AMG, Inc. was commissioned to design a large soybean crush plant, complete with oil refining, packaging for consumer use, warehousing and shipping. The Preparation Building in this plant contained one floor that was completely occupied with cracking rolls. These are used to split beans after they are heated and before they are dehulled and flaked. The rolls are very large, heavy, high-energy, vibration-inducing machines. The client had a bad experience with massive floor vibrations in the past and wanted to ensure that this plant minimized vibrations as much as possible.
In order to investigate the response of the floor to the vibration energy input from the harmonic motions of the multiple cracking rolls, a finite element (FE) model of the concrete floor slab and the steel floor framing was made. Then the various factors which affected the floor’s response to the input vibrations where varied in the FE Model in order to find the optimum method for minimizing the floor’s response to the vibrations.
With assistance from the Vibrations Group at the University of Dayton Research Institute, three characteristics of the floor system were chosen to be varied in the FE Model. These parameters influenced the response of the floor structure to the vibration input energy of the machinery more than any others.
- The stiffness of the floor framing
- The mass of the floor slab
- The use of loose ballast material within the floor system for damping
The Model showed that increasing the stiffness of the floor framing was least effective in attenuating the floor vibrations. Adding mass to the floor system was much better in reducing the response of the floor but the most effective improvement in minimizing floor vibrations was to build chambers in the floor and fill them with a loose material like sand. This approach added mass to the system while the motion of the loose sand within the chambers absorbed the vibration energy, removing it from the system by means of plastic collisions between particles.
Although adding mass by means of a thick concrete slab was slightly less effective in absorbing vibration energy than using loose material, it was selected as the preferred approach to use because it involved a simpler construction technique and was less expensive. It also was less risky in that there was no threat of loose material leaking out, as would a chamber of sand if it was inadvertently pierced or cracked.