Classical flutter of suspended bridge decks can be avoided if the torsional frequencies are lower than the vertical. Wind tunnel tests of single boxes and twin box section models with torsional natural frequencies above and below the vertical frequency has been conducted. Flutter was avoided in all tests where the torsional frequency was lower than the vertical. But too low torsional stiffness caused large static displacements of the girder at medium–high wind speeds and steady state oscillations driven by a combination of torsional divergence and stalling behavior at the critical wind seed. In order to design aerodynamically stable suspension bridges with low torsional natural frequencies it is suggested to increase the mass moment of inertia and provide adequate torsional stiffness by the main cables spacing.

A new simplified framework to study flutter and assess the full scale flutter wind speed is suggested. The flutter instability problem is reduced from a problem involving 8 flutter derivatives to only 4 coefficients. With this method it is possible to estimate the self-excited forces with increased precision by using stability diagrams. Furthermore, the physical transparency of the aerodynamic damping and stiffness terms is increased because the development in vertical and torsional damping and stiffness is analysed instead of flutter derivatives.

Flipped classroom is a relatively new teaching form in higher education which is aimed at improving the learning process by engaging students in pre-class video activities fostering more active discussions between lecturer and student in-class. The concept is that the students watch recorded lectures (lower-order thinking skills) at home prior to class. During the class the students work on assignments and discuss the course content which can make better use of the teacher as a facilitator, so the focus can be on the more difficult things (higher-order cognitive skills) in the course content. In the present paper, the design and implementation using the flipped classroom technique for a course on experimental vibration analysis is presented. Some important experiences are discussed and an evaluation of student experiences is presented. The outcome is that the teaching technique is relatively time consuming to implement, but that the students appreciate being able to watch the lectures at their own pace and having more time working with the difficult things together with the teacher. Also, it turned out that the way the videos were used was very different among different students.

Shock Vibration, Aircraft /Aerospace, Energy Harvesting, Acoustics Optics, Volume 9. Proceedings of the 34th IMAC, A Conference and Exposition on Dynamics of Multiphysical Systems: From Active Materials to Vibroacoustics, 2016, the ninth volume of ten from the Conference brings together contributions to this important area of research and engineering. The collection presents early fi ndings and case studies on fundamental and applied aspects of Structural Dynamics, including papers on:• Energy Harvesting• Adaptive Support• Shock Calibration• Operating Data Applications• Aerospace Testing Techniques• Aircraft /Aerospace Applications• Joints• Acoustics & Optics• Other Sensing

Four exercises are presented to teach experimental vibration measurements: mass calibration; accelerometer mounting; singledegree-of-freedom vibration measurement and analysis; and fullscale experimental modal analysis.

For operational modal analysis (OMA) the multi-setup (or roving sensors) measurements strategy are often applied because the number of wanted measurement DOFs exceeds the number of available sensors. Studies of potential effects of applying the multi-setup strategy in contrast to the optimal simultaneous measurement strategy have, however rarely been reported. Classically, multi-setup datasets are merged together in a post-identification step, known as the patch based approach in experimental modal analysis. Recently, methods for merging the datasets in a pre-identification step has been suggested in the literature, but none of these methods have been compared with simultaneous measurement data. The present paper presents experimental results from an OMA obtained from a measurement consisting of 45 sensors simultaneously measured on a ship in operation. The modal parameters have been estimated for the full dataset, and for three artificially created multi-setup tests based on the full dataset. The first multi-setup test is composed of five datasets that are parallel in time, hence consistency between the datasets is guaranteed. The second test is also parallel in time but with two datasets, and the third test is composed of the same two datasets, but serial in time, hence consistency between the datasets is not guaranteed. For the multi-setup tests merging of the datasets have been done both with pre- and a post-identification methods and the modal parameters are compared with the ones estimated from the simultaneous measurement data. It is shown that using multi-setup tests systematic errors can occur especially in the damping estimates. The tested case where consistency between the datasets could not be guaranteed larger error was observed than when consistency was ensured. In addition, it was found that when mode shapes are of high importance the number of measured DOFs in each dataset should be sufficient larger in order to have a good estimate of the mode shapes.

Operational modal analysis (OMA) is widely used whenever the dynamic characteristics of structuresthat do not fit into a laboratory are needed. In addition, OMA offers a test of the structure under itsreal boundary conditions which can be preferable for validation of numerical models. Theoretically, atleast, the natural frequencies and damping ratios should be identically estimated by an OMA test andan experimental modal analysis (EMA) test. However it is still commonly believed that an EMA testis more reliable because of the available information and controlled environment. The present paperpresents an experimental study of EMA and OMA tests of a Plexiglas plate under free-free conditions.The plate which is a simple geometry is designed to have closely spaced modes and is seen as a goodstructure for comparison. Modal parameters are compared between the EMA and OMA test and it isfound that natural frequencies and mode shapes are similar within 0.2 % (MAC), whereas the dampingratios showed differences up to 7.7 % and are consistently lower for the OMA test. This differences indamping were possibly caused by slightly nonlinear boundary conditions combined with the fact that thevibration levels for the EMA test were higher than for the OMA test.

A novel modal analysis technique called Impact-Synchronous Modal Analysis (ISMA) was introduced in previous research. With the utilisation of Impact-Synchronous Time Averaging (ISTA), modal testing can be performed in presence of ambient forces whereas the conventional method requires machines to be shut down. However, lack of information of phase angles with respect to impact in ISTA has caused this modal testing method to be labour-intensive and time-consuming. The effect of the phase angle of the disturbance with respect to the impact is found to be very important when performing modal testing on structures with dominant periodic responses from cyclic loads. Synchronisation of phases between impacts and periodic response from cyclic loads should be avoided to enhance the effectiveness of ISTA. The assessment showed that a few averages are sufficient to eliminate the non-synchronous components when the phase angles with respect to the impact are inconsistent for every impact applied.