CMU wall reinforcing 3

[scottiesei]

Good afternoon gentleman,
I have designed a cantilevered masonry wall which will reach 6'-0" above grade. It is in Florida with a wind speed of 120 mph. The vertical reinforcing calculations are indicating that I need #5 bar at 24" o.c. The builder has built this wall in the past using a design from another engineer with the #5 bar at 48" o.c. I used ASCE 7 and ACI 530 for my design. If anyone has any experience with walls such as these, please chime in and offer a suggestion on spacing.
Thank you,
SS

 

[UcfSE]

Did you use the reinforcing steel stress increase, screen wall pressures and I=0.77?

 

[scottiesei]

My wall pressure was at around 17psf with the "I" reduction, etc. I did not think we could take the 1/3 increase for steel anymore.....

 

[DaveAtkins]

I don't believe you can use the 1/3 increase for steel anymore. However, the new IBC has a section on ultimate strength design for masonry, and sometimes you can get better results with USD.

DaveAtkins

 

[UcfSE]

The ACI 530 allows the increase for rebar stress.

 

[MarcbSE]

The IBC forbids the use of the 1/3 increase which overrides ACI 530.

However, the Florida Building Code still permits the 1/3 increase.

 

[scottiesei]

Well, 1/3 increase or not, how does everyone feel about the spacing being 24" vs. 48"?

 

[UcfSE]

It seems a bit conservative to me.

What was your moment, and soil cover to top of footing?

 

[scottiesei]

Moment was 310 ft-#, using 17.2psf at 6 feet and wl^2/2. Now that I am running numbers I am seeing that I may have miscalculated. How would the soil on top of the footing help with bending of the wall?

 

[DaveAtkins]

I get that #4 @ 48" oc works.

DaveAtkins

 

[jike]

I agree w/ #4 @ 48" Dave.

Is the 17.2 psf based upon C&C?

 

[scottiesei]

17.2 was based on ASCE 7 for "other structures" using the Cf from figure 6-20.
Thanks for the checks guys.

 

[UcfSE]

The moment arm of the resultant wind force on the base of the wall is half the height of the exposed wall plus the depth of soil. So the soil doesn't really help you unless you want to use the passive soil force, but it does affect the wall moment. I was just wondering to check what spacing I would get. I don't typically use the passive soil force for a foot or two or soil cover.

 

[scottiesei]

Cover is only about 8" or so. Thanks again everyone.