This weekend the damaged jib on the luffing tower crane at One57 in New York was secured against the building frame. Sailors might analogize the work to reefing a sail.
As I mentioned in my initial post of Oct. 29, the damage began when hurricane winds pushed the crane jib upward until it reached a vertical angle, then overcentered.
Here's a picture from yesterday's work (from RT, photo Michael Heiman, AFP)
The stabilization job, which started Saturday morning, sounds simple enough: use a manual crank to spin the superstructure of the Favelle Fabco Model 440 crane on its slewing ring so that the jib hangs close to the building, then temporarily secure the metal lattice work to the building with wire rope.
That way, the jib won't break off and in a few weeks contractors can erect a second crane to take down the first one (probably setting off a new wave of traffic closures, but smaller in scope.)
New York residents and business owners pushed out by safety orders were highly frustrated after six days of waiting for this. So why not have started carrying out such a seemingly simple plan as soon as Sandy's winds died down, rather than start six days later?
Because the crane had to be checked carefully for damage. New Yorkers have seen several crane catastrophes, such as the collapse of a tower crane in 2008 due to human error at the jobsite, and no public official wants to be the cause of another.
And I can see why they were so cautious about shifting this one around. Normally a tower crane is pretty well in balance. The balance is maintained by a heavy counterweight on a short jib at one end of the superstructure, offsetting the dead weight of a jib (also called the boom) at the other end.
The counterweight on such a tower crane is massive (I haven't seen the figure, but 40 tons is typical) because it has to offset the jib's weight, the weight of any cargo it's lifting on the hook, plus the dynamic loads from movements of jib and cargo. From MarineDigital, here's a closeup of the counterweight on a 440D model:
Along comes Hurricane Sandy, the jib hinges over the top, and suddenly the jib isn't balancing the counterweight anymore; now the boom has joined the counterweight on the same side of the slewing ring. The tower crane is now seriously off-balance.
Here's a picture from earlier this week showing how the storm shifted all the weight to one side (photo Allison Joyce, Getty):
Consider also the dynamic force that the slewing ring, mast, and uppermost tieback to the building had to absorb as the jib pivoted over the top and draped itself atop the counterweight. The tieback, made of temporary steel beams, is silhouetted at the bottom of the photo.
As I said in the earlier post, it's a testament to the Model 440's sturdy construction that the crane and uppermost tieback were able to absorb this shock load without dropping something heavy to the street.
Although from a distance the crane might have appeared strong enough to handle this weekend's simple procedure, safety experts had to consider not just the functionality of cranking mechanism itself but also whether shifting the crane might cause something big to break off -- whether the damage had been such that the original margins of safety were gone.
On the question of cause: News reports quote the principals at One57 as saying that all hurricane procedures were followed, such as jib set and brakes released so that the crane could weathervane with wind shifts; that's routine for off-duty tower cranes because it reduces the wind load greatly.
There's no official word yet on why the wind was able to push the jib over the top when other area cranes were not so affected, but the following is one possibility other than some freak of turbulence. While support cables hold a jib up, only gravity holds it down. The higher the angle the jib was set before the storm, the less the crane could weathervane to point the jib downwind. In short, for two reasons, a jib parked at high angle is more likely to be caught by a wind gust and go over the top.
The Tower Crane Interest Group in the UK has published a series of warnings about luffing jib tower cranes left at a high angle before a storm. Here's an excerpt from one of those publications, "The Effect of Wind on Tower Cranes in Service":
Putting the crane in the out of service condition generally includes ensuring that the jib is free to “weather vane” when out of service so that the minimum wind area is presented to the prevailing wind. On luffing jib tower cranes it is also important that the jib is left at the correct out of service radius, not the minimum radius, to ensure that there is sufficient wind area to ensure that the crane is able to “weathervane”.Here's a cellphone video on the Daily Mail website, of the One57 crane's jib going over the top. But the video is short so we don't know from this snip what the jib angle was at One57 prior to the storm. So the jury is still out.