Redundancy quantification in the safety assessment of existing concrete beam-and-slab bridges

In current practice the strength evaluation of a bridge system is typically based on firstly using elastic analysis to determine the distribution of load effects in the elements and then checking the ultimate section capacity of those elements. Ductility of the components in most bridge structures permits local yield and subsequent redistribution of the applied loads from the most heavily loaded elements. As a result a bridge can continue to carry additional loading even after one member has yielded, which has conventionally been adopted as the "failure criterion" in bridge strength evaluation. This means that a bridge with inherent redundancy has additional reserves of strength such that the failure of one element does not result in the failure of the complete system. For these bridges warning signs will show up and measures can be undertaken before the ultimate collapse is happening. This paper proposes a rational methodology for calculating the ultimate system strength and including in bridge evaluation the warning level due to redundancy. © 2004 Taylor & Francis Group, London.

Static response of UHPFRCC slab specimens

The aim of this study is to present the results of an extended work on the development of an Ultra High Performance Fibre Reinforced Cementitious Composite (UHPFRC) and the experimental determination of the mechanical properties of the produced material. Furthermore, the paper will present preliminary experimental results on the static response of Reinforced Concrete and UHPFRCC slab specimens.

Bayesian and L1 Approaches to Sparse Unsupervised Learning

The use of L1 regularisation for sparse learning has generated immense research interest, with successful application in such diverse areas as signal acquisition, image coding, genomics and collaborative filtering. While existing work highlights the many advantages of L1 methods, in this paper we find that L1 regularisation often dramatically underperforms in terms of predictive performance when compared with other methods for inferring sparsity. We focus on unsupervised latent variable models, and develop L1 minimising factor models, Bayesian variants of "L1", and Bayesian models with a stronger L0-like sparsity induced through spike-and-slab distributions. These spike-and-slab Bayesian factor models encourage sparsity while accounting for uncertainty in a principled manner and avoiding unnecessary shrinkage of non-zero values. We demonstrate on a number of data sets that in practice spike-and-slab Bayesian methods outperform L1 minimisation, even on a computational budget. We thus highlight the need to re-assess the wide use of L1 methods in sparsity-reliant applications, particularly when we care about generalising to previously unseen data, and provide an alternative that, over many varying conditions, provides improved generalisation performance.

Numerical modelling of lubricated foil rolling

A model of lubricated cold strip rolling (1, 2) is extended to the thin foil regime. The model considers the evolution of asperity geometry and lubricant pressure through the bite, treating the strip using a conventional slab model. The elastic deflections of the rolls are coupled into the problem using an elastic finite element model. Friction between the roll and the asperities on the strip is modelled using the Coulomb and Tresca friction factor approaches. The shear stress in the Coulomb friction model is limited to the shear yield stress of the strip. A novel modification to these standard friction laws is used to mimic slipping friction in the reduction regions and sticking friction in a central neutral zone. The model is able to reproduce the sticking and slipping zones predicted by Fleck et al. (3). The variation of rolling load, lubricant film thickness and asperity contact area with rolling speed is examined, for conditions typical of rolling aluminium foil from a thickness of 50 to 25 μm. T he contact area and hence friction rises as the speed drops, leading to a large increase in rolling load. This increase is considerably more marked using Coulomb friction as compared with the friction factor approach. Forward slip increases markedly as the speed falls and a significant sticking region develops.

Ground vibration generated by trains in underground tunnels

A popular method used to reduce vibration transmitted from underground railways into nearby buildings is floating-slab track, whereby a concrete slab supporting the two rails is mounted on rubber bearings or steel springs to isolate it from the tunnel invert. This paper adds a track model to a previously developed three-dimensional tunnel model in order to assess the effectiveness of floating-slab track. A slab beam coupled to the tunnel in the wavenumber domain, with the slab bearings represented by an elastic layer, is examined first. A second beam representing the two rails together is then coupled to the slab, and axle masses representing a train are added to the rail beam. Power-spectral densities and RMS levels of soil vibration due to random roughness-displacement excitation between the masses and the rail beam are calculated. Analytical techniques are used to minimise the computational requirements of the model. The results demonstrate the inadequacy of simple mass-spring and Winkler-beam models with rigid foundations for the assessment of the vibration-isolation performance of railway track. They suggest that the achievable insertion loss is modest and that floating the track slab may in fact cause increased transmission of vibration under certain conditions. © 2006 Elsevier Ltd. All rights reserved.

A computationally efficient software application for calculating vibration from underground railways

The PiP model is a software application with a user-friendly interface for calculating vibration from underground railways. This paper reports about the software with a focus on its latest version and the plans for future developments. The software calculates the Power Spectral Density of vibration due to a moving train on floating-slab track with track irregularity described by typical values of spectra for tracks with good, average and bad conditions. The latest version accounts for a tunnel embedded in a half space by employing a toolbox developed at K.U. Leuven which calculates Green's functions for a multi-layered half-space. © 2009 IOP Publishing Ltd.

Analysis of case histories on deep excavations in marine clay

Two case histories on deep excavation of marine clay are used to study the use of a decision-making tool based on a new deign method called the Mobilized Strength Design (MSD) method which allows the designer to use a simple method of predicting ground displacements during deep excavation. This application can approximately satisfy both safety and serviceability requirements by predicting stresses and displacements under working conditions by introducing the concept of "Mobilizable soil strength". The new method accommodates a number of features which are important to design of underground construction between retaining walls, including different deformation mechanism in different stages of excavation. The influence of wall depth, wall flexibility and stratified ground are the major focus of this paper. These developments should make it possible for a design engineer to take informed decisions on the influence of wall stiffness, or on the need for a jet-grouted base slab, for example, without having to conduct project-specific Finite Element Analysis.

Supporting excavations in clay - From analysis to decision-making

Finite Element Analysis (FEA) is used to calibrate a decision-making tool based on an extension of the Mobilized Strength Design (MSD) method which permits the designer an extremely simple method of predicting ground displacements during construction. This newly extended MSD approach accommodates a number of issues which are important in underground construction between in-situ walls, including: alternative base heave mechanisms suitable either for wide excavations in relatively shallow soft clay strata, or narrow excavations in relatively deep soft strata; the influence of support system stiffness in relation to the sequence of propping of the wall; and the capability of dealing with stratified ground. These developments should make it possible for a design engineer to take informed decisions on the relationship between prop spacing and ground movements, or the influence of wall stiffness, or on the need for and influence of a jet-grouted base slab, for example, without having to conduct project-specific FEA. © 2009 Taylor & Francis Group.

Pipe in Pipe PiP

PiP software is a powerful computational tool for calculating vibration from underground railways and for assessing the performance of vibration countermeasures. The software has a user-friendly interface and it uses the state-of-the-art techniques to perform quick calculations for the problem. The software employs a model of a slab track coupled to a circular tunnel embedded in the ground. The software calculates the Power Spectral Density (PSD) of the vertical displacement at any selected point in the soil. Excitation is assumed to be due to an infinitely-long train moving on a slab-track supported at the tunnel bed. The PSD is calculated for a roughness excitation of a unit value (i.e. "white noise"). The software also calculates the Insertion Gain (IG) which is the ratio between the PSD displacement after and before changing parameters of the track, tunnel or soil. Version 4 of the software accounts for important developments of the numerical model. The tunnel wall is modelled as a thick shell (using the elastic continuum theory) rather than a thin shell. More importantly, the numerical model accounts now for a tunnel embedded in a half space rather than a full space as done in the previous versions. The software can now be used to calculate vibration due to a number of typical PSD roughnesses for rails in good, average and bad conditions.

Shape-Based Retrieval of Construction Site Photographs

Digital photographs of construction site activities are gradually replacing their traditional paper based counterparts. Existing digital imaging technologies in hardware and software make it easy for site engineers to take numerous photographs of “interesting” processes and activities on a daily basis. The resulting photographic data are evidence of the “as-built” project, and can therefore be used in a number of project life cycle tasks. However, the task of retrieving the relevant photographs needed in these tasks is often burdened by the sheer volume of photographs accumulating in project databases over time and the numerous objects present in each photograph. To solve this problem, the writers have recently developed a number of complementary techniques that can automatically classify and retrieve construction site images according to a variety of criteria (materials, time, date, location, etc.). This paper presents a novel complementary technique that can automatically identify linear (i.e., beam, column) and nonlinear (i.e., wall, slab) construction objects within the image content and use that information to enhance the performance of the writers’ existing construction site image retrieval approach.

Modelling and Fragility Analysis of Non-Ductile Reinforced Concrete Buildings in Low-to-Moderate Seismic Zones

Reinforced concrete buildings in low-to-moderate seismic zones are often designed only for gravity loads in accordance with the non-seismic detailing provisions. Deficient detailing of columns and beam-column joints can lead to unpredictable brittle failures even under moderate earthquakes. Therefore, a reliable estimate of structural response is required for the seismic evaluation of these structures. For this purpose, analytical models for both interior and exterior slab-beam-column subassemblages and for a 1/3 scale model frame were implemented into the nonlinear finite element platform OpenSees. Comparison between the analytical results and experimental data available in the literature is carried out using nonlinear pushover analyses and nonlinear time history analysis for the subassemblages and the model frame, respectively. Furthermore, the seismic fragility assessment of reinforced concrete buildings is performed on a set of non-ductile frames using nonlinear time history analyses. The fragility curves, which are developed for various damage states for the maximum interstory drift ratio are characterized in terms of peak ground acceleration and spectral acceleration using a suite of ground motions representative of the seismic hazard in the region.

New approaches for enhancing light emission from Er-based materials and devices

In this work, we present some approaches recently developed for enhancing light emission from Er-based materials and devices. We have investigated the luminescence quenching processes limiting quantum efficiency in light-emitting devices based on Si nanoclusters (Si nc) or Er-doped Si nc. It is found that carrier injection, while needed to excite Si nc or Er ions through electron-hole recombination, at the same time produces an efficient non-radiative Auger de-excitation with trapped carriers. A strong light confinement and enhancement of Er emission at 1.54 μm in planar silicon-on-insulator waveguides containing a thin layer (slot) of SiO2 with Er-doped Si nc at the center of the Si core has been obtained. By measuring the guided photoluminescence from the cleaved edge of the sample, we have observed a more than fivefold enhancement of emission for the transverse magnetic mode over the transverse electric one at room temperature. Slot waveguides have also been integrated with a photonic crystal (PhC), consisting of a triangular lattice of holes. An enhancement by more than two orders of magnitude of the Er near-normal emission is observed when the transition is in resonance with an appropriate mode of the PhC slab. Finally, in order to increase the concentration of excitable Er ions, a completely different approach, based on Er disilicate thin films, has been explored. Under proper annealing conditions crystalline and chemically stable Er2Si2O7 films are obtained; these films exhibit a strong luminescence at 1.54 μm owing to the efficient reduction of the defect density. © 2008 Elsevier B.V. All rights reserved.