The corner wind zones will still have to be considered on this square building. The Florida Building Code, ASCE or IBC do not have any allowable reductions.
When the wind load is directed at the building and can't go through the building the air must go around. Therefore, the corner will...
In the first section of IBC, 1609.1.1, loads shall be determined in accordance with Section 6 of ASCE 7. You should use the simplified method for enclosed structures only. If in doubt do the calcs per ASCE to verify.
IBC 2003 has revised the introduction to 1609.6.1 to read... like....that the...
Providing that you are only looking at wind loads to provide support for the pipe. You don't need to be concerend with internal pressures since your pipe should be a closed system. using ASCE7-98, consider this an open structure to determine the wind force. Then all you need is the velocity...
I would agree that you should be using category 'C' for this mountinous site. The contour of the terrain can reduce the wind velocity more than open water. Category D should only be used for shore line conditions.
From what you explained you can consider your deck as an open lattice structure table 6-13. However, you should probably look at the tables to use a more conservative loading. I think that you will find that the design will be more governed by the vertical loading. Good luck