Thread: Ladder Concepts and Trusses
11-23-2005, 12:27 PM #1
- Join Date
- May 2000
- Lincoln, NE
Ladder Concepts and Trusses
Hey all...found this on the firetimes.com website:
What do you think? All I've ever been taught is that trusses are bad, gusset plates are bad, etc. This throws a wrench in to things. I know the topic has been discussed before, but this is the first time (that I know of) where someone in the fire service is actually saying trusses aren't as dangerous as everyone once thought.
11-23-2005, 12:51 PM #2
That is a very interesting article. But to me nothing is safe in a fire. Weather they curl up and pop off when they get hot, or burn away from the plates, no one will tell me that I do not have to worry about any building material in a structure which is what the article led me to believe. Although this may be true, you always need to be aware of your surroundings and only trust what you see with your own two eyes. Look what happened to the Trade Center. No one thought those buildings would ever collapse."Training doesn't make you a good fireman, fighting fire makes you a good fireman"
11-23-2005, 01:27 PM #3
I have a gusset plate in my locker that I found on the floor after a fire. It is curled on all four sides and fell off a truss that had not burned through.
I don't believe they are saying trusses are good, they are saying every type of buiding construction has it's pros and cons.
11-23-2005, 01:56 PM #4
- Join Date
- Jul 2005
They're selling a service and looks like they're trying to make $$$ helping FFs better read buildings. Like the previous comments, they're not saying trusses are safe, just that you need to look at more than just the fact that a building has trusses.
Most single family dwelling have truss roof assemblies....we don't see too many collapses in those fires, even though the attic and trusses often become invovled. Why? Because the interior walls in single family buildings will usually support anything disintegrating in the attic. On the other hand, if you have a commercial strip mall with 30' spans and fire licking the bottom of the steel bar joists I don't want to be under it.
You have to really look at buildings when you do preplans and walk throughs - think about where the fire will go and what parts of the building will fail. Mass isn't always our friend either. Glue-lam beams have huge mass, but they're held together with steel connectors. The connectors will fail and bring the roof down in a fire just as quickly as if it had light weight trusses.
11-23-2005, 03:20 PM #5Originally Posted by gunnyv
A great visual for teaching building construction as it relates to the fire service is to hold a tiger torch to a 2 x 4 for a minute or two and then show the folks how, although charred, the structural integrity is still good.
Then do the same to a gusset plate in a 2 x 4 for the same amount of time.
Pyrolysis occurs from outer surface in, including the outer surface surrounding the teeth of a gusset plate. pyrolysis occurs around those teeth, loosening the connection. Flame impingment that would not catastrophically reduce the integrity of the wood itself will cause gusset plates to be loosened and pop out, leading to failure of the truss system.
They used this demonstration during a strat/tac course I took and it was a real eye opener for everyone, including those who 'knew' this as fact but had never seen it demonstrated.
12-03-2005, 02:11 AM #6
Originally Posted by jmitchell
- Join Date
- Feb 2005
- Ft Worth, Tx
I have some pictures in my slide show of a pile of plates that came off in the church fire in 1999, that our department lost three men in .
12-03-2005, 02:58 PM #7
I took a building construction class related to the fire service and the main thing I remember out of it, is some batallion chief with a really heavy New York accent saying "NEVER TRUST THE TRUSS".
12-27-2005, 05:12 PM #8
- Join Date
- Jun 2003
- Irwin, PA
I downloaded the articles from the magazine and am in the process of ordering the training CD from this website http://www.carbeck.org. I want to look this over before I form an opinion. Here are links to all of the articles from Structural Building Components Magazine:
http://www.sbcmag.info/past/2004/04j...saveslives.phpThomas Anthony, PE
Structures Specialist PA-TF1 & PA-ST1
Paramedic / Rescue Tech North Huntington Twp EMS
The artist formerly known as Captain 10-2
No, I am not a water rescue technician, but I stayed in a Holiday Inn Express last night.
12-27-2005, 07:06 PM #9
- Join Date
- Feb 2005
- Ft Worth, Tx
I studyed these out when I was working on the class I am putting on the web site. I agree with some of them. my concern is most FF are not trained in which ones are which and we train them to do ventalation and the first thing they want to do is run for the roof.
12-27-2005, 07:38 PM #10
There is much debate over whether fire immediately weakens or loosens the connecting gusset plates. Some researchers [Dunn 2001; Brannigan 1999] contend that these metal gusset plates can contribute to the degradation of wooden truss members through pyrolysis. Heat transferred through the metal fastener's teeth may destroy the wooden fibers held in tension by the gripping action of the metal teeth. This process loosens the plate and leads to a weakened truss and possible catastrophic failure if the gusset plate falls away and allows the weakened truss to pull apart. Other researchers [Grundahl 2003a; Meeks 2001; Cutter 1990] suggest that the metal plates protect the underlying wood during the initial stages of a fire. They suggest that the wooden members between truss joints may burn before the areas underneath the metal plates. The unprotected areas become charred to a depth that reduces the strength of the wooden member. Eventually, as the fire progresses, wood charring takes place underneath the metal connector plate. This causes the load-carrying capacity of the metal-plate-connected joint to be reduced. This reduction in the joint capacity eventually causes the metal connector plate to pu ll out and the joint to fail.
Lightweight wooden trusses are prefabricated at a factory and shipped to the construction site. If these trusses are improperly transported or stored at the site (exposed to the elements), or if they are dropped or handled improperly, the gusset plate or the entire truss can be significantly damaged. This can cause the plate to pull away from the wood surface or become weakened or loosened. In such cases, where the truss has not been properly repaired, the truss is weakened before installation and could fail under fire conditions much sooner than normally expected. Unexpected failure caused by mishandling is not unique to trusses and is difficult or impossible to predict during initial size-up.
FROM HOUSTON FIRE DEPT
The Houston Fire Department became involved with the WTCA shortly after the loss of Firefighters Lewis Mayo and Kim Smith. HFD set the goal of educating firefighters in wood truss construction and WTCA has offered its extensive information base along with technical assistance for this series of training videos. Getting first hand information on wood truss construction to the firefighters is the key to improving the knowledge base.
Also known as gusset plates or gang nails, metal connector plates have been improved significantly over the years. Plates have been tested with longer spikes, but were returned to the original length because too many of the spikes were being bent instead of being imbedded into the wood. Like some of the nails now used, the gusset plate spikes have a spiral twist for more gripping strength and are less likely to pop out.
Today's plates are thoroughly tested and have a design value of approximately 200 psi. This means a plate that is 3" x 4" applied to both sides of a web joint will have a working load of 2400 pounds and a safety design load of 7680 pounds. For comparison, one 16 penny nail has a load capacity of 100 pounds and would require 24 nails in the same 3" x 4" area to have the same strength capacity
The correct plate size is calculated in the truss design phase and is specified on the shop drawing. Assembly crews are required to use the specified plate or to use the next larger size if the specified size is not available. This is also done if a particular truss is known to have strength problems during shipping even though it performs well on the structure.
Connector plates in the 50's and 60's were flat plates that were bolted or nailed into place. Today's plates are pressed with modern machinery that press the plates. The two types of presses are roller presses and flat platen presses. The plates are tacked into place with a hammer and then pressed with over 1000 psi to ensure proper embedment. 1/32nd of an inch is the maximum allowed gap between board and plate. 1/8th of an inch is the maximum allowed distance between web members.
AND .... FROM THE MEMPHIS 7 MINUTE COLLAPSE
'The roof collapse can be directly attributed to failure of the top connection points of the trusses, where the kingpost meets the top chord sections. It cannot be determined precisely which truss failed first and whether the gusset plates came loose from the top connection or the kingpost snapped and pried the gusset plates loose. The top connection is a critical point in each truss and its failure would result in the failure of that truss. It appears that several trusses failed in rapid succession -almost simultaneously following the first. All of the trusses would have been close to the point of failure and unable to support any additional load, so the load shift that occurred with the first failure probably caused several more trusses to collapse' ....
12-29-2005, 04:23 PM #11
- Join Date
- Feb 2005
- Ft Worth, Tx
Thanks Paul You just almost thaught my class word for word, This same domino effect is what happened to the church in 1999 when we lost three men. I have studyed these and a lot more cases. My big concern is to get everyone knowedgible about what to look for before tacling the job. With our ventilation training, so many want to just run and do it, not knowing what they are dealing with. with proper training, these would not be near as dangerious as they are.
I have a lot of this on my web site now, and am working on a full class that will be finished soon.
Last edited by LeeJunkins; 12-29-2005 at 04:26 PM.
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