Boston "big dig" collapse

those 3 main contractors charged with using the substandard concrete should be hung; not only to make an example, but for endangering the safety of so many.

In most 3rd world countries they actually would be.

I still wanna know who even thought that hanging a 6000 lb piece of concrete over a road was the right thing to do. These could have / should have been part of the structure OR supported by columns of some kind...

Also being an engineer I know that the only reason you bolt stuff together is when you want to take it apart later...

94-302-vert said:

Also being an engineer I know that the only reason you bolt stuff together is when you want to take it apart later...

Click to expand...

That's why the bolts are fixed into place with epoxy.

Hanging concrete slabs like that is an accepted and proven method of tunnel construction. The procedure for using said epoxy is exact, and if used correctly, is literally stronger then the concrete its injected into. With the proper epoxy, the system can support over 100K pounds. If, however, it is applied incorrectly, like here, then its compromised.

I mean, imagine this: certified union workers slacked off on a job site? No way! That would never happen! Union workers are the best around! That's why they get paid so much! And why it takes 3 guys to operate a front end loader, 4 guys to clean up around the hole, and 5 guys to manage those 7 guys. Safety first.

For everyone that lives in Mass, I guess this puts into perspective all those pictures in the Globe of workers asleep in trucks or at the bar for 3 hour lunches, on our dime. Making 35 bucks an hour. Or that time they put a GPS tracking device on a Big Dig pickup truck and tracked the guy going to the ATM, grocery store, and getting a hair cut during business hours.

Adam

You use bolts becuase field welding is expensive and when it comes to concrete, it's very difficult to coordinate cast-in-place embed plates designed to be welded. Bolted connections are just as strong and are much easier to construct. And of course it can be fixed since it can be taken apart.

Another thing to note, a 5/8" bolt embeded 5" into a regular ol' concrete hole and epoxied can take close to 15,000 lbs of tension if done correctly. With safety factors, we only assume it can take around 6,000 lbs.

Most of the buildings being built today (if built exactly to code) are actually way over-designed. The idea is that there is roughly a 1/100,000 chance of some kind of failure. When we do our designs, we always consider failure modes and design a strength path into the system to provide warning and prevent sudden failures.

With that said, when I design connections, I almost always design them stronger than the members they support (like beam ties and bolted connections). This way a beam will sag and start falling apart long before the column or support gives way.