Natural Frequency of Foundation

[mlevario99]

Hi, I'm working in a foundation design for some kind of machinery, I was reading the requirements, and based on them you shall have a concrete floor system (on grade) with a first natural frequency higher than 200 Hz, I'm trying a 24-inch thick foundation, the dimensions of the foundation are 16'-6" wide by 31'-6" long. I try hand calculations based on soil spring values, and also Risa3D to calculate the natural frequency but I only get around 30 Hz. Has anyone done similar calculations.

Thank you for all your help

 

[civilperson]

Try adding either piles around the perimeter to encase a large mass of soil or a hollow box shape to use the soil mass enclosed. The greater the mass the better.

 

[flgulfcoasteng]

My dynamic machinery support experience has been limited to far above grade (steel structures) so I do not have a direct solution to draw advice from.

If the natural frequency (fn) needs to be > 200 Hz, I believe you need to lower your mass (do not contain soil) and increase your foundation stiffness. I believe a typical 12” x 40’ concrete pile in granular soil may have a vertical fn between 25 and 50 Hz and a lateral fn much lower, so it may not help much in this situation.

It will be interesting to see the solution with regards to the dynamic soil interaction.