Thanks -- again -- John, This is the third time that this arithmetical error escaped my clutches.
I will repair the damage on the archived copy on the website, and do some serious grovelling in the next issue of the Metrication matters newsletter. Cheers and thanks again, and again, and again, Pat Naughtin Geelong, Australia On 2011/02/10, at 05:08 , 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. Pat Naughtin LCAMS Author of the ebook, Metrication Leaders Guide, see http://metricationmatters.com/MetricationLeadersGuideInfo.html Hear Pat speak at: http://www.youtube.com/watch?v=_lshRAPvPZY PO Box 305 Belmont 3216, Geelong, Australia Phone: 61 3 5241 2008 Metric system consultant, writer, and speaker, Pat Naughtin, has helped thousands of people and hundreds of companies upgrade to the modern metric system smoothly, quickly, and so economically that they now save thousands each year when buying, processing, or selling for their businesses. Pat provides services and resources for many different trades, crafts, and professions for commercial, industrial and government metrication leaders in Asia, Europe, and in the USA. Pat's clients include the Australian Government, Google, NASA, NIST, and the metric associations of Canada, the UK, and the USA. See http://www.metricationmatters.com for more metrication information, contact Pat at [email protected] or to get the free 'Metrication matters' newsletter go to: http://www.metricationmatters.com/newsletter to subscribe.
