeth". My guess would be that they are engineered for efficiency in ideal conditions - not fire. If the fingers were longer, you'd likely need a thicker plate of metal to give the teeth sufficient stability. The more steel used, the lower the efficiency of the connection. It's probably a few bucks per house to make the gusset plates more substantial, but truss builders are focused on economies of hundreds or thousands of homes per year.

A change like that would likely need to be regulated through a change in the code requirements. It's the classic conflict between business (seeking to optimize efficiency) and government (needing to regulate for health, safety, and welfare).

Most houses now are built to burn, as long as the occupants have a reasonable chance of getting out unhurt. Rebuilding those homes that do burn is cheaper than requiring non-combustable construction for all homes. That said, charred roof framing is one thing - structural collapse is something else entirely.
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Thanks for the update. I guess the roof trusses are only one part of the overall problem. For example, those engineered wooden I beams with the thin center section will burn through long before the roof trusses give out.

Mike
 
Jeff Horton said:
If a fire gets to that point, does it really matter?? I would assume in most cases i's going to have to be torn down anyway to repair or replace it. Just as long as no one is inside, and suspect at that point they wouldn't be alive anyway.
Click to expand...

I believe the tests and the video was done for the fire service people, to teach them that a standard trussed roof, will fall in on top of the fire service people in a VERY short time.

Cheers!
 
Jeff Horton said:
Absolutely. I would think they wouldn't be in a building that was that bad engulfed.
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Jeff
From what I read it being on the roof where the most deaths have happened.
But I may have that wrong.


If it gets that engulfed in 5 minutes that is the problem. I am not a fire fighter but know they often open a roof just to get water to the fire. And its not only about saving the house that is burning it about keeping the fire from moving on to the next house. That all said I have truss roof and never gave it a thought until I saw this video.

I don't like them because they steal almost all of your usable attic space. But thats a whole seperate topic..

Garry
 
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