Effect of vehicular loading on suspension bridge dynamic properties

Journal ArticleSince the 1970s, many researchers have attempted to use changes in natural frequencies as means for condition assessment of large civil engineering structures such as bridges, but have faced the challenge of decoupling frequency variations apparently caused by changing operational conditions. In the case of the Tamar Bridge in southwest England, the time series of natural frequencies exhibit diurnal variations resulting from a combination of thermal and vehicular loading, the effects of which would need to be compensated for in dynamics-based assessment. By examining monitored data for several years, the effects of traffic mass have been characterised and compared with other operational effects. While temperature changes appear to have a greater influence for lateral modes, traffic mass is a strong factor in all modes and the dominant factor for the vertical and torsional modes evaluated. Physics-based explanations for the variable effects of vehicle mass have been sought using a finite element model calibrated against experimental data. As a caution for performance prediction in structural dynamics, while acceptable reconciliation of natural frequencies from FE model and measurements was achievable, reconciling simulated effects of changing mass with observed behaviour has not been straightforward due to the complexity of the retrofitted suspension bridge structure studied.EPSRCEU Framework

Cable supported immersed inversed bridge: A challenging proposal

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Numerical investigation of the effects of pedestrian barriers on aeroelastic stability of a proposed footbridge

A numerical investigation into the aerodynamic characteristics and aeroelastic stability of a proposed footbridge across a motorway in the north of England has been undertaken. The longer than usual span, along with the unusual nature of the pedestrian barriers, indicated that the deck configuration was likely to be beyond the reliable limits of the British design code BD 49/01. In particular, the investigation focussed on the susceptibility of the bridge due to flutter, and to assess if the design wind speeds could be met satisfactorily. The calculations were performed using the discrete vortex method, DIVEX, developed at the Universities of Glasgow and Strathclyde. DIVEX has been successfully validated on a wide range of problems, including the aeroelastic response of bridge deck sections. The proposed deck configuration, which incorporated a pedestrian barrier comprised of angled flat plates, was found to be unstable at low wind speeds with the plates having a strong turning effect on the flow at the leading edge of the deck. DIVEX was used to assess a number of alternative design options, investigating the stability with respect to flutter for each configuration. Reducing the number of flat plates and their angle to the deck lessened the effect of the barrier on the overall aerodynamic characteristics and increased the stability of the bridge to an acceptable level, with the critical flutter speed in excess of the specified design speed

Structural performance of prestressed precast high speed railway bridges using high performance concrete

Bridges often need to conform to strict alignment rules for high speed railway (HSR) lines. Generally, the bridges are constructed either from prestressed concrete or steel-concrete composite. Prestressed concrete bridges can be constructed by precast methods, which offer benefits in economies of scale, quality and construction times for long repetitive viaducts. However, currently precast construction utilises conventional concrete strengths, leading to thicker, heavier cross sections to resist the load. High performance concrete (HPC), with its increased strength, can be implemented to reduce the precast segment weights, subsequently reducing substructure and transportation capacities. However, lighter sections could lead to decks more prone to vibrations exceeding acceleration limits. Therefore, the implementation of HPC requires further research, addressed in this thesis, using the most sophisticated and realistic numerical models of the bridge, vehicle, track, wheel-rail interaction and rail irregularities, identified in literature. A suitable benchmark bridge is selected and analysed from a database of concrete HSR bridges. This analysis finds that using track irregularities with wheel-rail contact is mandatory for accurate bridge accelerations, leading to up to 3.75 times larger accelerations than equivalent moving load models. Furthermore, sectional deformations have been found to be non-negligible, with beam element bridge models incapable of exhibiting the wide frequency content of the acceleration response seen in shell elements. A subsequent parametric analysis reduces the geometrical cross sectional dimensions of the precast components, implementing HPC to maintain the structural capacities. The applicability of the acceptable parametric analyses are tested on other bridges, determining more general conclusions for HPC inclusion in HSR bridges. Appropriate reductions in geometry (web, bottom flange and top flange thicknesses down to 66, 75 and 75% of the original value respectively), are identified from the response of the bridge and vehicle, by using HPC up to 96 MPa, contributing to up to 22% lighter precast elements. Appropriate design guidance is subsequently made for better design of HSR bridges to incorporate HPC into precast solutions.Open Acces

Analysis of the performance of cable-stayed bridges under extreme events

University of Technology Sydney. Faculty of Engineering and Information Technology.In bridge structures, loss of critical members (e.g. cables or piers) and associated collapse may occur due to several reasons, such as wind (e.g. Tacoma narrow bridge), earthquakes (e.g. Hanshin highway) traffic loads (e.g. I-35W Mississippi River Bridge) and potentially some blast loadings. One of the most infamous bridge collapses is the Tacoma Narrow Bridge in United States. This suspension bridge collapsed into the Tacoma Narrow due to excessive vibration of the deck induced by the wind. The collapse mechanism of this bridge is called "zipper-type collapse", in which the first stay snapped due excessive wind-induced distortional vibration of the deck and subsequently the entire girder peeled off from the stays and suspension cables. The zipper-type collapse initiated by rupture of cable(s) also may occur in cable-stayed bridges and accordingly guideline, such as PTI, recommends considering the probable cable loss scenarios during design phase. Moreover, the possible extreme scenario which can trigger the progressive collapse of a cable-stayed bridge should be studied. Thus, there are three main objectives for this research, which are the effect of sudden loss of critical cable(s), cable loss due to blast loadings and progressive collapse triggered by the earthquake. A finite element (FE) model for a cable-stayed bridge designed according to Australian standards is developed and analysed statically and dynamically for this research purpose. It is noted that an existing bridge drawing in Australia cannot be used due to a confidential reason. The bridge model has steel deck which is supported by total of 120 stays. Total length of this bridge is 1070m with 600m mid-span. This thesis contains 8 chapters starting with the introduction as chapter 1. In chapter 2, comprehensive literature review is presented regarding three main objectives. In chapter 3 to 5, results of the cable loss analyses are presented. In chapter 3, the dynamic amplification factor (DAF) for sudden loss of cable and demand-to-capacity ratio (DCR), which indicate the potential progressive collapse, in different structural components including cables, towers and the deck are calculated corresponding with the most critical cable. The 2D linear-elastic FE model with/without geometrical nonlinearity is used for this analysis. It is shown that DCR usually remains below one (no material nonlinearity occurs) in the scenarios studied for the bridge under investigation, however, DAF can take values larger than 2 which is higher than the values recommended in several standards. Moreover, effects of location, duration and number of cable(s) loss as well as effect of damping level on the progressive collapse resistance of the bridge are studied and importance of each factor on the potential progressive collapse response of the bridges investigated. As it was shown in chapter 3, a 2D linear-elastic model is used commonly to determine the loss of cable. However, there is a need to study the accuracy and reliability of commonly-used linear elastic models compared with detailed nonlinear finite element (FE) models, since cable loss scenarios are associated with material as well as geometrical nonlinearities which may trigger progressive collapse of the entire bridge. In chapter 4, 2D and 3D finite element models of a cable-stayed bridge with and without considering material and geometrical nonlinearities are developed and analysed. The progressive collapse response of the bridge subjected to two different cable loss scenarios at global and local levels are investigated. It is shown that the linear elastic 2D FE models can adequately predict the dynamic response (i.e. deflections and main stresses within the deck, tower and cables) of the bridge subject to cable loss. Material nonlinearities, which occurred at different locations, were found to be localized and did not trigger progressive collapse of the entire bridge. In chapter 5, using a detailed 3D model developed in the previous chapter, a parametric study is undertaken and effect of cable loss scenarios (symmetric and un-symmetric) and two different deck configurations, i.e. steel box girder and open orthotropic deck on the progressive collapse response of the bridge at global and local level is investigated. With regard to the results of FE analysis, it is concluded that deck configuration can affect the potential progressive collapse response of cable-stayed bridges and the stress levels in orthotropic open decks are higher than box girders. Material nonlinearities occurred at different locations were found to be localized and therefore cannot trigger progressive collapse of the entire bridge. Furthermore, effect of geometrical nonlinearities within cables (partly reflected in Ernst’s modulus) is demonstrated to have some effect on the progressive collapse response of the cable-stayed bridges and accordingly should be considered. In chapter 6, the blast loads are applied on the bridge model and determined the bridge responses, since the blast load is one of the most concerned situations after 911 terrorist attacks. The effect of blast loadings with different amount of explosive materials and locations along the deck is investigated to determine the local deck damage corresponding to the number of cable loss. Moreover, the results obtained from the cable loss due to blast loadings are compared with simple cable loss scenarios (which are shown in chapter 3 to 5). In addition, the potential of the progressive collapse response of the bridge at global and local level is investigated. With regard to the results of FE analysis, it is concluded that the maximum 3 cables would be lost by the large amount of TNT equivalent material due to damage of the anchorage zone. Simple cable loss analysis can capture the results of loss of cable due to blast loadings including with local damages adequately. Short cables near the tower are affected by blast loadings, while they are not sensitive for the loss of cables. Furthermore, loss of three cables with damaged area did not lead progressive collapses. Finally, in chapter 7, dynamic behaviour of cable-stayed bridges subjected to seismic loadings is researched using 3D finite element models, because large earthquakes can lead to significant damages or even fully collapse of the bridge structures. Effects of the type (far- or near-field) and directions of seismic loadings are studied in several scenarios on the potential progressive collapse response of the bridge at global and local level. According to the case studies in this chapter, it is shown that near filed earthquakes applied along the bridge affected to deck and cables significantly. Moreover, the mechanism of bridge collapsed due to longitudinal excitation is analysed by an explicit analysis, which showed the high plastic strain occurring around the pin support created the permanent damage. The summary and suggestions for this research are shown in final chapter 8

System identification of bridge and vehicle based on their coupled vibration

Most current techniques used for system identification of bridges and vehicles are static-test-based methods. Methodologies that can use bridge dynamic responses or modal information are highly desirable and under development. This dissertation aims to develop new identification methodologies for bridge-vehicle systems using the bridge dynamic responses and modal information. A new bridge model updating method using the response surface method (RSM) was proposed in this dissertation. The RSM was used to design experiments in order to find out the relationships between the bridge responses and parameters to be updated. Results from numerical simulations and a field study show that the proposed methodology can effectively update bridge models with reasonable explanations available. A new methodology of identifying dynamic vehicle wheel loads was developed using only the measured bridge responses. The proposed methodology has demonstrated its ability to successfully identify dynamic vehicle loads by both numerical simulations and field tests conducted. This methodology can be used to improve the existing weigh-in-motion techniques which usually require slow vehicle movement or good road surface conditions. A new methodology of identifying the parameters of vehicles traveling on bridges was proposed in this dissertation. The proposed methodology uses the genetic algorithm to search the optimal vehicle parameter values in order to produce satisfactory agreements between the measured bridge responses and predicted bridge responses from the identified vehicle parameters. This methodology can also be used to improve the existing weigh-in-motion techniques with the ability to identify the static axle weights of vehicles. The dynamic impact factors for multi-girder concrete bridges were investigated in this dissertation. Relationships between the dynamic impact factor and bridge length, vehicle velocity, and road surface condition were investigated. Statistical properties of the impact factor were obtained. Simple expressions for dynamic impact factor were proposed, which can be used as modifications to the LRFD code regarding short bridges and bridges with poor road surface conditions

Simplified Vehicle-Bridge Interaction for Medium to Long-span Bridges Subject to Random Traffic Load

This study introduces a simplified model for bridge-vehicle interaction for medium- to long-span bridges subject to random traffic loads. Previous studies have focused on calculating the exact response of the vehicle or the bridge based on an interaction force derived from the compatibility between two systems. This process requires multiple iterations per time step per vehicle until the compatibility is reached. When a network of vehicles is considered, the compatibility equation turns to a system of coupled equations which dramatically increases the complexity of the convergence process. In this study, we simplify the problem into two sub-problems that are decoupled: (a) a bridge subject to random Gaussian excitation, and (b) individual sensing agents that are subject to a linear superposition of the bridge response and the road profile roughness. The study provides sufficient evidence to confirm the simulation approach is valid with a minimal error when the bridge span is medium to long, and the spatio-temporal load pattern can be modeled as random Gaussian. Quantitatively, the proposed approach is over 1,000 times more computationally efficient when compared to the conventional approach for a 500 m long bridge, with response prediction errors below $0.1\%$.Comment: submitted to the Journal of Civil Structural Health Monitorin

Determining the presence of scour around bridge foundations using vehicle-induced vibrations

Bridge scour is the number one cause of failure in bridges located over waterways. Scour leads to rapid losses in foundation stiffness and can cause sudden collapse. Previous research on bridge health monitoring has used changes in natural frequency to identify damage in bridge beams. The possibility of using a similar approach to identifying scour is investigated in this paper. To assess if this approach is feasible, it is necessary to establish how scour affects the natural frequency of a bridge, and if it is possible to measure changes in frequency using the bridge dynamic response to a passing vehicle. To address these questions, a novel vehicle–bridge–soil interaction (VBSI) model was developed. By carrying out a modal study in this model, it is shown that for a wide range of possible soil states, there is a clear reduction in the natural frequency of the first mode of the bridge with scour. Moreover, it is shown that the response signals on the bridge from vehicular loading are sufficient to allow these changes in frequency to be detected

Suspension bridge response due to extreme vehicle loads

Journal ArticleThis is an Accepted Manuscript of an article published online by Taylor & Francis Group in Structure and Infrastructure Engineering on 5th March 2003, available online: http://www.tandfonline.com/10.1080/15732479.2013.767844A 269 tonne trailer travelled across the Tamar Suspension Bridge in October 2010, and the authors monitored the response of the structure to the load. The following investigation documents the deflection of towers and the deck during the vehicle's passage, as well as the change in cable tensions. This was achieved by studying monitored data from the bridge collected by accelerometers and strain gauges attached to the stay cables, as well as two robotic total stations that measured the deflection of the mid-span and the sway of the tower saddle. These results were subsequently compared to the response predicted by a finite element (FE) model of the bridge, indicating an accurate match. The FE model was also used to simulate the variation of the dynamic response of the structure, which suggests that the natural frequencies vary depending on the vehicle's location to each mode shape's anti-nodes. © 2013 © 2013 Taylor & Francis

Multi-body dynamic and finite element modeling of ultra-large dump truck - haul road interactions for machine health and haul road structural integrity

Haul truck capacities have increased due to their economies of scale in large-scale surface mine production systems. Ultra-large trucks impose high dynamic loads on haul roads. The dynamic loads are exacerbated by road surface roughness and truck over-loading. The dynamic forces also subject trucks to high torsional stresses, which affect truck health. Current haul road response models are 2D and use static truckloads for low capacity trucks. Existing 3D models consider the road as a two-layer system. No models capture the truck dynamic effects on haul roads and predict strut pressures during haulage. Lagrangian mechanics was used to formulate the governing equations of the truck-haul road system. The equations were solved in MSC.ADAMS, based on multi-body dynamics, to generate the truck dynamic forces, which were verified and validated using data obtained from an open-pit mine. These forces were used in an FE model developed, verified and validated in ABAQUS to model the response of the haul road to the truck dynamic forces. The road was modeled using an elastoplastic Mohr-Coulomb model. The results showed that the maximum truck tire dynamic forces were 2.86 and 3.02 times the static force at rated payload and 20% over-loading, respectively. The trucks were exposed to torsional stresses that were up to 2.9 times the recommended threshold. Road deformation decreased with increasing layer modulus and increased with increasing payload. This study proposed novel multivariate models for predicting dynamic truck strut pressures. The novel 3D FE model and empirical relations for calculating truck dynamic forces incorporate truck dynamic forces into haul road design. This study forms a basis for designing structurally competent haul roads and improving truck health --Abstract, page iii