Wood Joist Failure Help 1

[bookowski]

I am looking at a wood roof joist that has failed. The failure looks peculiar, any input would be appreciated.

Here is the situation - This is an old flat roof (~100yrs) over a one story commercial kitchen. There is a large framed opening in the roof, approx. 8'x8', for numerous kitchen hoods and vents etc. The joist in question is a trimmer that frames the opening. To begin with the joist is very undersized as is typical for this type of building, in addition the header is framed into the trimmer with a mortise and tenon joint.

See attached photo. The joist failed in a very clean/brittle manner. The wood around and inside the failure is hard - extremely hard in fact. In the picture you can see that it is black, the owner is saying that this is from smoke in the kitchen but I don't think that's what it is. There is no water or fire damage. There is no splitting of fibers - just a very brittle hard break. (ignore the other framing in the picture, that is ceiling framing)

After this joist failed the adjacent joists partially failed and those look more typical - splitting fibers and clear wood.

I have read that long term exposure to heat can cause a brash failure in joists, could that be what caused this? The joist is directly next to kitchen hoods and vents. What about the blackness - is this some type of fungi that can also cause brittleness? The failure occurred approximately one month ago but this picture is from today.

Any thoughts are appreciated.

 

[bookowski]

I found this in an old USDA paper on wood failures. They are referring to 'extremely high temperatures' though, I doubt this was experiencing temps that high.

"When abrupt fractures occur in wood as a result of exposure to extremely high temperatures it becomes darkened throughout so much and its odor changes to such an extent that the cause of brashness can be readily recognized"

 

[Ron]

Looks like heat/fire damage. Does not appear to be fungal.


dik? What do you think?

 

[dik]

I was going to say the same...

A more distant photo might better show the overall collapse or failure.

I couldn't tell from the photo if the darkened wood was due to heat/rot. It would be a simple matter of taking a small sample and confirming the damage. It did not, however, appear to be from a rot condition.

It is possible that the prolonged heat may have damaged or deteriorated the cellulose fibres leaving the lignin which tends to be darker or possibly oxidising/discolouring some of the extractives... I couldn't tell. A microscopic examination should likely clear up the issue of the cause... check for burn, dirt particulate, cell/structure damage, mold/mildew/fungi etc...

A couple of items about wood... wood that is 100 years old can be extremely 'brittle'. There is little ductility to it and the ability to absorb energy diminishes dramatically (a measure of the area under the stress-strain curve).

Bookowski: Extremely high temperature doesn't have to occur; damage can occur with prolonged heat at a relatively low temperatures.

Prolonged heating (less than a couple of hundred degrees Fahrenheit) can cause a fractional distillation of some of the wood constituents. Many components of wood are made up of water and carbon dioxide based compounds. Moderately low heat can alter the structure of the wood fibres and varies with the species.

Dik

 

[dik]

Forgot to add that there are a couple of recent posts regarding fire damage that have some good links...

Dik

 

[woodman88]

Here is some information (chapter 18 of the Wood Handbook, you can get a free pdf of the book from af&pa website)on Thermal Degradation of Wood. See page 18-8.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[bookowski]

Thanks dik and woodman, very helpful. I had never seen anything like that.

There was no evidence of rot or water damage and no evidence of a fire. It seems like the age combined with a very long term exposure to high heat was enough.

 

[dik]

woodsman88... can you post a link to the page with the entire book? I cannot locate it on the site.

Dik

 

[woodman88]

Sorry they moved it. Here is a link to the book.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[MiketheEngineer]

When wood is exposed to over 150F degrees for long periods of time (and I don't know the definition of "long") - it will degrade and behave like what you are experiencing. It gets HARD and BRITTLE!!.

Replace it - with STEEL!!

 

[dik]

Thanks, woodsman...

Dik