Masonry Wall Construction

[SteelPE]

I have a project that a client is asking me to prepare a proposal for. The building is a single-story building in an area with a wind speed of 131 mph. The building is anticipated to have an eave height of 20'-0" with dimensions of 146'x68' and utilize open-web steel joists and masonry bearing walls. The project is pretty straight forward with the exception that the client is looking to use 8" CMU.

Our company standards have us moving away from 8" CMU around 16'-0" to 18'-0". There is no real reason for our limit other than this is what has historically worked for us (other than the old h/t limit of 30 outlined in ACI 530 which limits 8" CMU to 20'-0" before you have to take a 75% reduction in axial capacity for the bearing wall). For this project we are trying to figure out a way to get the clients system to work before we put in a proposal. We are considering using the depth of the roof joists system (24" deep roof joists) and support the walls off the bottom chord of the joist. This would require a system of kickers and bridging in order to properly transfer this force up o the roof diaphragm. It's an unorthodox solution for us. Has anyone attempted to brace masonry walls in this manner before?

To me, as long as we can transfer the loads to the diaphragm properly, I don't see why it wouldn't work. Would it be cheaper than switching to 12" CMU.... unfortunately that is above my head.

 

[JoshPlumSE]

You should look at slender walls if you haven't already. That approach eliminates the h/30 issue.

I agree with EZ Building, SE2607, Strucbells and others. Anytime you've got an h/t > 30 you really need to consider it a "slender wall" and do extra due diligence. Strucbells' link seems like it is probably the most up to date method for doing this.

However, I might start with the older iterative method from the older codes like the1997 UBC.... based on the SEAOC "green book" research on the subject. I think it's probably a little more conservative than what's done today.

 

[bIockhead]

Yeah this should all be feasible with 8" block using strength design, add pilasters at large garage door openings if needed.

A couple of methods to consider the 2nd order effects: https://www.structuremag.org/article/masonry-to-the-second-order/

This matches the Canadian standards. It's mostly a function of axial load relative to critical buckling load (stiffness) but second order effects can explode with the span gets too high. 30' and that buckling load comes way down.

Seems like a totally doable design, and what an engineer SHOULD be considering to make the best use of materials and keep the design economical.

 

[SWComposites]

Why? There isn't 10x more work or risk.

well, it sounds like a lot more work and risk for the 8" cmu wall. I just made up 10x. but pick a conservative number that drives them to the better design, or covers one for the extra work / headaches for the non preferred design.

 

[canwesteng]

This matches the Canadian standards. It's mostly a function of axial load relative to critical buckling load (stiffness) but second order effects can explode with the span gets too high. 30' and that buckling load comes way down.

Seems like a totally doable design, and what an engineer SHOULD be considering to make the best use of materials and keep the design economical.

I don't do much masonry granted, but I though the canadian code had a hard limit at kh/t<30

 

[Aesur]

Guess I'll be the jerk - why not just check a wall in Enercalc or other software for the wind speed and an approximate dead/live loading. This should take less than 5 minutes. To me, using 8" or 12" walls shouldn't change the fee one bit.

I'll add that I have used 8" CMU up to about 25' before with 115 (now 105 mph) and at that height it was difficult fitting reinforcing for wind, but 20' with our wind speeds I wouldn't have any issues, but I am not 131 mph so can't comment on that.

One additional consideration I would think about is how the architect intends to insulate the wall. I have been seeing more and more that architects want to use insulated grout which has no structural capacity (at least to date due to lack of testing). This doesn't work if you have lots of reinforcing as you need real grout in those cells and end up with half the insulation or hot spots.

 

[bIockhead]

I don't do much masonry granted, but I though the canadian code had a hard limit at kh/t<30

Good question, and it does in some ways. URM is completely out over 30, similar to seismic thresholds. Reinforced is fine, but there are added restrictions (can't count on a fixed base, can't use very small units, nominal axial load limits) but the biggest one is a requirement that tension steel needs to yield. This can be a pain for designs with even nominal axial loads because it moves the strain profile, and also means that you can't keep adding steel to add capacity.

Anytime using a smaller unit is involved, it's going to make it more likely to be a "slender wall" (kh/t over 30) but the cost savings in labour are often worth it. I just stick it in MASS and see if it works or not, since that software checks these for Canadian load combos

 

[canwesteng]

Ah yeah, you're right, I was just using that as a limit to keep the designs simple to avoid all the headaches, but it is allowed.