Burned wood capacity check

[gasma1975]

Hi,

I have to verify a wood structure that was affected by fire. I was wondering if there was a normalized code verification. How do you evaluate the resistance of a wood section affected by fire. Do you just remove a the burned layer and re evaluate the good wood section ?

Any experience to share ?

Thank you

Gasma1975

 

[bridgebuster]

I've never dealt with this but I have two articles on the subject

 

[bridgebuster]

here's the other

 

[IsaacStructural]

I believe the general method is to do like you've described, remove the char and evaluate at the new minimum section. The Timber Construction manual has a section on this.

Wood will ignite and burn at temperatures above approximately 500deg F. The residue create by the combustion of wood is referred to as Char. As wood burns, char develops at a rate of approximately 1.5inches per hour. The growing char layer acts as an insulator, greatly reducing the temperature of the underlying wood surface. A typical building fire will reach temperatures of 1290deg F to 1650deg F, but the interface between the char and the wood will be reduced to approximately 550deg F. Due to the insulating nature of the wood itself, the temperature drops to 360deg F at a distance of 1/4in. ahead of the char front and drops to 210deg F at 1/2in ahead of the char front. The core of the timber remains relatively cool and maintains its ability to carry loads. The capacity of the member is reduced only as the outer layer of material is lost due to the charing. Consequently, large timbers perform well in fires.

M.S. Structural Engineering
Licensed Structural Engineer and Licensed Professional Engineer (Illinois)

 

[racookpe1978]

On the other hand, the heavy wooden beam tower foundations above the top of the East caisson of the Brooklyn Bridge caught fire, burned for a long time under the high-pressure atmosphere in the caisson, and burned a great deal and a long ways down along the insides of the beams.

Caused a LOT of problems digging out the "coal and charcoal" from the wood. Biook says they refilled the holes with concrete and grout.

Bottom line? Check very carefully you have found and excavated ALL of the charred volume out - not only on the surface but down inside the beams.

 

[dik]

What is the floor system... I joists or dimensioned lumber? Depending on the extent and duration of the fire, serious damage may have occurred due to the destructive distallation of the wood products. Do a search on eng-tips for fire and wood... There have been several threads.

Dik

 

[gasma1975]

The floor is made of [2x12] @ 12" o/c, What happened is that the contractor has just replaced the elements which were heavily damaged and for the rest he sprayed an inflammable paint covering some burned elements very hard to identify. When a city inspector saw this he stopped the whole worked and asked for an engineer. See the picture attached, I think it is much better just to add a new joist in parallel rather than repairing it.

 

[RFreund]

yikes, those look pretty charred.

I have asked this question many times in the past and it seems like for light framed members (i.e. 2x's) there is not much information and/or testing. Why? Well I believe this is because once there is char on a 2x member there is basically nothing left. If it is only smoke damage than that may be a different story.

Response I received from AWC (American Wood Concuil aka the NDS people):

I forwarded your request to our fire technologist. Here's his response:

Lets assume light frame to be minimum dimension of 1.5"
If there is any sign of surface charing, the guide is to remove 1/3" for calculation purposes. Assuming 1/16" char layer we are down to 1.1" (1.5"-1/3" -1/16")
The structural properties of 1.5" vs 1.1" are likely too far apart to recommend anything other than replacement. Besides 1.5" material has a higher tendency to get exposed from both sides. Then we are down to 3/4" instead of 1.5"

Proposed recommendation: If there are signs of char on 1.5" material it should be replaced.

For SCL type products (LVL, etc) typical response:

Ryan:

Per our phone conversation, if the product only exhibits smoke damage then the product hasn't been exposed to extreme heat and the TJI joists and engineered wood products we manufacture will retain their structural properties per our ICC evaluation/code reports.

If there is any sign of char on the product, then it has seen extreme temperatures and the product should be replaced.

EIT

http://www.HowToEngineer.com

 

[dik]

They're pretty charred... I'd sister them.

The coating is to encapsulate the smell of the burned wood... Also, there are specialised contractors that deal with fire damage. It just takes a small area not treated to provide a burned smell and there is specialised treatment that is continually improving and a contractor should be up on these products.

Dik

 

[BridgeFixer]

I would agree with dik. Don't spend the time crunching the numbers for the burnt sections. Sister the three that suffered section loss due to fire. This should eliminate the possibility of problems with serviceability and ultimate loading. It shouldn't be that difficult for the contractor, although the photo does not show the situation on the opposite end of the joist.