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Fw: PROBLEM ON USING CORRECT Rw FACTORS

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----- Original Message -----
To: SEAINT
Sent: Wednesday, January 17, 2001 6:38 PM
Subject: PROBLEM ON USING CORRECT Rw FACTORS

I am in hesitation to decide on correct clasification type and consequently correct Rw factor of high steel structure in one petroleum refinery.
 
This steel structure is 77 m tall and and it has 5 columns on one alignment and 3 columns on the other alignment (therefore, there are 15 columns). The width of structure is 30.4 m where 5 columns are and 12 m in the other direction (where 3 columns are). The structure is eccentrically braced with vertical bracing in all directions. There are 18 platforms on different levels which are composed of steel beams and horizontal bracing covered with grating. These platforms are for supporting some equipment and for reaching different levels of reactor (long vertical vessel) inside the structure. (The reactor is not supported by the steel structure). Structure is not covered with any cladding material (open structure). There is a stair tower and elevator in the structure.
 
What should be the correct clasification type for this structure? Can it be accepted as steel building frame system with eccentrical bracing or should it be considered as trussed tower nonbuilding structure?
 
If it is accepted as steel building frame with eccentrical bracing, Rw factor is 10 according to UBC-1994. However, if it is considered as nonbuilding trussed tower structure, Rw factor is 4 according to UBC-1994, regardless of whether eccentrically braced or not. What is the reason having so much different Rw factors in building and nonbuilding structures? 
 
Any comments on this topic will be highly appreciated.
 
Best Regards,
Fuat TINIS