How can I model pit using STAAD pro? What is the correct way of computing natural frequency of pit?

[Nayan67]

Hi everyone,

I have to design an irregular underground pit structure which supports rotating pump. I need to know the correct method of modeling pit using STAAD Pro connect edition.
At the moment, I am using plate-element to model all the walls and base slabs. For that, I need answers to the following questions:

a) How can I calculate horizontal spring constant? Do I use same influence area as we do for obtaining vertical spring constant or if there is another way? Also, I would be grateful if you could assist me in computing horizontal subgrade modulus given that vertical subgrade modulus is already known.

b) What is the correct way of finding natural frequency of whole pit? Should I use same spring constant for the base or provide fixity in all direction? And is it necessary to provide spring support to pit wall to model surrounding soil? If so, how? This is because I need to design pit structure so that its natural frequency exceeds 2.5 times the pump's vibrating frequency.

Thank you in advance.