Vibration serviceability of Helix Bridge, Singapore

This is the author accepted manuscript. The final version is available from the publisher via the DOI in this record. Published Online: May 18, 2016The Helix Bridge is a key feature of the iconic Marina Bay Sands development in Singapore. It usually functions as a pedestrian link between the Esplanade and Sands Casino/Hotel, but is occasionally used as a viewing platform for events in Marina Bay that have centred on a small purpose built stadium opposite the bridge. To supplement the stadium capacity, a number of integral cantilevered 'pods' have been built into Helix Bridge. Because of its dual role Land Transport Authority, Singapore commissioned a vibration serviceability evaluation of the bridge following a specification developed by Arup Australia. The vibration serviceability evaluation was carried out in three stages. First, an experimental campaign comprising multi-shaker modal testing was used to estimate modal properties. Next, limited pedestrian and crowd testing directly evaluated the dynamic response to individuals and small groups walking, running or jumping. Finally, modal properties were utilised, with bespoke simulation software, to predict the performance of the bridge under extreme crowd loading, using models specified in the most up-to-date design guidance on crowd loading for pedestrian bridges and stadia. The bridge performance proved to be acceptable, both in the direct testing with small groups and the simulations of large crowds

Long span steel pedestrian bridge at Singapore Changi Airport - part 1: Prediction of vibration serviceability problems

Changi Mezzanine Bridge is a 140m span flat arch footbridge constructed from welded tubular steel sections inside a tunnel that connects two passenger terminals at Changi Airport, Singapore. A series of vibration measurements were made on the bridge during construction, showing that non-structural cladding added mass and reduced the natural frequencies while also increasing the modal damping, from as little as 0.2% originally to around 0.4% for critical vibration modes. From these preliminary studies leading up to the opening of the bridge in early 2002, it was clear that the first symmetric lateral vibration mode (LS1) at approximately 0.9Hz and the first symmetric torsional vibration mode (TS1) at approximately 1.64Hz could be excited easily by pedestrian movement. The modal parameters for mode LS1 suggested that the bridge could suffer from synchronous lateral excitation for a walking pace of 1.8Hz while for TS1 the potential problem was the coincidence of the mode frequency with the lower range of predominant footfall frequencies together with a very low modal mass. These possibilities had been identified by the consultant who advised that an experimental study of the characteristics of low frequency vibration modes should be conducted to check vibration serviceability predictions based on analytical modeling. Forced vibration testing using a combination of shakers and humans was used to determine in a very short time scale, the properties for modes below 3Hz. The mode shapes and frequencies compared favourably with predictions from the consultant’s finite element model that had been used to show that with a large number of pedestrians, comfort levels would be exceeded and the bridge would be unserviceable

Long span steel pedestrian bridge at Singapore Changi Airport. Part 2: Crowd loading tests and vibration mitigation measures

issue: 16article© Copyright The Institution of Structural EngineersFollowing experimental and analytical studies of vibration serviceability of a 140m span steel footbridge which predicted excessive and uncomfortable vertical and lateral vibration levels due to crowd loading, a series of walking tests involving up to 150 pedestrians was aimed at assessing the prototype behaviour under ‘limiting typical’ pedestrian loads in two vibration modes judged to be critical. In a walking test for possible instability resulting from so-called ‘synchronous lateral excitation’ (SLE), pedestrian volunteers were fed onto the bridge and told to walk casually. With all 150 available pedestrians circulating for several minutes, a steady increase in lateral vibrations was observed. This divergent response resembled the phenomenon observed during tests on the London Millennium Bridge (LMB), and while the maximum response reported here was an order of magnitude smaller than the largest levels reported for LMB on its opening day it was, apparently, uncomfortable for pedestrians. On the other hand, due to the apparent lack of synchronisation and random character of vertical loads together with enhanced damping due to the pedestrians themselves, vertical response levels were within acceptable comfort limits. From observations of pedestrian movement and the nature of response in vertical and lateral modes there appears to be an open question about the nature of any possible synchronisation of forces and the manner of generating and building up relatively large lateral amplitudes. To mitigate the potential for strong and unsafe lateral oscillation in the unlikely event of larger numbers of pedestrians, a tuned mass damping system has been installed. The damping in LS1 has been increased by a factor of approximately four, so that SLE is effectively prevented for any foreseeable reasonable pedestrian loading

A Study of the Factors Influencing the Synthesis of Tobacco Mosaic Viral RNA in a Partially Purified Synthesizing System

Research on biosynthesis of tobacco mosaic virus (TMV) ribonucleic acid (RNA) in vitro has been reported by Cochran, et al; Karasek and Schramm; Kim and Wildman; Cornuet and Astier; and Tongur and Baland in. It has been postulated that the replication of a number of viruses containing single-stranded RNA is accompanied by the formation of a virus-specific double-stranded helical RNA, the replicative form. This has been demonstrated both for animal and bacterial viruses including MS2. The double helical structure of purified replicative form of MS2 was established by X-ray diffraction studies. One of the strands was shown to be a viral RNA strand of the parental type ( plus strand), the other being complementary to it ( minus strand). Studies on Escherichia coli infected with RNA phages suggest that a structure containing both a plus 11 and a minus strand is an obligatory intermediate in viral reproduction

Mathematical models for vulnerable plaques

A plaque is an accumulation and swelling in the artery walls and typically consists of cells, cell debris, lipids, calcium deposits and fibrous connective tissue. A person is likely to have many plaques inside his/her body even if they are healthy. However plaques may become "vulnerable", "high-risk" or "thrombosis-prone" if the person engages in a high-fat diet and does not exercise regularly. In this study group, we proposed two mathematical models to describe plaque growth and rupture. The first model is a mechanical one that approximately treats the plaque as an inflating elastic balloon. In this model, the pressure inside the core increases and then decreases suggesting that plaque stabilization and prevention of rupture is possible. The second model is a biochemical one that focuses on the role of MMPs in degrading the fibrous plaque cap. The cap stress, MMP concentration, plaque volume and cap thickness are coupled together in a system of phenomenological equations. The equations always predict an eventual rupture since the volume, stresses and MMP concentrations generally grow without bound. The main weakness of the model is that many of the important parameters that control the behavior of the plaque are unknown. The two simple models suggested by this group could serve as a springboard for more realistic theoretical studies. But most importantly, we hope they will motivate more experimental work to quantify some of the important mechanical and biochemical properties of vulnerable plaques

Anisotropic diffusion in continuum relaxation of stepped crystal surfaces

We study the continuum limit in 2+1 dimensions of nanoscale anisotropic diffusion processes on crystal surfaces relaxing to become flat below roughening. Our main result is a continuum law for the surface flux in terms of a new continuum-scale tensor mobility. The starting point is the Burton, Cabrera and Frank (BCF) theory, which offers a discrete scheme for atomic steps whose motion drives surface evolution. Our derivation is based on the separation of local space variables into fast and slow. The model includes: (i) anisotropic diffusion of adsorbed atoms (adatoms) on terraces separating steps; (ii) diffusion of atoms along step edges; and (iii) attachment-detachment of atoms at step edges. We derive a parabolic fourth-order, fully nonlinear partial differential equation (PDE) for the continuum surface height profile. An ingredient of this PDE is the surface mobility for the adatom flux, which is a nontrivial extension of the tensor mobility for isotropic terrace diffusion derived previously by Margetis and Kohn. Approximate, separable solutions of the PDE are discussed.Comment: 14 pages, 1 figur

Development and optimization of a "water window" microscope based on a gas-puff target laser-produced plasma source

A laser-plasma double stream gas-puff target source coupled with Fresnel zone plate (FZP) optics, operating at He-like nitrogen spectral line λ=2.88nm, is capable of acquire complementary information in respect to optical and electron microscopy, allowing to obtain high resolution imaging, compared to the traditional visible light microscopes, with an exposition time of a few seconds. The compact size and versatility of the microscope offers the possibility to perform imaging experiments in the university laboratories, previously restricted to large-scale photon facilities. Source and microscope optimization, and examples of applications will be presented and discussed

Microscale Effects from Global Hot Plasma Imagery

We have used a three-dimensional model of recovery phase storm hot plasmas to explore the signatures of pitch angle distributions (PADS) in global fast atom imagery of the magnetosphere. The model computes mass, energy, and position-dependent PADs based on drift effects, charge exchange losses, and Coulomb drag. The hot plasma PAD strongly influences both the storm current system carried by the hot plasma and its time evolution. In turn, the PAD is strongly influenced by plasma waves through pitch angle diffusion, a microscale effect. We report the first simulated neutral atom images that account for anisotropic PADs within the hot plasma. They exhibit spatial distribution features that correspond directly to the PADs along the lines of sight. We investigate the use of image brightness distributions along tangent-shell field lines to infer equatorial PADS. In tangent-shell regions with minimal spatial gradients, reasonably accurate PADs are inferred from simulated images. They demonstrate the importance of modeling PADs for image inversion and show that comparisons of models with real storm plasma images will reveal the global effects of these microscale processes