CMU Vertical bar spacing vs dowel spacing

[redsouther]

On the vast majority of load bearing CMU wall design we do in our office, the vertical bar spacing is established and all vertical bars are extend to the foundation via dowels. In other words, the vert bar spacing matches the dowel spacing. I am wondering if anyone has experience with only providing dowels at 50% of the verticals? In my particular case, I'm looking at specifying wall verticals at 24" o.c. and dowels at 48" o.c. Technically, the wall is fine with bars spaced at 48" all around, but would there be any benefit to providing additional bars that do not extend to the footing?

 

[frogit22]

I would consider it good practice to match the wall bar spacing, and you will not save much by omitting them.

 

[cliff234]

I don't see any problems with what you are proposing. Actually you seem to be saying that the wall works with #4 @ 48" vertical. Why not just use #4 @ 48"?

 

[redsouther]

Thanks for the responses. In this instance the owner is requesting tighter grout/rebar spacing for security reasons. Strength-wise, the wall is fine with a 48" spacing, so I considered not using dowels at the bars that aren't required for strength.

 

[slickdeals]

Are the verticals needed for flexure? Does the wall work for axial and bending at mid-height, assuming pinned ends?

 

[redsouther]

Yes, the flexure w/ axial load controls, and the wall works with #5 verticals at 48" assuming pinned ends. My main question is whether the additional intermediate verticals, which wouldn't extend to the footing, would actually "do" anything.

 

[a2mfk]

I don't have my ACI 530 available, but I remember it specifically requires a certain percentage of the vertical reinforcement of walls to be anchored into the foundation. This allows you to skip some, which is very useful in renovations of existing walls, such as upgrading old buildings to new wind/seismic codes. I have never opted to do this on new construction.

By doubling the reinforcement you are doubling the lateral strength, since each filled cell is only carrying half the load. I seriously doubt the controlling factor in this wall or any other reinforced CMU wall is the connection to the foundation. I would guess in most instances friction and the bond strength of the mortar would calc out to meet your in- and out-of-plane shear requirements. Throw in your bars at 48" o.c. and you are WAY over designed for those two loading conditions. In my experience in high wind conditions, net uplift at the foundation can be a concern for shorter walls (less dead weight), and then the capacity of the bond beams since they span between the filled cells and have an uplift force applied to them.

All of these things in my experience end up normally being cursory checks of your standard calcs and details, the real issue is lateral and axial combined loading of the CMU itself.

HTH,
a2k

 

[msquared48]

If #4@48 is all you need for strength and the remainder are non-structural at the request of the owner, I see no problems with dowels at 48 too.

Mike McCann
MMC Engineering