Post-fire framing evalutation

[a7x1984]

We perform a ton of post-fire reconstruction and evaluation (mostly conventional framing) - I am sure a lot of people here have performed them frequently, as well.

After a few hundred of evaluations, I have begun to question our techniques in our evaluation/replacement/strengthening process. It has been quite simple so far - if wood is found charred, it is replaced in some manner and move on.

Some research I performed 'clued me in' that something *may* be missing from some of our more critical evaluations. The replacement of charred members is fine and dandy, but what about adjacent members who's temperatures did not reach the combustion point, but were potentially subjected to some significantly elevated temperatures for the 1 hour or more of exposure time. We never perform reduced capacity analysis of any remaining uncharred members.

My research has confirmed the NDS justification for a permanent linear reduction in MOR and MOE up to and including 150 deg.F. That same research mentioned after 150 def.F (up to combustion) the reduction of MOE & MOR varies non-linearly with increase of temp (amongst a host of other variables e.g. species, MC, exposure time, etc)- not surprising at all.

Understanding the mechanics of fire and impact to material and structural stability is covered by entire courses and degrees, yet all of us are called upon to make these assessments in a timely manner. My question is in the neighborhood of how others are performing their post-fire assessments, and to what degree in depth.

unrelated: I love that there are people in this forum with 30+ years of experience - your comments and recommendations are invaluable.




In Russia building design you!

 

[SLTA]

I definitely don't have the amount of experience you have with fire damaged structures, but I usually replace at least one member further out from the clearly damaged wood. At times I recommend replacing an entire roof system, if it's just too messy to get a partial replacement to tie in well with the other. I also stab at the wood around with my awl, and if it's remotely brittle or seems "off", I replace it, too. Not particularly analytical, but engineering judgement is critical, too.

 

[a7x1984]

Thanks, slta. Partial replacement can definitely be impractical (and risky) - trusses for example. I agree that the engineering judgement is critical, especially seeing as there is rarely (I want to say never) money for testing. I think the practical answer lies somewhere between developing a better intuition for the nature of fire and the material response.

For instance, determining the temperature gradient around different types of fires. It seems reasonable to want to know the extend of temperatures at or over some threshold that would induce permanent reductions of wood properties. I had thought about developing my own strength reduction factors for adjacent, uncharred members, based on as much research as I could find. Maybe something like this already exists? I have to do some digging...

In Russia building design you!

 

[dik]

There have been some threads related to fire damage... might want to do a site search... I'll see if I can dig up a couple...

Dik

 

[dik]

thread1066-255252

as one example, Dik

 

[a7x1984]

Thanks, dik!

In Russia building design you!

 

[dik]

and another that has another link...

thread337-309234

Dik

 

[RFreund]

This is always an interesting question. And it seems that your current practice along with what slta has mentioned is what I typical hear/see. However after doing several hundred I think you/your company is a worthy expert, so I ask you, have you had any complaints comeback?

For members not charred it usually seems that the hard questions are - how hot did it get and for how long.

EIT

http://www.HowToEngineer.com

 

[dik]

With your background of numerous investigations, I think there is sufficient information in the threads noted to help you on your way.

The problem that I generally wrestle with is that a building subjected to fire damage is not improved and has suffered some damage. Other than an initial improvement in some properties, albeit, small. The structure has been transformed. Anything more than a slight 'toasting' and the structure can be significantly diminished, and more prone to future damage in the event of a fire... the homeowner has 'lost' something in the transformation.

Dik

 

[TomDOT]

Interestingly, the Jersey Lilly saloon burned in 1897 and the remaining timbers were re-used in the rebuild (No trees nearby.) It is little changed or repaired since then and is still standing today. If you poke your head through the trap door over the bar (bring a flashlight) you can still see the charred timbers. Roof felt pretty sturdy when I walked around on it in 2007.

Of course, there's not much ELSE in Langtry, TX (population is around 20) - so you may as well have a look in the visitors center and Jersey Lilly if you are taking US 90.

 

[dik]

Thanks Tom... if I'm ever near there...

The large timbers likely formed a 'char surface' that somewhat insulates and protects the inner material. If the duration of burn and the heat generated wasn't too great, it may have even enhanced the strength of the material a tad.

In general, the new (burned) beams are not as good as the original (non-burned) ones... The timbers may be 'huge' by today's standards... I'm glad they have been reused... adds to the history of the establishment...

Dik

 

[a7x1984]

Thanks, guys!


In Russia building design you!

 

[TomDOT]

Since this thread popped back up again - I probably gave the impression of something larger than is out there with the term "timbers" - from my recollection of 2007, the "timbers" weren't all that large.