Delaminating Glulam Beam Repair 1

[Sam165]

Working on a rehab project of a four story multi-family apartment building. The front balcony beam (running parallel to railing) was water damaged to the point of needing to be replaced. Contractor replaced the beam per my recommendation and I was out there doing a QA this morning. While doing my QA I noticed the perpendicular glulam beam that face hangers onto the replacement beam is delaminating at the bottom plies and has a diagonal crack propagating from the over-fastened replacement hanger. The beam is a 3.5”x9.5” Anthony Power Beam that spans 7’-0”. It is supporting perpendicular top chord bearing floor trusses that span 9’ on either end. Live load is 100psf and dead load is 40psf.

I’m thinking of specifying lag screws that are driven from the bottom of the beam to fasten the plies back together. I’m not too worried about the diagonal crack. To figure out the required spacing of the screws, I calculated the max horizontal shear by determining the max bending stress and resolved that into a force couple. I feel like I need to take only a portion of the bending stress that is being resisted by the bottom plies (delaminating plies) but I wanted to get some input from you on how best to proceed. Thanks in advance!

Left face of beam looking into floor cavity.

Left face of beam:

Right face of beam:

 

[msquared48]

Is this a delamination, or an extra 2x?
Is this a transfer beam critical in shear?

Could add LVL’s to each side and thru-bolt...

Mike McCann, PE, SE (WA, HI)

 

[Sam165]

It is very surprisingly a delamination. At first glance, I thought they had used a 2x filler plate at the bottom but that’s not the case.

I’m not sure what you mean by transfer beam critical in shear? It’s supporting perpendicular floor trusses on both sides that span the balcony width.

 

[jayrod12]

There are methods to repair using dowel type fasteners (screws or gluing in FRP rods). They should also epoxy the gap after installation of the fasteners.

Are you confident in the beam capacity? I recently had to look at a project with Glulam beams from pre-1970. As it turns out, most glulam used in those days is undersized by up to 20%. There was an AITC technical note released about this.

In my situation, the costs to repair/reinforce all of the beams with this issue is likely to outweigh the costs of peeling the entire roof off and replacing it in full.

 

[Sam165]

Yes, it’s at 60% of allowable moment and 69% of allowable shear. I’m pretty confident that lag screws driven from the bottom to fasten the plies back together will suffice. I’m getting around 12.2 kips for the resultant tension force resolved from bending stress. I’m thinking I’ll take half of that force since I’m only looking at fastening the bottom ply and not the whole beam.

 

[jdcollins]

Sam, I don't think I agree with your approach on the calculations for the fasteners. The intent of the fasteners is to resist the slip between plies (as if you were adding a ply at the bottom to increase an existing beam depth). That force is a function of your shear, not your bending moment.

 

[molibden]

I would use diagonal full thread screws to reinforce for shear.

 

[STrctPono]

I have actually gone through the calculations for what it would take to laminate boards of wood together as a beam to justify them as composite using fasteners (screws or nails). The number of screws/nails you would need to resist the horizontal shear is through the roof. An insane amount that would not be practical. My takeaway is that you really can't justify it on paper but that the values provided in Table 11 in the NDS are ridiculously low. 141 lbs of shear capacity (Z) from a 16d nail attaching 2 pieces of douglas fir together....

 

[STrctPono]

Shouldn't you be bringing this issue up with the glulam manufacturer? If properly designed, how could this have happened?

 

[molibden]

Is there something hanged from bottom that would cause tension perp. to grain?

 

[jayrod12]

You need to design it for the shear flow at that ply. Not the total tension force expected in the moment calculation.

 

[dik]

Can you use lag screws to pull the plies together and use epoxy injection or Resorcinol adhesive to effect the fix? It will be difficult to transfer the shear with the lag screws.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?
-Dik

 

[Compositepro]

An adhesive bond to wood with any decent adhesive (epoxy is best) is much stronger than the wood. Fasteners are mainly for clamping the bonded joint. Water based adhesives (white glue) shrink a great deal and are not for gap filling.

 

[dik]

Some epoxies creep... I would not use a PVC PVA or cross-linked PVC PVA(white or yellow) glue... they also creep and are not water resistant.

A neat feature about white glues is that you can use them for gluing veneers... like contact cement... let them dry and iron them on... less sensitive to intial fitup as with contact cement... my brother showed me that about 30 years back.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?
-Dik

 

[STrctPono]

dik,

Do you mean PVA glue? It's pretty well established that you should not use PVA glue for a structural member..... with that being said I have built glulam beams before using PVA glue for my own home. I would never specify or recommend to anyone else to do so. PVA glue wouldn't work well in this scenario anyway since there appears to be a very large gap at the delam point.

Polyurethanes also wouldn't work due to the surface prep requirements and the fact that those also need a tight fit. I had read awhile back that polyurethane glues behave very poorly in shear. Can someone validate this understanding of mine?

I think injecting an epoxy into that crack is your best bet (choosing the right epoxy will require some homework as I don't have a brand off the top of my head that would be appropriate. Not sure if you're going to find any manufacturer, however, that will condone their product for use in this way.

Again, I would be in discussion with the glulam manufacturer looking for a way to resolve this is it sounds like this beam was specified by the OP and is fairly new.

 

[Sam165]

The beam in question is not new. The one it is hangering off of is (perpendicular to it). I was able to discuss this with the manufacturer today and they said that this type of delamination is very unusual. They recommended screwing the bottom ply back to the beam with SDS screws and adhesive.

I'm not too worried about it because the ply is only 1-3/8" of the total 9-1/2" depth and the beam works without that ply for the loads imposed. I'm going to specify 5/8"x6" hex head lag screws that are driven from the bottom of the beam at 6" o.c. staggered. I will also be specifying a construction adhesive to fill in all gaps.

As far as the calculation goes, I agree with STrctPono and dik, there is no way you are able to resist the horizontal shear with any dowel type fastener. It just doesn't calc out based off my calculations.

jdcollins and jayrod12, I agree with you in regards to it being a function of shear, however, it is not a function of vertical shear but horizontal shear. Thus, it is tied into the bending stress that is resisted by the beam. How would you go about calculating the shear flow at that particular ply?

Thanks for all the responses so far!

 

[dik]

sorry...pva. thanks...corrected... Resorcinals work well, too... but are not gap filling.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?
-Dik

 

[3DDave]

There isn't a shear load in that gap now, where will it come from? It looks like there was a ton of toe-nailing which may have split the ends before being pulled out and putting in the hanger.

 

[STrctPono]

Sam165, Did they advise you on a recommended adhesive?

however, it is not a function of vertical shear but horizontal shear. Thus, it is tied into the bending stress that is resisted by the beam. How would you go about calculating the shear flow at that particular ply?

...tied into the bending stress as in it is directly related to it as its derivative, thus it is related to the shear. Remember Mohr's circle? Horizontal shear stress (shear flow) is the same as the vertical shear stress. f = VQ/I

 

[dik]

3DD... the shear load comes from the load applied to the beam and at the end is equivalent to the reaction. If you have a notepad and flex it, the pages slide against each other... if you have a 'cardboard stick' the same thickness, it resists flexing. If a load is applied, the lack of continuity can precipitate failure... that's why it's good to have continuity... often works without it... seen many timberframed structures with significant checks...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?
-Dik