Comments about technological history, system fractures, and human resilience from James R. Chiles, the author of Inviting Disaster: Lessons from the Edge of Technology (HarperBusiness 2001; paperback 2002) and The God Machine: From Boomerangs to Black Hawks, the Story of the Helicopter (Random House, 2007, paperback 2008)

Monday, October 29, 2012

Hurricane Sandy and its Sandbags

News photos show a great range of sandbagging work on the Mid-Atlantic seacoast in the last few days -- this montage from Google images:
New York Magazine named Goldman Sachs as Top Sandbagger for this fortification (photo, Greg Roumeliotis)
I've heaved my share of sandbags, most recently at a flood zone along the Mississippi, but it's been a few years, so I needed to refresh my recollection of sandbagging dos and don'ts. Here's a website with very good info.

The following is gleaned from North Dakota State U at Fargo, an area that sees almost annual sandbagging due to spring flooding along the Red River of the North

Because most news photos of Sandy-sandbagging don't indicate whether a particular job was underway or thought to be complete and ready for the storm surge, I won't pass judgment on the attempts, but here are a few sandbag pointers.

Good sandbags are polypropylene weave and filled only a little more than half full of sand. And they don't need to be tied shut. Why? When half full and left untied, or half-full and tied at the top with lots of room left inside, the sand can shift to close up voids between the bags, which prevents leaks through the wall. The sand in a full bag, like a plastic bag of sand bought from the home-improvement store, can't do that. 

They're laid down parallel with the water flow, the open ends pointing downstream:
If a wall is to stand less than a foot high, construction can be as simple as a long row of bags, stacked atop each other, braced intermittently with columns of bags stacked on the dry side. 

But ... the labor requirement goes up exponentially if the bag wall is to stand over a foot high. That's because the sandbag wall has to take on a pyramidical cross section -- otherwise it will topple from the water pressure. And if it's to stand more than three or four feet high, it probably needs a "key" trench to keep the whole mass from sliding due to the water pressure:
The number of sandbags required for a neck-high wall is sobering. A hundred-foot stretch of wall that is five feet high, with a base ten feet wide, will need at least 9,000 bags weighing 35 to 40 pounds. And that's the bare minimum: the US Army Corps of Engineers recommends that the base be closer to fifteen feet wide.

Here's a diagram on making a sandbag wall more water-resistant, with a length of anchored plastic sheeting on the wet side:

One57: One flopped crane

Note for machine watchers: the storm-damaged crane in the news, atop One57 tower on 57th Street in NYC, is a luffing tower crane.

 Here's a photo of the damaged unit, from Melania Trump:
"Luffing" means the boom (also called a jib) can be raised above the horizontal, for more agility and to avoid dangling weights over neighboring buildings.  

Here's an image of a typical luffing crane from Wiki:
It looks to me from the news photos of One57's crane that a wind gust caught the boom from underneath and lifted up the free end; the boom over-centered and flopped backwards. This left the collapsed boom draped over the superstructure and counterweight, with the rest hanging off the back. 

Here's a closeup of the wreckage atop the counterweight, by Anna Holmes:
The structural steel of the boom will develop fatigue cracks as it sways in the wind, but the wire ropes responsible for supporting the boom in its working position (called guys, see the generic crane in the Wiki photo above) may be partially supporting the weight of the portion left hanging off the back end. 

The event must have been violent, with a lot of weight shifting sides in a few seconds, so it's a tribute to the strength of the upper, unbraced length of vertical mast (the latticed support below the cab) that the mast didn't snap off somewhere around the uppermost attachment to the building.

If there are crane experts out there, I'd be interested in their opinions on how to avoid this kind of thing. As an amateur I'd think that a safer course prior to a hurricane would have been to lower the boom to a near-horizontal position, and free to weathervane in the wind, but maybe that wasn't possible at this location.