Thick Footing at Heavy Machine (No impact - High Deflection Control) 1

[structural3]

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

 

[msquared48]

Why not use a pile foundation here to minimize the slab deflections with variations in loading?

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

 

[structural3]

As I mentioned, deflection criteria can not be met using pile system at this time. (Too much differential settlement)

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.

 

[PMR06]

I still think this is an exercise in futility!

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?

 

[msquared48]

OK. That seems odd, but OK.

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

 

[tai99]

It will be very difficult to achieve the goal directly through structural means, rather, you should consider the inclusion of mechanical means. For instance, set equipment on hydraulic system to maintain its balance. Once a suitable device/setup is determined, the remaining task is merely to design the foundation to externally support that system.

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.

 

[paddingtongreen]

This doesn't sound as though the initial finish of the slab has to be set with such low tolerance, only that once the equipment is set, that leveling must be maintained to those narrow limits.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[StructuralEd]

I keep reading deflection but are you referring to settlement, perhaps differential settlement?

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.

 

[BAretired]

Tell the vendor that such precision is not possible and that his equipment will be required to accommodate somewhat larger deflections. Alternatively, get a quote from a different vendor.

BA