SLENDERNESS RATIOS FOR REINF. CONC WALLS? 1

[Engineerataltitude]

I've been asked by an architect client to shave down the thickness of my standard 8" concrete walls to 6" thick for architectural reasons, but the walls are 8'-4" high. They look so tall and thin. I've never needed to push the envelope in concrete wall slenderness this much before in my practice and wondered if ACI318 or any other paper might cover limits to the slenderness ratio of reinf. concrete walls? Anyone know?

 

[dik]

If laterally braced and minimal moment, it should be no issue... I've done several 6 storey buildings with HC slabs on 6" CIP walls that were 8' high...

Dik

 

[Engineerataltitude]

Unfortuneately, my wall isn't laterally braced. It is a 50 ft long wall seperating two bays of an electric shop. The conc wall itself is 8'4" high but has another 12' of 2x6 stud wall on top of it. Luckily since it is an interior wall, the only lateral loading it might be subject to is wind loading (20 psf) when the garage doors open. I've run the lateral anyalysis on the wall with the wind loading and it seems fine. Am I OK? Should I just stop worrying?

 

[Lion06]

If you've run the analysis and you can get the design checks to work out, then you're ok.

 

[Engineerataltitude]

Excellent. Thanks for the help.

 

[csd72]

One thing i would point out about a 6" wall is that it is well near impossible to get 2 layers of reinforcing bar into it and still properly compact it. You then have a single layer of rebar with almost 3" cover each side. You are therefore much more likely to get cracking with the 6" wall than with the 8".

Just a thought.

 

[JoshPlumSE]

Take a look at the old slender wall provisions of the UBC. I believe these kick in at an H/t ratio of about 30. So, you should be in pretty good shape.

Even so, you might consider some assumed "moment amplification" due to slenderness if you feel uncomfortable. Though, my guess is that it will be not as significant as you would think it would be.

 

[BAretired]

Just to be clear...the 6" concrete wall is fixed at the base, reinforced at the middle and will be designed for 20 psf lateral pressure plus 120 plf horizontal at the top?

BA

 

[Engineerataltitude]

I've actually analyzed the wall with 150 plf at the top of the wall. It has a tall framed stud wall on top of it and the 150 plf accounts for the lateral loading from the bottom pl of the wall due to wind. There is no horizontal diaphragm at the top of the concrete wall. No horizontal support until the roof above the stud wall.

 

[hokie66]

I would hang out for the 8" wall. As csd72 said, you can't really doubly reinforce a 6" wall, and your wall is really 20' high, albeit the top part is studs. Look at it this way...if the concrete wall were 20' high, you would make it 8" thick, and if the steel stud wall were 20' high, it would likely be 8" thick as well. Introducing a hinge in the middle doesn't logically allow for reducing the wall thickness. In a light industrial setting like you have, a bit of extra robustness is more important than satisfying an architect's sense of airy/fairy lightness.

 

[Engineerataltitude]

I agree. I think this is one of those times I need to go with my gut. My architect may respond best to the thin wall cracking issue csd72 mentioned. Any architectural gain that is made in getting the slender wall is lost by introducing the possibility of poor consolidation and compaction.

 

[msquared48]

You can get two layers in a 6" wall if you stagger the vertical reinforcing horizontally. This should give better concrete flow and, if you use #3 horizontals, 4" or so structural depth for the vertical bars. Just use one end bar vertical.

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

 

[csd72]

msquared48,

how do you tie those? and where do the horizontal bars go?

 

[msquared48]

CSD:

With wire ties.

Just place the horizontal bars between the staggered verticals, tying one bar off to one side, the next horizontal bar the opposite side, and so on up the height of the wall. Works if the vertical bars are no larger than a #5, with #3 horizontals.

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

 

[hokie66]

"horizontal bars between the staggered verticals"? I can see that staggering the verticals helps, but only if the horizontals are outer face both sides. You can place it, but I still prefer the 8" wall. An 8" wall would require less vertical reinforcement.

 

[a2mfk]

Why is this CIP concrete and not CMU?

 

[ron9876]

Don't see a problem with two layers of steel--3/4" + 1/2" + 1/2" =1.75". 6" - 2 x 1.75" = 2.5" clear between layers. Two layers of #4's each way each face with 3/4" cover (unless more needed for fire rating).

ACI 318 gives criteria for designing walls as compression members. I would use column criteria with the appropriate k and I. I would check the design against the empirical method in ACI.

I would be more concerned with what size footing to use than with the wall thickness.

I would guess that 6" would work fairly easily.

 

[hawkaz]

With only 2.5" clear between layers- the stinger is going to be bouncing off the steel while vibrating.

I wouldn't worry about double mats- just put one in the middle.
You will probably need #5 verts, spaced around 10" o.c Use #4 at 12" o.c. horizontally. Your cracking should be minimal.

 

[hokie66]

It's not a compression member, it is a cantilevered wall.

 

[msquared48]

Hokie:

You are correct - horizontals on the outside, but staggered face to face - sorry - I had a brain freeze. It's what I was thinking with the 4" d, but didn't come out that way.

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