Isn't the *residual* load at any moment the critical measurement, or its slightly delayed accumulated distortion (strain, displacement) of roof trusts?
Load can be measured as mass per unit area, or force per unit area (stress), independent of its state, water plus melted snow of any *initial density*. Gene. ---- Original message ---- >Date: Wed, 09 Feb 2011 14:32:01 -0600 >From: "James R. Frysinger" <[email protected]> >Subject: [USMA:49831] Re: MM93-Item 3, Aircraft Hanger >To: "U.S. Metric Association" <[email protected]> >Cc: "U.S. Metric Association" <[email protected]> > >> 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 >...
