ACI 318-08 Section 14.8.2.6

[tolchijb]

Can anyone shed some light on the requirement, "Vertical stress Pu/Ag at the midheight section shall not exceed 0.06f'c"? This seems extremely restrictive. For 4000psi concrete, this value is 240psi. This is something I might expect to see in masonry, but not concrete. Your thoughts on this provision are appreciated.

 

[JAE]

I don't know - for 240 psi - an 8" x 12" strip of wall would allow 23 kips.

 

[JoshPlumSE]

This is for slender walls. The ones where the height tothickness ratio is greater than 25. Second order effects (i.e. P-Delta) can have a major effect for these walls, especially when you combine that with increased cracking and reduced flexural stiffness.

This goes way back to the UBC days and the slender wall testing done by SEAoC. If you really want to go back through the history of it, take a look at the 1980's publication: "Report of the Task Committee on Slender Walls". This was published by SEAoC and the Southern California Chapter of ACI.

P.S. It's one of my favorite research reports because it was commissioned by and performed by engineers.... Not academics.

 

[tolchijb]

I searched for the document you mentioned. I did not find the document, but I did find several links to articles about the document.

My question is this, is 6% of f'c a hard number based on empirical evidence or did someone say 6% late one night and they went home for the day. I have a situation where I am trying to make a 6" (ICF) wall support hollowcore planks. My P/A at the midheight is closer to 10% of f'c. Is this completely out of bounds? Do I have to design by Chapter 10?

Thanks for all your input, past and future.

 

[msquared48]

6" ICF seems awful thin to support hollowcore of any normal span, especially with any lateral earth, wind or seismic load.

I think it would be best to bite the bullet here and use a thicker wall, or different product.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[TXStructural]

6 inch flat/solid ICF is not an unusual wall to support one floor above. Is there a particular reason you are using the alternative method, rather than the other options per 14.4 or 14.5?

You do not mention retained soil or similar indication that it is a basement, so the empirical method should be acceptable. 14.4 allows design by the strength methods in chapter 10.

The alternative method is a simple method, which is why its requirements are more restrictive than necessary under chapter 10.

 

[JAE]

The alternative method is a simple method, which is why its requirements are more restrictive than necessary under chapter

Hit the nail on the head. Agree with that.

 

[tolchijb]

Thanks for your responses. My boss has promised this client we can make 6" exterior walls work on a four-story hotel. So I was left with the task of making it work. I did get it to work using chapter 10. All of the simpler, empirical methods failed me. I generally only use section 14.8 for tilt-up construction, but this being so thin, it seemed like a reasonable approach. Thanks again.

 

[JoshPlumSE]

Sorry for the slow response. I was in Orlando last week and was not checking the Eng-Tips threads. Below is the URL where this book can be purchased if desired:

http://www.seaoc.org/bookstore.html

Test Report on Slender Walls
"Green Book." Research report on testing of 12 concrete wall panels, nine concrete block masonry panels, and nine block panels ranging in h/t ratios between 30 and 60.
September 1982 129 pp.
Order SC #82-1 | Member $20.0 | Nonmember $25.00

Now, a quick review of what I see is that the axial load limit may reflect a limitation in what was tested. They say they really only tested the axial load up to a value of 1/10th the "short column axial capacity". That corresponds to approximately 0.06*f'c.

Hard to believe that no one else has done any significant testing on this over the years to see how the behavior changes with increase axial load. At least to see how high you can go before the procedures stop being valid. But, that is entirely possible.