All-Thread Rebars with Mechanical/Headed Anchor on End?? 1

[abusementpark]

I have a situation where I want to use all-thread A615 reinforcement bars (aka Dywidags) as anchor rods for a steel column with a high uplift load due to wind. I am trying to develop these bars (#9's) into a pile cap that is 3 feet deep. However, I am limited from developing the full length of the all-thread bar through straight length development due to the depth of the cap. So, I would like to put a mechanical anchor on the end in accordance with ASTM A970 to reduce the development length.

Does anyone have any experience doing this? I know there are a lot of different products out there for rebar end anchors, but they all seem to be for a regular A615 deformed bar. Can these products also be used for all-thread rebars? How are the threads usually forged at the end of the bars? Can this be done onsite with a special machine?

Any help would be greatly appreciated!

 

[msquared48]

Is this an augercast pile where you could just add a coupler to the center rebar and extend it up?

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[slickdeals]

What kind of uplift do you have? I assume you have 4 of these bars? That's a helluva lot of uplift...

 

[BAretired]

For ordinary all-thread bars, I have read that a nut and standard washer is adequate to fully develop the bar. Not sure about Dywidag bars, but Dywidag must have technical information available. If in doubt, use an anchor plate sized in accordance with the load to be resisted.


BA

 

[msquared48]

What I do not get here is 100 K or so of uplift to be resisted by one pile? Seems like an awful lot of uplift to me, let alone for only one pile.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[abusementpark]

Is this an augercast pile where you could just add a coupler to the center rebar and extend it up?

The piles in the cap are augercast, but I need to develop the force into the cap to distribute the force to all the piles.

What kind of uplift do you have? I assume you have 4 of these bars? That's a helluva lot of uplift...

It's about 280 kips. I plan on using 8 bars.

I had a job recently where the column uplift was a good bit more and we used #11 all-thread A615 anchors. But development was simpler, because each wind brace column was on top of a single 5'-0" diameter drilled shaft and the rods extended 25' down into the shaft.

For ordinary all-thread bars, I have read that a nut and standard washer is adequate to fully develop the bar.

If you are talking about regular threaded rods, I already tried to use those and can't make something reasonable work for the the ACI Appendix D anchorage provisions. I've found it easier to transfer large uplift forces through "development" of rebars as opposed to "anchorage" of regular threaded and nutted rods, despite intuition to the contrary.

Not sure about Dywidag bars, but Dywidag must have technical information available. If in doubt, use an anchor plate sized in accordance with the load to be resisted.

I looked on their website, but couldn't find anything for my specific application. I don't think they are in the rebar end anchor business. But I will try to give them a call next week to check.

My intuition tells me that using a nut and adequately sized anchor plate will be sufficient, but ACI notes that the anchors must be in accordance with ASTM A970 or be tested to prove their capacity or something like that (I forget the exact wording). I am just trying to ensure I do this by the book.

 

[msquared48]

Wow! And this is the net uplift?

Can you post a detail/plan of the rebar arrangement showing the pile, pilecap, and column positions?

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[abusementpark]

What I do not get here is 100 K or so of uplift to be resisted by one pile? Seems like an awful lot of uplift to me, let alone for only one pile.

The most amount of uplift going into a single pile is about 55 k. The actual capacity the geotechical engineer gave us for uplift is about 90 k, but the number of piles in the caps were governed by downward loading.

 

[msquared48]

So you only need to develop three or four of the pile in the group for uplift?

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[abusementpark]

Wow! And this is the net uplift?

Yes, it is from a major braced frame on a 4 story building in hurricane-prone region. The frame is along the exterior edge, so there is not that much dead load to counter the overturning. 0.6D will get you.

Can you post a detail/plan of the rebar arrangement showing the pile, pilecap, and column positions?

I won't be at the office for a few days, but the column is centered on a standard 5 pile cap with 3'-6" pile spacing for 16" diameter augercast piles. The cap is reinforced with a layer of steel in each direction top and bottom.

 

[abusementpark]

So you only need to develop three or four of the pile in the group for uplift?

Well, that could be a true statement assuming I provide enough anchorage from the pile to the cap to develop the full tension capacity of the pile. But I might just size the pile anchorage for the max pile uplift reaction.

 

[TXStructural]

The requirement for end anchors would not necessarily be the same as a full strength nut and plate washer on thread-deformed bar embedded the full depth of the pile cap. The ASTM and ACI specs for headed bars are not strictly applicable. Once you establish that the nut and washer are sufficiently strong to develop Fy of the bar (DYWIDAG makes full strength nuts), look at ACI 318 for details on development lengths of headed bars (22" or less for #9). While this is most likely unnecessarily conservative, it is what is available in the code.

Also, have you considered simply bending the bars into standard hooks? If they meet A615, they can be bent, and the development length is essentially the same for hooks and headed bars.

As to the selection of thread-deformed bars to hold down steel columns, this is done infrequently. It fine adjustment for height of columns more difficult. The threads are pitched such that the nuts will need to be jammed (DO NOT TACK WELD NUTS TO BARS, ANCHOR RODS, OR BOLTS, EVER) to prevent load cycling from backing the nuts off the bars. Also, the length of the nuts will make your column bases rather large.
I would be very surprised that you cannot make a standard anchor rod arrangement work properly.

 

[USGeotech]

I agree with TXStructural on this. I've used nuts on the tops of micropile core bars to develop the full tension load of the pile within "relatively" thin pile caps or into grade beams. The same mechanics apply as given in ACI 318 for headed bars. The nuts produced for threadbars by all of the major manufacturers' (DSI, Williams, SAS) all are capable of developing 125% of the bar strength.

In ACI 318 2002, the section on embeds was contained in an appendix. In all of the later versions of ACI 318, including 2011, the requirements have been placed in the main body.

BTW - We've tested the pull-out of an embedded threadbar with a nut because the structural engineer didn't believe it would work. Results appear to be very close to ACI31. If we had more than one test, we would have published the results.

If only I had the time and resources to do the basic research needed to answer these re-occurring questions – I'd be an academician.

 

[amec2004]

I disagree with the points that ACI specs for headed bars are not strictly applicable --> it's actually applicable.

There are two parts of capacity, always, when comes to any application of anchor bolt design or DYWIDAG bar anchorage design.

One is the material capacity, which is always adequate as DYWIDAG or DSI use high-strength threadbar, and their nut or coupling can develop 125% of the threadbar capacity.

The other one is the anchor bolt or DYWIDAG bar head pullout, breakout, punching (micropile case) etc resistance in concrete, which is always the bottle neck.

The second part *MUST* be checked to ensure adequate anchorage and 99% of the time it's the governing case.

Attached is a sample calc for micropile head anchorage into thin concrete pilecap for punching (compression) and pullout and breakout (tension).


anchor bolt design crane beam design

http://www.civilbay.com

 

[TXStructural]

Regarding embedment depth, ACI Committee 355 has an ongoing discussion regarding this application. 318 Appendix D is intended to address anchor rods, but AISC considers this overly conservative. I was told today by a 355/318 committee member to expect continuing discussions in the industry.