Torsion from spring isolators

[smb4050]

How are engineers treating spring isolators used to support exterior mechanical equipment supported by steel framing? Specifically is there a torsional force in the steel supporting framing from either vertical or horizontal loading? In the applications I'm referring to a spring isolator is installed between 2 steel beams. The top beam has the mechanical unit attached to it and is sometimes supplied by the spring isolator company, and the bottom steel beam is the one I am concerned about and is the member we design. In my opinion the answer is yes there is a torsion in the supporting steel beam from both horizontal and vertical loads. Torsion from the horizontal is the distance from the top of the supporting beam to the bottom of the mechanical unit x the horizontal force (wind/seismic). I'm less sure if there is torsion from the vertical loading. My assumption is the spring isolator deflects laterally, so the torsion would be the lateral deflection x vertical loads. All comments are appriciated. Thanks,

 

[MikeHalloran]

Coil springs will introduce a bit of torsion as they are stressed. I.e., the ends rotate relative to one another as the typical compression spring deflects. Some isolators include guides that slide against one another to keep the torsion within the particular isolator. Other isolators just assume that you have installed two or more in parallel, so the torsion resolves to some lateral deflection or twist of the overall assembly, or the spring ends are closed and/or ground so the spring ends can rotate against a flat surface within the isolator itself.

There's also some lateral deflection induced because the actual forces involved are transferred to the wire itself, not to the theoretical centerline of the spring, and the ends are not usually 'clocked' so that the forces are perfectly aligned.

It's the sort of problem that you probably should analyze in detail, and test, for a representative cheap spring isolator, time permitting, once in a lifetime, so you develop a feel for what's going on in the real hardware.






Mike Halloran
Pembroke Pines, FL, USA

 

[smb4050]

Thanks for the reply and comments. It's always helpful to get another perspective. I've done some elementary analysis, but as you pointed out it depends on how the isolator is made. I wrote the post in part because I don't recall seeing torsion being addressed in a structure with isolators, and thought perhaps I'm being overly conservative. Thanks agian