More Strut and Tie Questions

ron9876 · Feb 8, 2017

I have a couple of questions please.

First I am designing some very heavy transfer beams. The beams are wider than the columns/piers. ACI 318-11 commentary for section A.2.3 in the last paragraph discusses the possible need for reinforcement perpendicular to the beam face thru the nodal zone to prevent splitting of the nodal zone as the load spreads from the column/pier to the width of the transfer beam. It says you "may" need reinforcement which is an odd thing to put in a code in my opinion. My problem is that the quantity of reinforcement required--maybe 20 sq. in.--is prohibitive.

Second ACI 318-11 seems to imply that the member you are designing should be large enough to have less than 10 x sq. rt. f'c shear stress due to the applied load. But what about a condition where you have an applied load whose center is above but near the edge of a drilled pier and all you are doing is moving the applied load to the center of the drilled pier. From this loading there isn't any real shear in the conventional sense. It doesn't make sense to me to increase the depth of the element for a condition that doesn't really exist.

Does anyone have any background or any input about these conditions?

Thanks in advance.

slickdeals · Feb 8, 2017

For your first question, you would need to create another S&T model. I would treat this similar to what you would need to do in an anchorage zone of a PT tendon. I believe ACI Chapter 18 provides some guidance.

For your second question, would imagine you could treat it similar to what CRSI does for deep beam shear, which I believe even lets you go up as high as 32 root f'c. In your scenario, I would be hard pressed to imagine that you will need to provide a depth to meet the 10 root f'c requirement.

rapt · Feb 8, 2017

RE the maximum shear question, I am not sure about the background to ACI on this but other codes (Eurocode, BS8110, AS3600) require the maximum shear to be checked as vertical shear at the face of the support, disregarding any direct load transfer that might occur due to the short shear span.

ron9876 · Feb 8, 2017

Slickdeals I did the model you mentioned and that is where the large area of steel comes from. You can't place that much steel within the nodal zone in this condition. That left me wondering what other folks do in this situation.

Rapt in my case the face the outside face of the vertical element is slightly inboard of the face of a drilled pier. The pier is curved so are you go around the curve some will be outside the footprint of the pier but not much. It just can't really create shear stresses. That what seems crazy to me.

KootK · Feb 8, 2017

Sketches below show how I might view things.

OP said:
My problem is that the quantity of reinforcement required--maybe 20 sq. in.--is prohibitive.

Does this not come down to essentially the bottle strut problem? If so, I'd proportion the beam to the limit of A3.2.2b and eliminate the need for the transverse reinforcing. It's only a 20% hit.

OP said:
It doesn't make sense to me to increase the depth of the element for a condition that doesn't really exist.

See my recommendation in the second sketch below. Shouldn't be a big deal.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

ron9876 · Feb 9, 2017

KootK,

I have analyzed the struts with the 0.6 factor as you recommend. Had to put piers on some columns to enlarge the nodal zones so the struts would work but those are resolved. But the statement in the commentary is about nodal zones. I analyzed the nodal zone as you show in your first sketch and that is what resulted in the large quantity of reinforcement. There is so much steel that you can't get it in the nodal zone. If it were added it would have to extend well away from the nodal zone to meet spacing criteria or just simply looking at it and asking can they get concrete around the steel.

Your second sketch seems like a good idea and I will check that approach. This part of the design seems crazy to me. If the entire load is above the support the code seems to imply that the shear criteria would be required but that doesn't make sense to me--if a column was directly over a support none of us would think that there is any shear to talk about.

Thanks for the input.

KootK · Feb 9, 2017

ron9876 said:
But the statement in the commentary is about nodal zones.

That's the thing, I'm not sure that it is about nodal zones in the way that you're thinking. I read it as straight up bottle strut business. As such, I contend that the 0.6 relieves you of the obligation to install any transverse reinforcing.

ron9876 said:
If the entire load is above the support the code seems to imply that the shear criteria would be required but that doesn't make sense to me--if a column was directly over a support none of us would think that there is any shear to talk about.

Does none of the column extend beyond the pier below? The "x" dimension in my sketch is <=0? If so, then I'd agree. You described your situation as the center of the column being very near the edge which lead me to believe that there was a positive "x" dimension in play.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

ron9876 · Feb 9, 2017

KootK,

Yes most of the column is over the pier. I looked at like you said and I think that works.

You may be right about the transverse reinforcement. But it looks like to me that they are talking about the node itself. And it says may be required. The strut and tie method is nice to have for these kinds of conditions but the code sure leaves a lot to the imagination.

KootK · Feb 9, 2017

Yeah, the particular code clause that you referenced could certainly use some clarification. As not quite evidence that the clause isn't meant to apply to the node proper, I submit the following.

1) It's very common to have load delivered to STM designed members over only part of those members' widths. If that clause applied only to the node area, I'd think that we'd be seeing atrociously congested transverse reinforcement in members all over the place. I've not seen it once.

2) The purpose of the transverse reinforcement is to increase the concrete strength locally through confinement. At an externally loaded node, you've often got pretty good confinement available on the loaded face by way of friction with, and reinforcement in, the loading member.

3) The code cause suggests that an STM model be used to deal with the splitting issue. What kind of STM would one use over just the space available in a single node? You'd have to put nodes within the node wouldn't you?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

ron9876 · Feb 9, 2017

Good points one and all. Yes you would have to put nodes in the nodal zone and the reinforcement would have to go in the nodal zone to be effective. Thanks for all the input.

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