Wind and Seismic Drift

[MacGruber22]

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 V_LRFD = 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.

"It is imperative Cunth doesn't get his hands on those codes."

 

[MotorCity]

Your allowable drift will be whatever your structure can tolerate. In this case, it sounds like it will be determined by whatever the mechanical and electrical components/fittings can tolerate. I would consult your mechanical/electrical friends for input. If you are left to determine this yourself, I would say L/150 would be an absolute minimum and I would be tempted to tighten it up to L/200 or more. To find the elastic (actual) drift, simply re-analyze your structure using ASD wind pressure (multiply your wind pressure obtained from ASCE 7-10 (LRFD) by the ratio 90^2/115^2).

 

[MacGruber22]

I am not sure I communicated myself well enough. I understand how to check wind drift using the ratio of the squares you mentioned. I guess what I am saying is that when I use the seismic drift limits typical of buildings, it is controlling in the short direction of my platform and requiring beefing up columns and braces more than is preferred. I am saying that I do not believe that it is appropriate to use the seismic drift in that manner for a non-building structure and I want to reduce the seismic drift limit to something more in-line with the 50-year wind MRI of 90 MPH. I want to check a service drift load combination similar to ASCE 7 appendix, D+W_MRI=50 , but with a reduced seismic value (D+E_MRI=50). You could also say that I am looking for an apples-to-apples comparison.

"It is imperative Cunth doesn't get his hands on those codes."

 

[JAE]

I wonder if your attempt to "match" drifts with 50 year wind is missing the proper approach here.

Per MotorCity - [blue]"...it will be determined by whatever the mechanical and electrical components/fittings can tolerate"[/blue]
This is the proper approach.

You stated: [red]"when I use the seismic drift limits typical of [bold]buildings[/bold], it is controlling ... more than is preferred."[/red].
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.



Check out Eng-Tips Forum's Policies here:
faq731-376

 

[MacGruber22]

Thanks, JAE and Motor. I understand your comments.

"It is imperative Cunth doesn't get his hands on those codes."