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Delaminating Glulam Beam Repair

Delaminating Glulam Beam Repair

Delaminating Glulam Beam Repair

(OP)
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:

RE: Delaminating Glulam Beam Repair

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)


RE: Delaminating Glulam Beam Repair

(OP)
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.

RE: Delaminating Glulam Beam Repair

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.

RE: Delaminating Glulam Beam Repair

(OP)
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.

RE: Delaminating Glulam Beam Repair

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.

RE: Delaminating Glulam Beam Repair

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

RE: Delaminating Glulam Beam Repair

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....

RE: Delaminating Glulam Beam Repair

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

RE: Delaminating Glulam Beam Repair

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

RE: Delaminating Glulam Beam Repair

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

RE: Delaminating Glulam Beam Repair

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

RE: Delaminating Glulam Beam Repair

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.

RE: Delaminating Glulam Beam Repair

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

RE: Delaminating Glulam Beam Repair

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.

RE: Delaminating Glulam Beam Repair

(OP)
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!



RE: Delaminating Glulam Beam Repair

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

RE: Delaminating Glulam Beam Repair

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.

RE: Delaminating Glulam Beam Repair

Sam165, Did they advise you on a recommended adhesive?

Quote (Sam165)

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

RE: Delaminating Glulam Beam Repair

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

RE: Delaminating Glulam Beam Repair

That happens when there is deflection - the deflection is done. It's deflected. So gluing an additional item won't change that. I didn't see anyone saying they would first jack all the load off the beam to allow it to be undeflected before any of the repairs, so gluing a gap that currently carries no shear won't magically make it carry shear.

RE: Delaminating Glulam Beam Repair

3D, your assumption is based on the premise that this member is already carrying its maximum load. Live load is transient and can come in at any time.

You are correct, the proper thing would be to jack it, fix it, and then release it. However, this is probably not practical. Easier way is for the OP to see if the current condition meets dead load only and then check repaired condition for full Strength Level loads.

RE: Delaminating Glulam Beam Repair

3DD... with old structures jacking is nearly pointless... the deformed wood acts as a flat arch and the in plane resistance is generally very high.

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

RE: Delaminating Glulam Beam Repair

(OP)
STrctPono, yes of course, good ol’ Mohr’s circle! I definitely need to dust off my mechanics of materials book to refresh my memory!

RE: Delaminating Glulam Beam Repair

Shear flow is a function of bending. But you aren't designing the connection for the maximum tension force from bending, but rather a portion of it. It is the horizontal shear stress at the top of that ply that you need to design the fasteners for.

Can I suggest using SDS screws or GRK screws instead of lag screws? There's no pre-drilling required and are much nicer to work with. Your contractor will thank you.

RE: Delaminating Glulam Beam Repair

Just curious. Can you place the metal truss connecting plate on both faces of the beam, then glue the crack?

RE: Delaminating Glulam Beam Repair

I think your solution is perfectly reasonable. A little conservative probably. If you wanted to reduce the spacing of your repair screws, then you might calculate the "shear floor" at that location and design the screw spacing to resist that force. That's the "theoretically correct" way to determine the required shear. But, your method works and is conservative.

I like the manufacturers suggesting of combining this with adhesive. Though I'm not sure how to spec that out. If there is a "deflection" issue already, then you would want to consider jacking up the beam first. In fact that's not a bad suggestion if you're going to be adding additional loading to the beam as well.

RE: Delaminating Glulam Beam Repair

Quote (going to specify 5/8"x6" hex head lag screws that are driven from the bottom of the beam at 6" o.c. staggered)


Make sure you use a clutch that prevents overdriving... on one of the Mike Holmes show episodes... his worker was installing lag screws with a hammer...

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

RE: Delaminating Glulam Beam Repair

When jacking up the beam is considered, I think the beam should be lifted rather than jacking from below, which would close up the gap before applying adhesive, or squeeze the adhesive out after application, and create uneven bond along the beam.

RE: Delaminating Glulam Beam Repair

Speaking of a hammer, what about glulam rivets? Usually they are used with a steel plate, but in this case they would be connecting wood to wood.

Whatever method is used, it is worthwhile to jack first to relieve beam stresses, then slowly release the jacks after repairs are complete.

VQ/Ib is the theoretical shear stress at any point in the span. For uniform load, the spacing of fasteners should be variable throughout the span because shear varies linearly from a maximum at the end to zero at midspan..

The total shear force required between the bottom two laminations in each half of the beam is fb*A where fb is the average fibre stress of the bottom lamination at midspan and A is its area. For uniform load, it varies linearly as mentioned above.

BA

RE: Delaminating Glulam Beam Repair

Quote (r13)

When jacking up the beam is considered, I think the beam should be lifted rather than jacking from below, which would close up the gap before applying adhesive, or squeeze the adhesive out after application, and create uneven bond along the beam.

I don't believe that is practical. Better to inject the adhesive, then jack to close the gap, then add fasteners.

BA

RE: Delaminating Glulam Beam Repair

Quote (then jack to close the gap)


subject to the caveat that jacking up may not close the gap, where actual fasteners may...

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

RE: Delaminating Glulam Beam Repair

@dik,

Well, I thought about using an adhesive, but rejected it because of the time required to install the fasteners. It would be nice to have adhesive and screws, but I don't know if the contractor could install all of the fasteners before the adhesive set. Maybe it would be better to forget about adhesive.

BA

RE: Delaminating Glulam Beam Repair

BA,

The lift idea was a steal from external post tensioning. Yet it is easy said than done, as it requires the setup of good lifting points. Jacks should be designed to press the wood uniformly, but not push too hard. Just thinking.

RE: Delaminating Glulam Beam Repair

If closing the gap is the question, then that is best achieved by placing intermittent pipe clamps from top to bottom of the beam just after applying the adhesive. Will close the gap and squeeze out the excess adhesive.

RE: Delaminating Glulam Beam Repair

BART... most fasteners do not develop sufficient shear transfer... that's why I like glue...

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

RE: Delaminating Glulam Beam Repair

I like glue too, dik, except when I don't. Glulam beams have generally performed well over the years without the need for screws or other devices to hold the laminations together. In some cases, finger joints were used whereby a lamination was cut on a shallow angle and spliced somewhere in the span in order to save material.

We don't know why the bottom lamination separated from the rest. The most likely cause was that it had insufficient pressure during the period the glue was curing, but it is also possible there was something wrong with the glue. If the adjoining surfaces are coated with the old glue, the new adhesive may not bond with the wood but rather with the old glue which has already shown itself to be unreliable.

If glue is to be used, it should not be relied upon to carry horizontal shear, which is largest in the outer laminations because they are the most highly stressed in bending.

EDIT: After looking at r13's sketch, it occurs to me that the last comment (highlighted in orange) is wrong. Shear stress is maximum at the neutral axis.
BA

RE: Delaminating Glulam Beam Repair

Shear and bending stresses in a beam, from text of mechanics of materials.

RE: Delaminating Glulam Beam Repair

Can this works without nailing? BA has a valid concern regarding the condition of the existing adhesive though.

RE: Delaminating Glulam Beam Repair

Quote (Shear stress is maximum at the neutral axis.)


Correct... with rectangular beam it is a parabolic distribution... with average shear * 1.5 to give you the max parabolic stress... reason for the 1.5 factor in wood shear code. About 30 years back, in Canada, there was a bunch of delaminations... used to know the reason... but have forgotten...

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

RE: Delaminating Glulam Beam Repair

Quote (dik)

About 30 years back, in Canada, there was a bunch of delaminations... used to know the reason... but have forgotten...

I remember that too. There was a bit of a panic when the government warned everyone about glulam delamination and suggested inspections be carried out on buildings containing glulam beams. I can't recall too many details, but it had something to do with the Hot Press Method of applying glue. I looked for it in existing buildings, but didn't find anything on any of my projects.

https://cdnsciencepub.com/doi/abs/10.1139/l81-064

BA

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