Floor Vibration Experts 3

[whymrg]

I am working on a project for a hospital with an eye surgery units. I designed the framing using RAM-Steel software and started checking for floor vibration.

I am using AISC Steel Design Guide # 11 "Floor Vibrations Due to Human Activity" chapter 6 "Design for Sensitive Equipment". According to Table 6.1 on p. 46, Vibration Velocity for eye surgery use shall be less than 1000 microinches/sec. My framing is very similar to Example 6.4 on p. 53.

In this Example after decreasing the span to 25 ft (p.54) Neff is determined, but applicability of Eq. 4.7 is not checked. However, the value Lj^4/S equal to 4.4*10^6 is outside of range.

In the last statement in this Example it says that this framing is still not acceptable. If I try to increase the beam sizes or reduce the span, I will be even more outside of range for Eq. 4.7.

Reducing the walking pace in the hospital where emergencies may occur is not a good option.

I would like to know your opinion about this.

Thanks.

 

[lutein]

Check on Floor Vibration Program by Professor Murray
- DESIGN ADVISOR

 

[ishvaaag]

If you can't solve it with steel, try Reinforced COncrete, or if not feasible, modify through encasing for bigger rigidity.

 

[JAE]

whymrg,
Altering the spacing or size of the beam won't really help you much in reducing the human perception of vibration. The best way is to either:
1. Add damping - For your surgery areas you could specify installation of mechanical dampers in just that area. This might be the most cost effective way.
2. Add stiffness and mass by increasing slab thickness.

According to most published reports, bridging between beams does not help much. I really find this hard to believe in one respect: Plain, small angle bridging doesn't really addd much structural stiffness in the orthogonal direction compared to the beam stiffnesses. However, if you add a significant cross-truss system that creates a kind of two way system, I would think you'd decrease your amplitude to a greater degree.

 

[whymrg]

Thanks, guys.
I will try.

 

[GregLocock]

Build an isolated floor. The biggest I've seen is 200 tonnes, but for a theatre I'd guess you could get away with 10-20 tonnes. Mounting frequency should be less than 0.3 Hz, at a guess. Dynamic (tuned) absorbers may work, but then again they may not, as designed.

Cheers

Greg Locock

 

[JSA]

In my limited experience, adding mass is the most efficient way to reduce vibrations.

 

my best advise to you is to do an eigenvalue analysis (mode shapes) and workout the lowest frequency (vertical), and compare the result to the recommended criteria.
increasing the concrete thickness is one way but that is not really very efficient because mass will reduce frequency but the increased stifness will increase it, not by the same amount of course but they counter each other. However the advantage is that you gain damping.
hope that helps