Modal testing

About: Modal testing is a research topic. Over the lifetime, 4047 publications have been published within this topic receiving 64772 citations.

Scaling Factor Estimation by the Mass Change Method

Abstract: When natural input modal analysis is performed, the acting forces are unknown; by this reason only un-scaled mode shapes may be obtained so that the FRF matrix can not be constructed. If the structure is modified and a new modal testing is carried out, the scaling factors can be determined using the modal parameters (natural frequencies and mode shapes) from both the modified and the unmodified structure. Mass change is in many cases the simplest way to perform structural modification, which involves repeated testing implying mass change in different points of the structure where the mode shapes are known. In this paper, several methods to estimate the scaling factors, based on the mass change method, are presented. The accuracy obtained through the methods proposed depends on the type of normalization used in the mode shapes, the mass change magnitude and the number and the location of the masses attached to the structure, which effect is also analyzed. Finally, it is shown how the scaling factors can be used to improve the modal updating procedures.

Observations from Vibration Testing of In-Situ Structures

Abstract: Low-frequency floor and footbridge vibration serviceability problems typically arise when the structure is excited in resonance due to a walking excitation and the resulting accelerations exceed human comfort levels. The measures required to resolve an annoying vibration problem after the structure is constructed can be very difficult and expensive to implement. In most cases, the costs of fixing the problem in-situ are much greater than tackling the problem in the design phase, prior to the structure's construction, considering the potential cost to building owners from possible legal expenses, loss of rental revenue, and consultation fees. Design guidance for composite steel framed floor systems and footbridges is available in the AISC/CISC Steel Design Guide Series 11: Floor Vibrations Due to Human Acitivity . Although the current design guidance is generally acceptable, there is a need to continue characterizing the often-complicated vibration behavior of these structures in an effort to refine current design and analysis techniques, particularly as researchers gain a better understanding of behavior by collecting high-quality experimental data on in-situ floor and footbridge structures. This paper presents observations from such efforts to further characterize behavior through experimental modal testing of a large in-situ composite steel office floor and a laboratory constructed multi-span footbridge. While not entirely inclusive, some general observations are noted on the dynamic behavior, problems encountered, and the consistency/reliability of the applied testing/analysis techniques employed by the researchers.