model a solar panel as a solid sign or monoslope roof in ASCE 7 1

[SRO]

I have to design a single post support for a solar panel which is essentially a 10'x14' sign that is tilted 45 degrees. I’m torn between ASCE 7 section 6.5.14 “wind load on a solid sign”, & section 6.5.13 “monoslope roof”.

Sect: 6.5.14 (solid sign) doesn’t account for any uplift resulting from the tilt of the panel which may come into play with the overturning of the footing. However, Sect: 6.5.13 (monoslope roof) doesn’t account for eccentric loading.

Does anyone have any recommendations?

 

[kslee1000]

Combine the applicables. Any problem doing that?

 

[SRO]

That was going to be plan-B if i couldn't find a more direct approach.

 

[bdSE]

It is possible that the uplift from the mono slope roof occurs because of the pressure differential between the outside of the building and the inside of the building. Think of aerodynamic principals that make the area above the roof a low pressure zone.

If that is the reason for the uplift, then the panel will not create much (if any) uplift for this particular reason.

There will be uplift when the wind hits the "back" side of the panel. Imagine the forces on your hand as you put it out the car window at 60 mph.

I think this type of uplift could be calculated using a vector analysis approach. The numbers should be conservative since the energy transfer will not be 100% efficient.

Don't forget to apply both windward and leeward forces if you are using a method 2 type calculation.

 

[SRO]

damronb

Do you see any problem with using vector analysis with the solid sign approach? I could use the vertical projection of the panel as a sign, then break the forces into components that are perpendicular to the face of the panel.

 

[bdSE]

SRO,
I don't see a problem with vector analysis and vertical projection. I believe that is how I would analyze the model. Of course, if I have learned anything in engineering, most problems can be solved in a number of ways... just know your reasons for why you solved it that way.

Although uplift should be considered carefully, I am thinking that the overturning moment will control most of the design. Large sail on a small base can make for a large withdrawal forces on your anchor bolts and bending in your base plate. Plus the post will likely be sized for bending rather than compression.

Ultimately, I think the solid sign / vector analysis should give you appropriate forces with a little safety buffer in the end.

 

[racookpe1978]

I like the idea of combining vector analysis (to find out forces) with "sign wind load" to figure out how to resist thosee forces.

A "perfectly steady" north wind (a wind from due north) will tend to "lift" the sloped panel, bending the post and its foundation "down". A "perfect" south wind will push the top of the panel "down" towards the north creating an opposite torque and moment load. Uplifting forces from that north wind need to be checked carefully because your anchor bolts, anchor mass are all that holds the panel down.

Unfortunately, no wind is either perfect nor steady nor directly from the north or south - so the "sign" requirements against twisting and vibrations and wind gusts are more controlling than the sloping roof Code assumptions for a solid building and flat walls under a sloped roof.

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Careful! A 45 degree slope is WRONG for most areas of the country! What is your latitude for the installation? When do you "need" maximum power? (Winter, spring/fall, or summer?)
When is cloud cover (or storm conditions) worst?
When are cloud conditions "acceptable"?

For example, in Mass or upstate NY or NH or Washington or OR, solar panels get very, very little clear sunlight in winter, so they are aligned up there to be most effective (perpendicular to the sun!) for summer conditions of a high solar angle at midday.

In FL or TX or NM or AZ or southern CA, summer heat loads (high AC demands) require good reception spring (at the latitude of the panel), summer (at latitude -23 degrees), and fall (at latitude). Winter reception (at a higher slope of latitude +23 degrees) is less important in those areas.

Best reception year-round will be with a manually adjustable angle panel changed each quarter halfway between each soltices - most expensive of course, but it will pay off quickest. An automatically adjusting panel is most exotic, most elaborate, and most effective. (And almost unaffordable by any real person.)

 

[SRO]

The actual panel is designed by another firm, so location, angle, efficiency... is beyond my scope of work. The actual angle varies between 35-45 degrees, which was given in the information provided to me.


Thanks for the help.