Wind and Seismic Drift
Wind and Seismic Drift
(OP)
I am working on a fairly tall (and heavy) non-building structure (outside elevated mechanical support) and I am trying to minimize the requirement for drift limits. To give some background, the platform is 20' tall, 20'x30' in plan and supported by 4 square tube columns. No diaphragm available (relying on infill beams to distribute to LFRS) and the LFRS is the cantilevered tube columns with some kickers at the top. SDC = B and VLRFD = 115 mph. My understanding of ASCE 7-10 is that there is no specific seismic drift limitation for non-building structures such as mine. Obviously, wind drift has always been the discretion of the engineer depending on building-specific material, drift detailing, comfort, etc. Thus, I am left determining both limits to work within in terms of allowable service drift and the MRI at which they will occur. To me, there is an endless number of combinations of drift limit values and their associated MRI (provide high MRI and low drift limit, or the inverse).
I am no expert as to the deflection limits that mechanical equipment, conduit, fittings, etc. can safely handle to remain serviceable, so I have to assume something. Arbitrarily, I was going to run with L/150 and have a note on my drawing that puts the responsibility for the above on others. The question is then to decide the lowest reasonable MRI for that drift. For wind, I may choose something like 50-year MRI at that drift level. The main question is how do I go about manipulating the elastic seismic MRI to approximately match that chosen for wind drift?
I realize that is may be more or less an exercise in thought, rather than being practically useful.
I am no expert as to the deflection limits that mechanical equipment, conduit, fittings, etc. can safely handle to remain serviceable, so I have to assume something. Arbitrarily, I was going to run with L/150 and have a note on my drawing that puts the responsibility for the above on others. The question is then to decide the lowest reasonable MRI for that drift. For wind, I may choose something like 50-year MRI at that drift level. The main question is how do I go about manipulating the elastic seismic MRI to approximately match that chosen for wind drift?
I realize that is may be more or less an exercise in thought, rather than being practically useful.
"It is imperative Cunth doesn't get his hands on those codes."






RE: Wind and Seismic Drift
RE: Wind and Seismic Drift
"It is imperative Cunth doesn't get his hands on those codes."
RE: Wind and Seismic Drift
Per MotorCity - "...it will be determined by whatever the mechanical and electrical components/fittings can tolerate"
This is the proper approach.
You stated: "when I use the seismic drift limits typical of buildings, it is controlling ... more than is preferred.".
This is correct in that a non-building structure shouldn't use or rely on drift limits for buildings as the "reasons" for those limits are specific to buildings, building cladding, etc.
It seems to me that you should review the components on your platform and see what "real" deflections can be tolerated without significant damage to them. This would be much more rational than trying to use a 50 year or other nebulous drift criteria.
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RE: Wind and Seismic Drift
"It is imperative Cunth doesn't get his hands on those codes."