Machine Foundation 2

[marinaman]

I've been asked to design a foundation for a printing press. The press is about 60' long and about 7' wide. I've been collecting data before I begin any design. I've gotten with a local geotechnical engineer who will provide me the allowable static bearing capacity of the soil, the subgrade modulus, and tell me the natural frequency of the bearing soils.

I've been collecting data on the press itself from the manufacturer. He's given me the static mass of the machine (in kilograms) at each press component, the "basic fequency" of the machine as 5 Hz (300 cycles per minute), and has given me a dynamic force of about 2100 Newtons at each of the (8) printing press component locations. I've also been given criteria from the manufacturer regarding maximum movement of the foundation as 0.05mm/meter.

I typically do not work in metric units, so I have to convert the geometry and loads of the machine to US/Engish units. Converting the mass of each component of the machine to pounds is fairly simple as (Kilograms)(2.2) = Pounds. Converting a force in Newtons to a force in pounds is also fairly simple as (Newtons)(0.2248) = Pounds. So I have my loads in US/English units.

I have a question regarding both the frequency of vibration of the machine, and, the dynamic force application. I'm using RISA Foundation as my FEA software to work with this foundation. I will be using RISA to develop my soil pressures, deflections, etc of the mat slab foundation.

Frequency Question - If my geotechnical engineer determines that my soil has a natural frequency thats 3 or 4 times the natural frequency of the machine (frequency ratio < 0.333), is there any need to further review the basic frequency of the machine with the foundation? Is there anything else to analyze regrading vibration?

Dynamic Force Application - If all eight print components have a potential dynamic force of 2100 Newtons each, in addition to their static weight, besides the load combos of 1. All static plus all eight dynamic force down 2. All static plus half dynamic up and half dynamic down 3. All static plus all dynamic up.......is there any other load case I'm forgetting on this foundation?

If any of you guys know of any pitfalls to this type design that you've experienced in the past, I'd like to have your input so that I can avoid any problems with the design.

I am aware of the "over the thumb" design estimates for machine foundation design being the mass of the mat slab being about (3) times the weight of the machine and that the actual bearing pressure (static plus dynamic) should be only about 1/4 of the allowable static bearing pressure provided me by the geotechnical engineer.

Any input would be greatly appreciated.

 

[JStephen]

I'm not even sure what the natural frequency of the soil would be.

Seems to me you'd want to make sure the natural frequency of the equipment moving on/with the soil wasn't close to the operating frequency.

I recall another recent post where the equipment supplier had specified a minimum stiffness for the foundation, and that would be easier to work with.

 

[boo1]

review

http://yoga10.org/Documents/SPVKPSERCpaper.pdf

http://www.slideruleera.net/foundations.html

 

[dcarr82775]

I have never seen a geotech give a natural frequency for soil, just some more detailed stiffness information than normal when it comes to machine foundations. The total mass of the machine and foundation is used in conjunction to get something akin to the natural frequency of the system. So to answer your first question, I think you need to run the numbers to see if it is a problem 1st, then go from there. I would be skeptical of a geotech who gave you a natural frequency for the soil and not the system.

Don't forget the rocking and sliding modes either, it isn't necessarily all up and down

 

[WARose]

Like the others have pointed out you will probably need some dynamic spring constants [and related info] from the geotech. But first you need to decide how you will model it….and that depends on the system itself. The most common thing to do with what you seem to be describing is to represent the foundation as a Single Degree of Freedom System (SDOF) and calculate the natural frequency and displacements for the 4 (typically considered) degrees of freedom: vertical, horizontal, rocking, and torsional. (See ‘Design of Structures and Foundations for Vibrating Machines’ by: Arya, et al) You’ll need to calculate the amplitudes at the operating frequency of the machine and (if the foundation is “low-tuned” [i.e. the equipment passes through the resonance frequency of the foundation during start up for one of the vibrating modes]), you’ll need to figure the amplitude at resonance. You will compare these displacements to allowables from either the manufacturer or a general machinery chart. (I check for human perceptibility would be good too; there are charts for that.) You will also compare the operating frequency to the natural frequency of the various degrees of freedom/modes: you’ll want to keep them about 20% higher or lower than the operating frequency.

There are a couple of pitfalls with that approach however: if the equipment is sitting on tall (slender) pedestals, reducing the system to a SDOF (based on soil stiffness alone) may not capture how the pedestals displace (especially at higher modes). And along those lines you have to be careful to couple certain degrees of freedom that are related (i.e. rocking and horizontal) in hand calculations. It is very popular at this point however to do a FEA model of the system (and if it’s supported on piles: it’s much cleaner to do it that way).

The rule of thumb you mention is pretty good, but I normally start out with 5 times the mass of the piece of equipment as my first foundation weight. (Or 5 times the mass of the moving parts if that is not practical.)

Another thing here is to get a handle on the forcing function. With fans and equipment like that, it’s almost always a sinusoidal function. But with a printing press it may be different (it could be more of a pulse loading; I wish I could offer more here but almost all the dynamically loaded foundations I have done have been for compressors and fans). It may be a question to bounce off of the manufacturer.

Something else, you said you were going limit the dynamic+static pressure to ¼ of the allowable static pressure given to you by the geotech. I don’t know if you have told him that or not but be sure to do so (and get his approval). Reason being that some soils cannot take sudden increases in bearing pressure.