Relying on construction adhesive for structural capacity 2

[DTS419]

Especially in residential construction, it can be common in practice to use construction adhesive in addition to mechanical fasteners to make connections. Examples of this would include sill plates on foundation walls, wood sheathing to support members, sistering joists, etc. As a structural engineer, do you or would you ever account for the capacity of construction adhesive? If so, how do you do it an acceptable manner?

 

[XR250]

I only consider it when I am right on the edge of making something work and I want a warmer, fuzzier feeling.

 

[Eng16080]

I wouldn't. The only exception might be if it were being applied and tested in a factory setting with strict quality control measures. I'm picturing something like a glulam fabrication facility or something like that.

There's too much uncertainty otherwise in terms of whether the correct glue was applied correctly per the instructions, under the right temperature conditions, between the correct materials, etc. If the builder forgot to apply the glue, how would you ever know? It's tough to inspect something that's hidden. With mechanical fasteners, at least the connection is partially visible.

 

[BridgeSmith]

We've had so many problems getting the high-strength epoxy anchors for rebar and threaded rod installed correctly, we've ceased using them for tension applications. Despite the installation being done by heavy construction contractors, overseen by DOT field engineers, we still had anchor bolts popping right out of the concrete because they either didn't get the holes cleaned out or didn't mix the epoxy correctly. I can't imagine the QC/QA is better for residential, so if you're not going to be there to make sure the surface of the concrete doesn't have dust on it when the adhesive is applied, I wouldn't think it's prudent to count on any bond from it.

 

[phamENG]

No. Too unreliable as others have stated. It can be good have - the bond is real if not readily/reliably quantifiable, and so it can certainly limit slip and improve the overall quality of the connection. I think of it more as serviceability insurance and size the mechanical connection for the full load.

 

[Eng16080]

I read a post somewhere on here not long ago where the person posting mentioned having a discussion with a Hilti rep. The rep. said something like only 25% of their adhesive anchors which they tested were being installed correctly! That doesn't give me much hope. And presumably these are for projects where the installer is aware that the installation will possibly be tested so they have an incentive to be careful.

I can't imagine the QC/QA is better for residential

Definitely not! I've had enough projects now where the contractor was either disregarding the structural plans outright, not using the correct (most recent) plans, or wasn't even aware that there were plans, that trusting them to execute a non-simple adhesive connection is not reasonable.

 

[Eng16080]

Also, I do think that adhesives can still be beneficial to a project, like in gluing subfloor to floor joists, which can help minimize floor noise. But I wouldn't want to rely on the glue for any additional structural capacity. It would strictly be used to improve serviceability, as phamENG mentioned.

 

[dik]

I occasionally have when securing steel BAR material to wood joists to increase the capacity. A few months back a client wanted to secure some medical equipment to a finished masonry wall, and I suggested a product. They decided to secure it 'for real'.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[ChorasDen]

In the wood industry, we do not rely on construction adhesive for any strength based calculation (moment, shear, etc.), too many things on a jobsite prevent a consistent and reliable bond, dirt, dust, and adhesive skinning/curing before placement are key factors. However, we do rely on construction adhesive for non-strength based checks, i.e. deflection, in that you develop partial composite action between the wood members and sheathing, which is reflected in deflection calculations.

 

[Aesur]

If we are talking Epoxy anchors, I will use those all day long as almost no anchor bolts get installed in the right locations now days as long as they calc out; if we are talking gluing things together (which is how I read the OP's question) there is no chance I would rely on it.

As for proper installation, I have long questioned it, and seen many and rejected many that were not. However that doesn't mean they shouldn't be used, it just means SSI needs to actually be done/required and a pre-construction meeting discussing proper installation should be considered. Unfortunately I'm sure this isn't the only things that don't get installed correctly and if we start going down the rabbit hole of not using things because we have seen it not installed correctly we will soon stop designing anything.

 

[BridgeSmith]

...we do rely on construction adhesive for non-strength based checks, i.e. deflection, in that you develop partial composite action between the wood members and sheathing, which is reflected in deflection calculations.

Do you have test data that shows the construction adhesive is stiff enough to provide meaningful composite action? I've always assumed the modulus of elasticity would be too low for it to have much effect on bending stiffness in a sheathing to framing connection.

 

[ChorasDen]

Do you have test data that shows the construction adhesive is stiff enough to provide meaningful composite action? I've always assumed the modulus of elasticity would be too low for it to have much effect on bending stiffness in a sheathing to framing connection.

I cannot publish raw test data directly, but I can assure you we've tested it to verify. As an industry, we publish a document that discusses the limitations and how the calculations are performed, hosted on WIJMA's website, see link below.

https://i-joist.org/wp-content/uploads/2022/12/CompositeEI-Final-2022-10a.pdf

The references at the bottom of the article may contain information on the test data, but I have not reviewed them.

 

[BridgeSmith]

Thanks, ChorasDen. Being able to account for 45% of the composite EI is higher than I would have expected.

I didn't really know what to look for before, so can you tell me how difficult is it to find adhesives that meet that ASTM D3498 standard?

 

[ChorasDen]

The key is ASTM D3498-19, ASTM D3498 is an older standard, that does not include OSB as an option, and had less stringent gap filling requirements.

I believe at this point in time that many subfloor adhesives aim to meet D3498-19, because of the selling point that many engineered wood product manufacturers require it. However, the only one I know offhand is titebond, because I came across some of their literature a year or two ago that advertised D3498-19. I would assume that it's fairly easy to find product in most markets that meet that standard, but I have not evaluated that statement either....

 

[dik]

...some of us don't.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[Greenalleycat]

We use epoxy anchors a lot in our office, though we've been speccing more and more cast-in anchors for significant applications recently due to a lack of faith in epoxy for critical applications

If you mean glue for timber etc, I've never relied on it
I think there is some provision in the timber code for limited applications? Maybe? But if so, it's highly specific and I've never had cause to use it in my day to day
I would never rely on random 'structural glue' in my designs...though I echo XR250s sentiments of warm fuzzies

 

[Tomfh]

The effectiveness of construction adhesives varies with the situation and the product quality. In a personal project, I found that Sika 227, a high-grade structural adhesive, created a stronger connection than screw/nails. I had to reposition one ceiling joist, side fixed to a stud, and the glue refused to yield. In the end the side of the joist tore out. However, a cheaper adhesive used in another location provided little to no strength. So, depending on the product and situation, high-quality glues can be effective. For all around robustness a glue and screwed joint is hard to beat.

Another thing to consider with construction adhesives is their elasticity; many are brittle and can fail due to differential timber shrinkage, leading to detachment as the wood contracts and shears the adhesive.

 

[BridgeSmith]

The key is ASTM D3498-19, ASTM D3498 is an older standard,

The latest version is 19a (amended 2019 version). There are quite a few older versions. Looks like OSB was added for the 2018 versions (D3498-18, -18e1, and -18a)

 

[ChorasDen]

Yes, but also check gap filling of 1/8" and adhesive spread rates that align with in-field application with tested assemblies.

 

[XR250]

In the wood industry, we do not rely on construction adhesive for any strength based calculation (moment, shear, etc.), too many things on a jobsite prevent a consistent and reliable bond, dirt, dust, and adhesive skinning/curing before placement are key factors. However, we do rely on construction adhesive for non-strength based checks, i.e. deflection, in that you develop partial composite action between the wood members and sheathing, which is reflected in deflection calculations.

I don't think I would count on it for deflection as you have no control in the field how much and how well it is applied. Go on a job site in the winter and watch The Framers struggle to squeeze that cold adhesive out of a giant caulk gun and then get back to me. Happened on my house. They ended up going back and screwing the crap out of the floor due to squeaks before the hardwood was installed. Has been fine since.