Design Analysis

Analysis of HEB Platform Addition

Created By Meadows Analysis & Design, LLC Project created on 4/19/2006. Last updated on 4/21/2006. Checked By Marc A. Meadows, P.E. Project checked on 4/21/2006.

Summary

Description

A platform addition to existing structure at HEB Foods in San Antonio, Texas is being designed by TNA Robag North America. Meadows Analysis & Design, LLC has been retained to assure the structural integrity and usefulness of the platform. As part of this assurrance, we are performing a frequency analysis. This platform has oscillating machinery on it and therefore is subject to resonant loading situations. We must make sure that the natural frequency of the structure is much higher than the driven frequency of the conveyors on it.

Model Information

Analysis Type - Natural Frequency (Modal)
Units - English (in) - (lbf, in, s, deg F, deg R, V, ohm, A, in*lbf)
Model location - C:\Temp36\hertz

Comments from Reviewer

All things vibrate. Think of musical instruments, think of riding in a car, think of the tires being out of balance, think of the rattles in an airplane when the pilot is revving up the engines or the vibration under your feet when a train goes by. Usually, however, vibration is bad and frequently unavoidable. It may cause gradual weakening of structures and the deterioration of metals (fatigue) in cars and airplanes. Vibration is about frequencies. By its very nature, vibration involves repetitive motion. Each occurrence of a complete motion sequence is called a cycle. Frequency is defined as so many cycles in a given time period. One cycle per second is equivalent to one Hertz. Individual parts have natural frequencies. For example, a violin string at a certain tension will vibrate only at a set number of frequencies, which is why you can produce specific musical tones. There is a base frequency in which the entire string is going back and forth in a simple bow shape. Harmonics and overtones occur because individual sections of the string can vibrate independently within the larger vibration. These various shapes are called modes. The base frequency is said to vibrate in the first mode, and so on up the ladder. Each mode shape will have an associated frequency. Higher mode shapes have higher frequencies. The most disastrous consequences occur when a power-driven device, such as a motor for example, produces a frequency at which an attached structure naturally vibrates. This event is called resonance. When vibration causes resonance in an object, destruction will result unless it has been designed to withstand the stress. A wine glass, for example, is not sound enough to withstand the resonance caused by the frequencies produced by an opera singer. Engineers must design so that resonance does not occur during regular operation of machines. This is a major purpose of natural frequency (modal) analysis. Ideally, the first mode has a frequency higher than any potential driving frequency.

Analysis Parameters Information

Processor Information

Part Information

Element Properties used for:

• 6x6

Element Properties used for:

• 2x10

Element Properties used for:

• 2x2

Material Information

Steel (ASTM - A36) -Beam

Load and Constraint Information

Constraints

Constraint Set 1: Boundary Conditions

Nodal Boundary Conditions

Appendix

Stucture as built without floorplate.

We will attempt to analyze without the floorplate as a structural member. Because we are not optimizing for weight, we would rather not rely on the floorplate for structure so that we do not require specific attachment methods.

First Mode

As stated in the summary, we are looking for a structure that has a mode higher than the driven frequency of the machinery vibrating on it. In this case our maximum driven frequency is 2 Hz and our first mode is 13 Hz, so we are good to go. Now, this mode is simply the base frequency, we will investigate the first five which will all have different directions. This one is directed in the z-direction. None of our machinery is vibrating much in this direction, but this mode is important for the personnel walking on the platform. Studies show that the minimum frequency mode for walking platforms should be 8 Hz. Since we are above that, then the floor should offer a comfortable, and safe walking surface with no chance for resonance. Please refer to the movie below for an animation. You will not that the area most susceptable to vibration is the area not supported be near columns. This analysis shows that the structure is still stiff enough to handle the vibrations.

First Mode Animation

Second Mode

This mode happens in the y direction. This would be the cross feed conveyor direction. You can see that the frequency is 14.7 Hz which is over 7 times the driven frequency of 2 Hz. The animation follows below.

Second Mode Animation

Third Mode

This mode happens about the x axis as a rotation. Animation follows below.

Third Mode Animation

Fourth Mode

This mode occurs about the z axis as a rotation. Animation follows below.

Fourth Mode Animation

Fifth Mode

This mode occurs about the z axis as a rotation. Animation follows below.

Fifth Mode Animation

Second Mode of Unattached Structure

This model was run without attachment to existing platform to see if our structure was stiff enough without having to rely on the adjoining structure. While our margin is much lower, there is a higher frequency for the structure than the driven frequency, and therefore a good design. I have chosen to show the second mode because it was in the direction of the main body of Roflo conveyors. Animation follows below.

Unattached Animation