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 


Raza said...


I just wanted to know the standard size and grade of the Bolts and Nuts used for the connection of the Tower Masts together. I really need help with this, Thank you,


Gaytor Rasmussen said...

Hey Raza. It varies. The bolt that you are looking at on this post has a Nut diameter of 90mm. That's an older design from the 70's and 80's. For the masts that are bolted together, a very common size is 55mm. Grades of 10 or 12 will be required. On a Liebherr you'll find that they torque up to 4300 ft lbs. Most cranes that need larger bolts switch to a horizontal pin which may or may not include a nut. I hope that this helps.

Raza said...

Awesome!! Thanks a lot, helps a lot. I was making detailed design on cranes, but couldnt find the bolt size.. was gonna take a shot and use 20mm diameter.. That would have been a disaster.. ! Thanks once again! Really appreciate this ! :)