Denso tape around anchor bolts 1

[ABA102]

In steel structure part of our stadium project, the designer required anchor bolts of fully threaded. However, the contractor procured anchor bolts which are threaded at the ends only. So, they wrap the anchor bolts with denso tape along the unthreaded portion. They are using this procedure of denso tape on those anchor bolts which shall be subjected pre-tensioned of 75% of the specified design force after erection.

I am just newbie to this kind of system. Is this procedure of providing denso tape around anchor bolts really a practice in lieu to the full threaded anchor bolts? And is this really effective?

 

[JStructsteel]

Im not sure what the tape does to mimic threads?

 

[rdubs]

Not sure why full-length threading would be required as long as the threaded length is sufficient to tension the bolts. I don't see the tape being used to compensate for lack of threading. Could be that the tape is being used as a bond-breaker so that the bolt can be properly stressed along its full length.

 

[JStephen]

I think I've seen details where the bolt is anchored with a nut or welded washer plate on the bottom, and then tape or similar product is used on the shaft, the idea being to avoid developing any bond on the shaft and allowing additional (and known) length to elongate in preloading. Is this what they're doing?

 

[sandman21]

Even if it was a bond breaker, not the best way to allow for ductile failure of anchors, its the EOR's call on what to do not the contractors.

 

[ABA102]

To Jstephen,

Yes, exactly. Only that the bottom plate were not welded instead provided with nut on each side.
I'm bit confused because of the threaded shaft of anchor bolt against denso tape idea. Correct me if I'm, my personal impression is, having threaded shaft even make better contact on grout than the denso surface. So, why designer told that it was originally full threaded that denso tape can even be avoided.

 

[dhengr]

ABA102:
These days, people are inclined to call out all-thread rod, which is a commodity piece, in 10' lengths. You cut it to length and it has threads on both ends for exactly your type of application. The alternatives would be spec’ing. a 24" long rod (xxx grade) then having a shop thread both ends, one end 4-5" long and the other end 2" long and then applying the washer and nuts on the correct end; or ordering an extra long bolt (not a stock item) and being content with a loose washer. But, both of these latter items are more expensive per foot than the all-thread, because of the extra steps involved. Then, to get proper tensioning and elongation you have to wrap all of them with some form of bond breaker. I don’t know what Denso tape is, but most any bond breaker will work. Obviously, the threads would cause more bond and trouble than a smooth rod, much like the deformations on rebar.

 

[dik]

From my General Notes, used for 100 projects (maybe):

ANCHOR RODS (GENERAL)

ANCHOR RODS SHALL BE WELDABLE U/N

ALL ANCHOR RODS SHALL BE [3/4 | 19]%%C MIN U/N

3/4"%%C ANCHOR RODS SHALL BE FABRICATED FROM ROUND BAR x [20" | 24"] LONG U/N. THE BOTTOM OF THE BAR SHALL HAVE A 1" LONG UNC THREADED PORTION TO ACCEPT A HEAVY HEX HEAD NUT WELDED AT THE BOTTOM WITH 1/4" FILLET WELD 3/4" LONG. THE TOP OF THE BAR SHALL HAVE A 4" LONG UNC THREADED PORTION TO ACCEPT A HEAVY HEX HEAD NUT, AND WASHER. EMBEDMENT LENGTH SHALL BE 19" MIN. LENGTH OF PROJECTION SHALL BE 4"

Dik

 

[hokie66]

The threaded part of a rod will bond. A rod wrapped with Denso tape will not bond. Perhaps you misunderstood?

 

[IFRs]

dik - Your note seems ambiguous as to where exactly the weld between the nut and the threaded rod should be located. It should be on the underside of the nut at the end of the threaded rod as a method to keep the nut from becoming loose. If it is on the topside of the nut it will weaken and/or embrittle the threaded rod.

 

[dik]

IFRS:
"ACCEPT A HEAVY HEX HEAD NUT WELDED AT THE BOTTOM WITH 1/4" FILLET WELD 3/4" LONG"... welded at the bottom and never had a problem with location, but, will clarify my notes. Thanks

Dik

 

[IFRs]

I've inspected work done and found welds above the nut. The guys said they welded the nut on the bottom not the nut on the top, because they knew the nut on the top needed to be tightened. They welded on top of the nut on the bottom because getting to the underside was hard - they were already hung in the forms. I told them "good thinking" but "please replace all those anchor rods before pouring the concrete".

Might have been a once-in-a-lifetime occurrence but I've changed my drawings to show the weld symbol and not rely on just a note. I also ran into welds on top when there were no threads or too few threads showing below the nut. Again, maybe just a one-off deal.

 

[dik]

My detail has the weld symbol to the underside.

 

[KootK]

3/4"%%C ANCHOR RODS

Is the %%C about carbon content?

Even if it was a bond breaker, not the best way to allow for ductile failure of anchors

Can you describe some better ways? I'm curious to know more.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[IFRs]

%%C is AutoCAD speak for a diameter symbol.

 

[dik]

Yup... another part of the notes specs the material types...

Dik

 

[Leftwow]

We tack weld the nut to the bottom of anchor bolt. This way the nut can't slip off the end during construction, also ensures placement of the nut. We use all thread rods because it's easy to call out.

 

[ABA102]

http://www.denso.net/densotape/

 

[dik]

I've revised my notes to reflect IFRS comment. Another caution, make sure the concrete is sufficiently hard to prevent rotating the nut in the concrete.

Dik