Tension Lap Splice 1

[miecz]

I'm often faced with this situation:

At the bottom of a tank or retaining wall, I need #10 bars at 10" O.C. At 4 feet above the base mat or footing, I need #8 bars at 10" O.C. So, I'm splicing #8s to #10s. What's the splice length based on, a #8 or a #10? Is there a published solution to this condition?

 

[csd72]

go back to first principles and figure out where you need to develop the strength of a #8 and where you need to develop the strength of the #10 this will tell you the answer to your question.

This is a fundamental detailing issue that can occur thousands of times on the one building, so you really should try and understand it yourself rather than being told how it should work.

 

[WillisV]

Typically it is based on whichever is greater: the development length of the larger bar or the lap length of the smaller bar.

 

[miecz]

WillisV

Thank you, as this is what I've always done. Only, this practice has been called into question by my superiors, management types who have lost touch. (I'm being asked to provide the splice length of the larger bar.) I sure could use a reference to back up your response. All of my references manage to skirt the issue.

 

[csd72]

jmiec,

Sorry if I sounded condescending, it wasnt my intention.

A couple of points:

1. the management types probably mean splice length not development length - some foreign codes do not differentiate between the 2 and perhaps the old ACI codes did not either.
2. Think about how this is going to be built - the builder will probably rest the bottom of the bar on the pour joint at the top of the slab giving a greater lap than necessary.
3. Do not argue with management if they are being conservative - you will have plenty of opportunity to argue with them when they ask you to do something that is unsafe!
3. Even the out of it management types understand first principles, so if you need to prove your point do method of sections at the top of the footing and at the top of the #10 bar(the two critical sections for bar stress) and show what is required to obtain the bar stress (with little stress diagrams if necessary).

Regards
csd

 

[miecz]

csd72-

1. They do mean splice length: "Why not just treat it like two #10s?"

2. I detail the splice from the top of the footing.

3. Very good point.

4. I tried this, but it was, uh, too technical. That's why I need a reference.

 

[WillisV]

Concrete Reinforcing Steel Institute (CRSI) - Manual of Standard Practice (MSP-1-86) - Chapter 4, p4-3 - "the detailer will use the appropriate tension lap splice for the smaller bar, or tension development length for the larger bar, unless otherwise noted."

This is from an older version (1986) but I would think the newer printings have a similiar statement.

 

[WillisV]

And in a practical sense the absolute most force that can transfer from one bar to another would be based on the maximum force that the smaller bar could handle - therefore it makes no sense to use a lap length any longer than that for the smaller bar.

 

[miecz]

WillisV-

Thank you, just what I need. Often, when an issue is raised, the first question is "which way is more conservative?". What "makes sense to me" isn't always enough to counter that. But a respected reference is.

 

[swivel63]

splice with the #10.

i've always spliced with the bigger bar. but i guess you can check if you actually need that length.

 

[jike]

A detailing idea that you might consider in the future is to use #8 foundation dowels at 6" oc and then #8 at 12" oc above the base rather than to introduce another bar size.

 

[henri2]

ACI 318 Sec 12.16.2 deals with lap splices for bars of different sizes in compression. There must be a similar provisions for tension lap splices...no?

 

[miecz]

jike-

Yes, that works in many cases. Sometimes, though, I need to step bar sizes.

henri2-

If there's an explicit provision for tension lap splices, I'm not aware of it. As you can see from the above responses, engineers have different solutions.

 

[mitchelon]

jmiec,

For the usual case of dowel bars which are smaller in diam than the column bars, the minimum dowel embedment length into the column must the larger of the compression development length of the column bar (ACI 12.3.2) or the compression lap splice length of the dowel bar (ACI 12.16.2). In addition, the foundation-vertical member connection shall be adequate to transfer/resist any computed tensile forces (ACI 15.8.1.2). I can tell you right now that it is not going to be based on the larger or smaller size bar. There are different requirements for steel in compression, dowels or mechanical connectors, and steel in compression and you have to choose the largest of all development and splice lengths.