[zchini]

I've been trying to setup a modal analysis test for a floor I've already experimented on in real life. The test I did in real life found the natural frequency of the floor to be about 8.02 HZ, but every time I run a simulation in ANSYS, I can't get the natural frequency to be below 30 Hz.

I have attached my Solidworks files for the floor I'm working on, as well as a picture showing the model in ANSYS. The ANSYS model has point masses of 33.5 lbm at coordinates (22in,27.79in,6.834in) and (22in,27.79in,17.084in) to represent the shaker that was put on the floor to initiate vibrations. The 4 top and bottom sheets of the floor, labelled "al_floor_sheet", are AL 6061-T6, and every other part is structural steel.

I have tried:
-making the bottom of each of the legs fixed supports
-making the each of the legs fixed cylindrical supports
-making 2 of the legs displacement supports free in the vertical direction, the two others fixed
-making 3 of the legs displacement supports free in vertical, other one fixed (this gave me my closest result to the real life experiment, but I don't know if it makes sense)
-many, many others

I don't believe it is an issue with the dimensions of the floor, as I have double-checked them by reading the parts order forms and by measuring them myself, so the only thing I can think is wrong is how I'm setting up the experiment. Any insight would be greatly appreciated.

[Christophe]

What are the actual mode shapes in both the test and the analysis? Did you do a bump test, an ODS, etc?

[zchini]

We did a frequency sweep test from 1 to 10 Hz using the shaker placed on the floor. To measure readings, we used a laser sensor aimed as close to the center of the floor as possible, as that is where maximum displacement occurred. For the analysis in ANSYS, I just used the automatically generated mesh.

[zchini]

My university was lucky enough to have some engineers from DRD, the company that makes ANSYS, come out to do trainings on the program. I managed to ask the engineers who came out for advice on my simulation, and with their help, I worked out a solution.

The main thing was that my connections in my simulation were wrong. I had the middle plates that made up the internal structure bonded to the surface plates, when in fact they only had frictional bonds. Also, since bolts were used to hold down the surface plates, I added forces on the tops and bottoms of the bolts to simulate the forces. I tried using bolt pretensions, but that got to be to complicated, so I decided to opt for a longer run time with correct results.

Main thing to get out of this, always check to make sure your connections are accurate.