Parallels between wind and crowd loading of bridges

Parallels between the dynamic response of flexible bridges under the action of wind and under the forces induced by crowds allow each field to inform the other.Wind-induced behaviour has been traditionally classified into categories such as flutter, galloping, vortex-induced vibration and buffeting. However, computational advances such as the vortex particle method have led to a more general picture where effects may occur simultaneously and interact, such that the simple semantic demarcations break down. Similarly, the modelling of individual pedestrians has progressed the understanding of human–structure interaction, particularly for large amplitude lateral oscillations under crowd loading. In this paper, guided by the interaction of flutter and vortexinduced vibration in wind engineering, a framework is presented, which allows various human–structure interaction effects to coexist and interact, thereby providing a possible synthesis of previously disparate experimental and theoretical results.

An efficient model for the dynamic behaviour of a single pile in viscoelastic soil

This paper presents a novel, three-dimensional, single-pile model, formulated in the wavenumber domain and adapted to account for boundary conditions using the superposition of loading cases. The pile is modelled as a column in axial vibration, and a Euler-Bernoulli beam in lateral vibration. The surrounding soil is treated as a viscoelastic continuum. The response of the pile is presented in terms of the stiffness and damping coefficients, and also the magnitude and phase of the pile-head frequency-response function. Comparison with existing models shows that excellent agreement is observed between this model, a boundary-element formulation, and an elastic-continuum-type formulation. This three-dimensional model has an accuracy equivalent to a 3D boundary-element model, and a runtime similar to a 2D plane-strain analytical model. Analysis of the response of the single pile illustrates a difference in axial and lateral vibration behaviour; the displacement along the pile is relatively invariant under axial loads, but in lateral vibration the pile exhibits localised deformations. This implies that a plane-strain assumption is valid for axial loadings and only at higher frequencies for lateral loadings. © 2013 Elsevier Ltd.

Re-mobilization of pile shaft friction after an earthquake

During strong earthquakes, significant excess pore pressures can develop in saturated soils. After shaking ceases, the dissipation of these pressures can cause significant soil settlement, creating downward-acting frictional loads on piled foundations. Additionally, if the piles do not support the full axial load at the end of shaking, then the proportion of the superstructure's vertical loading carried by the piles may change as a result of the soil settlement, further altering the axial load distribution on piles as the soil consolidates. In this paper, the effect of hydraulic conductivity and initial post-shaking pile head loading is investigated in terms of the changing axial load distribution and settlement responses. The investigation is carried out by considering the results from four dynamic centrifuge experiments in which a 2 × 2 pile group was embedded in a two-layer profile and subjected to strong shaking. It is found that large contrasts in hydraulic conductivity between the two layers of the soil model affected both the pile group settlements and axial load distribution. Both these results stem from the differences in excess pore pressure dissipation, part of which took place very rapidly when the underlying soil layer had a large hydraulic conductivity.

Water supply to a geotechnical centrifuge for pile jetting in sand

The supply of water is often required during a centrifuge experiment. For the case of pile jetting, significant flow volumes and pressures are required from the water supply. This paper aims to detail the successful provision of water at high pressures and large flow rates to a centrifuge, using a novel water supply system. An impeller pump was used to pressurise the water in advance of the slip rings, with further pressure provided by the fluid accelerating along the centrifuge beam arm. A maximum pressure of 2 MPa and continuous flow rate of 6 litres per minute were achieved. The calculation of water pressure away from the measurement location is presented, offering a repeatable solution for the pressure at any point in the pipe work. © 2010 Taylor & Francis Group, London.

Effect of pile spacing on pile demand for small pile groups in laterally spreading soil

Liquefaction-induced lateral spreading has been responsible for widespread damage to pile foundations in many large earthquakes. The specification of inertial and kinematic pile and pile cap demands is a particularly challenging aspect of the analysis of pile foundations in laterally spreading soils. This paper presents and examines the results from a pair of dynamic centrifuge tests focusing on pile and pile cap demands for small pile groups with different pile spacings. Inertial and kinematic pile cap forces and lateral pile group interaction are examined with regard to the overturning mechanism that dominated the pile group response. © 2014 Taylor & Francis Group.

Lateral spreading-induced abutment rotation in the 2011 Christchurch earthquake: Observations and analysis

A series of strong earthquakes near Christchurch, New Zealand, occurred between September 2010 and December 2011, causing widespread liquefaction throughout the city's suburbs. Lateral spreading developed along the city's Avon River, damaging many of the bridges east of the city centre. The short-to medium-span bridges exhibited a similar pattern of deformation, involving back-rotation of their abutments and compression of their decks. By explicitly considering the rotational equilibrium of the abutments about their point of contact with the rigid bridge decks, it is shown that relatively small kinematic demands from the laterally spreading backfill soil are needed to initiate pile yielding, and that this mode of deformation should be taken into account in the design of the abutments and abutment piles.

A role of monitoring to reduce the uncertainty in the performance of pile foundations

In typical conventional foundation design, the inherent variability of soil properties, model uncertainty and construction variability are not modeled explicitly. A main drawback of this is that the effect of each variability on the probability of an unfavorable event cannot be evaluated quantitatively. In this paper, a method to evaluate the uncertainty-reduction effect on the performance of a vertically-loaded pile foundation by monitoring the pile performance (such as pile load testing or placing sensors in piles) is proposed. The effectiveness of the proposed method is examined based on the investigation of a 120-pile foundation placed on three different ground profiles. The computed results show the capability of evaluating the uncertainty-reduction effect on the performance of a pile foundation by monitoring. © 2014 Taylor & Francis Group, London.

Fabrication of nanoscale metallic air-bridges by introducing a SiO2 sacrificial layer

A new method to fabricate nanoscale metallic air-bridges has been investigated. The pillar patterns of the air-bridge were defined on a SiO2, sacrificial layer by electron-beam lithography combined with inductively coupled plasma etching. Thereafter, the span (suspended part between the pillars) patterns were defined with a second electron-beam exposure on a PMMA/PMMA-MAA resist system. The fabrication process was completed by subsequent metal electron-beam evaporation, lift-off in acetone, and removal of the sacrificial layer in a buffered hydrofluoric (HF) solution. Air-bridges with two different geometries (line-shaped and cross-shaped) were studied in detail. The narrowest width of the air-bridges was around 200 nm, and the typical length of the air-bridges was 2-5 mu m. The advantages of our method are the simplicity of carrying out electron-beam exposure with good reproducibility and the capability of more accurate control of the pillar sizes and shapes of the air-bridge. (C) 2007 Elsevier Ltd. All rights reserved.

Fabrication of metallic air bridges using multiple-dose electron beam lithography

The techniques of fabricating metallic air bridges using different resists in a one-step electron beam lithography are presented. The exposure process employed a single-layer polymethyl methacrylate (PMMA) or photoresists with either different doses in the span and feet areas or with varying acceleration voltage of the electron beam. The process using photoresists with different doses has produced air bridges more stable than what the PMMA method using various acceleration voltages would achieve. Using this method, air bridges up to 12 mu m long have been fabricated. The length and height of these metallic air bridges vary with the photoresist thickness. (c) 2006 American Institute of Physics.

Effect of volume ratio on thermocapillary flow in liquid bridges of high-Prandtl-number fluids

In present study, the transition of thermocapillary convection from the axisymmetric stationary flow to oscillatory flow in liquid bridges of 5cst silicon oil (aspect ratio 1.0 and 1.6) is investigated in microgravity conditions by the linear instability analysis. The corresponding marginal instability boundary is closely related to the gas/liquid configuration of the liquid bridge noted as volume ratio. With the increasing volume ratio, the marginal instability boundary consists of the increasing branch and the decreasing branch. A gap region exists between the branches where the critical Marangoni number of the corresponding axisymmetric stationary flow increases drastically. Particularly, a unique axisymmetric oscillatory flow (the critical azimuthal wave number is m=0) in the gap region is reported for the liquid bridge of aspect ratio 1.6. Moreover, the energy transfer between the basic state and the disturbance fields of the thermocapillary convection is analyzed at the corresponding critical Marangoni number, which reveals different major sources of the energy transfer for the development of the disturbances in regimes of the increasing branch, the gap region and the decreasing branch, respectively.

Syntheses, structures, and magnetic properties of two novel three-dimensional frameworks built from M4O4 cubanes and dicyanamide bridges

Two novel coordination polymers Ni-4(CH3O)(4)(CH3OH)(4)(dca)(4) (1) and Co-4(CH3O)(4)(CH3OH)(4)(dca)(4) (2) have been synthesized by solvethermal reaction. X-ray single-crystal analysis reveals that the two complexes are isostrutural and possess 3D frameworks that are built from the M4O4(M= Ni (1) and Co (2)) cubanelike building blocks linked by dicyanamide (dca) bridges. The temperature dependence of the magnetic susceptibility was measured and the DC experiment data were fitted using the Heisenberg spin Hamiltonian.

Liquid crystalline and light emitting polyacetylenes: Synthesis and properties of biphenyl-containing poly(1-alkynes) with different functional bridges and spacer lengths

Biphenyl- (Biph-) containing 1-alkynes (3 and 4) and their polymers (1 and 2) with varying bridge groups and spacer lengths were synthesized and the effects of the structural variation on their properties, especially their mesomorphism and photoluminescence behaviors, were studied. The acetylene monomers 3(3) [HCdropC(CH2)(3)O-Biph-OCO(CH2)(10)CH3] and 4(m) [HCdropC(CH2)(m)OCO-Biph-OCO(CH2)(10)-CH3, m = 3, 4] were prepared by sequential etherization and esterification reactions of 1-alkynes. While 3(3) exhibits enantiotropic crystal E and SmB mesophases, its structural cousin 4(3) displays only a monotropic SmB phase. Enantiotropic SmA and SmB mesophases are, however, developed when the spacer length is increased to 4. Polymerizations of the monomers are effected by Mo-, W-, Rh-, and Fe-based catalysts, with the WCl6-Ph4Sn catalyst giving the best results (isolation yield up to 85% and M-w up to 59000). The polymers were characterized by IR, UV, NMR, TGA, DSC, POM, XRD, and PL analyses. Compared to 1(3), 2(3) shows a red-shifted absorption, a higher T-i, and a better packed interdigitated bilayer SmA(d) structure, while the mesophase of 2(4) involves monolayer-packing arrangements of the mesogens. Upon photoexcitation, 1(3) emits almost no light but 2(m) gives a strong ultraviolet emission (lambda(max) similar to 350 nm), whose intensity increases with the spacer length.