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Wind Pressures on Horizontal Tubes

Wind Pressures on Horizontal Tubes

Wind Pressures on Horizontal Tubes

(OP)
Hi,

I am designing framing for a solar collector unit much like this one:

https://www.google.com/search?q=solar+chill+solar+...:

The framing is essentially four columns and four girders connecting the columns. The columns will need to be 7' tall in order to fit a generator underneath.

However, the unit is only slightly inclined, much closer to horizontal. The framing will not be attached to any other structure. From my understanding, in order to analyze the wind pressure acting on the tubes per Chapter 29 in ASCE 7-10, the tubes must be normal to the direction of the wind. This is not going to be the case. I am left to analyze the collector as if it were the "roof" of an open structure per Chapter 27, in order to size my frame members. Because of the generator underneath, I am assuming obstructed wind flow, thus increasing my uplift wind pressures. I feel that my wind analysis is too conservative if done this way.

Is there any other provision in the code which allows me to analyze the uplift pressures on the tubes similarly to that of lateral pressures on a chimney stack as in Chapter 29?

Thanks

RE: Wind Pressures on Horizontal Tubes

Why would you be able assume the wind (at some time or another) would NOT be perpendicular to the horizontal tube? Wind load MUST be analyzed for the worst case condition at the required gust levels, not "the most streamlined" condition.

RE: Wind Pressures on Horizontal Tubes

(OP)
racookpe1978

You misunderstood my question. Chapter 29 in the ASCE analyzes vertical cylindrical structures such as chimneys (they are perpendicular to the flow of wind). The solar collectors i am placing are going to be lying down horizontally, parallel to the plane of the ground. Therefore, I am not able to analyze the collectors as cylindrical elements per Chapter 29 with gaps in between and must instead analyze them as a "roof" structure per Chapter 27. I am not analyzing them in their most streamlined state. From my understanding, even though there is uplift due to wind on roofs, the "wind direction" is always defined as being parallel to the ground.

RE: Wind Pressures on Horizontal Tubes

I don't know if it is explicitly in the code somewhere but I see no problem in taking your open structure uplift pressures and applying the same principles you would if you were analyzing lattice work with horizontal pressures and only apply the pressures to the projected face of each member using the appropriate shape factor. I can't think of any reason why this method is not applicable.

RE: Wind Pressures on Horizontal Tubes

What latitude are you installing the collectors at?
Fixed angle, or will you rotate them each month through the year to improve efficiency?
At that interval between the row of collector tubes, it is much more accurate to assume a complete single flat plate as the wind surface, not a series of individual tubes. reason is, the very small air gap between tubes stalls and blocks flow between the tubes, so it all acts as one big flat plate.

You indicate there will be four verticals, a side frame around the tubes, and a pump (or eqpt) below the array, right? So the exposed surface needs to be calculated like an inclined open roofed structure that height above ground. "Lift" occurs with winds trapped below at low angles to the ground (small latitudes) and push-over forces dominate at high angles to the ground (high latitudes).

RE: Wind Pressures on Horizontal Tubes

(OP)
dnlv,

If I use Figure 29.5-2 in ASCE 7-10 for Open Signs and Lattice Frameworks, Note 2 says "The calculation of the design wind forces shall be based on the area of all exposed members and elements projected on a plane normal to the wind direction. Forces shall be assumed to act parallel to the wind direction." I interpret this as I can only use this figure if my sign or lattice framework is vertical as "wind direction" is lateral, but not necessarily the pressure derived from the wind. Do you agree with this interpretation? My solar collectors are sloped 20 degrees from the horizontal.

racookpe1978,

Latitude is 13.4443 degrees North. Not sure, but I think the angle of 20 degrees is fixed.

This is what I have been doing, using Figure 27.4-4. I am assuming it is an open monoslope building with a "roof" slope of 20 degrees. My client wants to use a certain structural tubing that he has already purchased, but it is failing in my analysis. I wanted to see if there is another less conservative way of doing the analysis before breaking the bad news.

RE: Wind Pressures on Horizontal Tubes

in reality, wind is not always parallel to the ground especially in the layer influenced by the terrain....

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