Retaining Wall Footing Constructed Wrong? 9

[XR250]

The contractor has the hook pointing toward the heel but add similar bottom bars to compensate. Is there a reason this is not OK?

 

[Celt83]

you'll get reduced joint performance.

I'm making a thing:

http://www.thestructuraltoolbox.com

(It's no Kootware and it will probably break but it's alive!)

 

[WARose]

Great post Celt83

 

[XR250]

Which case do you consider is mine, Celt83?

 

[JoshPlumSE]

Just make sure you've got enough development length for that bar... Based on the demand at the wall / footing interface. If so, then I think you should be good.

 

[BridgeSmith]

Which case do you consider is mine, Celt83?

I believe the configuration that's effectively closest to yours is U74, which has only about 60% of the strength at failure of U77, which is what you would have if the hook was turned into the toe, where the stem provides compression force into the concrete over the hook.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[Celt83]

BridgeSmith beat me too it. I’d probably say your condition is marginally better than U74 since the bottom bar even though not fully developed will provide some minimal tie restraint for the compression strut formed from the stem compression block.

I'm making a thing:

http://www.thestructuraltoolbox.com

(It's no Kootware and it will probably break but it's alive!)

 

[Tomfh]

Yes a sharper bend with a corner bar would likely be better than the U74 bend. It's likely in that U73/U74 territory, i.e. ~60-70% of the calculated capacity.

All that being said, the hook pointing towards the heel is a fairly common detail. It all depends how hard it's working.

 

[KootK]

Is there a reason this is not OK?

It's fundamentally bass-awards concrete detailing and the subject of one of the most contentious threads we've ever had here: Link. I guessing that's where Celt pulled the test data from. I wouldn't let this configuration remain if it were a more significant retaining wall but, at the scale of this wall, I can get behind the reduction business.

In my opinion, the extra bottom bars -- while well intentioned -- accomplish very little and the 40% reduction associated with U74 reflects the fundamental nature of the situation having changed: it's now an anchorage problem rather than a reinforced concrete problem. That doesn't mean that it's worthless... just worth less.

 

[BridgeSmith]

It's fundamentally bass-awards concrete detailing...

I agree. I haven't read the other thread you linked to, but for the life of me I can't understand the advantage of turning the hook into the heel, weakening the connection, and then adding reinforcement in the bottom of the footing, that would be unnecessary if the hook extended into the toe instead.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[Tomfh]

In smaller walls with small toes there’s often little room to run a hook into the toe.

 

[kissymoose]

In smaller walls with small toes there’s often little room to run a hook into the toe.

U76 has a toe length of just 4in (though says 10in in the picture, nvm it's 10cm), and still sits at 94%.

 

[Tomfh]

Yeah fair enough. I guess there's no excuse.

 

[Settingsun]

U73 and U74 have no reinforcement on the tension face of the toe? I wonder what Nilsson was trying to prove.

 

[Tomfh]

no reinforcement on the tension face of the toe?

Good point. XR250's bottom bar crosses the main crack. It'd be nice to see those test results. It's not hard to imagine that bar adding the missing ~30%

 

[Celt83]

KootK: that's a solid thread added to my archive thanks, I had the Nilsson paper for some detailing research recently.

It's not hard to imagine that bar adding the missing ~30%

I don't believe so, in my understanding of a strut-tie approach turning the hook the other direction creates a compression strut from the curved bar node into the heel leading to a required vertical concrete tension tie around where the mid-depth crack is shown on the U73 specimen.

I'm making a thing:

http://www.thestructuraltoolbox.com

(It's no Kootware and it will probably break but it's alive!)

 

[MotorCity]

If ACI's not worried about it, neither am I. In reality, if you had X-ray vision I think you would find that there are installations with the hooks in both directions and the walls are working satisfactorily as designed.

 

[KootK]

If ACI's not worried about it, neither am I.

ACI is "not worried" about this because, for the most part, they don't consider it their business to teach concrete designers the fundamentals of concrete detailing. That's the job of a concrete designer herself, in concert with her educational and professional mentors. I feel that it is spurious to imply that ACI somehow condones this detailing practice simply because they haven't explicitly prohibited it.

To my knowledge, ACI only really addresses moment joint resistance in the context of beam to column joints. There are entire ACI publications dedicated to the topic, as you surely know. And in none of them, anywhere, will you see anything like the sketch below.

 

[KootK]

...the 40% reduction associated with U74 reflects the fundamental nature of the situation having changed: it's now an anchorage problem rather than a reinforced concrete problem.

I'd not made this connection explicitly prior to this thread. And I feel that it's both accurate and significant. If the bass-akwards hooking transmogrifies the connection into appendix D style anchorage rather than reinforced concrete design, then the efficiency number should really be treated like this:

Joint Moment Capacity = Cross Sectional Moment Capacity x 0.6 / 0.9 x 0.55 = 0.37.

This, because the reliability of an anchorage joint is much less than that of a true reinforced concrete joint.

And, really, 0.37 aligns much better with my expectations for joint performance than does 0.6. This make intuitive sense to me.

 

[KootK]

Moreover, I feel that a reasonable way to evaluate the joint would be simply as a tension breakout condition on post installed rebar helped by the presence of the wall flexural compression reaction falling on the breakout frustum. We have tools for this, annoying and unpopular as they may be.