Development of column reinforcing in to footing 1

[JIMEY]

I have a concrete column which has tension in it when under seismic load. Obviously I am using the vertical reinforcement in the columns to carry all of the tension. The column is pin-connected to a pad footing at its base. The footing has a double mat of reinforcing (top and bottom) since it could be in positive or negative bending depending on the circumstance. Normally I would just hook the column reinforcing into the bottom of the footing (basically, I lap the hook with the bottom footing reinforcing). Since this column has tension in it, my question is about development length in this case. I need the column vertical steel to be fully developed because they are maxed out in their tension capacity. For the development length in the footing, do I take it as the distance from the bottom of the hook to the top of the footing? Or do I take it as the distance from the bottom of the hook to the top mat of footing reinforcing?

As an example, suppose I have a 14" thick footing, and the top and bottom mats of reinforcing are both 3" from the top/bottom of the footing. So the column vertical reinforcing is hooked (14" - 3") 11" into the footing. Am I safe to assume my development length for tension is 11"? Or do I have to take off another 3" to account for the inset of the top reinforcing as well?

I've always thought that in this case, the development length should only be 8" because the tension in the column steel needs to transfer to the footing steel, so it wouldn't start to be developed until it crosses it. But maybe I'm overthinking it. I'd appreciate other people's opinions. Thanks.

 

[KootK]

...basically, I lap the hook with the bottom footing reinforcing

That won't hurt anything but is probably unnecessary for the uplift case.

...the development length should only be 8" because the tension in the column steel needs to transfer to the footing steel, so it wouldn't start to be developed until it crosses it.

I don't think that's the case for the uplift situation. Here, I would consider the development length to extend to the top of the footing concrete.

This is an example of a situation that arises commonly here: a failure to recognize that bar development is not equivalent to bar anchorage. I believe that the failure mode shown below will be your critical check. I'd look at evaluating that as either:

1) A form of punching shear and/or;

2) An appendix D style anchorage problem.

 

[A.F]

The development starts at peak stress hence from top of the footing .
As per ACI Ldh[for hooks] depends on bar size,

 

[Celt83]

Article just published last month that now covers this topic:
[URL unfurl="true"]https://www.concrete.org/publications/internationalconcreteabstractsportal.aspx?m=details&id=51728058[/url]


My Personal Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

Open Source Structural GitHub Group:

https://github.com/open-struct-engineer

 

[JIMEY]

Thanks for the responses. After thinking about this some more, I believe you guys are totally right. It's a question of anchorage and whether or not the concrete is going to "break out". It's interesting how ACI 318 doesn't seem to directly address break out of cast-in hooked rebar (unless I'm missing it somewhere). To be conservative, I suppose I could assume it behaves similar to a grouted straight bar, and design it based on Appendix D.

 

[r13]

For footing design, shear reinforcement is usually not preferred, so the first step in design is to check one, and two way shear capacity of the concrete without reinforcement, with, or without moment. IMO, those checks eliminate the potential for concrete breakout, so there is no need to check it using App. D. However, since the column subjects to the moment at the base, thus the tension bars need to be developed per ACI provision regarding "development for hooked bars in tension". Then, to be classified as a standard hook with known capacity, you need to have adequate development length, L[sub]DH[/sub], measured from the yop face of tha base mat to the bottom of the hook tail, and a tail with code specified length. Missing either, the capacity of the connection is everybody's guess.

 

[dik]

I generally have the hooks going the other way...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[r13]

dik,

I believe your approach belongs to the category of "good wish and practice", that is something ACI cares little, at least for now.

 

[dik]

been doing it for decades... other than the first few projects... had a German engineer, Paul Krauss ask me why (I used to do it the other way) about 1970 and I explained it was the only way I had seen it... he then suggested the bars go into the compression area rather than the tension area... and I've done it the 'correct' way since... Paul has long since passed on... but, I still have a warm spot in my heart for him.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[r13]

dik,

He was correct, which has been upheld/confirmed by tests and papers. I believe the reason for ACI's inaction is due largely to the practical concern has outweigh the benefit in making this change. Since failure due to the tail directing outward is not a prevalent phenomenon thus far.

 

[STrctPono]

I am confused. OP states that this is a pinned connection with the foundation yet all the details being discussed leads me to believe otherwise....

If this is a pin, wouldn't you stop your vertical column reinforcing short of the joint and run one dowel down into the footing?

 

[r13]

I have a concrete column which has tension in it when under seismic load. Obviously I am using the vertical reinforcement in the columns to carry all of the tension.

Because the bars are required to resist tension.

 

[STrctPono]

r13, I am not sure if that response was directed toward me.... I am aware that there is tension in the column. My question still stands, "why not stop your vertical column reinforcing short of the joint and run one dowel down into the footing?"

You don't think you can transfer the tension load from the outside column steel to the dowels? You can develop a strut and tie mechanism if you add horizontal tie bars in the bottom of the column.

 

[dik]

A single bar is more of a 'pin'... but, maybe just slightly. it depends on the axial load, also...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[r13]

STrctPono,

Yes, it was mean to remind you why the subsequent discussion has shifted (not really, as the OP also talked about hook). The straight bar is fine, but it will require a thicker base mat than hooked bar. Also note the OP needs all bars in tension (I could be wrong on this), the equivalent single bar would be quite large. Hope this makes sense.

Don't get me wrong, I think your detail is excellent in achieving a near-pinned column support, but given the loading condition, I wonder why to make such connection, unless the uplift is small, or other concerns unknown to us.

 

[STrctPono]

dik, you're correct. There is still some moment capacity in this detail. You can approach this 2 ways.

1. Check that the plastic moment capacity of the connection does not exceed the limiting overturning or bearing capacity of the footing. Not sure if building code allows you to design for plastic hinges in column/footing connections like this. Something tells me no.
2. You place pre-molded joint filler around the perimeter of the column to as great a degree as needed to force the hinge. You essentially eliminate the ability to develop a compression stress block. I have done this for other designs before but it becomes a pain as the columns need to be braced during construction until the framing element above can be constructed so as to provide it support. This also might not work if your axial compression loads are very high.

All I'm saying is that the OP needs to address the pinned column to footing behavior. He's not getting it with the details he proposed.

 

[dik]

Because of potentially high compressive loads, there might be a moment transfer without any rebar...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik