Reuben Hale, P.E.
Ph: (510) 507-1300
Resonance identification, dynamic testing, modal analysis and operating deflection shape analysis form the basis of our set of expertise in vibration analysis, and are described on this page, in the embedded video below, and in the links above in more detail. This is the heart of vibration testing and key to making a confounding vibration issues understandable. Our vibration testing, structural dynamics and modal analysis consulting services have helped push the limits of technologies and cutting edge designs for over 30 years.
- How is the structure deforming physically as it vibrates at a problem resonance?
- Is the vibration problem caused by excessively large forces, do we have a structural resonance problem, or some of both?
- How will vibration levels change if we move the force excitation to a new location on the structure, and/or if we change the force axis?
- Which structural modifications should we expect to reduce vibration levels, and by how much?
Step back and consider what is Resonance? - Resonance is energy trapped between two different forms of energy storage such as strain in a spring and mass in motion. Resonance is involved in most every thing we hear and feel. Resonant oscillations occur at specific frequencies that are unique to a structure and depend on its mass, stiffness and damping distribution. A system can be excited to large and problematic amplitudes at resonance (and lead to many unintended and interesting engineering problems).
The fascinating thing about resonant oscillations is that each resonance corresponds to a unique deformed shape called a mode shape. Like the ovaling of a bell when struck with a hammer, the cantilevered twang of a diving board, the half-sine shape of the air pressure distribution in a horn, or the twisting of a microscope structure, the mode shape is the unique deformed shape a structure will take as it oscillates at resonance, and tells us how the structure will respond with vibration at location A in direction 1 to a force at locaton B acting in direction 2.
Consider the fact that we can naturally tell the difference between the sound of a block of wood dropping on the floor and that of a steel bar. We characterize the sound by the frequency content of the response to excitation, allowing us to identify thousands of objects in our world. What most people don’t think about as often is what the deformation shape looks like as the system oscillates. We spend much of our time thinking about just that.
|Video Sample of Our Work and Modal Analysis Background|
The resonant frequency and mode shape are collectively referred to as a mode. A structure will have an infinite number of modes. The modes can be superimposed to approximate the possible deformed motions of the structure. However, usually just a handful of carefully identified mode shape estimates are enough to understand a complex vibration problem. As a vibration consultant we enjoy sharing our knowlege and experience with modal analysis concepts so that we can communicate with our clients and help give the team an appreciation for these important concepts and how they can affect the quality of their designs.
In our work we find the sub-set of modes that are sufficient to characterize and gain a full understanding of a vibration and/or acoustic problem. The most fundamental concept we use in all of our work is the concept of structural modes. If you don’t understand the mode shape you can’t fix the resonance problem.
Modal analysis and ODS testing go hand in hand to create the full picture of the forced response of a system with structural resonances below and near the operating frequency. These resonances amplify motion at some frequencies and attenuate it at others. The mode shapes from modal analysis also give us the physical, geometric, dependence of the structural response to a force at a given location on the structure acting through a specific axis. Some times the force levels are perfectly reasonable but the resonances have changed to become problematic making modal testing very important. Unfortunately, quite often we cannot shut down the vibrating structure to preform high quality modal testing to characterize the structural resonances because shutting the equipment in questing means shutting down the whole factory. This is costly (although sometimes we perform limited modal tests with the structure in operation).
Operating deflection shape analysis (ODS analysis) provides the deformation shape of a structure in response to the forces that are applied to the structure during its operation. Often it is the forced shape during operation that we want to see, as when we characterize an issue with a unstable control system as the ODS shape is often non-linear. Thus, ODS testing alone, however, still allows us to visualize the way the structure deforming due to the forces inherent in operation. This is very valuable and can give us very important clues as to where the structure is weak, where the structure is moving abnormally, and suggesting where we might consider modifications to the structure. We would still like to see the resonant modes that are found using experimental modal analysis but this will often have to wait until there is a scheduled plant shutdown.
Example of project involving Modal, ODS, and strain gage testing - The animation below is a modal analysis representation of the top three floors of 6 story building supporting a multi-ton piece of process equipment located on the 5th floor (represented as the red elements with large exaggerated motions to make the floor system deformations visible, all levels were included in the modal). We performed strain gage testing at critical locations on many of the structural members based on our original ODS Testing. The stain data allowed a structural engineering team to insure that the building was no in danger of structural failure. The floor system vibrated enough, however, that workers were not comfortable in the building and this was a problem for the client. We came up with a suitable design specification for allowable floor vibration levels for worker confort and safety and went to work engineering solution options for the structural team to meet those specifications.
|Modal Analysis of a Building and Process Floor System|
We developed an FEA model for this 6 story building which was guided by our on-site experimental modal analysis, many dynamic stiffness tests, and ODS testing. We evaluated structural modifications that included major stiffening members to achieve the desired floor vibration levels. We evaluated custom vibration isolation options to for mounting of the vibration source on the 5th floor. The most cost effective solution, however, was the use of tuned reaction masses (TRMs).
Our extensive Client List represents hundreds of projects, some involving days of work, others lasting several months, over the last 30+ years. Descriptions of many of our projects can be found in the left hand links under the various topics that describe our test, analysis, and design work. Our website is put together by our engineers.
- Modal & Resonance Testing Topics Include:
Overview, Modal Analysis Testing, Operating Deflection Shape Testing (ODS)