In this Case Study, a vibration problem in a vertical pump is investigated. ODS and Modal tests were performed to identify the problem, then a correlated Finite Element model was used to determine a solution to the problem. This case study is an excellent example of how both experimental and analytical techniques can be used together to identify and solve vibration problems.
By Surendra N. Ganeriwala (Suri), Spectra Quest Inc. and Mark Richardson, Vibrant Technology, Inc.
In this paper, we present three case studies of the use of operational deflection shapes (ODS’s) of a rotating machine as a means of detecting shaft misalignment and unbalance in its rotating components. Our purpose is to show that a significant change in the ODS can be used as an early warning indicator of mechanical faults in rotating machines.
By Chris D. Powell, Structural Technology Corporation
A 100 hp air handling unit exhibited high vibration at two times fan shaft rotation (2X) near the upper end of its variable speed range. The fan was a 32” diameter single inlet design with nine blades.
The problem was investigated by first generating a startup waterfall to identify if 2X vibration was due to poor mechanical health or if it was being enhanced by structural resonances. The latter were identified in the waterfall. However, impact testing did not find the same frequencies as in the waterfall. Modal testing was performed using ME’scopeVES.
By M. Setarah & M. Lovelace, Virginia Polytechnic Institute and State University
Excessive vibrations of building floors have become a serious serviceability issue in recent years. This has been mainly due to decrease in the system mass resulting from the use of higher strength materials; use of computer-assisted design and the Load and Resistance Factor Design Method to optimize the structure based on the strength requirements; and more innovative designs by architects achieving long, column free spans resulting in a reduction in the natural frequency.
This paper provides details of the vibration analysis and design of a novel office building. Three-dimensional computer models of the structure were created and various modifications were made to the original structure, designed based on static loads, to reduce the possible excessive floor vibrations when subjected to walking excitations. Tuned mass dampers were also designed as a back-up vibration control system. A series of dynamic tests were conducted on the building floor to identify the dynamic properties of the structure. Finally, various forcing functions representing human walks and the updated computer model of the structure were used to evaluate the accuracy of the walking excitation force models to predict the structural response. Conclusions are made on the validity of each forcing function studied here.
By Mehdi Setarah, Ph.D., P.E. & Xiaoyao Wang, Virginia Polytechnic Institute and State University
Excessive and annoying vibrations in buildings, stadiums, and footbridges have become more common in the past two decades due to several reasons including the tendency to optimize the use of building materials, use of higher strength and lighter structural properties, etc. Stair vibrations have also become an important design issue mainly due to the architects’ desire to create more innovative, slender, light, and flexible monumental structures. Large vibrations and movements of stairs can become serious safety and serviceability problems, as they have the potential to disrupt the evacuation during an emergency and also people may feel unsafe during the normal use. Even though over the past two decades a large number of research studies on the floor vibrations have been conducted, very little information is available for stairs.
This paper presents experimental and analytical studies of a large monumental stair. Using an electrodynamic shaker and a series of seismic accelerometers, a set of modal tests was performed on the structure. This was done to estimate the dynamic properties of the stair. A computer model of the structure was created using the common practices used in structural engineering design offices. Comparison of the measured stair responses and the results of computer analysis showed reasonable agreement. A Cumulative Modal Assurance Criterion (CMAC) was introduced, and was used to identify the degrees of freedom with low quality measured responses. It was found that CMAC performs better than Enhanced Coordinate Modal Assurance Criterion (ECOMAC) and Coordinate Modal Assurance Criterion (COMAC) when the response is due to the excitation of a single mode. Several walking and running tests were conducted on the structure, which showed the structure may not be susceptible to large levels of vibrations for everyday use.
By Mehdi Setarah, Ph.D., P.E., Mico Woolard & Amanda Schlichting, Virginia Polytechnic Institute and State University
Slender footbridges can be susceptible to large vibrations due to human movements. The low natural frequencies and damping of these systems can result in excessive or annoying movements. This paper presents details of the vibrations analysis of a two-span steel footbridge, designed and built by a group of architecture students at Virginia Tech, Blacksburg, Virginia, USA.
By Surendra N. Ganeriwala & Jun Yang, Spectra Quest, Inc. and Mark Richardson, Vibrant Technology, Inc.
The structural integrity of blades is critical to the continued operation of a wind turbine. Resonant or modal properties of a mechanical structure are directly influenced by its physical properties. So any change in the physical properties of a structure should cause a change in its modal parameters. In this article, we present test results from a wind turbine blade with different induced cracks. Each result shows that some of the modes of the blade are significantly affected by a crack and that the modal parameters change more significantly with a more severe crack.