ASCE 7-05 Solar array on single post Area for oblique wind direction

[RHTPE]

I will post this question because I'm sure that someone out there has encountered this before.

I am analyzing wind forces on a solar array that will be mounted on a single post. When considering wind at an oblique angle, should one use the area projected onto a plane normal to the wind direction or should one use the actual area?

See Figure 6-20 Case B or Case C.

As I read 6.5.14, I suspect it is the actual gross area no matter what the wind direction is relative to the plane of the surface.


Ralph
Structures Consulting
Northeast USA

 

[racookpe1978]

You should plan on the wind coming from any direction.

And, since the solar panel will be inclined to the vertical - you will have both a lifting and a downwards force from any direction.

 

[Ron]

Projected area, normal to the wind direction.

 

[RHTPE]

ra - I agree that the wind will come from any direction. The point is moot I suppose, since with s monopole mount, Case A would control. The array is intended to be inclined to the vertical during heavy snow to avoid snow load on the panels. This obviously gives the largest "sail" area. I do realize that the monoslope condition (its normal position) must also be considered - Figures 6-18A & 6-18D.

Ron - With a monopole mount, the question is probably moot.

Using 10" Std pipe for the pole I now have dig into other resources to calculate a realistic Gust Response Factor, as this really must be considered a flexible structure.

Thanks for the comments.


Ralph
Structures Consulting
Northeast USA

 

[Ron]

Ralph,
Good point about the flexible structure, although it's possible that it is not, depending on the column section and height.

As for the other issue of projection, you have to consider the oblique condition because of the torsion it will induce in the column.

We won't even get into fluttering!!

Ron

 

[RHTPE]

Ron - I went to view a similar installation yesterday with my client. Fortunately for me, my task is to design the foundation (it will be founded on bedrock) and not the entire support structure for the array. Fluttering, as you put it, is definitely something to be considered, as it takes little force applied to a corner of the array to effect movement of the entire assembly. Certainly a lot must be considered when designing the foundation, but at least I'm not checking every member.


Ralph
Structures Consulting
Northeast USA

 

[racookpe1978]

Double check your "assumptions" and calc's by comparing the result to a several existing monopole advertisement or road signs of comparable cross-section. Fluttering will be an irregular amplified vibration, so that adds cyclical fatigue twisting the member. Yuccch.

You appear to be northeast-based from your login ID, but just because you don't face Gulf-Coast hurricanes doesn't minimize your storms. Plus the ice-weight when they do blow in ...

 

[wannabeSE]

RHTPE,

ASCE 7-05 Chapter 6 commentary provides guidance for approximating the fundemental frequency of poles which can be used to determine the gust factor.

If you are trying to decipher the sketches for case B and C, I would apply the wind force normal to the surface rather than the projected area. In the sketch, the F and square block depicts a force which is normal to the surface.

 

[RHTPE]

ra - You are correct, I am Northeast based. While our hurricane frequency is minimal, we are not immune. I remember 1955 (mostly heavy rain, but some areas experienced high wind). I'm also old enough to have heard the 1938 hurricane stories. Our most common high winds come from micro bursts associated with thunderstorms. Tornadic winds are rare - I would put them in the "Act of God" category. Ice load combined with wind certainly needs to be considered. Thankfully, this particular project is in my hometown, so I do have reasonable insight on what to expect.

wannabeEIT - I'm aware of Eqn C6-22a in the Commentary - I'm somewhat confused about the units though - I would expect it should be inches & pounds, but for 10" or 12" Sched 40 the results don't feel right.

Ralph
Structures Consulting
Northeast USA

 

[wannabeSE]

Perhaps you are having problems with the units for mass(W/g). The results of the frequency equations are time^-1.