For Wind loading on external pipe racks (1 tier of piping with Cable trays at the top, about 20 ft high) is it considered adequate to apply transverse and longitudinal wind loads (no uplift) to the structure?
Yes, pipe racks typically don't consider any type of uplift from wind load. There is an ASCE document, "Wind Loads for Petrochemical and Other Industrial Facilities" that goes through this type of structure loading.
Also, with a pipe rack don't assume that whatever obstructed wind area you have today will be the same next year. For pipe rack design I was always taught, if there is available space someone will put something there. Plan for that additional potential loading.