Footbridge dynamic performance assessment using inertial measurement units

This is the author accepted manuscriptDynamic performance of footbridges is still a great concern to designers, operators and users, with many structures requiring investigation before, during and after construction to manage performance. We have been investigating the use of wireless inertial measurement units (IMUs) designed for biomechanics, health and sports science application for estimating human dynamic loads or ground reaction forces (GRFs) on structures. The aim has been to move from direct measurements using force plates and treadmills, via optical motion capture in the laboratory (with application of Newton’s Second law), to unconstrained field conditions. Initially we used IMUs to evaluate pedestrian synchronisation, but we found that a single IMU attached to the C7 neck vertebra can provide a remarkably accurate estimate of vertical GRF. With an ability to communicate and synchronise within a group wirelessly, to identify orientation and transform accelerations into world coordinates, IMUs can identify both the GRF force vectors and their time varying location with a moving pedestrian. As a side-benefit, the signal to noise ratio and synchronisation accuracy are sufficient to enable low-cost wireless footbridge ambient vibration testing and monitoring. So far we have used IMUs for ambient and forced vibration testing (the latter using a human shaker), moving pedestrian load and response measurement and crowd tracking. There are many more possibilities

Results of heat transfer tests on sharp and spherically blunted 4 deg half angle conical models in a plasma jet and in a hypervelocity wind tunnel

Heat transfer tests on sharp and spherically blunted conical graphite nose cones in plasma jet and in hypervelocity wind tunne

Circularly polarized resonant soft x-ray diffraction study of helical magnetism in hexaferrite

Magnetic spiral structures can exhibit ferroelectric moments as recently demonstrated in various multiferroic materials. In such cases the helicity of the magnetic spiral is directly correlated with the direction of the ferroelectric moment and measurement of the helicity of magnetic structures is of current interest. Soft x-ray resonant diffraction is particularly advantageous because it combines element selectivity with a large magnetic cross-section. We calculate the polarization dependence of the resonant magnetic x-ray cross-section (electric dipole transition) for the basal plane magnetic spiral in hexaferrite Ba0.8Sr1.2Zn2Fe12O22 and deduce its domain population using circular polarized incident radiation. We demonstrate there is a direct correlation between the diffracted radiation and the helicity of the magnetic spiral.Comment: 4 pages, 4 figure

Time-dependence in Relativistic Collisionless Shocks: Theory of the Variable "Wisps" in the Crab Nebula

We describe results from time-dependent numerical modeling of the collisionless reverse shock terminating the pulsar wind in the Crab Nebula. We treat the upstream relativistic wind as composed of ions and electron-positron plasma embedded in a toroidal magnetic field, flowing radially outward from the pulsar in a sector around the rotational equator. The relativistic cyclotron instability of the ion gyrational orbit downstream of the leading shock in the electron-positron pairs launches outward propagating magnetosonic waves. Because of the fresh supply of ions crossing the shock, this time-dependent process achieves a limit-cycle, in which the waves are launched with periodicity on the order of the ion Larmor time. Compressions in the magnetic field and pair density associated with these waves, as well as their propagation speed, semi-quantitatively reproduce the behavior of the wisp and ring features described in recent observations obtained using the Hubble Space Telescope and the Chandra X-Ray Observatory. By selecting the parameters of the ion orbits to fit the spatial separation of the wisps, we predict the period of time variability of the wisps that is consistent with the data. When coupled with a mechanism for non-thermal acceleration of the pairs, the compressions in the magnetic field and plasma density associated with the optical wisp structure naturally account for the location of X-ray features in the Crab. We also discuss the origin of the high energy ions and their acceleration in the equatorial current sheet of the pulsar wind.Comment: 13 pages, 4 figures, accepted to ApJ. High-resolution figures and mpeg movies available at http://astron.berkeley.edu/~anatoly/wisp

Evaluating a video gauge for deformation measurements of two UK long span bridges

Technical program paper presented at Breakout Session 5B: Structural Health Monitoring, Tuesday June 21st 2016. The 9th International Cable Supported Bridge Operator’s Conference Halifax, Nova Scotia, Canada June 19-22, 2016 icsboc2016.comFull technical program papers available via: http://icsboc2016.com/technical-program/Bridge deformation measurements provide, through time and space derivatives, a rich set of information on cable stayed bridge (CSB) performance. Direct measurements of deformation of major components of CSBs i.e. deck, pylons, cables, are difficult due to problems with access and reference points. Compared to use of GPS and accelerometers which have more fundamental limitations, optical measurements offer many advantages such as multi-target tracking with limitations on resolution and accuracy in space and time being steadily eroded through developments in software capabilities. Hence we have revisited a research technology initially developed to monitor the Humber Bridge in 1990 and which now, in the form of a robust combination of hardware and Video Gauge software, provides a robust and effective solution for field monitoring

A framework for experimental determination of localised vertical pedestrian forces on full-scale structures using wireless attitude and heading reference systems

This is the author accepted manuscript. The final version is available from Elsevier via the DOI in this record.A major weakness among loading models for pedestrians walking on flexible structures proposed in recent years is the various uncorroborated assumptions made in their development. This applies to spatio- temporal characteristics of pedestrian loading and the nature of multi-object interactions. To alleviate this problem, a framework for the determination of localised pedestrian forces on full-scale structures is presented using a wireless attitude and heading reference systems (AHRS). An AHRS comprises a triad of tri-axial accelerometers, gyroscopes and magnetometers managed by a dedicated data processing unit, allowing motion in three-dimensional space to be reconstructed. A pedestrian loading model based on a single point inertial measurement from an AHRS is derived and shown to perform well against benchmark data collected on an instrumented treadmill. Unlike other models, the current model does not take any predefined form nor does it require any extrapolations as to the timing and amplitude of pedestrian loading. In order to assess correctly the influence of the moving pedestrian on behaviour of a structure, an algorithm for tracking the point of application of pedestrian force is developed based on data from a single AHRS attached to a foot. A set of controlled walking tests with a single pedestrian is conducted on a real footbridge for validation purposes. A remarkably good match between the measured and simulated bridge response is found, indeed confirming applicability of the proposed framework.The research presented here was funded by EPSRC (grant EP/I029567/2). Authors thank Devon County Council for permitting the experimental campaign to be conducted on Baker Bridge in Exeter, UK, and Dr Erfan Shahabpour (supported by EPSRC grant EP/K03877X/1) for providing access to and assisting with measurements on the ADAL-3D treadmill at the University of Sheffield (funded by EPSRC grant EP/E018734/1)

The Crab Nebula's Wisps in Radio and Optical

We present four new, high-resolution VLA radio images of the Crab nebula, taken between 2001 Feb. and Apr. The radio images show systematic variability in the Crab's radio emission throughout the region near the pulsar. The principal geometry of the variable features is that of elliptical ripples very similar to the optical wisps. The radio wisps are seen to move systematically outward with projected speeds of up to 0.3c. Comparing the new radio images to our earlier ones from 1998 and 2000, we show there are also more slowly moving features somewhat farther away from the pulsar. In particular, there is a prominent moving feature to the northwest of the pulsar which has a projected speed of order 10,000 km/s. Striation is seen throughout the nebula, suggesting the presence of wave-like disturbances propagating through the synchrotron bubble. The radio images were taken simultaneously with HST optical observations. Comparing the radio to the optical images, we find that the radio wisps are sometimes displaced from the optical ones or have no optical counterparts. We also find that some optical wisps in particular, the brightest optical wisps near the pulsar, do not seem to have radio counterparts. In the exterior of the nebula, by contrast, there is generally a good correspondence between the radio and optical features.Comment: LaTeX, 15 pages, 6 figures, Fig. 4 in colour. Supplementary material: mpeg animations accompanying Figs. 1 and 3 (download source from "other formats" to get mpegs). Accepted for publication in the Astrophysical Journal Typos fixed from previous versio