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 undercut section. 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 of $?
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 "installation adaptor".
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 as intended.
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.)
here's more info than you're probably interested in about them
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Post installed (after the concrete is cured) anchors are a huge industry....
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.
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...
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