Looks like a flat roof with a meter of fresh snow. http://www.thekathrynreport.com/2010/02/dulles-airport-hangar-roof-collapse.html
Mike On 09/02/2011, at 17:21 , John M. Steele wrote: > Jim, > > It is a density, and based on a specific condition of snow, but not on depth. > It needs to be multiplied by depth. > > Like any density, it is dependent on the material meeting the condition. It > is based on fully saturated but draining snow (basically it needs to be > melting). However, I would assert that is the general condition when roofs > are at risk. If you have icicles hanging off the roof, at least some of the > snowpack is melting. The condition also applies if the snowpack has been > rained on, but on a sloped roof, is able to drain. > > On a flat roof, there is a significant risk of appraoching the slush > condition. I don't know whether the hanger had a sloped or flat roof, as I > have not seen a picture. > > --- On Wed, 2/9/11, James R. Frysinger <[email protected]> wrote: > > From: James R. Frysinger <[email protected]> > Subject: Re: [USMA:49829] MM93-Item 3, Aircraft Hanger > To: [email protected] > Cc: "U.S. Metric Association" <[email protected]> > Date: Wednesday, February 9, 2011, 3:32 PM > > > If anybody is worried about their roofs, please use at least 320 kg/m³ for > > wet snow on sloped roofs. If you have a flat roof, determine whether the > > drain is working, it will be the difference between 320 kg/m³ wet snow and > > 960 kg/m³ slush. > > I believe that those figures assume an accumulation of 80 cm of "snow", > whether it be light and fluffy, wet and dense, slushy, or icy. If 80 cm of > snow falls, then, due to insolation (not insulation!) and warming by air or > by conduction from below, the depth will no longer be 80 cm. > > And of course, not all snow accumulations are 80 cm in depth. > > Rather than recommending a load estimate figure (that is based on 80 cm of > accumulation of whatever sort -- snow, slush, ice), it might be preferable to > teach the method, which then can be adapted to any given precipitation amount. > > I collect and report daily precipitation data for CoCoRaHS > http://www.cocorahs.org > including snowfalls. For snow I measure the depth of the accumulation, > collect snow from an area of known size, and weigh it to determine the > "rainfall equivalent". Working backwards for this winter for some of our > snowfalls, I observed that the snow:rain ratio might be better stated as > 1 cm:0.7 mm > 1 cm:0.75 mm > 1 cm:1.2 mm (followed rain and freezing rain) > 1 cm:0.26 mm (notes indicate unusually light & fluffy snow > with large "flakes") > > As you can see, there is quite a bit of variation in those ratios for my > location. Generally, I would tend to characterize our snows as averaging 0.7 > mm to 0.8 mm rainfall equivalent in 1 cm of snow from what I have seen the > last 4 years. As John suggests, other areas might typically see snow of a > different average density. > > This variation is exactly why meteorologists melt (or weigh) fallen and > accumulated snow to determine its actual water content. If one is concerned > and capable enough to estimate roof loading, they probably should do likewise. > > By the way, some architects might show maximum snow loadings on the plans for > the structures built from those plans. > > Jim > > On 2011-02-09 1208, John M. Steele wrote: > > I just received Metrication Matters 93, and saw the aircraft hanger > > example again. As it is snow season in the US, and people need to worry > > about their roofs, I have to point out two huge errors in the example as > > I don't believe anyone should rely on that example. > > 80 cm of snow != 8 mm of rain > > Even if the 10% rule were true, it would imply 80 cm of snow is 80 mm of > > rain. Doing some Googling on snow load and roof designs, I find the > > density of wet, heavy snow is more like 32-33% water density, 320 - 330 > > kg/m³. The Washington (DC) area is not noted for light, fluffy powder, > > and light fluffy powder isn't what collapses roofs. Using the 320 kg/m³ > > x 0.8 m, the actual roof load was more like 256 kg/m³ if the drainage > > system was still working, not the 8 kg/m² of the worked example. > > Flat roofs are a particular problem as snow tends to clog drainage and > > then you get slush, a mixture of ice and water. Not surprisingly, the > > density of slush lies between 920 kg/m³ (ice) and 1000 kg/m³ (water). > > The figure above of 320 kg/m³ is for drained (but wet) snow - imagine > > snow on a screen so any water melt can drip out. > > I can't find the spec for flat roofs, a lot of local codes in the > > northern US are 35-40 lb/ft² for sloped roofs. That converts to 170 > > kg/m². Some extreme snow areas are higher, and I would expect flat roofs > > to be higher. > > If anybody is worried about their roofs, please use at least 320 kg/m³ > > for wet snow on sloped roofs. If you have a flat roof, determine whether > > the drain is working, it will be the difference between 320 kg/m³ wet > > snow and 960 kg/m³ slush. > > > > -- James R. Frysinger > 632 Stony Point Mountain Road > Doyle, TN 38559-3030 > > (C) 931.212.0267 > (H) 931.657.3107 > (F) 931.657.3108 >
