Friday, September 25, 2009
I found this vid online and the text states that this demolition was to take down the building only. The location doesn't make any sense for anything that I can find. I would certainly hope that no one was in the crane at the time and that everyone was clear on the ground.
Posted by Gaytor Rasmussen at 7:36 AM
Thursday, September 24, 2009
Bogota Columbia A tower crane was being used to remove a tree. The tree cutters didn't know the weight of the pieces being cut off and overload the crane. Most crane manuals say no hoisting of "stuck" loads. Cutting away a tree while not knowing what the load is or will be fits in this category.
I don't know that the operator survived, but it looked like an easy fall into the tree, then the jib, then the tower top and stopped. We can only hope that all involved were all ok. Any details on these, pass them along and I'll post them.
Link to video of the accident
Posted by Gaytor Rasmussen at 11:59 PM
Russia? The title of this video indicates that Russia was the location of this accident. Russia is slightly large and just because it was a Slavic accent doesn't prove that it was Russia or establish where in Russia. But that's not really important.
The link is to a vid of a self erector tower crane coming down in the wind. It also strikes the luffing pendant of a second crane taking the jib down on it.
What I see are cranes weather vaned together. The winds are flat out howling. Then we have a quick change of direction that side loads one of the cranes. The side loading overwhelms the tower and/or it's connections and down comes the crane.
Tower cranes are designed to spin in the direction of the wind. The front jib should be pointing with the wind, indication where the wind is going. This allows the back moment of the crane to face the wind. Essentially the crane is leaning into the wind like you would when you walk in 60 mph winds. The change of direction at the rate shown in the vid appears to be incredible. Sometimes the brakes drag a bit but the crane normally weather vanes say after 20 mph. The rate of direction change may have simply been too much for this crane. Another possibility is that the crane was not put in weather vane mode and the crane was not spinning with the change of direction... or at least not fast enough.
It's a cool vid and if you can't see the link... go up to the word link above, and click on it. I know it's hard to see the highlight, but the link is there.
Posted by Gaytor Rasmussen at 11:42 PM
Sunday, September 20, 2009
I've found a crane accident from Regensburg Germany. A Self-Erecting Tower Crane has collapsed due to an apparent luffing rope failure. Unfortunately this is not an uncommon cause of self-erector accidents. The solution is simple, inspect the ropes during the crane erection. No one was injured and the damages are apparently are around 50,000 Euros. Sorry for the delay but only searching in German found the accident... Kran Unfall.
So the rope. The jibs on many of the self erecting tower cranes are pulled up via a luffing winch with a rope. The winch is locked in place and the jib is held up by the rope. After the crane is erected it's difficult to inspect the rope. It would require a man basket or other powered access such as a snorkel lift. While the rope is on the drum you are only inspecting a third of the visible rope. So as the crane's mast is being extended you have to inspect the rope. I like to stand over the hoist with a tower wrapped around the rope. The towel protects my hands from wires, yet it lets me feel the shape of the rope and any broken wires will snag the towel clearly showing the problem and it's exact location. This is the trick I use for hoist rope inspections as well. I can't tell you how much time it saves and how much more effective this is over a visual inspection of one side of the rope.
I cringe when I get a call to inspect self-erector tower cranes after they are erected. The luffing ropes are critical but the least inspected item. I spend extra time on site to see the whole crane erected because of this item. I sometimes spend more time on a small self-erector than I do on a large crane because of this time. It must be done.
Posted by Gaytor Rasmussen at 9:36 PM
Monday, September 14, 2009
The collapse of the Liverpool crane in July is focusing on the foundation of the crane. The claim is that the base is to be excavated so I am working on the assumption that the base is a concrete structure which would be common. There isn't a lot that we can do about this in the field, but let's talk about the issue anyways. (I apologize for the tardiness, I was on vacation.) Story
Generally accepted engineering practices, most manufacturers, and in the US ASME B 30.3 require that foundations for cranes be capable of supporting 150% of the intended load. Occasionally I'll see higher numbers, but 150% is the requirement. These numbers are developed off of Vertical Load, Torsional Moment (twisting induced in the horizontal plane) and Overturning Moment (the forces the structure apply to the foundation in the vertical plane in an attempt to become horizontal). The concrete foundation must have enough surface area for the given soil to resist the foundation twisting during operation and have enough surface area to spread the load along the soil provided. In the Liverpool case the crane appears to have changed from a Wolff 100 to a Wolff 500. While the torsional moment and vertical loads changed, the pictures indicate that the overturning moment is what was exceeded. If you increase the size of the crane, you have to increase the size of the foundation as well.
The base is being left as it was according to the report. I can only assume that the base did not significantly dislodge. What can happen here is the underlying soil is not able to resist the overturning moment while the luffing crane is boomed up. If the crane starts to overturn ever so slowly, the mast sections of the crane may eventually buckle and foundation may drop right back into place with minimal disturbance to the soil.
Cranes are to be erected to a degree of 1:500 in the vertical plane (commonly called 1 inch in 40 feet in the US). Geometry makes this true for the foundation as well. So if the base goes out of level by just a couple of inches, with the boom at a high angle, the base doesn't have to move much to induce forces on the mast beyond it's designed intended load. The crane that collapsed in Bellevue Washington was out of horizontal by 1 inch in 8 feet. That translated to 28 inches in the vertical, not include the normal tower deflection of around 10 inches. So you can see how two or three inches out in the horizontal would quickly cause some buckling.
The base of a crane whether on steel or concrete, foundations must be properly sized, as near horizontal as possible (I've always used 1/8 inch for 8 feet to stay safe), and they cannot be moving beyond this while in use either. The design of the foundation isn't the only relevant factor. The design of the crane's mast is for the plumb factor of 1:500 as well. No engineer designs both, so without the manufacturer issuing a letter, saying it's ok to be out of plumb, you cannot erect the crane out of plumb.
If it's a base design issue as noted, then the finger must be pointed at either the engineer of record, or the contractor for changing the crane after the base was designed. It's like building a building foundation for a ten story building then throwing a 30 story on top of it. Who made that choice and who will write the check?
I look at the foundations as far as exposed size, concrete breaks, and soil reports. Beyond that, I'm not the engineer or contractor and it's difficult to determine if things were done correctly.
Posted by Gaytor Rasmussen at 8:21 AM