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Epoxy Set Anchors - Drilling Thru Re-bar Concern 3

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FLCraneBuilder

FLCraneBuilder

Industrial
Sep 10, 2006
93
For setting Overhead crane columns, We prefer to Epoxy set anchors after the Slab (or footers) are cured
This is because cast in place anchors are rarely done correctly - crane system alignment is critical to function of crane.
A situation has arisen where the foundation installer cannot tell us where the rebar is, and the foundation engineer forbids us from drilling thru the rebar if we encounter it. (when we encounter it).
I'm sure in the past, we ended up drilling thru the rebar more often than I should admit... and its never been a problem (yet?)

I feel that if one drills thru (basically cuts) the bar, the combined presence of the steel anchor and the epoxy material "re-creates" the continuity of the bar.

I recognize the shear strength of the epoxy is less than the rebar steel, but the Epoxy is usually stronger than the anchors... so....

A few more details:
-- 7/8" dia steel threaded rods for anchors
-- B-7 Material (higher strength material)
-- 8" imbed
-- Anchor pull out is approx 5200# - (The columns see moment loads)
-- Rated capacity of anchor/epoxy combination is 9600#
-- Our guys are meticulous about clean holes, proper epoxy setting to achieve bond etc
-- First layer of rebar is 3" below top of footer, second layer 13" below top of footer

Any experiences or opinions on this would be appreciated
BTW - I did a thread search and did not find anything
 
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Force in the rebar is generally tension epoxy touching just the exposed end of the bar will not be adequate to transfer the tension between the bar ends. In general you should never be cutting or damaging the reinforcement without explicit direction of being allowed to do so by the EOR.

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Open Source Structural Applications:
 
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It's typically not the shear or compression strength of the rebar that is needed - it's the tension strength, which will not be mitigated by the epoxy. A bar that is drilled through is discontinuous and not effective for transferring tension. If the rebar is required to provide tensile resistance at the drilling location, it will have to be avoided.

There are several scanning techniques available to map the locations of reinforcing steel in concrete.
 
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OP said:
I feel that if one drills thru (basically cuts) the bar, the combined presence of the steel anchor and the epoxy material "re-creates" the continuity of the bar.
Click to expand...

Hell no. The bar is shot for tension. As the other guys have mentioned, there's just no way that you'll be able to develop the cut segments of bar into the epoxy sufficiently. Other comments:

1) A good foundation designer would have designed the foundation such that a couple of bars could be sacrificed if need be.

2) In the absence of #1, I guess you've got to scan the rebar and, if necessary, create a custom base plate that dodges it.

3) If it's appropriate for your situation, you can sometimes do some creative slotting of the base plate holes to make the odds of being forced to hit rebar very low with a stock base plate. This may still require scanning, however, as you probably don't want abandoned holes near your final hole.

 
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Since when is the steel in the top of a typical footing in tension?
 
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It happens for overturning and uplift loads sometimes. Of course, that makes it a non-typical footing.
 
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Get it scanned. This is precisely what rebar scanners are typically utilized for.

The real trick will be what do you do when you find a conflict? Larger baseplate? Variable spacing on the columns? Slotted baseplate holes? Etc.

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL)
American Concrete Industries
 
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Thank you guys for your thoughts on this
The way you look at it makes perfect sense to me

Oldest Guy - we see top matt of bars quite often for over turning loads due to free standing columns

Agreed on Scanning, but no one wants to pay for it - our problem & we will deal with it somehow
there are over 100 anchors n this job - (ugh!)

We are considering extra holes in our base plates - but appearance is questionable... and we have to rely on installer to not hit the bar

Again - Thanks !!
 
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I'd lean on the EOR to see if they can let a couple of bars go. I struggle to imagine a practical footing design that couldn't tolerate that.
 
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There are devices commercially available that will instantly cut power to the drill if the bit makes contact with rebar. Might be worth investing in one of those if you do this kind of thing a lot.

I worked on a project that had hundreds of post installed wall plates. The process was scan the wall and design custom plates to avoid the rebar if necessary. Also used the device mentioned above.

If the rebar is for temperature/shrinkage only, I generally allow the contractor to cut the bars. Curious how others feel about that.
 
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Agree with others that you cannot just cut bars willy-nilly. But if you use percussion drilling rather than coring, steel bars will stop the bit, making drilling through very difficult.

The most important concept here is that the moment capacity at the column base depends not just on the column and its anchorage, but on the footing as well. And the footing's ability to resist moment depends on its size and its reinforcement.

 
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I assume the foundation is already cast for this project, but if it comes up again, I'd suggest:

1) If the exact location for the anchor bolts is known, make them provide and use a template for the base plate, with the anchor bolts attached, positioned precisely and secured in place, before pouring.

2) If the exact location for the anchors is not known, provide a base plate with oversized holes and plate washer of sufficient size and thickness to accommodate the tolerance on placement.
 
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"There are devices commercially available that will instantly cut power to the drill if the bit makes contact with rebar."

Besides not damaging the rebar, using one of these also has the advantage of being much safer for the guy handling the drill. I've seen a worker end up in the hospital when the rotary hammer drill he was operating hit a rebar, locked up, and twisted around.
 
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Prior to engineering, i spent a few years in contracting that put me in frequent use of a rotary impact drill (AKA hammer drill)

1) most trades drill into concrete without asking. most engineers seem unaware of this.

2) if you hit steel with your drill bit, you aren't going to be drilling through it. you will just fry the drill bit. hammer drill bits aren'tn that cheap, especially the big ones.

HotRod10, I have never seen a drill lock when it hit steel. steel on steel is smooth contact. I've seen the drill lock when the side of the bit snags a particularily tough bit of aggregate protruding from the side of the hole. without knowing the specifics of the injury you're discussing, I'm inclined to blame the worker for that one, for not knowing how to operate a drill. likely a good ribbing from his workmates would follow once he got back to the jobsite.

These comments apply to drilling, not coring, which is an entirely different beast and is treated with much more care from my experience.
 
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As others have noted, do not cut the rebar. As hokie66 noted, it all has to work as a system and what the OP suggested breaches that system.

Further, with post-installed epoxy anchors you have to pay attention to both the air temperature and concrete temperature at installation. Assuming this application is in Florida, that can even be a problem here. I did a failure investigation on a series of post-installed epoxy anchors that were installed in Central Florida in the winter. The temperature was low enough that the epoxy had not set up properly when the contractor started to plumb the columns. One of the columns was pulled over in the process and killed one of the workers. My point is pay attention to ALL the details when using epoxy set anchors.
 
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