Frank...a couple of comments:

When the rack is loaded, the net wind load will likely not be critical.  

When unloaded, with the panels at the top, the lateral net wind load will likely be at its most critical.  In general, the windward side of the structure would have a factor of about 1.25 to 1.50 and the leeward side would have a -0.75 to -1.0, therefore both acting in the same direction.  If the top of the rack is open, you would need to consider windward and leeward pressure on each of the panel walls, as the wind could "dip" over the top of the leading panel wall and impact the lagging wall, thus exerting pressures on both.  This, I believe, answers your shading question.

You might also check the rack with only a few pipes on it, as this could conceivably give you a higher wind pressure from the side, though likely the dead load of the pipes will be adequate to stabilize for overturning.

Ron

Check your building code for wind load requirements on Components and Cladding.  Then, like Ron said, you may have to consider pressure on one face and suction on the other acting at the same time.

Using ASCE 7-95 as the guide to most building codes your threshold for Components and cladding is a trib area of 750 square feet.  A 30 spacing of bents would only need to be 25 feet high to meet this.  You can use the main wind force resisting system to determine your total load.  When calculating this, you can leave out the internal wind coefficient, GCpi, because you have no internal pressure.  Also look to see if you could qualify as an open structure since your cladding could concievably be on one side.  Finally don't forget to check the column for overturning compression, where the worst case would be fully loaded.
Hope this helps.

If there is a cladding 18' deep, it certainly will provide a lot of
shielding to a similar cladding 20' on the leeward side. You
should consider a shape coefficient of 1.5 for the windward
cladding. The shielding coefficient for your case should be
available in some codes. (I know Indian Standard has, others
should have; and some texts should. Sorry, I get no time for
search these days!). It would be of the order of 2/3, and your
total force would be about 2 times the basic wind pressure
times the area.

Windward = 1.5 * force
Leeward = (1 - 2/3) * windward = 0.5 * force
Total = 2 * force, where force = pressure * area

The load on the elements not covered by the cladding should be
considered separately!

HTH,

M. Hariharan

I have found that wind loads and seismic loads can often dominate the design for pipe racks but then my designs are often in cyclone (hurricane) regions and/or seismic regions.
The sheeting on the windward side should shield the pipes and leeward sheeting and will probably be in your wind loading code.
You will have to check different load combinations to see which is the most critical for design, including uplift on footings.
Pressure coefficients are likely to be different between your code and mine so I won't quote any.