Rebar Detailing

[JRosso01]

I am on a job in NYC so all rebar bending is done on site.

I have a rebar detailer who detailed a bar with two bends. The drawing shows a 31'-08" bar. The delivery ticket shows a 31'-08" bar but the actual length is 31'-5.75". The bar tag says 31'-5.75" as well. He is telling me that the fabrication yard accounted for the "creep" when bars are bent and trimmed them accordingly.

Has anyone ever heard of this? I thought the bars were cut as detailed.
Thank you.

 

[DST148]

On bar bending schedules, the dimensions are given out-to-out of bar except on standard hooks. Assuming bar shape 17, the theoretical cutting length of the bar would be (B + C + D - curvature effect @ each bend). In practice the curvature effects are neglected.

 

[dik]

Are they setting the length based on allowable tolerances for the final bar size? Providing a bar that will just 'fit' when permissable tolerances are included. If it fits drawings and specs, contractually, you have bought a bunch of rebar... any that are non-compliant can be rejected outright.

Dik

 

[JRosso01]

We will find out when we bend and install. Have you ever heard of this though? Is there a structural standard that allows for this reduction? I am not familiar with CRSI standards.

 

[Teguci]

unless your bend radii for the 2 90 degs are less than 2 5/8", you should be OK. When I detail bars, I use out to out dimensions. As DST noted though, if you provide a bar at that length, the final bent bar will exceed those dimensions due to curvature effects and you will lose your cover.

I think you'll be OK.

 

[TXStructural]

You say the bars have two bends - they aren't bent on both ends are they? This a typically a poor practice, since doing so requires cuts and bends to be very precise to maintain the CRSI and ACI 117 tolerances, and well as requiring formwork to be very precise, so that cover is not compromised. Better practice is to use either corner bars or a splice in the bar to allow easier placement and for adjustment to maintain required cover. Obviously, some designs may not work with laps, but there are frequently reasons that double-ended bars may not work well on a construction site.

As to the detailed length versus the delivered length, I'm not sure why they would ship a bar which was different from the length specified, plus or minus the tolerance. It would not be unusual for bars to be cut at the minimum allowed length (contract length minus tolerance.)

As for the allowable tolerances, the finished overall length of the bar is +/- 1" and the finished length of the bent ends (for 90 degree ends) is +/- 1", for #3 through #11 bars, The tolerances on #14 is +/- 2-1/2", and for #18 the tolerance is +/- 3-1/2".
If you are inspecting rebar, you should have a copy of appropriate publications handy.

CRSI/ACI 315 is being revised to make clear how/where to measure dimensions and tolerances (generally out-to-out.) The bending tolerances reported in ACI 117 and expected in the design provisions of 318 originate in committee 315 (joint ACI/CRSI committee.)

The bend radius is as specified in CRSI and ACI, and should never be smaller than the code permits. Tighter bends risk cracking and inadequate continuity of the bar through the bend radius, as well as encouraging crushing of the concrete on the inside of the bend.

 

[DST148]

@JRosso01 - For basic understanding you can calculate the theoretical cutting length of the bar measured along the center line of the bent bar. I have made some assumptions here:
Bar Shape 17, #8 Bar, 2 x 90 degrees standard bends, internal radius of bend = 3 x diam. of bar
B = D = 16 x diam = 16 x 1" = 1' - 4" , C = 29' - 0".
Reduction @ 90 bend, measured along the center line of the bar = 2.5 x diam (curvature effect. You can calculate this mathematically)
Theoretical cutting length of the bar = B + C + D - 2(2.5 x diam)
= 1' - 4" + 29' - 0" + 1' - 4" - 2(2.5 x 1") = (31' - 8") - (5") = 31' - 3".
Tolerances apart, you would get correct covers etc. only if the bar is cut to length 31' - 3" and not 31' - 8".