Vibration controlled span - is this a "failure"

[skeletron]

I'm running a load rating for an existing mezzanine floor. The existing joists (effectively 2x12 @ 12 o/c) are ok for strength, borderline for deflection, but exceed vibration by 15-20%. So, I've gone down the rabbit hole of understanding the vibration calculation (holy smokes is it a long one) and also some Virginia Tech research regarding floor vibration and frequencies.

This is a mezzanine over top of a tenant's office. They want to use it for storage.

My question:
Is exceeding the vibration controlled span a "failure"? Or is this more of a serviceability nuisance in this setting?

In a residential setting, I wouldn't allow the span. But in an industrial/office setting, is this less of a concern?

 

[JoshPlumSE]

This is a mezzanine over top of a tenant's office. They want to use it for storage.

Well, vibration as a failure state is usually related to walking excitation due to human activity.... Like walking across the floor system. It is technically possible for energy to transfer up or down columns to adjacent floors, this is pretty rare. I wouldn't think it would be a concern for a space used as storage.

 

[Tomfh]

It's a servicability limit state. Whether it's a "failure" depend on how bad it is. The London millenium bridge was a good example of a severe serviceability failure.

 

[phamENG]

I don't check vibrations for storage unless it's a frequently used space. I really don't bother much with vibration for wood - I just go to L/480 live load for joists and L/600 live load for beams/girders on high end residential. There are some wood vibration papers out there that go through the whole thing and land on "designing for vibration in wood is usually not economical, but using these higher deflection criteria usually result in satisfactory performance."

 

[GeorgeTheCivilEngineer]

It's a serviceability failure but I'd try and put it back on the Client, or at least only 'pass' it if the proposed uses are clearly defined as not being adversely affected by the vibration. A light duty storage area that is not heavily used would be a good candidate for that.

I'd also consider brittle finishes - if the deflection is borderline and there may be excess vibration then plaster ceilings could be at higher risk of cracking.

 

[ThomasH]

When you say the the vibration "exceeds by 15-20%", what is it that is exceeded?

Usually vibrations for a floor is in my experience calculated as accelerations and compared to comfort criteria. Then it is SLS and the resulting stresses as often very low. In these cases safety is not an issue even if people may find the situation uncomfortable.

But I have also seen situations where the stresses have been high and the accelerations are in those cases usually huge. Then it becomes a ULS problem and safety can be an issue.

 

[XR250]

As Pham stated, I wouldn't worry about this for a storage area. Similarly, I don't worry about it at all for wood construction. I believe the I-joist software checks it however. Most of my jobs use dimensional lumber though.

Ironically, a homeowner had an engineer write a report about a house I had designed years ago. There was concerns that things on top of the dresser were rattling in the bedroom. The engineer's conclusion was that the floor was too stiff - WTF? It was 2x10 @ 16" O.C. spanning 12 ft.

 

[KootK]

1) It's certainly not a failure in the life safety sense. And it's only a failure from a serviceability perspective of the vibration interferes with the owner's intended used of their building.

2) I'd expect the stored stuff to actually improve the vibration performance in many cases. Lengthen the period and add some damping. As you noted, however, it's complex.

3) I'd also put this back on the owner. I doubt that many owners of this kind of building would be willing to alter their usage plans or reinforce the existing system to deal with a modest vibration issue.

 

[Canuck65]

As others have said above, this is a serviceability issue. Likely not an issue as soon as some stored materials are in place.

In the prescriptive tables of the NBCC (Canada) it is interesting to note that various joist configurations, i.e. 2x10 @ 16" o/c spanning 'X' feet, that the various configurations can span further when they have a concrete topping SDL on the floor. The floor joist configurations without the concrete topping are limited by vibration.

 

[JoshPlumSE]

One thing to add. If you're talking about vibration (and most serviceability issues). The loads you include when you calculate vibration need to be the ACTUAL loading you expect. That may be considerably lower than the estimated dead load and live load (which are normally based on STRENGTH design).