Anchored CMU Veneer Above 30' Height Limit

[Guastavino]

All,

I saw a couple old threads on this that were great, but they are closed and I'd like to take up the issue again. I'm designing a wood framed building (including wood shear walls) with a CMU decorative veneer. The top of wall elevation is consistent (small sloped roof) at 42'. I understand the use of the 30' limit in Chapter 6 of TMS-402, but I'm curious about what people consider the "Alternative design" in TMS-402-08 section 6.2.1 allowing unlimited height.

My thoughts:

1. 42' doesn't concern me as long as I take in to account wood shrinkage, CMU thermal expansion, CMU shrinkage, and inform the architect to account for such details.
2. I know shelf angles to wood are used, but I feel like that is potentially more problematic than going an additional 12'. Those detail introduce wood creep, proper angle installation, expensive difficult to build details, etc.

I'm curious what other things you all would consider for the rational design using 6.2.1, and if you've used it successfully?

Thanks,

Nick

 

[mike20793]

I'd be more worried about lateral deflection compatibility than strength of the brick, especially under reduced MRI wind loads. Any lateral deflection can cause strain around the openings where the wood is deflecting much more than the brick will. Not to mention the difficulty with differential shrinkage over that much height. In my experience, it's much better to support the brick at each level for the lateral deflection issue alone. I typically will talk to the architect about lipped bricks at relief angles to "hide" the wood shrinkage.

 

[XR250]

I would rather go the extra 12' than install shelf angles that likely rotate enough to just load up the veneer anyway. Use adjustable brick ties - not the corrugated ones.

 

[structSU10]

I had to get into this a bit, in a more general basis recently. Technically the engineering approach used for veneers higher than 30' needs to be approved by the building code official, (this was based on ACI 530-05, section 6.2.1 leading to section 1.3). In general, as long as thought is put into detailing around openings and such, I have never had a problem. I also have never seen this be reviewed by a code official in any atypical way prior to the start of a project. Its an awkward code provision basically that I have not seen truly utilized.

 

[EngineeringEric]

How would the architect account for such details? How would all the penetrations work when the wood shrinks 1" at the top and the brick expands 1/4"? Not argumentative, just curious

I have once, once, only once, informed the architect and they agreed to use fiber cement above the third story with a veneer expansion joint at the transitions.

But typically it is brick shelves for me, hopefully you have a small wall cavity. And hopefully they do not require continuous insulation on the exterior of the sheathing (2 more inches of cantilever! => L8x8 all around!)

 

[Guastavino]

@structSU10,

I agree. It's almost a license to do what you want. Rare you see that in the code, which is one reason I want to think through it a long time before I utilize it.

@mike20793,

I'm in a 90mph wind area (still on ASCE7-05 here), and the pressures aren't too bad. As for supporting the brick at each level. I worry about the contractor not installing the detail well more than shrinkage. Have you had success with contractors doing that well?

@XR250,

Thanks!

 

[mike20793]

Yes, I haven't really had any issues with them supporting the brick off each level when I've coordinated it with the architect ahead of time. I've actually found (on my most recent wood buildings) that the contractor is much more skilled than I give them credit for. I'm used to assuming a housing type contractor on these projects, but enough have been built, that there are plenty of good contractors out there now. That being said, there are certainly bad/unskilled contractors out there.

 

[manstrom]

Detail for the shrinkage. Assume about 1/4"/floor. Use brick anchors that accommodate movement. If the county asks to justify it, you can cite the "engineered design" over "empirical design" option in the ACI. I do it all the time. I would not go 5 stories ever, 4 is fine.

I would not recommend using relieving angles. That is a lot of steel that will not be properly installed.

When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.

-R. Buckminster Fuller

 

[JAE]

Don't forget that there is also the lateral sway of the building to consider as well, not just the vertical shrinkage issues.

The wood framing, with presumed wood shearwalls, will sway some distance horizontally under wind loads and the brick veneer won't tend to move at all.
So at top floor window openings you have the interior stud walls and window framing moving laterally while the brick around the windows is not moving much at all.

Check out Eng-Tips Forum's Policies here:
faq731-376

 

[manstrom]

FWIW, I can say that I've never seen a problem with lateral movement between wood and 4 story brick. That's not to say that it couldn't happen, but it's not a concern that has ever been on my radar. I've seen a lot of these.

When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.

-R. Buckminster Fuller

 

[XR250]

I believe the OP stated it was CMU veneer - which I believe moves lees than brick. I too have never seen a problem doing this.

 

[Guastavino]

It's actually 3-stories with 13'-4" floor to floor heights (~12' stud length due to platform framing style). The wall studs will be significant to keep the deflection down. I will want to keep it to L/600 based on the 0.7*Wind load combination, as "recommended" in the ASCE7-05 commentary for serviceability. I'm fully prepared for the blank looks I'm going to get when they see the wall requirements.

 

[manstrom]

You don't need L/600 for out of plane deflection of wall studs supporting veneer. Code minimum in L/240. L/600 is for metal studs or vertical deflection of a member supporting masonry.

When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.

-R. Buckminster Fuller

 

[JAE]

manstrom - I think you are right that usually, for most traditional 3 story buildings the sway doesn't seem to be an issue with brick veneer.
But have you've seen a lot of 42 ft. tall veneers not get affected?

My comment was just a suggestion that with a much higher building than the typical 30 ft. veneer, then lateral sway might be a problem to at least consider.
I don't think it is a good idea to just ignore it.

On the vertical shrinkage aspect, I've seen wood shrinkage wreak havoc with brick veneer when the architect failed to separate the veneer from second floor windows.
The exterior wood studs and plates all shrunk and the brick veneer, set tight to the bottom of the windows didn't want to move down with the wall. The EIFS system above
also moved down with the studs and crushed itself at the brick line all around the building.

This kind of effect is also accentuated higher up.


Check out Eng-Tips Forum's Policies here:
faq731-376

 

[Guastavino]

JAE,

We are actually using decorative CMU, not that it matters much in terms of load/anchorage. However, at least it won't be growing while the wood wall shrinks. With that said, I also plan to inform the architect of the need to detail joints for the wood to shrink and not negatively affect the wall.

JAE, what do you think about the fact that if I set the CMU veneer on a steel ledge beam above, I have wood creep and shrinkage to detail for also. I guess I don't see the difference between how important the details are between keeping the veneer solid to the foundation versus hanging it off the wall at 30'. Either way the details are critical. It's just a matter of where you detail the joints. My thought would be to avoid hanging it and detail joints elsewhere. That would save money and eliminate the CMU adding to wall creep. But I'm open to being wrong on this, just my reasoning to this point.

 

[mike20793]

Detailing CMU to allow for the wood shrinkage is much easier at each floor than it is from the ground up, not to mention the lateral drift compatibility. I have seen several buildings locally in the last year that have had window sill issues from lateral drift at lower than designed wind forces. Is brick falling off the building? No, but it is cracked and shifted to the point the owner called in someone to look at it within 5 years of being built. FWIW, I always support the brick off each level, regardless of how high it is going. This is based off recommendations by my superiors based on their experience and Terry Malone, who I have talked to in great depth about these issues. The detailing is actually pretty easy if everyone in the project knows what to expect (contractor, all engineers, and architect).

It's also worth noting that architects have an easier time detailing the shrinkage if you give them a range of expected shrinkage. Since the change in moisture content isn't always known, I usually give a range of expected shrinkage. Also, you're not really going to have to worry about wall creep. The amount it will shorten based on the stud loads is usually accounted for in the range of shrinkage expectation.

 

[JAE]

Either way the details are critical.

That's all I'm saying.

Check out Eng-Tips Forum's Policies here:
faq731-376

 

[manstrom]

@JAE

In my experience, I have never seen any drift related issues with 4 story brick on a wood framed building. I've designed maybe a hundred of these now in one shape or form and my office(s) have done another few hundred 4+ story stick with brick. Of course, this is in a low seismic, 90mph zone. Still, nothing after the last 15 years of storms and a minor east coast earthquake.

Of course, nothing is a problem until it is. I don't know everything, but I know a safe bet.

The masonry problems that I have seen are usually due to shrinkage and lack of detailing / joints.



When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.

-R. Buckminster Fuller

 

[rwpe]

Isn't the 30 foot limit for an unbraced height of 30 feet? Since you have floors at 13'-4", tie the veneer back to the floor diaphragm at each of those levels. And at least one or each of the floors, provide a shelf angle to "break" the veneer and reduce the impact of wood shrinkage.

 

[Guastavino]

Great discussion folks. I really appreciate the feedback. I'm still undecided but I don't think there is a "wrong" answer. I think the "right" answer is detailing effectively.