Lap Splice Requirements
Lap Splice Requirements
(OP)
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?
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?






RE: Lap Splice Requirements
Mike McCann
MMC Engineering
RE: Lap Splice Requirements
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.
RE: Lap Splice Requirements
RE: Lap Splice Requirements
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
RE: Lap Splice Requirements
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?
RE: Lap Splice Requirements
If it is further than that, then yes, some sort of cross ties might be in order.
RE: Lap Splice Requirements
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.
RE: Lap Splice Requirements
Since no EOR has ever made them demo a footing and start over to correct this they will keep doing it.