Following up on Harold's comments; I would tend to guess that you're looking
at details that were generated with not a lot of analysis effort and are
"aged". That doesn't necessarily make it unacceptable, but on the other
hand, just because its in a standard detail doesn't necessarily make it
acceptable. ACI's newest publication "Strength Design of Anchorage to
Concrete", and the forerunner's to this (ACI 355.1 - State-of-the-Art Report
on Anchorage to Concrete & Appendix "B" of ACI 349 Code Requirements for
Nuclear Safety Related Concrete Structures). As we all should know now,
when you embed a steel plate at the end of anchor bolts it actually weakens
its resistance to pullout since it makes the failure cone larger (i.e., the
plate doesn't make it any stronger). I would guess Harold's suspicion is
right, it's just a template that gets left in place.
With regard to tower construction (EIA/TIA 222F) I've witness the use of
wood templates used above the concrete placement to hold the anchor bolts in
place while pouring the caisson (i.e., cast in place pier). I have a hard
time fathoming this "corrosion" issue since grout is place under building
column base plates all the time. I'm with you Bill; still puzzled about the
engineering analysis behind all of this. If you don't count on grout
contact surface area to distribute the compressive load you have three some
issues to deal with:
(1) You have a contact stress problem at the interface between the top of
the nut and the bottom of the baseplate. I would like to know how this is
analyzed by hand calcs. If you're not taking advantage of the base plate to
distribute the load out to the concrete why make the bloomin base plate 2 to
3 inches thick everywhere ?
(2) You gotta check the nut for stripping (i.e., thread failure).
(3) Once into the concrete, what about the end of the anchor bolt in
compression against the concrete (i.e., allowable concrete bearing stress) ?
If you're putting the anchor bolt into compression, don't you need some
confinement steel due to the unbraced length of the anchor bolt (depending
on the compressive force) ?
I don't know exactly what the analysis approach is without utilizing the
distribution of the compressive force. I asked the question once before on
this list to a proponent of the "no grout" design method but never got a
reply. It may be one of those things that's been used but never quantified
by hard numbers. Of course the argument goes... if it hasn't failed it must
be okay. I disagree since just because something doesn't fall down doesn't
mean that the item is overstressed and may be fatigued by such
Just my thoughts...I'm with you Bill....would someone enlighten us ?
Robert C. Rogers, PE
From: Sprague, Harold O. [mailto:SpragueHO(--nospam--at)bv.com]
Sent: Tuesday, May 16, 2000 2:54 PM
Subject: RE: More On Ungrouted Baseplates for Traffic Signal/Signage
Struc ture s
I would be very surprised if the Texas DOT assumes the embedded template is
serving as a mechanical anchor. The Texas DOT was the sponsor of several
studies by Lee, Breen, Cook, Doerr, Klinger, Collins and Polyzois on anchor
bolts at the Center for Highway Research and the Center for Transportation
Research at the University of Texas in Austin that concluded that the
embedded nut was more than adequate for anchor bolt anchorage.
In the TIA and EIA world of tower construction, the embedded template is
just to keep the bolts plumb and in the right position. I would suspect the
Texas DOT is using the templates for the same reason.
I can only speculate that the omission of the grout is for corrosion
considerations or for frangible connections or both.