Performance of in situ stitches in precast concrete segmental bridges

In-situ concrete stitching with subsequent prestressing is often used in construction to join precast concrete segments together. An example of application is the stitching of the approaching decks of precast segmental bridges constructed using the balanced cantilever method. The relatively narrow stitches used in segmental bridges are often provided with nominal reinforcement only. Therefore, the in-situ stitch is a location of potential weakness but relatively little work in this area has been done. In light of this, extensive experimental and numerical studies on the behaviour of precast concrete members with in-situ concrete stitches have been conducted. The effects of stitch width, prestressing force, concrete strength, bonding of tendon and provision of shear keys on the performance of such members are presented in this paper. Unlike members that are continuously cast, cracking initiates along the construction joint between the precast segment and the stitch at low load level for members with in-situ stitches while no cracking is observed elsewhere. The cracks widen extensively as load level increases and can become detrimental to the serviceability of member. Such members should be designed such that the joints are subject to no tension under service load.published_or_final_versio

Damage Detection of a Continuous Bridge from Response of a Moving Vehicle

This paper presents a multi-stage multi-pass method to identify the location of damage of a continuous bridge from the vertical response of a moving vehicle. The damage can be detected from this response by some existing methods if the road surface of the bridge is smooth, which implies that the vibration of the vehicle contains information of the damage. The detection becomes difficult if the road surface of the bridge is rough as it will affect the response of the vehicle and may mask the information of the damage. To deal with this problem, a multi-stage multi-pass method is proposed. A vehicle runs over the bridge several times at different velocities and the corresponding responses of the vehicle can be obtained. The vertical acceleration records of the vehicle running on the intact and damaged bridges are used for the identification. The multi-stage damage detection method is implemented by application of the modal strain energy based method, followed by a modified genetic algorithm. The modal strain energy method extracts frequencies from the responses of both the intact and damaged states of the bridge before calculation of a damage indicator which can be used to roughly estimate the location of damage. At the second stage, the identification problem is transformed into a global optimization problem and is solved by the genetic algorithm techniques. For each pass of the vehicle, the method can narrow down the possible locations of the damage until it is determined with acceptable accuracy. A two-span continuous bridge is used to verify the method. The bridge is modeled using the finite element method while the vehicle is modeled as a spring-mass-damper system. The damage is inflicted by reducing the stiffness of an element. In the numerical simulation, the vertical acceleration of the vehicle is computed using a vehicle-bridge-interaction system. The results show that this method can identify the location of damage reasonably well.published_or_final_versio

An efficient method for time-dependent analysis of composite beams

This journal vol. is Proceedings of the Twelfth East Asia-Pacific Conference on Structural Engineering and Construction, EASEC12Composite beams have found wide applications in buildings and bridges. As the time-dependent effects, including the ageing, creep and shrinkage of concrete, affect the interaction between concrete and steel, it is essential to account for them properly to ensure satisfactory service and safety. This paper presents an efficient method to investigate the time-dependent performance of composite beams. Firstly, the age-adjusted elasticity modulus to account for concrete creep and the shrinkage-adjusted elasticity modulus to account for concrete shrinkage and its interaction with creep are introduced. Then an effective single-step numerical method for time-dependent analysis is developed in conjunction with the finite element method. Results obtained for the time-dependent behaviour and long-term redistribution of stresses agree well with published results. Numerical results show that the interaction among concrete ageing, creep and shrinkage increases the long-term deformation and results in stress redistribution, particularly at the interface.postprin

Finite element formulae for internal forces of Bernoulli-Euler beams under moving vehicles

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Time-dependent effects on dynamic properties of cable-stayed bridges

Structural health monitoring systems are often installed on bridges to provide assessments of the need for structural maintenance and repair. Damage or deterioration may be detected by observation of changes in bridge characteristics evaluated from measured structural responses. However, construction materials such as concrete and steel cables exhibit certain time-dependent behaviour, which also results in changes in structural characteristics. If these are not accounted for properly, false alarms may arise. This paper proposes a systematic and efficient method to study the time-dependent effects on the dynamic properties of cable-stayed bridges. After establishing the finite element model of a cable-stayed bridge taking into account geometric nonlinearities and time-dependent behaviour, long-term time-dependent analysis is carried out by time integration. Then the dynamic properties of the bridge after a certain period can be obtained. The effects of time-dependent behaviour of construction materials on the dynamic properties of typical cable-stayed bridges are investigated in detail.link_to_subscribed_fulltex

Estimation of ultimate stress in external FRP tendons

In a prestressed concrete beam with external tendons, the tendon stress depends on the member deformation, and it cannot be determined from section analysis alone as in the bonded case. Previous work has been mainly on the ultimate stress in unbonded steel tendons, with little on unbonded fibre-reinforced polymer (FRP) tendons. To account for the relative slip between the unbonded tendon and concrete, the ratio of the equivalent plastic hinge length to the neutral axis depth is analysed using available test results. It is found that this ratio for unbonded partially prestressed concrete (UPPC) beams with external FRP tendons can also be treated as a constant as for those with unbonded steel tendons. A simple method for evaluation of the ultimate stress in either steel or FRP external tendons is therefore proposed. After suitable modifications, the equations currently adopted by various design codes can still be used to predict the ultimate stress in external FRP tendons of UPPC beams.published_or_final_versio

Numerical modelling of prestresssing steel tendons under fire and post-fire conditions

The behaviour of prestressing steel tendons under fire and post-fire conditions is investigated numerically employing the ABAQUS package and its user-defined material subroutine UMAT which allows comprehensive constitutive relationships to be implemented. Based on an existing thermal creep model, a new set of parameters are proposed and calibrated against available test results in the literature. The proposed numerical model is versatile in predicting thermal creep, isothermal response and transient thermal relaxation of prestressing steel tendons. Moreover, parametric studies are conducted to investigate the effects of heating rate, heated length ratio, initial stress and various boundary conditions on the thermal relaxation of prestressing steel tendons. Besides, transient state analyses are carried out with constant heating rate and initial stress to investigate the rupture properties of prestressing steel tendons.postprin

Prediction of ultimate stress in unbonded prestressed tendons

The behaviour of prestressed concrete members with unbonded tendons is different from that of members with bonded tendons. While the behaviour of prestressed concrete beams with bonded tendons is characterised by that existing at individual sections, analysis of the entire member is necessary for beams with unbonded tendons. This paper examines various design methods for the determination of ultimate tendon stress at flexural failure of prestressed concrete beams with unbonded tendons. Two broad categories of deformation-based approaches have been identified, namely those based on the span-depth ratio together with loading type, and those based on the neutral axis depth. These methods are reviewed critically. A new design formula has been proposed in the light of the available experimental data. It is applicable not only to the conventional high-strength steel prestressing tendons, but also to those made of other materials such as fibre-reinforced polymer.published_or_final_versio

Full-range analysis of multi-span prestressed concrete segmental bridges

This journal vol. is Proceedings of the Twelfth East Asia-Pacific Conference on Structural Engineering and Construction, EASEC12The in-situ concrete stitches of prestressed concrete segmental bridge are locations of potential weakness for the entire bridge deck but relatively little work has been carried out in this area. The effects of the performance of in-situ stitches on the global behaviour of bridge deck are not well understood. As most existing techniques cannot cope with such full-range analyses, a numerical technique has been developed for conducting full-range analyses of continuous prestressed concrete bridges under incremental loads or displacements. The bridge is modelled as a series of beam elements each of which is governed by the corresponding moment-curvature relationship of a representative section within it. While most of the existing techniques are only capable of analysing the behaviour of continuous prestressed concrete beams up to the peak load-carrying capacity, the present technique can extend well into the postpeak range, which is crucial to the investigation of ductility or deformability. The development and verification of the technique are presented in this paper.postprintThe 12th East Asia-Pacific Conference on Structural Engineering and Construction (EASEC-12), Hong Kong, 26-28 January 2011. In Procedia Engineering, 2011, v. 14, p. 1425-143

Collapse mechanism and robustness of precast segmental bridges

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