Monday, February 21, 2011

February 20th, 2011

Stuttgart, Germany A Tower Crane collapsed inside of a 40,000 seat stadium. The crane was a mere 14 meters high, and the cab of the crane was dislodged in a way that it seapartated from the wreckage which allowed the 31 year old operator to clear himself from the cab with broken ribs. He was immediately taken to the hospital.

The crane appears to have broken away at the Cat Head to turntable connection. Sometimes these connections are made with through bolts. We have seen a number (1, 2)of Comedils come down due to poor bolting and bolts that have never been changed out after being torqued. I have personally found loose bolts on similar connections. I'm not familiar with the particular crane in use here so I can't really know what type of connection it is that we are talking about. Is it a pin that maybe had a keeper missing? A set of shouldered bolts that were not tightened due to being forgotten?

You should be putting your hands on the bolts of your cranes. I can't tell you how often I find hand loose bolts that should be tightened well beyond my physical ability to loosen them without a tool, let alone by hand. This is how we can prevent these problems from getting to the point of failure. Daily Checks should be done prior to each shift. Don't take the extra time your employer gives you to just go up in the crane and get yourself set up, walk that jib. Look at those pins. Put hands on those bolts. It's your life that we might be talking about.

Kran Unfall Stuttgart

Saturday, February 19, 2011

February 19th, 2011

Plymouth, Devon, UK A man in his 30"s fell from a tower crane. The fall was about 200 feet in length and resulted in death.
Vertikal reports the story as a man whom climbed over a security fence then climbed the crane without authorization. When he reached the turntable he found that he couldn't get any higher and then fell or jumped to his death.
The picture shows what appears to be a Liebherr 420 (US designation). These cranes have a hatch door from the turntable to cab transition. If you are the operator, you can place a lock that would prevent a person from getting past this area. The height of the cab and it's sheer sheet metal face would make it impossible to climb around without setting up ropes.
This is one of those things that we in the industry often endeavor to prevent, but it seems like a uphill battle. I've seen cranes with 10 feet of plywood walls built with razor wire at the top. The crazies still get by. People stealing copper show up to take the large power cords that feed 480v with a 200 amp service up to the crane. Operators will show up and find the cord gone. We've seen it where they've just taken bolt cutters and cut right through the live wires. I've heard a story about it happening at a crane service yard where they found drag marks  as if the person who did it was injured and couldn't walk out. Sorry, I find that hilarious. Sometimes justice moves at the speed of light.
You should do what you can to prevent people from getting themselves hurt, but if they are jumping fences, clearing plywood walls while getting through razor wire, they are determined and short of putting a couple of trained pit bulls on the second mast landing, we can't save these people from injuring themselves. Even at that, they would certainly sue here in the US for the bites that they would receive, and sadly they'd often win.

Monday, February 14, 2011

February 13th, 2011

Buenos Aires, Argentina (Palermo Las Canitas) A tower crane on a specially designed pedestal collapsed killing no one. It fell over four buildings, destroyed the ceiling of one apartment, which would have likely killed the occupants if they were in that spot, but everyone walks away from this one.

Looking at the design, I've never seen anything like it. It's a large concrete footing on top of four concrete pillars. At first my thought was that that they were trying to make it easier to finish the building at the end of the job. Normally you would pull the tower sections out, then go back and fill in the 3x3 meter holes in the floors that allowed the crane through. But if you had four concrete pillars close together like that, what could you put in that area of the building? So ease of removal couldn't have been the case. It actually looks like it would make it 100 times worse because you would have to remove all of that concrete. The only thing even remotely close to this that I've ever seen is a crane that was anchored to a large structural concrete beam that then had Aluminium Screw Jack Shoring for the next two floors down.

Looking at the base, I just don't like the way it's set up. You have a crane that is sitting on top of this large concrete block as if it's a raised footing. Then you have two floors of concrete columns. It would be one thing if the concrete portion had something to resist the torsional loading, but I don't see much that would resist that. I see some wood, but a compression or two on soft wood and you simply have wood sitting there and providing no resistance. We've all felt concrete floors move under us when a car drives by in a parking garage, and frankly a crane being loaded and torquing up to swing is doing the same thing to concrete. Since even flexible concretes are terrible in shear, what is stopping this concrete from moving? In the case of the crane on the large structural beam that I've seen, it was all tied into the floor of a building with enough mass to resist it. Here, there is little mass, and the mass that is there, only makes it worse because it's so top heavy.

For us in the field, this strongly points out the need to inspect your cranes, daily, and prior to your shift. Get out that flash light and look for concrete cracks. Don't show up to your crane and just run up the ladders. You might get the odd person whom doesn't understand. But here is the reality. That concrete, it didn't crack overnight. It was moving, and those cracks could have been seen. The same is true with most weld cracks. They spend a while moving and tearing just a bit more each time until enough of the weld has failed that it can't take the stress at all and let's go. Dig around, look. One other important thing, notice that the failure is on the side that would normally be considered loaded because the limited size of the jobsite on that side of the crane? The counterweights load that side of the crane nearly as much as the maximum pick on the jib side. So be sure to look at all sides of your crane thoroughly.

This story was brought to my attention by another blogger. Gustavo with Gruas y Tranportes (Cranes and Transports) alerted me to the story as it was in his home town. He has written a few blogs on it and there is some more information that can be found there. Thank you Gustavo.

This morning a video from inside one of the apartments came out on this website. Can you imagine the fear of watching TV while sitting on that couch! If you were under it, it might be deadly. I also found it interesting to see so clearly where the rebar was helping the concrete hold together.


Colapso de grúas en la Argentina

Wednesday, February 2, 2011

April 18th, 2009

Harbin China. A Tower Crane Collapsed due to a broken bolt, apparently in the mast of the crane. The fracture was 20 meters below the turntable. The mass of the superstructure came down killing 1 and injuring 4 others. This may be a fortunate result considering the fallen scaffolding around the building. Many more could have been on the scaffolding.

Bolt inspection is often overlooked. It's been my experience that this was an area in need of improvement when I got into the inspection side. It has greatly improved since the problems were pointed out. I'll give a couple of examples.

Markings. ASTM standards in the US require that the manufacturer and the grade of the bolt be marked and visible on the bolt. I found some bolts had no markings at all. Over the years some of the bolts would get ruined or lost. Rather than purchasing new bolts which are quite expensive, one company was making their own. I don't know of it being a problem and the machinist whom did it is quite good. The quality of the materials is claimed to be adequate, but if that bolt fails, whom is now responsible? Certainly there isn't a manufacturer to blame or pay for the damages. So I chose to have them removed in every case.

Dings in the landings and threads lessen the cross section of the bolt. This lessens the ability of the bolt to withstand the load. Personally, If I found one, or two, I might not get too excited. But if I found lots of them, I'm going to start rejecting them. These bolts should be handled with a modicum of care.

I don't believe that I have a picture of these, but I found stretch bolts to be hour glassed in the threads once. Any hour glassing is an indication of failure and internal, if not external, cracking. The bolts have been tightened to their yield and cannot be used. In this case, it stopped the crane from being erected as the section with the bolts was not able to be inspected until it arrived on site, the day of the crane erection.

A crazy one that I found was soft washers under hardened tower bolts over a large loop bolt hole. The washers were dished out and collapsed in. Even after seeing the picture, the crane owner asserted that the washers were correct. It pissed me off so I went back with a saws-all and cut a section off. A hardness test showed that it wasn't hardened at all. We eventually replaced them all. The crane in question got so hammered by me that it was never erected again. I may have to blog just about this one crane one day. It was absurd. The tower leg had even been cut into with a torch and it was just painted over as if loosing a 1/2 inch of material was no problem.

Poor Install: Washer @ Yellow Arrow Belongs where Orange Arrow is.

But back to the bolts. Bolts should be periodically NDT tested. They should be cleaned, spin freely and have oil on them when torqued. It's not something to be overlooked.

Original Harbin Story Link