Seismic Requirements for Non-Building Structures

[TJW]

Hi all, and thanks for reading. I am looking for some thoughts as to how folks apply seismic design requirements to non building structures (ASCE 7 Chapter 15). I'll try to make this as brief as possible:
IBC Section 1613 says "every structure, and portion thereof.... shall be designed to resist effects of earthquake motions in accordance with ASCE 7."
In ASCE 7, for non building structures, Chapter 15.1.1 states that "Nonbuilding structures include all self supporting structures that carry gravity loads and that may be required to resist the effects of earthquake..."
When ASCE 7 says "structures that carry gravity loads", are they referring to structures that support something else? Where I am getting into a gray area (at least in my mind) is related to equipment that is not supporting anything other than its own weight.
Example:
Consider a composite layup mandrel. Picture a drum (let's say it is the size of an airplane fuselage) with trunnions or bearings on each end. This gets mounted into a machine that spins it to layup the composite layers. While this mandrel supports it's own gravity weight, it is not supporting anything else. I would think that seismic design for this would be concerned with the attachment of the mandrel to the base machine so that in a seismic event it doesn't become detached and fly away. However, I wouldn't think that the mandrel itself would require a seismic design. If the mandrel gets destroyed in an earthquake do I care as long as it doesn't become detached?
Another example would be a piece of machinery in a factory (say a milling machine). Again, I would think that you would be concerned with the connection of the machine to the floor so it doesn't tip over and hurt someone, but the milling machine itself would not require seismic analysis.
Am I looking at this correctly? How do others apply these requirements?
Thanks for reading through this and I'd appreciate any feedback.

Tom W

 

[hawkaz]

Sounds like chapter 13 may be more appropriate for these

 

[calvinandhobbes10]

The nature of seismic requirements is to make sure structures don't rack or sway over when lateral loads are imposed on them. It's a blurry line how far to take this (for instance, the desk I'm typing on). My two-cents is that most pieces of equipment with moving parts tend to have the necessary structural integrity to remain stable under significant lateral loads, while the base-mounting often gets overlooked. Such as the HVAC units mounted on and in the buildings I deal with.

My advice would probably center on whether your task is to design this building/facility (site-specific); or to design the mandrel frame/assembly itself (presumably not site-specific). If the former, look at base mounting and leave it at that---the mandrel inner workings and connectivity is not your scope. If the latter, I'm not sure IBC/ACSE governs intra-equipment framing/loads, and would suggest you look into codes and criteria governing mechanical unit design.

 

[JAE]

ASCE 7 and the IBC and other building codes in general deal with life safety issues, not equipment/building survival after the EQ.
So your assumptions listed above where you are concerned with the equipment not tipping over and hurting someone is valid.

The equipment itself is many times designed by others and its internal integrity with respect to seismic would be the equipment manufacturer's responsibility, I would think.

Some equipment is compact and discreet such that an internal failure of its "structure" might not ever occur.

However, for larger systems and equipment, the internal stability of the piece may in fact involve life safety...i.e. it's held down OK to avoid tipping over but it laterally fails and collapses - putting nearby occupants at risk. You'd have to evaluate each situation independently I would think.



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[SlideRuleEra]

At our electric generating stations, in a high seismic area, anchoring of large equipment, including top-heavy coal pulverizers is accommodated. Per JAE's comment, the equipment itself is not addressed.

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[JStephen]

One problem is lack of a usable definition of a "structure". If I remember right, it's defined as "that which is built" so it includes automobiles, hamburgers, anthills, beaver dams, and just about any physical object. So it's a non-definitive definition, unfortunately.

Traditionally, some items are designed for seismic, some are not, and there is not really any logical difference to distinguish the two in many cases. So, for example, vehicles are subject to wind and seismic forces as much as any building, but aren't traditionally designed for it. More commonly, something that's custom-designed for a specific application and that will not be moved will include seismic and wind design. Something that is liable to be moved from place to place, and the same identical item is used in different areas, is less likely to be designed for wind and seismic. So typically, a barrel is not designed for seismic, a tank is, even though they may be functionally identical.

If the specific piece of equipment is included in the tables in ASCE 7, that's a good sign that it's commonly designed for wind or seismic.

Does the item require a building permit, come to think of it?

 

[TJW]

Thanks to everyone for your insight. Seems like it may be a good opportunity to exercise some "common sense" rather than black and white Code pages.
JStephen, the scope of these projects typically would be permitted, but I'm thinking in more general terms than a specific project as the theory applies to many projects we have.
Thanks again for the feedback.

Tom W