CMU Veneer Height 5

[SteelPE]

I am currently in the preliminary design phase of a warehouse project. This will be the 3 addition to an existing building that was originally building in the mid 80's. The last addition was completed in 2006. The building falls under IBC 2015 requirements.

All of the original buildings were steel framed with a masonry skin. The building in 2006 was 38' tall with 12" CMU around the perimeter . This CMU was not load bearing and had the backing WF structural girts at the 1/3 points.

Now the owner is looking for a 4th addition. The owner would like to maintain the look of the original building, but due to energy code requirements they are not interested in using the CMU perimeter façade. The initial thought process has us using a 12" CMU knee wall to an elevation of 4'-8' and then metal stud/masonry veneer from this elevation up to the eave height of approximately 38'-0" (the height of the knee wall will be dictated by the energy code requirements). The code seems to have limits on the height of masonry veneer (ACI 530 12.2.2.3.1.3). The issue is, this section of the code seems to be written for a different case (where we have a multi story building).

The GC the owner has selected to complete the work is insistent that he has successfully completed projects in the pas with veneer of this height without any type of structural support for the veneer. He is also telling the client that the veneer height is 40' not the 30' we are specifying.

Do any others have opinions on this?

 

[Rabbit12]

I think the reason to limit the height of the veneer isn't because it can't support itself it's that there's a risk of falling during a fire.

 

[phamENG]

The height limit you cited is in the prescriptive veneer portion of the code. You can go higher, but you have to use rational analysis to get there. As long as you can balance stresses in the veneer, shrinkage, etc., then you should be fine - you just won't have the standard to fall back on if it doesn't work.

 

[KootK]

I violate the 30' regularly and have first hand knowledge of many others doing the same. That, particularly for the multi-story light frame over podium buildings, where nobody wants to get into relief angles etc suspended from the wood/CFM. Like phamENG said, if you've given it some critical thought and deem it okay, you're at liberty to run with that I believe.

In your particular case, you probably won't much care about any dimensional changes occurring above the level of the tops of the windows and doors (single story). As such, even though your physical wall may be 38' tall, your wall height for the purpose of working about the masonry dimensional stability is probably less than 12' to 16'.

 

[SteelPE]

I violate the 30' regularly and have first hand knowledge of many others doing the same. That, particularly for the multi-story light frame over podium buildings, where nobody wants to get into relief angles etc suspended from the wood/CFM. Like phamENG said, if you've given it some critical thought and deem it okay, you're at liberty to run with that I believe.

In your particular case, you probably won't much care about any dimensional changes occurring above the level of the tops of the windows and doors (single story). As such, even though your physical wall may be 38' tall, your wall height for the purpose of working about the masonry dimensional stability is probably less than 12' to 16'.

At this point, there is only one drive in door along this wall.... no other windows or doors along this wall. Just a massive wall of a veneer. Currently we plan on supporting the veneer with metal studs and a structural girt at mid height (between the knee wall and eave).... so the metal studs will be around 15'-17' tall.

 

[KootK]

Yeah, these kinds of buildings tend not to have a lot of windows. Taken to the extreme, in a building with no openings, you could stack the brick to the moon or until:

1) It crushed under it's own weight or;

2) You started stretching your brick ties too much.

 

[Rabbit12]

Guys, is my memory on one of the reasons for the height limitations to prevent falling on first responders foggy? It's been years since I did a building with veneer over 30' but I seem to remember that being a reason that forced us to use a shelf angle. Of course this was a steel building so it wasn't a huge deal.

 

[phamENG]

Rabbit12 - seems like it would be a concern either way. True, if there are no intermediate supports you could have a massive slab come down, but it seems more likely that by the time that happened the mortar would be badly damaged by heat and it would crumble. Don't know for sure though - I haven't studied masonry failure modes in a fire in great detail. Say your fire was contained to a single floor - if the masonry is supported full height, then it seems it would be more likely to stand. If it was supported at each floor, a fire at a single floor could damage the connections enough to drop 10ft or so of veneer.

 

[KootK]

If you wanted a little more ammo, or to dodge the rational evaluation, there's the 38' allowance on gables shown below. I can only think of one reason for this exception on gables: they don't often have big doors and windows in them above the ceiling plane.

 

[masonrygeek]

Generally, the height limitation for veneers has to do with differential movement of the veneer vs the backing. While I have heard of fire concerns being brought up like rabbit12 mentioned, pham shows that may really not be the main concern (one story height of brick falling vs the entire facade). And Koot is right about the theoretical limit although I would add to #2 that its the brick ties not being able to deal with the differential movement over that height. So the veneer height limitation forces the designer to consider differential movement and putting in a shelf angle is one of the easiest ways of dealing with that. Since this is a prescriptive requirement as pham points out, you can engineer the veneer to go higher. When an architect approaches me about going 35' or 40' without a shelf angle. I ask them three things: 1) can their adjustable wall ties take the amount of differential movement expected? 2) how are you detailing around windows or other penetrations through the veneer to consider the differential movement? and 3) what are you doing at the top of the wall to deal with the increased amount of movement? In most cases going 40' is not a big deal, but these issues need to be addressed. Most of my comments are dealing with a clay brick veneer, but I think the OP was mentioning concrete masonry veneer (?). In that case you have shrinkage of the concrete masonry veneer and that might match the backing.

Its interesting to note that these requirements are for backings of steel studs or wood studs, but not concrete masonry backings. We can debate that in another thread, but that's the current state of the requirements. One of the biggest issues we are seeing is wood stud backings/frames that are 4 and 5 stories high (40'-60') that have a clay brick veneer. When the shrinkage of the wood frame is added to the clay brick's expansion you could be talking 2" of differential movement. How do you easily accommodate that? Most designers aren't keen on using shelf angles on wood stud construction (although look at some of the literature coming out of Canada on that) and you start to see the brick extending up only so far (30' or so) and then switching to metal panels (eck!).

 

[KootK]

...although look at some of the literature coming out of Canada on that...

Could you point me to that? I'm dealing with the potential need for relief angles on a light frame building right now. And, despite being Canadian, I know nothing of this literature of which you speak.

 

[Celt83]

..In that case you have shrinkage of the concrete masonry veneer and that might match the backing.

Any chance you can point us to any documentation for shrinkage/expansion of clay and concrete veneers, my normal process has involved using the material thermal coefficients out of the steel manual and BIA document giving suggested material temperature ranges.

My Personal Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

Open Source Structural GitHub Group:

https://github.com/open-struct-engineer

 

[phamENG]

Celt - this is a good one for clay (at least to get you started): BIA Tech Note 18

 

[phamENG]

This one is fairly academic, but covers a lot of ground on differential movement and cracking.

 

[XR250]

Has anyone in practice ever witnessed this differential movement? I have not and have looked at a shit-ton of house/buildings. I suppose if the bricks and/or CMU were installed fresh out of the oven, I can see it happening.

 

[phamENG]

I've seen a little - nothing too terrible, though, and could usually be corrected with some new trim and caulk. Anecdotally, I have a client (or, rather, the client of a client) who owns several apartment buildings. When I brought this up on a recent project he thanked me and informed me that he was dealing with it on one of his other properties. So it can and does happen. It's probably not all that common in 1 and 2 and perhaps even 3 story buildings, but it happens just enough to make it worthwhile to consider. I think the code doesn't explicitly require consideration of differential movement between materials in general until you're greater than 3 stories, right?

 

[masonrygeek]

Here are a couple of documents that discuss brick veneer on tall wood frame buildings:

https://albertamasonrycouncil.ca/wp...atingMasonryinMidriseWoodFrameBuildings-2.pdf

https://www.woodworks.org/wp-conten...Brick-Veneer-Wood-Solution-Paper-Oct-2015.pdf

https://www.gobrick.com/docs/defaul...-above-30-feet-with-wood-framing.pdf?sfvrsn=0

There's a bit of controversy of attaching shelf angles (I prefer the term shelf angle over relieving angle) to a wood frame due to the various concerns mentioned above. It "can" be done - with a lot of design and detailing considerations - you just have to decide if you want to assume the risk involved.

XR250, while not every brick building has cracks in them, yes, there are a "shit load" of buildings that do. In most cases, it is because the designer (and sometimes the installer) hasn't considered movement of the brick and its supporting structure. All buildings move and all building materials move. You just have to determine the scale of the movement and design in movement joints (expansion joints, control joints, etc.). The BIA Tech Note 18 (along with its companion 18A) mentioned above is a great source of info on brick's expansion and movement.

 

[Mike Mike]

Could someone please point me to the source of fire concerns Rabbit12 suggested? I just spent a couple hours digging through literature and I can't find any mention in IBC 2015, ACI 530-13, or any BIA tek notes of veneer falling off of burning backing and landing on fire fighters, or any other fire related reasons for height limitation. However, the word non-combustible appears a few times in ACI 530-13 12.2.2, the prescriptive subchapter, and Shepherd posted this response in another post:

"Section 6.2.2.3 of ACI 530 regulates the vertical support of anchored masonry veneer. Section 6.2.2.3.1.2 and Table 6.2.2.3.1 limit the height of the veneer above noncombustible foundation. An exception is given for veneer with a backing of cold-formed steel backing, Section 6.2.2.3.1.3. There are no restrictions on the height of veneer backed by concrete or masonry. This is a fire protection requirement. The designer should also consider the effects of differential movement on the anchors and connection of the veneer to other building components. See Commentary Section 6.2.2.3.
As this restriction is fire safety related, additional calculations would probably not be a sufficient proof for an alternative approach
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