CMU/Concrete Foundation Dowels
CMU/Concrete Foundation Dowels
(OP)
Our office typically does not show reinforcing dowels between a concrete foundation wall and a reinforced CMU wall. This is for low seismic areas A or B.
We use the grout on top of the concrete wall to resist shear at the bottom of the wall. The rebar in the CMU wall is usually just there for bending strength mid-height in the wall. The CMU walls start at the floor height and are completely above grade.
I'm asking what other people think because I've gotten a lot of calls recently from contractors wondering if we missed something on our drawings. I assume everyone else must be using a dowel at the bottom and maybe they have a good reason for doing so.
We use the grout on top of the concrete wall to resist shear at the bottom of the wall. The rebar in the CMU wall is usually just there for bending strength mid-height in the wall. The CMU walls start at the floor height and are completely above grade.
I'm asking what other people think because I've gotten a lot of calls recently from contractors wondering if we missed something on our drawings. I assume everyone else must be using a dowel at the bottom and maybe they have a good reason for doing so.






RE: CMU/Concrete Foundation Dowels
ACI 530-99 2.1.8.3.2 requires 25% positive reinforcement be carried into the support. Although 2.1.8.3.1.3 says that it need not extend beyond the point it is needed at supports of simple spans.
I do not want the joint at the base of the wall opening under lateral load.
RE: CMU/Concrete Foundation Dowels
RE: CMU/Concrete Foundation Dowels
Personally, I usually match the dowels size and spacing to the size and spacing of the CMU vertical bars. Think this is just good practice.
RE: CMU/Concrete Foundation Dowels
How do you tie down the ends of the shear walls without dowels?
RE: CMU/Concrete Foundation Dowels
RE: CMU/Concrete Foundation Dowels
I understand the positive moment reinforcing needs a certain development length beyond where it is needed. I think for a lot of walls the vertical rebar probably isn't needed at the ends of the span for flexure.
Does anyone have a reference where dowel anchorage is needed if the wall is not a shearwall?
There is definitely some shear capacity in the mortar to concrete wall interface, right? Mortar holds CMU to CMU, so it must hold CMU to concrete, right? Otherwise how do unreinforced walls function?
I'm not saying I'm right. I'm just trying to figure out the code requirements, and why other engineers design the way they do.
Thanks
RE: CMU/Concrete Foundation Dowels
RE: CMU/Concrete Foundation Dowels
RE: CMU/Concrete Foundation Dowels
RE: CMU/Concrete Foundation Dowels
RE: CMU/Concrete Foundation Dowels
The horizontal dowels to tie the walls together help with any differential settlement should it want to occur at the juncture of the two wall types too.
Mike McCann
MMC Engineering
RE: CMU/Concrete Foundation Dowels
You should use the dowels to provide a positive shear resistance for all loading conditions. Because you probably assumed a pinned connection, the dowels would have to adequate to resist only the shear. In the spirit of good design and practical detailing, I would make sure the dowels were aligned with the flexural steel in the wall. Adding a little extra length to lap the fexural steel should not be detrimental, but I could be wrong on this.
Dick
RE: CMU/Concrete Foundation Dowels
Mike McCann
MMC Engineering
RE: CMU/Concrete Foundation Dowels
RE: CMU/Concrete Foundation Dowels
Looking at section 2.1.10.4.2 and section 1.14, it seems pretty clear that the code is asking for some kind of steel anchorage at floors and roofs for shearwalls.
If you have a masonry wall that is not a shearwall (maybe this is not common) I don't see any requirements that you need anchorage. Wouldn't you use section 2.2.5 to calculate the allowable shear stress at the CMU/Concrete interface?
I'll probably just try to get the office to switch and use dowels for all walls with reinforcing, just to make life simpler for myself.
Section 1.14.3.3 may imply that even unreinforced shearwalls in seismic category A need dowels, but it doesn't specifically mention foundation attachment.
RE: CMU/Concrete Foundation Dowels
RE: CMU/Concrete Foundation Dowels
WHOA! You have masonry walls with grout in the bottom of the wall so that it can resist shear?! What planet do you live on!? On our planet all the cells get filled up with mortar long before grout has a chance to get in there.
RE: CMU/Concrete Foundation Dowels
1. ACI 318 requires it,
2. Contractors question it (and you, I would assume) if you don't show it
3. It follows under good engineering practice in my opinion
RE: CMU/Concrete Foundation Dowels
No need to be dramatic... It often actually takes away from your point.
LowLax: Dowels are cheap, easily placed and do not add a great deal of cost to the peoject. And it's not your money, but it is your license. Save the client time and money where it is more worthwhile. My simple advice: Add dowels. If you want to skimp, add dowels sufficient to take serviceability loads under earthquake and wind, then rely upon the grout friction to provide the rest up to ULS loads.
Regards,
YS
B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...
RE: CMU/Concrete Foundation Dowels