Excess Reinforcement in TENSION member

[PostFrameSE]

I'm wondering if I'm looking at the ACI 318-05 correctly. Rather than using appendix D for connections I've seen where several authors have stated that that is too conservative when designing anchorage of steel to concrete. I have a pier with (16) #6 bars and I need to connect a steel tension member carrying 66kips. I'm using (6) 7/8" anchor bolts with nut and washer embedded in the concrete to accomplish this. What I'm attempting to do is calculate the overlap length of my pier reinforcing and my anchor bolts assuming a 45 degree failure plane of the concrete. So in other words, if my anchor bolts were embedded 30" in the concrete and they were 5" away from the pier reinforcement, then the overlap length of my anchor bolts/pier steel is 25". Due to the fact that the pier steel terminates 3" from the edge of the concrete, my effective steel overlap becomes 22".

The calculated development length for a Gr 60 #6 bar using 3,500psi concrete is 30.4" Am I correct in looking at Section 12.2.5 for flexural members and seeing that I can reduce my development length by As req'd / As provided? I have a tension member, but I'm thinking that this is applicable. If I do that and I calculate the amount of steel required to resist that 66kip load I come up with needing five #6 bars where (16) actually are provided. 5/16 = .3125. Can I take my 30.4" calculated development length and multiply that by .3125 and come up with a 9.5" development length? That translates to a dimension of actual embedment in concrete to the top of my nut/washer on my anchor bolt of 9.5" of development length plus 5" for my shear plane loss plus 3" for my pier coverage, or 17.5". Does that sound right? Am I looking at that correctly? My pier bars are not hooked at the top.

Thanks.

 

[KootK]

The prorating of the development length is fine although I don't normally do it as it as I prefer to keep my base connections as ductile as I can. The more important question in my mind is whether or not it's appropriate to use all of your rebar. Some is often further from the anchor bolts than is prudent. I've seen limitations to the tune of 6". Google a paper by Widianto. It's easy to find and is an excellent reference for this.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[structSU10]

If you haven't seen the attached paper, it may help answer some of your questions. You have to make sure the rebar is within a certain distance of the anchor rods to be considered effective.

 

[KootK]

I've probably read that paper a dozen times. I've just now noticed that the outermost stiffener in figure one is junk. The top isn't restrained by anything stiff.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[TXStructural]

You are spending dollars to save pennies. If you have 6 of these anchors, just embed the anchor rods sufficiently to develop the lap with the rebar and move along. The difference in cost is so very small that this is an academic exercise that creates risk to you and the contractor with no benefit to the owner.

That said, if you really want to minimize embed depth, use ACI 318 Appendix D and take credit for the reinforcement crossing the fracture planes. Again, probably a waste of your client/employer's money on small quantities of anchors.

 

[Hokie93]

You are on the right track but consider the following.
1.) The minimum development length per ACI 318 is 12".
2.) The maximum distance permitted between the centerline of the anchor rods and the pier reinforcing is 0.5*hef, where hef is the embedment depth of the anchor rods. This is covered graphically in ACI 318-08 Figure RD.5.2.9. This figure is not included in ACI 318-05.
3.) As an academic matter, section 12.2.3 will yield a lesser required development length than section 12.2.2 but this doesn't matter in your case since the 12" minimum value will govern.
4.) I would consider providing hooks on the tops of some of the pier longitudinal just to add a bit of redundancy and to help ensure their development.

 

[PostFrameSE]

Thanks all. I'm not wanting to shrink this down to a gnat's whisker.............I'm just wanting to make sure that I'm understanding this correctly. I will ultimately end up with more than enough overlap.

 

[KootK]

I don't recommend using hooks at the top of your pier reinforcement. They cause all manner of congestion problems and, for that reason, tend to get cut off in the field. If I can't get it done with straight bars, I'll consider headed rebar anchorage. The last thing you need at an important connection is lousy concrete.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[Hokie93]

I agree that congestion is a concern with hooks at the top of pier reinforcing. That is why I suggested adding hooks to some of the longitudinal bars. I would not add 90-degree hooks to 16 bars. One could also use 180-degree hooks or, as KootK suggested, headed reinforcement.

 

[TXStructural]

I was looking through a preview copy of ACI 318-14 and ran across this:

—Reduction of development length in accordance
with [excess reinforcement provisions]is not
permitted in calculating lap splice lengths

 

[RFreund]

Is it appropriate to extend the 35 degree failure line depicted in figure 3 of the Widianto paper to find Ld? What is the appropriate maximum distance from the anchor rod to the pier reinforcement?

To anyone: Don't you normally have a problem with the pier reinforcement being too far from the anchor bolts?

EIT

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