Was wondering if anyone knows anything about concrete bolts/anchors.
Have been reading about the replacement bolts made by Hilti that the Boston
big Dig is going to use to retrofit their ceiling.
What a fiasco !
Anyway, apparently you have to make an undecoat in the concrete hole at the
end using a
"special bit". Then, by turning the bots screw, the head expands into this
It's the head of the bolt in this undercut that therefore provides the
"hold", and not the bolt's shank, apparently.
Was curious how this undercut is done.
Or, it looks like turning the bolt's body may enable the head of the bolt to
do the actual
undercutting itself rather than using some special drill bit ? Is this
what's done ?
Seems interesting, and was just wondering about this undercutting.
Another posted a link to Hilti on the style of bolts...
The subject came up at the local intellectual center (aka The Donut
Shop :) ) the other morning--my immediate reaction when I first heard
of the problem was to suggest they should have gotten a bunch of coal
miners used to roof bolting in the beginning.
My first question is what the (bleeped) they're doing hanging very
heavy concrete assemblies from holes drilled into concrete. Why? Are
these panels providing additional fire protection or such, if they are
such a burden and danger to fix in place.
Or are they purely "architectural" (cosmetic) and a further huge waste
Concrete is very strong in compression, enormously less so in tension.
Don't need a C.E. to tell you that. Nor why so much rebar is used in
any concrete structure- essentially ALL the tensile strength.
Living in "Red Sox Nation" I've been following this fiasco daily. To
answer your specific question, the flat concrete ceiling panels are
suspended below the curved "ceiling" of the concrete lined tunnel bore
and produce an air channel which is used for ventilation and through
which large amounts of air can be blown to clear out smoke if a vehicle
should catch fire inside the tunnel.
The finger pointers are saying that a proposed alternate for the
concrete ceiling panels were alumunimum panels with box channel
stiffeners and perhaps a thin layer of concrete on their upper sides to
prevent "flutter" when large amounts of air had to be blown over them.
There's accusations flying that the much heavier all concrete panels
were cheaper and quicker to obtain than the aluminum ones, and there's
also been innuendos about favoring certain local contractors with the
ceiling panel business.
Regarding the OP's question about how the "undercuts" for the Hilti
"expanding head" bolts are formed, if you dig deeper into the Hilti
website you can find the installation instructions.
As I read that, the bolt hole is drilled with a hammer drill and a
pretty normal looking masonry drill. Then, there's a special Hilti
adaptor piece which couples a hammer drill to the back end of the bolt.
(Hilti sez you have to use THEIR hammer drill too.) The "petals" at the
far end of the bolt have carbide tips on them which do the drilling of
the undercut as they are expanded by second bolt running up the center
of the main bolt. I didn't learn whether that center bolt has to be
screwed in manually as you go along or if it somehow gets turned by the
Those look like a much better concept than the epoxied in bolts which
seem to be the proximate cause of of the ceiling collapse fiasco. I've
no doubt that when PROPERLY installed those glued in bolts can work, but
given the possibility of slipshod work done while rushing to meet a
schedule, or while working with epoxy in freezing weather, I'm not
suprised that there's a HUGE number of those epoxied in bolts which have
now been found to be bulled down out of their holes.
There's a lot of rebar in the concrete tunnel roof, and stories are
circulating that when an epoxied bolt installer ran his drill into a
piece of that bar before it had reached design depth, they just epoxied
the bolt into the "short hole" and cut off the "extra" length.
I've also heard that if the holes were drilled too deep (Allegations of
their being drilled without depth stop collars on the drill bits have
been mentioned in the press.) the epoxy ended up getting pushed into the
extra length of hole and didn;t squeeze down along the sides of the bolt
Our "hired hands" in state gummint are now assuring us that they will
back up all those epoxied in bolts with those Hilti expansion bolts
installed nearby them, and that if the drill hits rebar before it's deep
enough they'll start a new hole a short distance away.
Undoubtedly it'll be our state's taxpayers who will have to take it up
the backside for this mess. Starting with just the losses in turnpike
tolls caused by closing down parts of the system, which were stated to
be $100K per day.
(All my comments are based on what I've read in The Boston Globe and
seen on local TV.)
Great. I imagine if the anchors are failing on their own, if they had a
tunnel fire and tried to pressurize the plenum to blow the smoke out
they would have succeeded in extinguishing the fire by dropping nearly
all the roof sections on it.
AL costing a lot more than concrete, it's clear why it wasn't used.
Is anyone surprised?
Since Hilti is pretty much number one in the fastening world I wouldn't
argue with their instructions or the need to use their drills.
Hilti also makes very good epoxy anchor systems. The "properly
installed" qualifier is equally applicable to the undercut anchors as
well. Installed in holes drilled without the proper precise tolerance
drill with stop collar, or installed without the proper installation
tool, the undercut anchors are just as likely to fail as improperly
installed epoxy anchors.
Assuming they autopsy enough bolts to prove this it sounds like some
construction folks will be going to jail.
One would assume that the Hilti bolts will be applied in a quantity to
give a 10:1 safety factor based only on the Hilti bolts and ignoring the
existing failing anchors.
Pocket change. Compared to the Billions the Big Dig cost so far, a few
million more to fix it is nothing. Of course once the next problem
surfaces and it costs a few mil more to fix, and then the next one...
here's more info than you're probably interested in about them
Post installed (after the concrete is cured) anchors are a huge
there are chemical (epoxy, et al), mechanical (expansion & the odd
screw type) & undercut (most difficult to install but most closely
match the performnace of a cast inplace headed anchor)
I have installed & used & tested 100's of chemical anchors......hole
prep is KEY
too shallow, not clean, bad anchor material fill can all lead to very
reduced anchor capacity.
overhead installs are esp trouble.....gravity working against you.....
with careful install, no problems.
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