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Re: Am I in the ballpark for wind loading?
Re: Am I in the ballpark for wind loading? -- djinn	Post Reply	Top of thread	Forum

Posted by: Kelly Bramble ® 02/18/2007, 08:47:03 Author Profile eMail author Edit

Can you give more details regarding how AS1170.2 determines the worst case lifting load? For a 8m x 6m x 8m roof, this does seem large - for the USA.

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Re: Re: Am I in the ballpark for wind loading?
Re: Re: Am I in the ballpark for wind loading? -- Kelly Bramble	Post Reply	Top of thread	Forum

Posted by: djinn ® 02/18/2007, 15:55:39 Author Profile eMail author Edit

Hi Kelly, thanks for the reply. I daresay AS1170 is very similar to the american standard. It gives pressures on structures due to wind, used to determine loads for strength limit state design. I believe it is for wind gust speeds up to around 50m/s (180km/h). There doesn't appear to be any provision to reduce the design wind speed. There is a detailed procedure and a simplified procedure. The simplified procedure is only allowed for simple building shapes fitting certain dimensional criteria. I have used the simplified procedure as I don't have the experience to comprehend the detailed procedure. Free standing roof net basic wind pressure -- AS1170.2 2.4.4.2 Downward 0.85kPa Upward 1.6kPa Regional factor B1 = 1 -- AS1170.2 Figure 2.5.1 Terrain and height factor B2 = 1.3 -- AS1170.2 Table 2.5.2 Topographic factor, worst case assumed (top of a hill) B3 = 2.4 -- AS1170.2 Table 2.5.3 Forces are to be multiplied by a "temporary structure factor" of 0.65 -- AS1170.2 2.8.2 The worst case for my design is 1.6kPa x 1 x 1.3 x 2.4 x 0.65 = 3.25kPa upwards. 3.25kPa x 8.4m x 6.4m = 174kN upwards. I am more concerned with overturning than the strength. The standard allows a factor of 0.6 to be used for serviceability calculations (such as deflection), so I daresay I could use that for overturning, however I will need some small safety factor. The complete structure will be around 5000kg including the blocks. As this is a temporary structure to be erected for one day, several times a year, it can't be held down by bolts, hence the concrete blocks. Modified by djinn at Sun, Feb 18, 2007, 15:57:22

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Re: Re: Re: Am I in the ballpark for wind loading?
Re: Re: Re: Am I in the ballpark for wind loading? -- djinn	Post Reply	Top of thread	Forum

Posted by: swearingen ® 02/19/2007, 12:22:00 Author Profile eMail author Edit

Working from the American ASCE 7-05, the tables give coefficient values that vary with roof slope and wind direction. The maximum average pressure coefficient values for clear wind flow are -1.5 (at 30 degree slope) and 2.35 (at 45 degree slope), the negative value acting away from the top surface and the positive value acting towards the top surface. The other terms in the wind equation I won't go into here, but a high value would be 35psf (1.68kPa). Multiplying that out, I get 35psf x -1.5 x 8.4m x 6.4m = 30k (135kN) upwards and 35psf x 2.35 x 8.4m x 6.4m = 47.6k (212kN) down. That said, you never mentioned any slope at all. The average coefficient values for 0 degree roofs range from -0.6 to 0.75. Using ratios to the answers, I get an uplift of 12k (54kN) and 15.2k (67.7kN) down, which is considerably less than what you're working with. Hope this helps...

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Re: Re: Re: Re: Am I in the ballpark for wind loading?
Re: Re: Re: Re: Am I in the ballpark for wind loading? -- swearingen	Post Reply	Top of thread	Forum

Posted by: djinn ® 02/19/2007, 15:30:30 Author Profile eMail author Edit

Swearingen, thanks for your time. For free-standing roofs (no walls), the australian standard doesn't appear to give different values for different roof pitch. Judging by your results (135kN upward), I don't think I've made any major errors in my interpretation of the standard. I need to find a way to bring this value way down. I'll let you know how I get on.

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Re: Re: Re: Re: Re: Am I in the ballpark for wind loading?
Re: Re: Re: Re: Re: Am I in the ballpark for wind loading? -- djinn	Post Reply	Top of thread	Forum

Posted by: swearingen ® 02/20/2007, 07:30:24 Author Profile eMail author Edit

Please keep in mind that the 135kN figure is for a roof sloped at 30 degrees with the wind blowing towards, and perpindicular to, the higher eave. If your slope is less, the values will be less, far less if it's close to a 0 degree slope. I do like the 0.65 factor for a temporary structure in your code. Here, there is a minimum wind speed of 70mph (113kph, 31.3m/s) that you must use. I'm on a hurricane coastline, so it does give some relief to our local wind speeds in the hurricane off-season for temporary structures. Many of my contemporaries inland don't see any benefit, though.

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Re: Re: Re: Re: Re: Re: Am I in the ballpark for wind loading?
Re: Re: Re: Re: Re: Re: Am I in the ballpark for wind loading? -- swearingen	Post Reply	Top of thread	Forum

Posted by: djinn ® 02/20/2007, 21:16:00 Author Profile eMail author Edit

The roof is to be on a minimal pitch, say 5 or 10 degrees. I struggled through the "detailed procedure" of the code. I was able to use a design basic wind speed of 25m/s, which was factored down from a regional wind speed of 38m/s. I came up with a much more promising result of 41kN uplift. I asked an experienced engineer about it, a very clever guy. He did about 30 seconds of calculations on a scrap of paper, estimating the coefficient of lift and using Bernoulli's equation, and amazingly came up with 40kN. The price was being lectured on the Tacoma Narrows bridge. The temporary structure factor in the "detailed procedure" was only 0.8, but still very useful. It is for structures intended to be erected for 6 months or less.

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Re: Re: Re: Re: Re: Re: Re: Am I in the ballpark for wind loading?
Re: Re: Re: Re: Re: Re: Re: Am I in the ballpark for wind loading? -- djinn	Post Reply	Top of thread	Forum

Posted by: swearingen ® 02/21/2007, 08:28:34 Author Profile eMail author Edit

For grins, I ran the numbers for 10 degrees and got 18k (80kN) uplift and 20.9k (93.1kN) downforce at a 35psf loading. Please note that the 35psf is a very conservative number; 41kN sounds like a reasonable figure. Also, the numbers I gave act perpindicular to the roof surface, which means some of the load goes into the lateral bracing system of the structure. Make sure the structure can handle this lateral force - uplift is one thing, but the Big Bad Wolf didn't count on it when he blew those houses down.

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