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USFEngineer (Structural) (OP)
11 Sep 07 12:05
I was wondering if anyone can lead me in the right direction for some guidleines for structural condition asessment of fire damaged pre-engineered wood roof trusses? I need to either recommend a repair or replacement of several roof trusses in an apartment building. Some sections of the bottom chords and webs were completely burned out and the trusses are currently temporarily scabbed. With that being said, there was no damage at all to the top chords or to the roof sheathing just some minor smoke damage. Otherwise the fire was pretty much contained to a small area maybe 10'-0" x 12'-0".

Any help would be appreciated. Thanks
jike (Structural)
11 Sep 07 12:25
Contact the truss manufacturer for a repair scheme.
USFEngineer (Structural) (OP)
11 Sep 07 12:55
That was my initial thought as well. However, the building was constructed in the late 80's and unfortunately the truss manufacturer is out of business. That would be the second easiest option. The first being to replace the truss with new ones. But like I mentioned earlier, no damage took place to the roof sheathing so replacing the truss would be expensive and very time consuming.
msquared48 (Structural)
11 Sep 07 13:03
Could you convert the trusses to a collar tie format with a double chord at the bottom and new webs as necessary?  Might be able to bet Microlams long enough for the collar ties.  Connection at the end would be the most critical.  Upgrace the top chord - sister on - as necessary to make the span work.  If could make the collar tie arrangement work, could forget about the web members.

Mike McCann
McCann Engineering

Helpful Member!  twinnell (Structural)
11 Sep 07 13:22
We have done this type of survey and repair a few times in the past.  It seems convienence stores catch fire alot.

Anyway, the basic assesment is how much good member area is left, not counting the char.  We would scrape off the char at the worst location and measure the dimensions. then analyse the truss with the new member dimensions.  If it fails, then provide a fix.  Usually it was scabbing over the burnt section of the member,  no need to cut it out unless you want to, with plywood splice plates, 2x splice plates, or sometimes steel plate.  We didn't use any references except for NDS.  You know how much area you need, you know how much remains, provide the difference.
MiketheEngineer (Structural)
11 Sep 07 13:23
Could you install "sister" trusses??

Might use a two piece truss with field splices designed in.
USFEngineer (Structural) (OP)
11 Sep 07 14:04
twinnell,

I had a similiar thought. Possibly shoring the damaged trusses (possibly jacking up the top chords to relieve stresses and eliminate weight) cutting away all burnt members and replacing with new 2x4's (bottom chord and some webs) and providing plywood gusset plates nailed and glued.

The actual linear feet of damage is very little, but in the areas that are burnt the entire section of bottom chord and webs (2), particularly at the panel point are gone. It appears that the area smoldered for sometime. Maybe not massive flames. It appears very isolated to these areas. For example the trusses are 32'-0" long (clearspan) and a 3'-0" section of bottom chord is completely gone. It is weird I expected more charring but as I mentioned, if it burned its gone.

I would rather remove any burnt members and replace them.

Seem logical?
sundale (Structural)
11 Sep 07 14:21
Don't hang your license on someone else's pre-engineered trusses using an unknown proprietary metal tooth connector at the panel points.

I would not think twice about telling the owner's insurance company to replace these two trusses.

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