Basement Wall Corners: Horizontal Bar Development 1

[Delchi]

It is common practice to analyze basement walls using tank-tables, which usually indicate high negative moments around corners and at counterforts. My question is how the bars around corners can be used for tension reinforcement in walls that are too thin to allow the minimum hooked-bar development length.

For example, how can the negative moments be resisted at the corners of an 8" or 9" basement wall when the horizontal reinforcing consists of #5 bars? (In the case where the backfilling and compacting would be performed prior to framing the floor)

I would appreciate any thoughts on how the corner conditions can be detailed in accordance with the ACI 318. Thanks!

 

[msquared48]

Embredded plate in the wall with the #5 welded to the plate.

Mike McCann
MMC Engineering

 

[ishvaaag]

Respect curving rebar CTE is now more or less where ACI is, but for decades we stood in inner curvature diameters thrice the rebar diameter when under 20 mm and I haven't heard of a single case of structural failure of any kind that have been adscribed to this practice. This is not to say, of course, that softer curvatures are not better; yet when one considers the quite ugly detail of anchor plates one starts to wonder.

 

[Splitrings]

We build a fair share of rectangular tanks using the PCA reference, "Rectangular Concrete Tanks". A lot of times we will reduce the horizontal bar spacing in the corners to half of what it is along the face of the wall. We do this by placing an equal length leg "L" bar between the wall reinforcing. Sometimes the legs will be 4' or better, extending to a point where the wall reinforcing is sufficient for the moment at that point. As I read ACI 12.5.4 it appears we are not following the code. The only confining reinforcing we would have in our case would be vertical bars and they are certainly spaced greater than the required 3db.

 

[HeavyCivil]

I do same thing as Splitrings. 1/2ing spacing and running L's at Ld or .1b into wall usually is enough.

With a good length of wall to work with why are you using a hook? - Not saying you shouldn't , just curious..

If you're not using std hook, just running h bars into each wall an appropriate distance, I dont see why 12.5.4 applies

 

[DaveAtkins]

A corner bar, even if it extends many feet into each wall, is still just a standard hooked bar. If the embedment past the inside corner is less than the hooked development length, the bar will not be able to reach its full yield stress.

DaveAtkins

 

[miecz]

See the attached detail, and thread507-228698.

 

[Delchi]

Ah!--it's obviously an area of some dispute.

VTEIT: the hook is not to develop the bar out in the wall, but rather to develop a tension-controlled section at the corner itself, which resists *negative* moment.

SplitRings: I've also added corner bars in between the horizontal reinf to tighten the spacing by 1/2, since it's what I leared from the beginning from other engineers, but I've also wondered how it was possible to develop the bars.

Dave: You've hit my concern. I am concerned about the discrepancy between "standard practice" and the code requirements. It seems there is a danger of the bars spalling out the inside corner of concrete.

 

[Delchi]

miesc: I read throught the previous thread you referenced--Thanks. However, I don't think it addresses walls that are as thin as 8" - 10", unless I missed something.

 

[Delchi]

Mike: Yeah, I know that embed plates with weldable rebar would work. But I would end up slaughtered, butchered, and cast into the concrete wall during the reinf observation, and that that would cause voids in the wall...

 

[HeavyCivil]

Sure - I understand different detailing practices for corners. There was a brief article in Concrete International a couple issues ago on the subject.

But I'm still not seeing a problem. Lets say you have #5 H bars and have #5 crn bars extending 24" past each inside corner. Class b splice = 24" for that so it's good to go, correct? Why does it need confinement?

 

[DaveAtkins]

VTEIT,

The problem is you can't extend the bar 24" past the inside corner if the wall is only 8" or 9" thick. You will only be able to extend it 6" or 7" to within 2" of the opposite face. The hook development length of a #5 bar is 8.4", so you cannot fully develop the bar.

DaveAtkins

 

[miecz]

Delchi

Strictly speaking, the detail that I showed does not provide development according to any provision in ACI. However, the hairpin detail provides so much embedment, that I'm certain that it works.

 

[Splitrings]

Fig R12.10.6 on page 218 of ACI 318-08 and fig. R12.12 in ACI 318-05 show what is being discussed. The dimension ldh for a #5 bar with 3000 psi concrete is in the neighborhood of 12". Therefore your wall would need to be at least 14" thick. So, the question is what do you do when your wall is less than 14", like 8"?

 

[jike]

DaveAtkins hit upon the problem. You need a certain amount of depth in which to develop a standard hook. An 8 or 10" wall may not have enough depth to fully develop this bar.

Options are:
to reduce hook embedment by required by
As req'd / As provided
use smaller bars
provide thicker walls
increase corner thickness with triangular fillet

 

[HeavyCivil]

At the risk of beating a dead horse when everyone else gets it and I dont....


Attached is a detail I recently did for deep foundation (not actually a "deep" foundation but it has very high walls). Bars are not hooked - similar to hairpins in that they are just deeply embedded.

Are you saying that because they are not hooked @ corners that they are not developed there, and thus do not fully resist negative moments there?

 

[HeavyCivil]

Note that intersection is a rectangular tank connected to building but under very different loading.

 

[Delchi]

VTEIT, That's the concern. For a #8 bar, the required Ldh is at least 15.3" (if you have more than 2" cover on the back side). With an 8" bar with 2" end cover in a 16" wall, you only have 13" of available Ldh at the most, and less if you can't get those hooks right out to the 2" limit.

 

[Delchi]

Whoops-just re-read your question. The problem is not that the straight bars aren't hooked, but that the hooks themselves don't have sufficent Ldh. (And neither would the straight bars even if they were hooked.)

 

[DaveAtkins]

VTEIT's example just makes it. ldh = 13.3" for a #8 bar, if you use the 0.7 factor from 12.5.3(a).

DaveAtkins