Thick Footing at Heavy Machine (No impact - High Deflection Control)
Thick Footing at Heavy Machine (No impact - High Deflection Control)
(OP)
I need help to design a thick foundation to control equipment deflection, I understand that there are a lot of publications covering dynamics support design, but in this specific case, the impact load is very minimal from machine, and vendor specifies 10^-6 order of magnitude accuracy (in inches) for the equipment deflection. How can I achieve this requirement by design a thick concrete foundation (I am sure there are other way, if you know please recommend it)? (machine wt =150k and footing size is about 100'x40')
Assuming sombody uses very thick foundation as a model to design this base foundation, I would imagine that typical mat foundation design software could not handle the design due to the high thickness requirement. Since mat foot typically design footing as 4 noded plate element up to certain depth(I think thickness can go up to around 5'),I would need to model it as 8 noded solid element. Please recommed a software or approach to accomplish this goal.
What will be the design criteria for rebar design, assuming concrete thickness is 16' deep, can I still use 0.0018 for temperature/shrinkage design. I feel plain concrete may work.
IF you know any reference talk about this topic, please let me know.
Thanks very much
Janet
Assuming sombody uses very thick foundation as a model to design this base foundation, I would imagine that typical mat foundation design software could not handle the design due to the high thickness requirement. Since mat foot typically design footing as 4 noded plate element up to certain depth(I think thickness can go up to around 5'),I would need to model it as 8 noded solid element. Please recommed a software or approach to accomplish this goal.
What will be the design criteria for rebar design, assuming concrete thickness is 16' deep, can I still use 0.0018 for temperature/shrinkage design. I feel plain concrete may work.
IF you know any reference talk about this topic, please let me know.
Thanks very much
Janet






RE: Thick Footing at Heavy Machine (No impact - High Deflection Control)
What is the footprint of the machine?
RE: Thick Footing at Heavy Machine (No impact - High Deflection Control)
RE: Thick Footing at Heavy Machine (No impact - High Deflection Control)
I'd go back to the vendor and say, "Okay, you've told me what you want, now tell me what you actually need".
Michael.
Timing has a lot to do with the outcome of a rain dance.
RE: Thick Footing at Heavy Machine (No impact - High Deflection Control)
Just to get a feel for the magnitude of that... if a 200# man were to stand on a 1ft cube of concrete, he would "squash" it 0.0000046 inches... that's the level of accuracy they are talking about?
I hope the construction workers don't step near the final machine... might deflect it excessively!
RE: Thick Footing at Heavy Machine (No impact - High Deflection Control)
I am not sure the deflection requirement was so stringent, however.
But, for precision machining on mills where the workpiece and milling head are not on the same base, the support deflection needs to be VERY small.
RE: Thick Footing at Heavy Machine (No impact - High Deflection Control)
You need also very precise measures of the underground soil properties, especially if tilting by whatever the cause is paramount. Think that in foundations spans between columns, or in spans of beams themselves, center to column points, 1/250 distortion is one of the most commonly heard values to avoid non-structural cracking. If your statement was also center to end, you have a 4000 times more strict requirement; if deflection to span, a 2000 times more strict requirement. The mere natural variability in the ground, combined with thermal effects may forbid to deliver the wanted amobility.
Most likely you need to control thermal effects, to get only controlled exposure, or bracketed exposure to thermal movement. Think that heatflow is going to have a daily and seasonal pattern and excess of heat or cold from one side means tilt. Some of the named software, I would have to look RISA-3D manual, may not do this well. If you limit the temperature gradient by thermal insulation you will also limit the tilt. Also, for thicknesses over 1 m, this insulation also will prove extremely beneficial to forfeit inner shrinkage cracking. You need to control to under 20º C the gradient anywhere within the mass in the hydration process to do that. So enlisting someone with good mechanical and thermal FEM modeling abilities is almost a must.
In all, it maybe difficult to meet the requirement. Devices like the very large array telescope mitigates the imprecisions that a single telescope has by distance between supports. The same absolute imprecision gets minimized by the effect of such distance upon the measures. A degree of control of such precision for locale differential settlement of such degree for a 60 tonne machine with some degree of mechanical effects must be thought, I think, state of the art.
RE: Thick Footing at Heavy Machine (No impact - High Deflection Control)
RE: Thick Footing at Heavy Machine (No impact - High Deflection Control)
RE: Thick Footing at Heavy Machine (No impact - High Deflection Control)
As structural engineer, the typical way we handle equipment foundation is by specifying foundation weight to be 2.5 to 5 times of the equipment weight, this includes some units with dynamic load. adding damping pad will be effective for high vibration case.
In this case, vendor specification requires that machines sitting on the finish floor level (on our foundation) can not deflect more than 10^-6 inches. I reviewed some design which specified thick concrete foundation which may end up to be 20' thick of concrete. I feel if this is the correct approach, then just replace concrete with steel plate then thickness can be reduced to be 2' since E of steel is 10 times higher than concrete's E. But designer thinks that the foundation is not purely under compressive, it involves bending etc., the spring constant is given by geotechnical engineer. I have the same feeling this design is very questionable.
Ishvaasg seems know well about this kind of design, I agree, that this specification may be achieved by mechanical engineer's design, however, as a civil/structural engineer, I have not used Ansys and Abqus recently. I am very familiar with Risa-3D and know SAP 2000 may not handle this except use SAFE program (but SAFE is typically a MAT foundation design, and it typically will involve plate element instead of solid element), Risa 3D was used to model thick foundation but, the feature of solid element model is very limited. Can you make more detail suggestion by giving some good references. I want to know if it's possible to use RISA but at this time I could not persuade myself to be comfortable with this program. (think about my plate element is 5'Lengthx5'Widthx18'Depth, depths is three times of horizontal dimension, Risa technical support says it's ok that this ratio can go up to 3 to 5, based on FEM book from KJ Bathe (but I could not locate this definition in his book).
Attached at the bottom of this email is the similar data for machine I asked.
Thanks
Janet
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RE: Thick Footing at Heavy Machine (No impact - High Deflection Control)
Can you elaborate what you recommended as follows, especially about thermal effect. the machines is sitting inside of the building and is located in California w/o extreme weather condition. Please recommend a good reference to a structural engineer to handle this situation.
Thanks
Janet
--------------------------------------
Most likely you need to control thermal effects, to get only controlled exposure, or bracketed exposure to thermal movement. Think that heatflow is going to have a daily and seasonal pattern and excess of heat or cold from one side means tilt. Some of the named software, I would have to look RISA-3D manual, may not do this well. If you limit the temperature gradient by thermal insulation you will also limit the tilt. Also, for thicknesses over 1 m, this insulation also will prove extremely beneficial to forfeit inner shrinkage cracking. You need to control to under 20º C the gradient anywhere within the mass in the hydration process to do that(what does this mean, if concrete is 20' thick, how long will the curing process end so deflection will not be effected). So enlisting someone with good mechanical and thermal FEM modeling abilities is almost a must (Can you recommend someone?).
RE: Thick Footing at Heavy Machine (No impact - High Deflection Control)
SP-152, Design and Performance of Mat Foundations, State of the Art Review, Edward J. Ulrich editor, ACI 1995
the paper is SP 152-7 Mass Concrete Pour Techniques for the Mat Foundation of 1100 Alakea Plaza, by Dar, Bravo, Sarwar and Samada
Another, ACI SP-139, Durable Concrete in Hot Climates, Cameron McInnis editor,
paper SP 139-9, Concreting of thick section in the tropics, by Tam, Swaddiwudhipong, Mani and Lee
is a bit more specific on FEM approach then taken for modelization of a thick mat rebar included.
Respect modelization I think SAP 2000 can still be used, only that by not using something like SAFE you'll be left with the stresses. I once did the thermal analysis for a ceramic "arab" curved tile for roofs to ascertain what incidence the thermal exposure could be having in some ruptures of the same that were being studied. So you can have stresses, both in some axes and principal and decide if safe for the case for some criteria of maximum tensile strength, tresca or whatever.
Respect what happens with the heat passed or taken from the ground there is certainly some stational variation, that for horizontal homogenous soils should be vertical, and say for an embedded block that some exposed upper machine may be shading in part can make irregular and so some gradient extant from some high temperature point to some lower temperature point. By the description of the machine it turns that is likely it won't be in this kind of exposition but in a building enclosure. So, fortunately, in general, both seasonal and daily variations of temperature, that are seen not to be high in general nor penetrate much towards the conditioned parts of the building won't be much and I think will pale in comparison with the thermal ones of the hydration process. So placing the machine say 3 or 4 m from outer walls in a conditioned building will allow you to dismiss that part of the deformation since whatever it takes will stay stationary for a conditioned building, that is where such a precision machine must be except other conditions so mandate.
Respect Temperature variations and their effect see
Effects of Column Exposure in Tall Structures, Mark Fintel and Fazlur Khan, Portland Cement Association. unspecified year, taken from ACI Journal of 1965
Respect penetration of the variation of the temperature in the ground see the freely downladable program Climate Consultant.
Control of the hydration process will be helped by isolation of the pour. However, having compressive material in the sides of a tall footing, mat or pile is contrary to the favourable aspect that passive push must have against tilting of the foundation and so should in this particular case be discarded in favour of other lower heat of hydration processes, adding ice, whatever. Then you need to help yourself in your model adding also horizontal compressive only springs counteracting the tilt to represent the passive push reaction.
A review of what ACI has respect hot temperature pours, and a search in google for hot temperature concreting will help good advice about this thing, for certainly there are some books about (I have at least another, but not familiar enough with it to say in what would help to this design).
RE: Thick Footing at Heavy Machine (No impact - High Deflection Control)
RE: Thick Footing at Heavy Machine (No impact - High Deflection Control)
RE: Thick Footing at Heavy Machine (No impact - High Deflection Control)
ToadJones, can you tell me how do you solve the problem based on your experience working on horizontal boring mill.
Shall I post this message if Civil/Structural guy can not help for the solution?
Thanks very much for all your help.
Janet
RE: Thick Footing at Heavy Machine (No impact - High Deflection Control)
They offered no explanation as to why such mass was required.
RE: Thick Footing at Heavy Machine (No impact - High Deflection Control)
RE: Thick Footing at Heavy Machine (No impact - High Deflection Control)
Janet, please correct me if I'm mistaken about the terminology.
Is the vendor referring to the horizontal alignment of the machine? I don't see why it would matter if the machine foundation settled uniformly as long the horizontal alignment remained accurate.
RE: Thick Footing at Heavy Machine (No impact - High Deflection Control)
experts on this use in house excel programs..
get a hold of Arya book..
you should be very, very careful during construction as i know a case in my previous company that the foundation cracked.. they had to hire experts to redo the foundation.. the crack was caused by temperature.. if remember right they put some device to control the temperature and add ice whenever required..
RE: Thick Footing at Heavy Machine (No impact - High Deflection Control)
ToadJones, how much rebar are you putting in the foundation?
RE: Thick Footing at Heavy Machine (No impact - High Deflection Control)
Depending on the dynamic requirements of the system, the slab may be able to be much thinner than originally thought.
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Thick Footing at Heavy Machine (No impact - High Deflection Control)
Assume the deflection restriction to be reduced to be 10^-3inches, for me, the precision is still far beyond the anticipated tollerances of general civil-structural works, let along soild guy (not sure how accurate the subgrade modulus will be calculated) (I agree with Tai99) but at this time, there is no better solution to achieve this goal unless some expert make suggestions.
RE: Thick Footing at Heavy Machine (No impact - High Deflection Control)
Do you know E of concrete to that level of accuracy?
If the contractor builds the 16'-0" thick mat and it is actually 15'-11 255/256", how does that affect your assumption of A and I?
RE: Thick Footing at Heavy Machine (No impact - High Deflection Control)
Then how about adding concrete grade beams connected through the mat footing to an outrigger pile system set say 5 to 10 feet outboard of the mat extremeties, to take the differential movement. Should work ... in theory.
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Thick Footing at Heavy Machine (No impact - High Deflection Control)
Or, you may consider base plate with precision grout system. However, it's easier said than done though, as the settlement alone can mess it up.
RE: Thick Footing at Heavy Machine (No impact - High Deflection Control)
Michael.
Timing has a lot to do with the outcome of a rain dance.
RE: Thick Footing at Heavy Machine (No impact - High Deflection Control)
I will always remember what my Geotechnical Engineering professor once said in college: "In soils work, if you are only off by a factor of 100%, then you are in the ballpark".
That gives me great reservations to imply that a specific settlement criteria will not be exceeded unless the foundation for the item in question if supported by bedrock.
RE: Thick Footing at Heavy Machine (No impact - High Deflection Control)
BA