Posted on 12/24/2006 1:56:20 AM PST by SunkenCiv
As a drafter that produces shop drawings you have no idea how true your comment is. I save letters that I write to architects/engineers who have a fantasy how things are held in place. Everyday it seems the architects become more hesitant to issue an RFI because he knows it will be used as a basis for a change order.
I thought and thought and can't se how the length of the bolt has anything to do with something hanging from the ceiling except for how close to the ceiling do want the hanging panel.
"I thought and thought and can't se how the length of the bolt has anything to do with something hanging from the ceiling except for how close to the ceiling do want the hanging panel."
The concern has to do with the method used to attach the bolts to the concrete. If you were to thread a bolt into steel, then the maximum strength would be attained by making the thread length approximately 1.5 times the nominal diameter of the bolt. If you had a 1/2" diameter bolt then the maximum strength would be realized with a 3/4" long threaded hole.
In this case, the material that was being threaded into was much weaker than steel and in order to attain the strength required to hold the panels in place, the area of engagement would need to be much greater.
Not knowing what the design parameters or requirements were, I can only speak in generalities. If the strength of the base material were to be 10% of the strength of steel, then the length of engagement to attain the strenght of a steel base would be 10 times that required in steel. In other words, if peanut butter had 1/100 of the strength of steel, then you would have to put our 1/2" bolt into 75" of peanut butter in order to match the strength of a tapped hole in steel.
In this case, they were only trying to insure that the mounting strength of the bolt that was epoxyed into concrete was adequate (with an appropriate safety margin) to hold the panels in place assuming that the bolts were properly installed.
Whereas a 1/2" bolt installed in steel could fail with a load of perhaps 18,000 pounds. The same bolt epoxyed into concrete could fail at 1,200 pounds although the mode of failure would not be the same. In the first case, the bolt itself should be the weakest component whereas in the second, the interface between the bolt and the concrete would be the point of failure. Again I am simplifying, but the more epoxy that you have in contact with the bolt, the stronger the joint will be. Thus the deeper the bolt is imbedded into the concrete, the stronger the joint wil be.
With the technology used in this application however (an epoxy joint), it is impractical to get much more than 10% of the strength of a steel to steel joint. Moreover if the installation is improperly made, the theoretical strength goes out the window.
This example uses SWAGed numbers (Scientific Wild Assed Guess) and I make no claim as to their accuracy other than to say they are close enough for illustration.
I am an engineer who worked on the big dig. Mark, you have made yourself look amazingly ignorant, and belligerently so, on this subject.
Am I correct?
On drilling rigs, we have really big nuts to fasten our bolts.
Yes they were referring to the length of the fastener actually retained in the concrete.
Who was talking about sanitation engineers anyway?
I was referring to those engineers involved with the litigation, I still doubt they would be saying anything with out having it cleared.
What about the jerk who asked if I was a Big Dig engineer, what were his motives? Were his motives aimed at keeping the conversation flowing, or was his intent to hurl an insult?
Why is it that you call me ignorant for saying the length of the bolt should not matter after it is a certain length? After more discussion with others who are not of the ilk like you and your buddy gasolinewhatever, it turns out that the length of the bolt may not of been a factor as much as the strength of the plastic nut. The problems may be the result of the epoxy that formed the nut was not properly installed, and the whole idea of gluing in the bolts may of been flawed.
And if you were an engineer on the Big Dig, how much of the cost over runs and fleecing of the taxpayers were you responsible for? Someone else posted that the traffic is just as bad as before the project. Good Job.
Dont forget my original question was merely related to the bolt length, in my feeble little mind I could not see a 7" bolt holding with twice the strength of a 5" bolt. The article starts out by talking about saving money by using a slightly shorter bolt. I would guess that a longer bolt would of made little difference if the mating threads were flawed.
Despite your insults, I'll try to explain why a longer bolt will develop more strength. For now, ignore the nut/bolt situation, and think of the bolt more like a nail embedded into wood.
In a tensile application, you have to look at everything as a system of components. First, the bolt has to be of sufficient material strenght to handle the application, and of sufficient diameter to carry the intended load. Secondly, the epoxy used has to be of sufficient strength to carry the load through the bolt (anchor rod, really), through the epoxy, and into the base material. Several assumptions are made, though, in transferringthe load to the base material, like proper adhesion of the epoxy to both andhor rod and base material. It seems as though there were failures in both regards, as well as full drill depth not being able to be acheived due to bad planning and lots of rebar obstructions.
The key here is the base material. All the components expect the base material are relatively easy to get to exceed design loads, but the base material here is the weak link. I'm sure you understand how constete strength is measured in PSI, (pounds per square inch). Typically the concrete we used was supposed to be 4,000 psi, which is an ultimate strenght, meaning failure strength.
The load is applied from the bolt to the base material in a 3-dimensional cone, like a christmas tree. If you only insert the very tip of the cone, the base area is very small. As you go deeper, the cone of influence is much greater.
I'm sure you've seen where a masonry nail has been removed from concrete, leaving a conical spalling type failure. In order to develop the full strenth of all the material, you need to make sure you get the full penetration where the "christmas tree" loading is less than the ultimate strenght of the concrete at the base of the application. The deeper you insert the anchor rod into the concrete, the wider area the base of the cone of influence will act on the concrete, thereby reducing the stress on the concrete. Because the cone of influence is substantially larger with a 7" bolt, it could likely hold twice as much as a 5" bolt
If you were to hand a heavy picture on a wall, the half inch sheetrock would likely not hold it well even if you used a long screw, both because of the screw length, and the relatively thin sheetrock. However, if you had 4" of sheetrock and 4" screws, it would hold a hell of a lot more, because you could utilize a lot more of the bolts strength because you have much deeper development length.
I have my doubts about the cone theory, on one hand I can see what you are saying, on the other hand I could imagine the bolt stretching and breaking out the first threads, where the cone is smallest. This would progressively move the cone deeper in the hole.
I can understand how the epoxy method was a disaster waiting to happen. My first response to this accident would be to defend the engineering, because those who like to pick things apart are no where to be seen when there is a blank sheet of paper. The weak link in this chain was gluing the bolts in over head. Now if the holes were back tapered and a steel insert, like a big heli-coil was set in the epoxy, maybe it may of held up.
Great photo! LOL
Book marking to come back and see when you finally understand what these guys are telling you :)
"This is the core of this whole tragedy. It took thousands of excess (and unnecessary) hours to drill for and set these bolts, therefore more taxpayer dollars for worker payroll, foremen, contractors, and, of course, outstretched political palms on up the line......"
Placing anchor bolts through concrete formwork (prior to placement of concrete) may insure a more foolproof anchor, but it is very difficult, time consuming, and therefore expensive, to do it that way.
IMHO innovative engineering could have overcome the necessity of drilling and setting, possibly something like the Sepulveda viaduct below one of the LAX runways, where glass bottles were used as spacers between forms during the pours and were subsequently ripped out with cables, leaving a void space to separate the runway from the viaduct ceiling. However, I question the basic design decision to creat a plenum using heavyweight concrete panels below the ceiling of anything, no matter what the attachment method. Vibration, corrosion (particularly in a near marine environment) and thermal expansion and contraction can ultimately take their toll on fasteners. Lightweight stainless steel panels could have been used, or some other method, perhaps similar to ventilation schemes used in other tunnels.
The best way to have done it would have been a proper design from the get-go, which would include threaded inserts in th concrete, tied into the rebar, before the first yard of concrete was poured.
The problem is that their were so many different contracts, the design didn't mesh well. That also caused a whole lot of the overruns.
I was only involved in deep foundation construction. I was a project engineer and project manager, as well as field engineer for contractors only. I did no design on the Big Dig. I never saw any of the corruption people like to claim, as I was way low on the food chain.
The ineptitude of the designers and those responsible for oversight, while maybe not quite criminal, was certainly incredibly lacking.
That is exactly how i think it should have been done. Richmond anchors cost around $7-12 to buy, and I generally use $25 a piece in place. The hilti epoxy cost at least that without installation. At labor rates (all burden included) of around $60/hour, inserting richmond anchors would be the cheapest way to go, and they are a time tested method of anchoring in concrete.
I can't help but notice that your picture, while quite funny, fails the Too Many White Guys test.
Width does. lenght don't/
Actually I think i do know how true. I am one that has always had great respect for the smart people without advanced degrees. I later left my position as an engineer and became a hands on tech in a field I have no formal education for...mechanical systems. Now I have the reverse problem...completely uneducated people that think anyone with any education at all are idiots. In their defense, they assume that all college grads are students of psychology, history, and such other useless crap. In the world of industry, these such graduates really are imbeciles. I myself despise them almost as much.
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