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Masonry Control Joints - Rubber Gasket Shear Capacity
2

Masonry Control Joints - Rubber Gasket Shear Capacity

Masonry Control Joints - Rubber Gasket Shear Capacity

(OP)
For concrete masonry wall construction it is usually required to provide vertical control joints in the walls - perhaps at 25 feet on center.

For a wall that spans vertically from foundation to roof, the joints don't interfere with the wall's load paths.
We typically place the joints in clean areas (i.e. not near openings)

Some technical manuals recommend placing the joints off the upper corners of doors. However, if you do this, then the area of masonry above the door has no load path against perpendicular wind forces.

The joint would interrupt the ability of the wall area above the opening to shed the wind laterally to the adjacent full height, reinforced cells on that side of the door.

We've seen where NCMA and other literature imply that the cruciform shaped rubber or PVC joint gaskets (go here for a look at one: (Link) may provide lateral shear capacity to transfer shear loads across these gaskets.

The literature that we found always seems to suggest that the manufacturer should/can provide the engineer with the shear capacity of these gaskets.

We haven't found anything online - just wondering if any of you know where this sort of info might be found. Just a general shear capacity (range of values) would be adequate.

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RE: Masonry Control Joints - Rubber Gasket Shear Capacity

Just a random thought. What about using smooth dowels across the control joint? it would allow for movement along the length of the wall but would lock it in against out-of-plane wind loading. It would also have the added benefit of providing some vertical shear transfer across the control joint as well.

RE: Masonry Control Joints - Rubber Gasket Shear Capacity

I know nothing of the gaskets. For that matter, I'm not so good with masonry. Western Canada refuses to acknowledge that it's good for anything other than infill. I thought that door/window expansion joints did not interrupt the bond beams present at the window heads and the tops of the walls. Were that the case, the block over the door/window could still span vertically between bond beams. Is that not how it's done?

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Masonry Control Joints - Rubber Gasket Shear Capacity

I should have added to my comment I'm not very good with masonry either. Elevator shafts and infill is all I really used it for. The odd fire separation in a wood framed condo building.

RE: Masonry Control Joints - Rubber Gasket Shear Capacity

I found this:

Quote (Masonry Magazine)

Nonstructural reinforcement, such as horizontal joint reinforcement used to control shrinkage cracking, should be discontinued at the control joint. Any reinforcement crossing the joint effectively restrains the wall and may negate the benefit of the joint. Structural horizontal reinforcement, such as bond beam reinforcement at floor and roof diaphragms, however, may be required to be continuous across the control joint to maintain the structural integrity of the wall.

Got it here: Link

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Masonry Control Joints - Rubber Gasket Shear Capacity

(OP)
Thanks for the replies. My main question is about the rubber/PVC cruciform gaskets and what shear capacity they have.
I'm aware of smooth dowels, placement of joints, etc. Just want/need information on the gaskets.

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RE: Masonry Control Joints - Rubber Gasket Shear Capacity

I don't think the gaskets have any dependable structural strength. I agree with you that locating control joints at openings is not logical.

RE: Masonry Control Joints - Rubber Gasket Shear Capacity

These guys discuss shear and provide some values: Link. Would one be limited by shear across the 1/8" throat that extends into the wall or does the thicker part bond/key to the sides of the wall somehow?

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Masonry Control Joints - Rubber Gasket Shear Capacity

JAE...pre-formed or site constructed sealant systems for control joints have essentially no effective shear capacity. In actuality they have some (not the pre-formed joints) shear capacity; however it is so small compared to the potential lateral loads, it doesn't make much sense to use it.

RE: Masonry Control Joints - Rubber Gasket Shear Capacity

Never mind my last question. I was confused about the gasket orientation. Despite the published shear values, I'd feel better with values tested on an actual, constructed masonry joint.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Masonry Control Joints - Rubber Gasket Shear Capacity

Description of the ASTM testing: Link

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Masonry Control Joints - Rubber Gasket Shear Capacity

I have had the same question for years. It seems the code designers are not building designers. If you can run the bond beam reinf. thru, why not the ladder wire? (which is what I end up doing)

RE: Masonry Control Joints - Rubber Gasket Shear Capacity

(OP)
BAretired. - thanks for that. So if I use the 5/8" thick width of the tongue, times 1200 x 0.4 and convert to feet I get about 3600 plf of shear capacity across that material.
(The 0.4 is a made-up shear safety factor - could be anything I suppose - 2.0, 3.0, etc.)

That shear capacity value seems high to me (in light of Ron's suggestion to not count on them at all).

So if I had a 20 ft. tall wall with a 10 x 10 ft. door opening and a control joint on one side of the door - the wind load that I'd be trying to
transfer might be on the order of this:

25 psf (C&C Wind) x 5 ft. = 125 plf. That seems way below the 3600 plf capacity.

So the numbers here seem to suggest that the gasket has way more capacity than the applied shear.

Besides your link, BA, I can't seem to fine very much information on this so it seems the manufacturer's aren't pushing this aspect very much.

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RE: Masonry Control Joints - Rubber Gasket Shear Capacity

Here is another article dealing with the topic of control joints in masonry. Shear values are not mentioned, but it seems clear that the location of control joints beside openings would require some shear capacity.

http://www.rcpblock.com/images/pdf/TEK-10-2C-Contr...

BA

RE: Masonry Control Joints - Rubber Gasket Shear Capacity

The 1.125 lb/in was intended to be 1125 lb/in. It jives with the width and shear stress value of 1800 psi. Based on the numbers, it also appears to be an identical product to the manufacturer's spec sheet that I linked above. Perhaps it's the same product being distributed under two different names. In the first paragraph of the document that I linked to, they specifically mentioned that their product has sufficient shear strength in addition to its other fine properties. Clearly manufacturers are at least aware that designers expect the gaskets to provide shear transfer capacity.

Although the gasket shear capacity is very high (750 lb/in / FOS), I wouldn't expect the assembled joint to possess anywhere near that capacity. Among other issues, the masonry pocket that contains the gasket shear key wouldn't be able to sustain that level of localized stress. Here's how I think that the gaskets work:

1) Unless we think that the manufacturer's are lying, shearing of the gaskets is off the table. Definitely not the limiting failure mode.
2) It's a good thing that the gaskets are not particularly rigid. Their flexibility probably protects the surrounding masonry pocket from high localized stresses.
3) Since the pertinent source of deformation is shear strain across a gap of 1/8", the joint will be relatively stiff despite the constituent material being relatively flexible.

I think that I'm a believer now. It's really quite an ingenious setup.


The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

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