RC Slab Supporting VR Pump.

[mo74se]

Hi,

I'm looking at an existing concrete slab that is supporting a vertical water pump. The pump will be replaced w/ a newer one that weighs more, has more HP, and higher thrust force. The slab is a single-span simply-supported on concrete walls. The superimposed loads on the slab are the concrete housekeeping pad, the pump weight, the thrust force and a 100 psf of live load. My concerns are:
1- What load factor is appropriate for the thrust force? Would you treat it as live load or an impact load (50% more) or else?
2- What about the deflection limit? L/360 or is there a maximum limit to keep the pump running properly?
3- How about the vibration? The supplier's cut sheet report a reed frequency, is this the fundamental VERTICAL mode shape of vibration (up and down)? The salesman was not sure!!!!!
4- Would you evaluate the slab for the vibration of the pump's fundamental mode shape only or is there a RANGE of frequencies beyond which the slab's fundamental mode shape needs to clear?
5- Are you aware of anything in the ASME and/or the Hydraulic Institute standards/codes that addresses this issue in a more stringent way than ASCE 7-05 or the 2006 IBC?
6- What else would you consider/investigate?
7- Resources? Not your typical structural engineering textbook example.

Since it's a single-span, there is not much redundancy in the system. If anything goes haywire, there is no alternative load path! I'm trying to get as close as possible to the edge w/o falling off!

Your thoughts are greatly appreciated.

Cheers!

 

[youngstructural]

Hello;

A well-laid out question. I am well impressed... I'll try to help as much as possible, and will answer in the order you present if and only if I have something for you on that point.

1. Depends on your code. Normally this is considered a static load if the pump is always on (thus continously applied) and a live load if the pump comes on and off at some interval (thus continually applied). Check your code.

2. Depends on your code again, but almost certainly will not govern over your much more important question regarding vibration (#3).

3. This is typically a vertical frequency, from what I have seen. HOWEVER, this does not mean what you have been given IS a first mode fundamental vertical. Your supplier really must confirm this, and no one here is going to be able to tell you with 100% certainty. Oscilating frequencies can be tricky. BE SURE. And post the value, as that might help someone to give you a "probable" indication.

4. I will also mention that you should avoid the first fundamental by at least 15% each way, as well as the first two harmonics as well (minimum; I always try to avoid first three as a general rule). I also like to calculate my systems fundamental frequency at least two ways (typically by hand with simplified short-hand formulae, as well as by full computer modelling). Be sure to include your walls if they are tall in proportion to the span; You might be well surprised at how much it can effect your result. Also remember that you slab is very likely to work as a two way slab for vibration, since the loads will likely not approach the strength design loading (presuming you avoid matching the pump's frequency and harmonic modes).

7. I would recommend J. L. Humar's Dynamics of Structures text; Not only because I am familiar with the work, but also because it is a very comprehensive treatment of the subject. If you want something more hands on, look at the ATC manual on the subject (the name excapes me at the moment, however it would be by Thomas Murray and David Allen (this is principly a floor vibration text, however it should assist with a good understanding and would be easily extended to cover a point vibratory load).

Post more questions if you are still unsure... This is a problem I handle from time to time: I'm happy to help if I can.

Good luck,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[RMunoz1361]

Hey,

1. You should treat the thrust load as an impact load and increase the load similar to designing a crane rail system. (Minimum of 25%)

2. The deflection of the system should meet the limits set forth in ASCE, IBC and especially ACI 350. ACI 350-89 is the concrete design for environmental structures much like ACI 318 is for buildings. Table 2.9.2(b) in ACI 350 sets limits on elements supporting equipment at specific operating speeds.

3., 4., 5. Your structure should have a natural frequency at least 50% greater than the natural frequency of the pump. There is discussion in ACI 350-89 section 2.9 on impact, vibration, torque, and seismic loads.

6. You should also consider an environmental load factor when determining your design load (Mu) versus your nominal strength (oMn).

7. Do not gamble in environmental structures, pump foundations and try to keep a little in the bank, since you did not witness the construction. Just a note, the more displacement the pump is allowed to achieve you run into maintenance issues with the pump.

And I have not seen and any textbook examples, so if you see some send them my way.

Ram