Tilt Panel - Exceed 0.06f'c?

[LearningAlways]

Hello All.

ACI551 and ACI318 limit vertical stresses at mid-height of slender panels, as calculated as Pu/Ag, to not exceed 0.06f'c.

Does anyone know the background on this limit?

And, while its not mentioned in the code, I wonder if it is possible to count for any compression steel?

I'm looking at a panel that has a maximum allowable stress of 300 psi (5 ksi concrete) and I am currently at 306 psi, within 2% of the allowable. There's mention in the code about using compression steel to increase moment capacity but nothing in regard to using as part of an effective area.

I realize the limit is calling out gross area, Ag, but it seems feasible to take some of the steel into account when I am this close to the limit. I am very good in all other areas of the wall (flexure, deflection, etc.) so no worries there.

 

[jayrod12]

Not sure about tilt up panels specifically, but don't you have to tie reinforcing if you want to use it as compression steel? Seems like it would be much more economical to go to a higher concrete strength or a slightly thicker wall instead of having to add a bunch of ties to the vertical rebar.

 

[RPMG]

I'm not familiar with slender precast panel design, but for masonry:
[li]The steel must be tied to contribute to compressive strength. Pressing on slender steel bars causes elastic steel buckling.[/li]
[li]Stress limits exist for ductility. As the compressive stress gets higher, the flexural strength gets higher, until it doesn't, at the stress limit.[/li]
[li]The terms "Allowable" and "Pu" refer to ASD and LRFD, respectively, so they should never be used together. Obviously, you are referring to LRFD, but it should not be done.[/li]

 

[KootK]

Untied compression steel is routinely used in concrete wall design so long as various limits are adhered to. This was [ < 1.0% Ag ] at one point in time.

Does anyone know the background on this limit?

I don't know with certainty. I do have a theory however.

I believe that the [0.06 f'c] is an approximate means of keeping the axial load low enough, relative to the Euler buckling load, that P-Baby-Delta moment amplification will not be excessively severe. In rough terms, one might envision the moment amplification as [Af = 1 / (1 - P/Pe)]. In this sense, [0.06 f'c] serves as an index of [P/Pe].

If one accepts this premise, then it probably does not make sense to use the compression steel to satisfy the [0.06 f'c] limit. That, for a few reasons:

1) The issue is really cross sectional stiffness. Two layers of reinforcement may well help with this but single layer reinforcement won't help much at all.

2) Even with two layer reinforcement, it's hard to know how that should factor into the limit which, no doubt, assumes some reinforcement to begin with.

3) If anything, it will be the tension reinforcement that most impacts stiffness.

I'm looking at a panel that has a maximum allowable stress of 300 psi (5 ksi concrete) and I am currently at 306 psi, within 2% of the allowable.

2% is nothing for a nebulous "limit" like this. I'd be happy to let that go based on engineering judgment alone.

 

[LearningAlways]

jayrod12 - Yeah, we'd need ties into order to count if for compression steel. I'm trying to keep the same concrete strength and wall thickness as it is but thank you for the recommendations.

RPMG - I hear ya' on "allowable" being paired with LRFD, I'm just parroting ACI551 and they use "allowable" when referring to the ultimate loads applied to the section. I've never heard of it this way before but that's how its written.

KootK

1) The issue is really cross sectional stiffness. Two layers of reinforcement may well help with this but single layer reinforcement won't help much at all.

We're using two layers of reinforcement, it would be nice if this limit was increased somehow if using two layers instead of one because, like you said, the stiffness is increased significantly.

2) Even with two layer reinforcement, it's hard to know how that should factor into the limit which, no doubt, assumes some reinforcement to begin with.

That is an interesting take that the stress limit already assumes reinforcement. What makes you think so?

2% is nothing for a nebulous "limit" like this. I'd be happy to let that go based on engineering judgment alone.

Great! I myself felt very good about letting that go too. Just wanted someone to confirm my judgement.

 

[KootK]

That is an interesting take that the stress limit already assumes reinforcement. What makes you think so?

Just my intuition that it would be sensible to study a cracked concrete cross section when considering member stiffness. And, obviously, once you've chosen to study a cracked cross section, you've also usually chosen to study a reinforced cross section.

 

[LearningAlways]

once you've chosen to study a cracked cross section, you've also usually chosen to study a reinforced cross section.

Fair enough. Makes a lot of sense. Thanks for the input.

 

[southard2]

Working off of memory here. But I believe the compression stress limit was put in place because the tilt-up wall equations were based off a slender wall testing program done in California originally. Thus the equations used for tilt-up are also some what empirical. So the compression strength limit keeps you within the boundaries of what has been tested. Remember the tilt-up wall equations found in the Walls chapter of ACI adjust for the P-Delta effects automatically so staying within this limit is critical to the applicability of the equations.

I find that the .06 ratio is often exceeded on panels with large openings and | or heavy floor loading. If the compression stress limit is exceeded you must simply switch over to the regular ACI column equations and design it accordingly. This of course forces you to use column ties if you are spreading out the vertical reinforcement so that you have good confinement. If this is the case I do not recommend the use of 5 1/2" panels and recommend a minimum of 7 1/4" panels. Another trick is to just keep the reinforcement in the middle of the panel in the regions where you must use regular ACI Column and Beam Equations to avoid the column ties.

I believe UBC (Unified Building code) used a limit of .04 f'c for the compression stress limit but they used service loads not ultimate loads like ACI. I've always tried to comply with both limits. But once the compression stress is exceeded do not use the slender wall equations. Switch to the regular biaxial column equations for columns to design the wall.

A fantastic resource on everything tilt up is

"The Tilt-Up Design and Construction Manual" 6th Edition by Hugh Brooks. So freaking good. Sadly its getting to be impossible to find a copy of this. But I highly recommend it if you are going to be doing a lot of tilt-up wall design.





John Southard, M.S., P.E.

https://www.pdhlibrary.com/