Lap Splice Requirements

[slickdeals]

Folks,
I have come across this happening more than once. Dowels (starter bars) in foundations often end up being placed too far in into the column cage. The spliced bars are often 3-4" away from the dowels.

I looked in the ACI code but found no specific requirements for columns. It says that bars in flexural members should not be 6" away from each other (because the unreinforced region will crack during force transfer). I consider a column to be a flexural member (carrying compressive forces).

I presume that providing ties closely spaced over this splice region (say at 6" o.c) will mitigate any cracking by keeping the concrete locked in (analogous to shear friction). What is your take on this?

 

[msquared48]

If I understand what you are saying here, if there is a moment transter to be accomplished, then I would be concerned that enough "d" distance had been provided if the bars were mislocated to that extent, possibly drastically lowering the moment capacity.

Mike McCann
MMC Engineering

 

[JAE]

slickdeals,
Here's what I do:

The typical ACI detail for column vertical bars is to splice them just above each floor line. This is usually accomplished by having the main column corner bars bend in at a 1:6 pitch and then extend up into the floor structure, then above the floor line the required splice distance.

The next story cage is then set with its base fully on the outside of the cage such that the lower column bars are supposed to be just inside the upper column bars. What you are describing is a case where these offset splice bars are offset too much and thus you have the gap between the spliced bars.

ACI also allows an inverted splice detail where the lower column bars simply extend straight up through the floor and into the next column. The upper column bars are thus spliced inside the lower column bars and the 1:6 tapered pitch occurs just above the splice, usually near the middle 1/3 of the column height.

This inverted detail avoids a couple of problems with the traditional method:
1. The tapered vertical bars don't taper within the floor framing and thus don't interfere as much with the longitudinal floor bars.
2. The highest moment in a column is typically at the floor line and so your "d" distance of your column right at the floor line is not diminished by the tapered lower bars.
3. The splice occurs near the mid-height of the column where the moment typically is small.

In your case you have a foundation below and I presume "L" shaped dowels which simply get placed in the wrong spot. What you could possibly do is use longer dowels and the inverted splice. This way the contractor simply places the cage and corner bars (with fixed ties too) in the right spot, assuming 1 1/2" clear, etc.

Then, when they place the upper column bars they can simply tie them on right where they belong.

Don't know if this works out perfectly with every contractor but it makes some sense to me.

 

[hokie66]

Where a column bears on a footing, whether or not the starter bar placement is critical depends on whether there is tension in the bars in any load case. If not, the column will deliver the compressive load to the footing in bearing.

 

[BAretired]

JAE,

I like your idea of the inverted splice detail. Never thought about doing that before. I think I will use it on my next job. Oh, wait...I'm retired.

Slickdeals,

It might be a good idea to drill and epoxy some additional dowels a bit closer to the column steel.

BA

 

[slickdeals]

Let me ask you the following question.

Assuming that the column is over-designed considerably and having the starter bars further in (with reduced "d") still works, what additional steps will need to be taken such that the force from the bars above get transferred into concrete and then into the starter bars without cracking the concrete?

Will additional ties to prevent a 45 degree tensile crack from occurring in the transfer region be sufficient?

 

[JAE]

If you are within the 6" limit that ACI suggests for gapped splices (12.14.2.3), then you probably don't have to do anything.

If it is further than that, then yes, some sort of cross ties might be in order.

 

[doka1]

in foundation construction I usually offsett column bars in the footing. That way when columns are built out of place and stuffed, it makes it easier to place and looks the right way. The rebar detailer would only bend (taper) the column bars for a high rise, where we are building the same columns for multiple floors, or the second level of a column (not possible to offset since the bar is already coming out of concrete).

sometimes problems occur when the wrong stirrup is used in setup (as is probably your case) or if they used wooden templates, the carpenters didnt tell the ironworkers the right side of the template to use.

 

[boffintech]

What you probably have is a rodbuster internationally placing these dowels (starter-bars) too far in for ease of placement of the pre-tied column cage. The further in the starter bars are the easier it's going to be to drop on the column cage.

Since no EOR has ever made them demo a footing and start over to correct this they will keep doing it.