Natural Frequency of Floor Slab for Laser Cutter 1

[aws04]

We are installing a laser cutting machine at work and the factory requests a 1 piece floor slab 13.6 metres long x 3-4 metres wide with a minimum thickness of 300mm depending on the underlying foundations.

The factory requests that the natural frequency of this floor slab be = >30Hz and a max differential settlement of .1mm per metre.

Typical stability required is 0.05G max acceleration with .5 microns max amplitude.

How do I estimate the natural frequency of the slab and that a slab thickness of 300mm will be sufficient to meet the other requirements.

Regards

Steve

 

[hacksaw]

you have to model the floor slab and design it.

there are several structural dynamic analysis packages that will do that, but you'll require a complete structural-civil design to go with it.

 

[GregLocock]

First mode of a free-free beam with a uniformly distributed load is at

f=22.4/2/pi *sqrt((E*I)/mass per unit length/L^4) Hz

Which should get you in the ballpark. Use equivalent EI if you have a lot of reo.



Cheers

Greg Locock

 

[Tmoose]

At some point the soil conditions supporting the (perfect) slab can become very important. That "point" is shortly after the first (imperfect) part comes off the machine.

"Typical stability required is 0.05G max acceleration with .5 microns max amplitude." Can you feel any vibration on the floor in that location now?

 

[sreid]

There is a company named Fabreeka

http://www.fabreeka.com/

that sells high end vibration isolation equipment. They also do site surveys and can then design "Isolation Pits." As Tmoose points out, this may well be worth the money.

 

[aws04]

The exsisting floor does receive vibration from a punch
press,a large gillotine,gantry crane etc, most installations of this brand (bystronic byspeed) are put on
the standard factory floor slab - the factory just insists that the slab be one piece - no cracks or cuts - the machine is about 17000 kgs & does not bolt to the slab &
still manages 3g accel. It is the alignment between the laser unit & machine that will need constant adjustment if the foundation is not stable.

steve

 

[hacksaw]

the slab is not usually considered free-free, but a supported slab. for the the criticals and deflection modes are much more complex and can only be sorted out with FEA methods.

 

[GregLocock]

We mount vehicle dynos on floating foundations (rubber blocks) to isolate them from the building (and vice versa). That's anything up to a couple of hundred tons of concrete.



Cheers

Greg Locock