In-place retrofit/reinforcement for sawn lumber DFL roof beam failed in horizontal shear?

[Engineerataltitude]

I am working with a condo project located in CA in the Sierra that has seen huge roof snow overload conditions this winter. Roof was designed for 135 psf of snow (about 5.4 ft of snow) and had 12 ft of snow on it when the beam failure was noticed. The 8x14 roof beam has a longitudinal crack at the neutral axis of the section almost the full length of the beam. Fairly deep too. Not all the way through but almost. No obvious problems with mid-span bending failure, just horizontal shear.

I'm trying to work out some kind of minimalistic way to retrofit vertical shear reinforcement (something like the shear ties in a concrete beam) into this wood beam.

Is that a realistic approach? Any ideas? Any products?

 

[JAE]

I had a glue lam beam system that framed an outdoor entrance canopy that was damaged by a truck hitting it (truck too tall and canopy too low).

There were horizontal cracks in the beam similar to what you describe. The solution we came up with was to drill a series of vertical lag screws up into the bottom of the beam such that the lag would extend past the crack and take the horizontal shear force in the beam. Sort of a set of internal pins tying the top and bottom sections together internally.

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[dik]

Can you use epoxy or polyurethane injection?

Dik

 

[Engineerataltitude]

The long lags are a good idea.

I have access to use epoxy or polyurethane. Are there products with strength ratings?

 

[Ingenuity]

I have access to use epoxy or polyurethane. Are there products with strength ratings?

I have used epoxy injection on past projects to provide structural 'crack' repair to wood beams similar to what you would typically do with concrete crack repairs. A low viscosity resin of less than 500 cps would fill fine cracks. If the crack width is larger than 1/8" you may consider a paste resin. Both would provide similar structural bond, tensile and compressive strengths.

Lags screws, as JAE stated, coupled with epoxy injection, would be a 'belt and suspenders' approach.

Polyurethane has no structural strength - mainly used to seal cracks from water ingress etc.

 

[jdgengineer]

Just remember whatever you do you've bought the beam so make sure it's equal or better than what it was originally designed for.

 

[XR250]

Wouldn't it make more sense to replace the beam with one capable of carrying 12 ft. of snow?

 

[KootK]

I like wood screws installed at a 45 degree angle such that the shear loads them in tension.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Sawbux]

What is the span of the beam? It is unusual for a beam sized for bending to fail in horizontal shear before the bending stress reaches the failure point. Typically shear governs on short beams carrying a large point load.

Did you calculate the snow load required to fail the beam in shear and bending - which one governs?

The 8x14 is solid sawn - right? It may have had a seasoning check pre-existing which reduced it's capacity in H shear.

I agree with the repair methods with lags & epoxy, but you need to verify the capacity of the beam and reinforce it if it is marginal. Shouldn't get much resistance from owner, since it "failed" before.

 

[Triangled]

agree with every line of Sawbux
Feet of snow is not too reliable a measure of weight.

 

[Engineerataltitude]

I was also surprised to see the horizontal shear failure without bending failure in a roof overload condition. It was beam that had checking cracks in it prior to this winter, so it was prime crack propagation site.

It is an exposed interior beam by the way, so the fix needs to look good too.

I have not had a chance to calc any of the loads in the beam yet. I am triaging distressed structures all over town right now for temporary (get through the end of winter) shoring. This one is a bit lower in urgency than some others but I wanted to research options before I spec'd a fix. We put a temporary shoring post under the beam mid-span and removed the snow off the roof, so I bought myself some time. I am looking for the most effective, least intrusive long term fix for this. You guys are always a good resource on what has worked well in real life.

I have an email in to 3M's technical staff for a high strength wood-to-wood structural bonding agent recommendation. I thought I'd calc the shear flow value and try to find a bonding agent with enough margin to work for the loading condition. I am also researching Fastenmaster, that has some good long lag-style woodscrews that might work. Their shear values are higher than regular lagscrews. Once I calc the shear loads however I might have to go to big thru bolts if the loads are high enough.

Thanks for the help!

 

[bhiggins]

Have you thought about replacing the 8x14 beam with a 4-ply 14" LVL beam? Fv values are 285 PSI, much better than DFL.

 

[JAE]

On our lag screw fix we overdrilled the area where the lag head and washer ended up so we could put a wood plug in over them in an attempt to hide the fix from below.



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[Engineerataltitude]

I don't usually like the aesthetics of an LVL or parallam in interior exposed beam conditions, especially since in this case there are other exposed 8x14 beams in the rest of the roof in the open room and a new LVL wouldn't look very good. I'm trying hard to find a fix that will be safe and aesthetically pleasing as well. Trying to fix this beam in place is my top choice since it already matches the others in the roof. If possible.

 

[XR250]

Seems like all of them should be reinforced to handle today's non-standard climate change induced snow loads.
What if this happens again and one collapses and kills some people? I think the lawyers would have a field day with it if there is already evidence that you knew they saw a snow load in excess of their design capacity (if I am reading your original post correctly)

 

[Engineerataltitude]

You are absolutely right about designing the fix to a much higher snow load than before. But I'll do it because its the right design thing to do, not because I'm worried about lawyers.

 

[Tmoose]

http://www.ftet.biz/index.php?action=resources.wp01

I was thinking epoxy, then clamped, then angled lags added, plus some oak dowels epoxied in bored horizontal holes, oriented similar to the keys shown in the link above along the bonded split.

 

[Tmoose]

Plus a lovely wrought iron bar truss with cast iron stanchions and fittings added to the underside, like an old trolley car.

https://4.bp.blogspot.com/-o7UBnJUh2Vk/TzwPuAWZGGI/AAAAAAAABMA/ja0MOJq0FSY/s1600/blog-toledo.JPG

 

[Triangled]

curious as to the area/elevation you're project is in as I have hiked all over the Sierras

 

[Triangled]

we've done similar to JAE.... and (obviously) trying to reduce/eliminate horizontal shear reinforcements in midspan region so as to avoid reducing moment capacity