CMU/Concrete Foundation Dowels 3

[LowLax]

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.

 

[jike]

We dowel all our vertical wall reinforcement to the foundation including additional jamb reinforcement.

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.

 

[JAE]

We add dowels. How do you resist the lateral wind forces on your wall? Using friction isn't always a good idea.

 

[lkjh345]

We use 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.

 

[UcfSE]

I've always seen and shown dowels.

How do you tie down the ends of the shear walls without dowels?

 

[ron9876]

Dowels are required in our area. Allowing for friction is disallowed.

 

[LowLax]

I have ACI 530-05 in front of me and I see the section jike is referring too. It says the 25% requirement is for walls that are part of the -lateral- load system.

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

 

[spats]

Using your same logic, I guess there's no need to extend concrete beam bars into the support. You should always use dowels for reinforced masonry. You can't design for bending as reinforced masonry, and shear as unreinforced masonry. You are definitely out of the mainstream on this one!

 

[Taro]

See ACI 530 1.14.2.2.2 and 1.14.3.3. It seems that the intent of the code is to provide a mechanical connection between the wall and all locations of lateral support. You may be able avoid foundation dowels if you provide connection to a slab, but foundation dowels would be easier.

 

[Hemifun]

Now the question is, how many of you use dowels into the foundation wall for unreinforced CMU walls?

 

[civilperson]

When wind causes net uplift and horizontal shear, what shear strength remains at the junction of wall and footing if you are using grout with a horizontal surface?

 

[msquared48]

I always use horizontal and vertical dowels too.

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

 

[concretemasonry]

You are designing a reinforced wall (even if minimally reinforced), so you you cannot the use the empirical/unreinfrorced provisions and portions of the code (ACI 530) at whim.

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

 

[msquared48]

I am a high seismic area, as are many others in this forum. A lot of the soils here are prone to liquefaction, or sand boils. Consequently, if you have a typical stem wall over a strip footing without the extra interface of vertical dowels, there is the risk of the stem wall literally walking away from the strip footing in a seismic even under certain concrete foundation scenarios. This makes lawyers real happy. A few 2'6" long bent #4 dowels go a long way to CYA where I am.

Mike McCann
MMC Engineering

 

[Hemifun]

I'm not in a seismic zone and we design quite a few small unreinforced CMU buildings and do not typically show dowels into the foundation wall. Now I'm wondering if we should revise our detail to show dowels. Are dowels required by code??

 

[LowLax]

ACI 350-05 allows you to design your masonry shearwalls in seismic zones B & C using chapter 2 or 3 and any masonry walls that are not lateral force resisting can use empirical design chapter 5.

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.

 

[Taro]

Aside from parsing the code provisions to determine if you can get away with omitting anchorage, you should also consider the Standard of Care. That is, what do other engineers in your region typically do in similar situations? If your peers are using dowels, you better be using dowels (or equivalent method of attachment) also. In the event of a lawsuit, the Standard of Care typically carries much more weight than an individual's interpretation of the code.

 

[boffintech]

LowLax wrote, "We use the grout on top of the concrete wall to resist shear at the bottom of the wall."

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.

 

[tngolfer]

Our office uses (and shows) dowels matching the vertical steel. Honestly I don't know if I've ever seen the requirement in ACI 530 but:

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

 

[youngstructural]

Easy does it Boffintech; If you use masonry wall clean-outs (as many seismic codes require) you will be able to clean out the cores through the full height of the wall and ensure that the core is filled with grout.

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...