Effect of Buoyancy Load in Dynamic Analysis of Foundation for Vibrating Machinery

[AJMF]

Dear all,

Good day.

Do you have any material/idea/references regarding the effect of Buoyancy Load in Dynamic Analysis of the foundation for vibrating machinery?

On our previous design, we did not consider the effect of Buoyant Force in the dynamic analysis (using STAAD). But one of our senior engineer ask if Buoyant Force will have an effect since it will make a weight of the footing lighter (against the soil or pile).

It will be really helpful if you can provide some material/idea/references or thoughts.

Thanks!

AJMF

 

[robyengIT]

in 1986 I erected a spinning machine for the production of giant piles to be used for the pier of the port of Trieste in Italy (you can see the photos when you visit the WEB site

http://www.pisaniengineer.com).

Foundation of machine was on piles as per my request. Vibrations during spinning were really heavy (the total weight of steel mold + concrete of pile under construction was approx 120 metric tons at 150 RPM) : nothing happened.
two years later another spinning machine was installed 1 km apart by a different company (piles under spinning were similar). To save money they didn't do the foundation on piles but only a slab : every spinning cycle they had to align and level the machine during spinning because of the variable water-table

 

[SlideRuleEra]

...one of our senior engineer ask if Buoyant Force will have an effect since it will make a weight of the footing lighter (against the soil or pile).

Buoyancy will make the weight lighter... but mass, not weight, is important for a tuned foundation. Mass is not affected by buoyancy. Damping of vibration by soil may be affected by high water table, but that does not affect foundation tuning, either.

Our generating stations (in high water table locations) use high-tuned turbine-generator pedestals where the structure dedicated for tuning (piling, foundation, pedestal, and the machine) is considered part of the tuned mass.

http://www.SlideRuleEra.net

 

[AJMF]

robyengIT,

Can you please elaborate? I think I don't have an option to change the alignment of the equipment. I am thinking that as long as it is not fail on uplift condition under service load (static design), this alignment will not be a problem.


SlideRuleEra,

So if I did not consider damping in the soil/pile spring constant, am I on the conservative side?

In conclusion, damping of vibration of soil will be reduce if we will consider the effect of the water table.

 

[SlideRuleEra]

1) So if I did not consider damping in the soil/pile spring constant, am I on the conservative side?

2) In conclusion, damping of vibration of soil will be reduce if we will consider the effect of the water table.

1) Not necessarily. The structure, including the machine, has to be designed to vibrate exactly opposite (both frequency and magnitude) to the machine-caused vibration. Then the two sets of vibrations cancel out each other. The soil/piling spring constant has nothing to do with this.

2) No, as stated in 1), neither soil nor water table have anything to do with a tuned foundation.

There are other ways to design foundations for vibrating machines. Believe I misunderstood your question... I thought it was about tuned foundations. Sorry.

http://www.SlideRuleEra.net

 

[AJMF]

SlideRuleEra,

Oh, I see. So you don't have any experience for dynamic analysis of foundation for vibrating machinery?

Anyways, thank you!

To others,

Any ideas? Thanks!

 

[WARose]

I'd model the uplift force just like you model the vertical (downwards) dead load of the machine. (I.e. as point/pressure loads.)

But in this type of design you need to consider multiple scenarios. There should be a scenario (separate model/run) without this uplift.