non-building structures similar to buildings

huangyhg

non-building structures similar to buildings
i have a free standing, 4 column concentric braced frame stair access tower in a seismic design category d zone.  
it can be classified per asce 7-05 table 15.4-1 as an ordinary concentrically braced frame, whereby you have to either design it as a building or use a low r value of 1.5.  as far as i can tell it would be subject to the redundancy requirements in chapter 12 no matter what the r value.
or it can be classified per table 15.4-2 as a freestanding trussed tower with r=3 and minimal detailing requirements.
what defines a non-building structure similar to a building?
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the definition is a little unclear to me, we also design a lot of non building structures and this comes up often in discussion in my firm.  i would think your stair tower would definately be similar to buildings, as well as pipe racks, storage racks, structural towers supporting tanks, etc.  so aisc 341 detailing would be required.  but the ocbf requirements aren't that bad....
look at the forestry service fire watch tower designs.   
richard a. cornelius, p.e.
it sounds to me that the defining point of a building would be if the structure has a diaphragm at each level, with reasonable story heights 8ft-20ft (bearing in mind some relative building footprint)...if so, you could use table 15.4-1.  also, see 15.5.5 for the definition of a "structural tower" similar to a building.
since you are using r=1.5 for the ocbf, it sounds like it is really tall, otherwise you'd be using the r = 3.25.
bummer.
trussed tower seems appropriate if it's quite tall, and without diaphragms (multi-brace up the height = truss).
r=3 for tt... see 15.7.3:  i would still keep the principals of seismic design (ductility) in mind when designing these connections.  it's better to have ductility with forces of a low r, than no ductility with forces of a high r.  
basically, to slow down a building in an earthquake, something has to yield.  with minimal details, that weak link is failure (possible brittle failure) of your connections.  hence you are forced to have the higher forces  (strength development) of a low r.   
also, closely examine where you need to use the omega factor in 15.7.3, and elsewhere.
so, a little care in detailing would be important, and not that crazy to do, if it as repetitive as i imagine.
best wishes!