Development or splice length of reinforcement ? 2

[CivilSigma]

Hi everyone,

I am having a hard time understanding when to specify "development" or "splice length" in reinforced concrete design.

I understand that:
[ul]
[li]Development lengths are specified to prevent slippage failure, and transfer stress from reinforcement into concrete.[/li]
[li]Splice lengths are specified to prevent slippage between lapped reinforcement and transfer full stress across the bars.[/li]
[/ul]

I am trying to resist uplift forces in a pier cast ontop of slab using reinforcement (see sketch).

If we are analyzing the load path in tension (uplift):
[ol 1]
[li] Tension is resisted by the embed plate rods.[/li]
[li] Then force is resisted by the vertical pier reinforcement. [/li]
[li] Then force is resisted by dowels embeded into the slab.[/li]
[/ol]

Clause 12.1.1 of A23.3 says - "The calculated tension or compression in reinforcement at each section of reinforced concrete members shall be developed om each side of that section by embedment length, hook, or mechanical device, or by combination thereof. Hooks may be used in developing bars in tension only"

Doe that mean I need to specify development length of the reinforcement in the pier since it is considered a "section" other than the slab?

 

[KootK]

Usually you want to:

1) Effectively offset lap your anchors with your pier reinforcement.

2) Develop (hook or straight) your pier bars into your footing.

This paper by Widianto is a commonly used reference for this technique: Link

More or less the same information is presented in ASCE's guide for petrochemical facilities I believe.

AISC's design guide #1 also covers similar subject matter.

A hook developed into a footing doesn't necessarily mean that the rebar can't pull out of the footing but, in many practical cases where the pier bars are well spaced, it seems to get the job done.

 

[CivilSigma]

Kootk, thank you very much. I will study this paper and report back.

 

[DaveAtkins]

My understanding is that "splice" refers to two reinforcing bars lapped with one another. Because they are generally positioned close to one another, each bar is not surrounded by concrete as completely as it should be. Hence, the length to fully develop each bar is 30% longer.

When you have different elements which are not positioned close to one another, "development length" applies. Each element must be developed.

DaveAtkins

 

[DoubleStud]

1. this is appendix D because it is has a head at the end so it spreads the load more. I say you draw 135 degree from the head, that's where your hook development begins.
2. I am not really sure the detail. A bit confusing here. Where does the hook go?
3. you do hook development length.

 

[BridgeSmith]

I'm not sure of the exact shear flow that requires more length for a lap, but we have a similar situation to the anchor bolt to reinforcing overlap in our high mast pole and sign foundation structures. Under AASHTO, we have decided that the proper way to calculate the overlap is a lap length for the reinforcing + the distance between the anchor bolt circle radius and the reinforcing cage radius.

Not sure about other specs/codes, but my reading of AASHTO is fairly clear the reinforcing should be developed in the footing, so at least the hook development length should be provided measured from the top of the footing, assuming there's a cold joint between the footing and pier. I wouldn't do less, even if there isn't a cold joint.

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

 

[le99]

In your sketch, the "embedment" of the red main reinforcement is too short above the potential crack plane to develop the required strength. You can extend it to the top and fully "develop" the tension bars or, if it is still too short, "splice" it to the hooked bars. But I don't think the latter will work, because the required splice length is usually no less than the development length. As I am not looking at the equations, I could be wrong.

 

[Tomfh]

What’s the basis of the 30% figure? Is it derived from test results?

 

[DaveAtkins]

What’s the basis of the 30% figure? Is it derived from test results?

I thought I read that somewhere but I cannot find anything online or in ACI 318.

DaveAtkins

 

[KootK]

What’s the basis of the 30% figure? Is it derived from test results?

Definitely test results and intended to capture two effects primarily:

1) When the bars are literally in contact, part of the perimeter of the bars is no longer available for bond with the concrete.

2) When the bars are as far apart as code allows (6"), the tension in one bar is passed to the neighboring bar via compression struts in the concrete that require some to form. Like the sketch below but without the cross ties.

At the extremes, it's about one or the other of these effects. In between, it's a combination of the two potentially.

PS: I realize that you already know most, if not all, of this stuff already. This is mostly for the benefit of the conversation at large. Your question simply made for a convenient springboard.

 

[KootK]

When you have different elements which are not positioned close to one another, "development length" applies. Each element must be developed.

That statement makes me a bit uncomfortable without a corollary with respect to element offsets that cannot be considered code compliant lap splices:

By itself, development of each element does not guarantee that tension can be successfully passed between those elements. To do that, one must make recourse to either reinforced concrete design principles (STM, Widianto, AISC Design Guide 01) or concrete anchorage theory (appendix D) in addition to developing both elements.

 

[KootK]

1. this is appendix D because it is has a head at the end so it spreads the load more. I say you draw 135 degree from the head, that's where your hook development begins.

It doesn't have to be appendix D and usually is not as that tends to lead to ridiculously sized piers. Methods such as Widanto's strut and tie method and the methods described in AISC DG01 usually produce much more efficient designs. The one respect in which appendix D is a bit unavoidable with respect to the anchor heads is in checking side face splitting / bursting. Perhaps that is what you were referring too. And, as Widianto shows, even that can be addressed via some obnoxious, spiral reinforcing.

 

[le99]

A "splice" is when one bar in the bar group is the continuation of another bar. In other words, the two pieces must be equal to "one integral bar" strength-wise. The splice length is the overlapping length.

The "development" is usually pointing to the end of the bar that is embedded in the concrete to develop the yield strength of the bar across a defined/critical plane on the other end, whether it is embedded in the concrete or not.
The development length is the length passing the defined/critical plane and measured to the developing end.

For spread footing with anchorage, you might run into situations as shown below:

 

[KootK]

A "splice" is when one bar in the bar group is the continuation of another bar. In other words, the two pieces must be equal to "one integral bar" strength-wise. The splice length is the overlapping length.

I feel that is too restrictive. In my view, and in the context of OP's question, a "splice" is the passing of tension from one reinforcing element to another without resorting to the use of concrete breakout frustums to resist direct tension. Here, the elements being spliced are:

1) The pier rebar and;

2) The anchors.

The business about development across critical planes is all just poor man's strut and tie design. It doesn't in any way change the nature of the problem as being, fundamentally, about lapping the pier rebar with the anchors.