Anchoring to Brick and CMU Cavity Wall 1

[RFreund]

What detail do you typically use when anchoring (form the exterior) to a brick veneer with CMU (Hollow) back-up wall.

Here are a few options, your comments welcome:

1. Anchor to the brick. Not everyone favorite but if you could find testing for 4" brick would you do it?

2. Have the CMU grouted solid and have the bolts (or some other member) cantilever over the brick. Some provision would be made so that the nut does not get over tightened against the brick (i.e. some sort of double nut).

3. Through bolt the hollow CMU and have the bolts (or some other member) cantilever over the brick. Same provisions as mentioned above.

4. Use a screen tube into the face shell of the hollow cmu. You would still be bearing on the brick but maybe you could use the hollow CMU pullout values. Then you would have to establish some limit for the bolt bearing on the brick.

See attached for additional clarification and questions. I'm curious to hear any comments.

Thanks!

https://onedrive.live.com/redir?res...4221&authkey=!ABeP9AXkrXxk76E&ithint=file,pdf

EIT

http://www.HowToEngineer.com

 

[concretemasonry]

Is this a classic brick and block wall assembly with an air gap between them? - In this case the brick carries not vertical dead load, but can resist and transfer the wind load to the CMUs.

Normally this is built using an "eye and pintel" form of joint reinforcement. The joint reinforcement is laid as the block wall goes up and insulation (if any), leaving an "eye" projection out slightly beyond the block or insulation face. When the veneer is laid, a "pintel" is hooked down into the projecting eye and into the veneer as is being laid. Various lengths of eye projections are usually available for the common configurations depending on the spacing of the masonry wythes.

For the "Cadillac" of walls (8" block, 2" air and/or insulation and 4" of brick) provides thermal mass within the insulation plus an air gap between the veneer for moisture drainage and a pressure relief. It is a waste of time and money to fully a CMU wall if it doe not have closely spaced vertical reinforcement.

The vertical and horizontal spacing of the "eyes" on the joint reinforcement are spaced so that the veneer can withstand wind loads and act as a plate. Usually the drains/vents above any barrier are voided 3/8" wide head joints with SS wool of or manufactures screened 3/8" wide to eliminate bugs from getting into the void.

There are no vents or openings in the CMU portion of the wall, only in the veneer. The only hole/vents are in the brick veneer and there are no bolts to tighten and both wythes are built vertically along the exposed faces.

I built may lake home with this method and left the XPS foam exposed for a winter or two before I had the brick laid since the windows were attached to the CMU wall and wood "bucks" that form the window openings and carried the bond beams/lintels above for the roof to be put on.

There are many ways to design and build masonry cavity wall. I used 8" CMUs because they were readily available and fit the 8" module and the block supplier also had 14x8x16 CMUs for the below frade walls (48"+ frost depth). I have seen many cavity walls build using 6" CMUs, 2" gap/insulation and 4" brick, the wall thickness is 12".

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[concretemasonry]

RFreund -

Just an after-though -

If you only need a 3/8" gap, there are joint reinforcement manufacturers that make the joint reinforcement with wire taps projecting out to allow the wall to be built without "eye and pintel" details and separate pieces if you want to build the entire wall at once with no interior insulation.

Dick


Engineer and international traveler interested in construction techniques, problems and proper design.

 

[RFreund]

Dick - thanks for the input, very good point on their own. However my questions were really geared toward fastening to existing masonry cavity walls - Brick, air gap, CMU - from the exterior side. Sorry, I should have been more clear.

Thanks though!

EIT

http://www.HowToEngineer.com

 

[thaidavid40]

RFreund –

We often run into this problem when anchoring the support struts for small canopies on existing buildings (or even on most new construction, really, since by the time the canopy contractor gets there, the brick facing is usually already in place!). Our solution, for modest-sized strut reactions, is to use something like the attached sketch. We usually end up with at least a ¾”-diameter bolt size, given the shear and tension loads present on most angled support struts. Anyway, this detail takes the load off of the brick, and places it squarely on the backup CMU wall. The process is like this –

1) Drill a clearance hole through the face brick (or remove about a half-section of brick).
2) Assemble an all-thread rod through a steel backup plate with a nut.
3) Screw a coupling nut over the all-thread, with a steel flat washer captive underneath, and tighten securely. The clearance hole allows for a socket to tighten the coupling nut, and the hole can be sealed to prevent water intrusion. Usually we have a decorative cover plate on the outside face of the brick, sealed all around too.
4) Screw the eye bolt (or whatever) into the coupling nut, and you’re ready to go!


Thaidavid

 

[RFreund]

@Thaidavid - Great response, thank you. I'm curious about the analysis. Do you use a T/C couple between the face shells and assume uniform bearing on each face shell? If so what do you use for the allowable bearing stress?

Thanks!

EIT

http://www.HowToEngineer.com

 

[thaidavid40]

RFreund,
We have been using the ACI 530 allowable of 0.25 f'm for bearing (or, 375psi for f'm =1,500), with the distance between couple forces of C-to-C of the face shells, d=6-3/8”. Of course, the best results are when the cells are fully grouted. That essentially throws the weak link back to the cantilever strength of the selected bolt size.
Dave

Thaidavid

 

[RFreund]

To extend this question further -
What about the tension capacity of the hollow CMU? Do you treat this as a punching shear problem?
What sort of allowable shear would you use?

Thanks again!



EIT

http://www.HowToEngineer.com

 

[thaidavid40]

If the load becomes large enough to worry about punching shear, we grout the cells solid. If this is a retrofit installation, and post grout filling is impractical, then we use a backup channel instead of a flat backup washer, and this distributes the tension over a larger face area of the wall.
Dave

Thaidavid

 

[RFreund]

thaidavid - thanks for staying with this.

How do you determine if "punching shear is large enough to be an issue". Or how are you determining the surface area of the wall to distribute the tension force?

For punching shear I was thinking the some allowable shear multiplied by the flange thickness x the length of the unit x 2 + the flange thickness x the height of the unit x 4.
i.e. t_f x L_u x 2 + t_f x h_u x 4

Not to side track but for my application, I may need an hss pipe or something similar just get the cantilever thru the brick to work. So I may need a large plate on both sides anyway. or I need to accept some bearing on the brick, which really isn't practical because they will need to remove the brick in order to install the thru-bolt assembly. They may need to grout the cells. However I'd like to have some sort of calculation to guide me to this conclusion.

EIT

http://www.HowToEngineer.com

 

[thaidavid40]

Maybe I oversimplify this, but I assume the backup plate is infinitely stiff compared to the CMU face shell, and use the backup plate total peripheral length times the face shell thickness as my shear area. If that fails, then I select a backup channel length sufficient to span horizontally across at least two CMU webs.
Dave

Thaidavid