Actually, it is possible to search the NWS climate records for Dulles. The monthly climate reports are called CF6. Looking at the Dulles CF6 for February, 2010, they reported both the snow and eqv. water for this storm. Prior to the storm, they had 1" snow depth on the ground presumably icy and granular based on melting and max temps for a few preceding days. On Feb 5, they reported 14.9" snow (1.47" water) with additional 17.5" (1.98" water) on Feb 6. Allowing something for the initial snow depth and converting, they had 90-110 mm of water eqv. on the roof. Local loading from drifts may have been higher. I don't know what code is in Washington, but I don't think a roof that collapses at around 100 kg/m² would meet code here. As I mentioned before, code for sloped residential roofs converts to around 170 kg/m², and I would expect flat roofs to be a higher spec.
--- On Mon, 2/21/11, Michael Payne <[email protected]> wrote: From: Michael Payne <[email protected]> Subject: [USMA:49897] Re: MM93-Item 3, Aircraft Hanger To: "U.S. Metric Association" <[email protected]> Date: Monday, February 21, 2011, 6:45 AM 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
