Effect of Variation of Concrete Properties on the ...

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study on the dynamic characteristics of bridge superstructures with FRP composite ..... namely Katy Truss Bridge, Market Street Bridge, and Laurel Lick Bridge.
Composites in Construction 2005 – Third International Conference, Hamelin et al (eds) © 2005 Lyon, France, July 11 – 13, 2005

DYNAMIC CHARACTERISTICS OF BRIDGE SUPERSTRUCTURES WITH FRP COMPOSITE STRUCTURAL ELEMENTS A. Zhou, Y. Bai and T. Keller Composite Construction Laboratory (CCLab), Ecole Polytechnique Fédérale de Lausanne (EPFL) EPFL-CCLab, BAT. BP, CH-1015 Lausanne, Switzerland [email protected] [email protected] [email protected]

ABSTRACT: The lightweight nature and low stiffness of FRP structures combined with low material damping may lead to excessive structural vibration and may increase the dynamic amplification of deformations and stresses in bridge structures. This paper presents a preliminary study on the dynamic characteristics of bridge superstructures with FRP composite structural elements. It provides a review on considerations of dynamic load effects in design of highway and pedestrian bridges in different countries, the development of dynamic characterization of constructed bridge superstructures with FRP structural components, and the basic characteristics of constructed bridge superstructures with FRP components. It concludes by summarizing challenges related to structural dynamics of bridge superstructures with FRP structural components. 1.

INTRODUCTION

Lightweight structures from Fiber Reinforced Polymer (FRP) composites provide numerous opportunities and economically promising solutions for upgrading, rehabilitation and new construction of bridge and building structures. In bridge structures, three main types of FRP structural systems have been developed and implemented in highway and pedestrian bridges: deck-girder (or deck-stringer) system, truss system, and cable-stayed (or suspension) system. In the deck-girder system, the bridge superstructure usually comprises FRP deck and existing underneath girders (from traditional materials such as steel, concrete, and timber or FRP hybrid girders with FRP composites). The lightweight advantage and the ease of construction have helped the deck-girder system to be used in highway bridge deck rehabilitation and new construction. In the truss system, usually all the bridge components are made from FRP composites, therefore, a lightweight bridge superstructure can be obtained. The truss system has been mainly used in pedestrian FRP bridges. In the cable-stayed system, fiber reinforced polymers can be used as structural elements such as cable, pylon and deck. The lightweight advantage and elegant appearance of this system are attractive for long-span bridges. Most constructed FRP composite structural systems for bridge applications have been designed based on static or quasi-static characterisation and analysis. The design criterion is usually based on global stiffness analysis in conjunction with local strength analysis at critical locations [3, 4]. In the design stage, the effects of dynamic loads have been considered by adding a certain amount of equivalent load to the static design load through the concept of dynamic load allowance (impact) [3-5]. Therefore, the designer only deals with static load in their calculation. Compared to traditional concrete or steel structures, being lightweight has been considered as an important advantage of FRP composite structures. However, the lightweight nature of FRP structures also brings concerns in terms of dynamical characteristics. Due to its lightweight nature, an FRP structure has high live load to dead load ratio. Therefore, the use of FRP composites makes the structural response more live-load dependent. The live-load/dead-load ratio for FRP structures varies from three to five, while the same ratio for conventional concrete and steel

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structures varies from one to two [6]. FRP composites are brittle in nature. The material damping of FRP composites is relatively low compared to concrete or reinforced concrete structures [19]. The light weight and low stiffness of FRP structures combined with lower material damping (than concrete) can lead to excessive structural vibrations and may increase the dynamic amplification of stresses and deflections. Therefore, the dynamic characteristics of bridge superstructures consisting lightweight FRP structural systems should be investigated and analysed. In fact, if FRP bridge superstructure systems are to be used in long span bridges, the analysis of their dynamic characteristics is more demanding. Basic dynamic characteristics include natural frequencies, mode shapes, acceleration characteristics, and damping ratios. In addition, a global structural integrity and health condition evaluation is necessary to ensure the long term performance of the key structures made from FRP materials. Evaluating integrity and health condition through structural dynamic characterization could be a promising technique to be developed. The objective of this paper is to review considerations of dynamic loads in highway and pedestrian bridge design in various countries, the analysis and characterization process for bridge dynamic analysis, and the development of dynamic characterization of bridge superstructures incorporating FRP composite structural systems. It concludes by summarizing challenges related to structural dynamics of bridge superstructures with FRP components. 2.

DYNAMIC LOAD CONSIDERATIONS IN BRIDGE DESIGN

2.1

Dynamic Load Considerations in Highway Bridge Design

The consideration of the effects of dynamic loads on highway bridges usually focuses on the Ultimate Limit State (ULS). Dynamic loads to be considered in ULS of highway bridge design include: (1) Vertical loads exerted by a moving vehicle, (2) Longitudinal loads exerted from braking or accelerating vehicles, (3) Transverse horizontal centrifugal forces, (4) Forces from collision or strike of bodies, (5) Dynamic bridge loads from earthquake and wind etc. Constructed highway bridge superstructures with FRP structural systems usually only consider the vertical loads exerted by a moving vehicle [7-10]. The maximum vertical loads exerted by a moving vehicle often exceed those produced by an equivalent static or slow moving vehicle. The effect has commonly been called impact, interpreted by the Impact Factor (IM), Dynamic Amplification (DA), Dynamic Load Allowance (DLA), or Dynamic Amplification Factor (DAF). The definitions of IM, DA, DLA, and DAF are [3, 5]: IM = DA = DLA = R 'dyn /R stat =( R dyn − R stat ) /R stat

DAF = 1 + DA

(1)

where Rdyn is the maximum dynamic response (such as deflection or strain under dynamic loads) of the bridge; Rstat is the maximum static response (such as deflection or strain under static loads); R’dyn is the additional response due to the dynamic effects and R’dyn=Rdyn-Rstat. In many national codes, the vertical loads exerted by a moving vehicle are considered by DLA or DAF, as summarized in Table 1. It is seen that no worldwide consensus has been reached as to the design requirements for vertical loads exerted by a moving vehicle. Some disagreements exist between provisions of various national bridge codes. This is mainly because the DLA or DAF values depend on many other parameters that are difficult to take into account in addition to the maximum span and the natural frequency. Excessive vibrations can cause discomfort to passengers in highway bridges. The vibration limits established to ensure comfort to passengers can be stated as vibration Serviceability Limit States (SLS). AASHTO dealt with the problem of vibrations perceptible by pedestrians on highway bridges by limiting the deflection due to live load to span length ratio and the depth to span length ratio for a long time [8]. A person travelling across the bridge in a vehicle will be able to withstand higher amplitudes of vibration and accelerations because of the high damping afforded by the vehicle’s 2

suspension system. However, this problem is more critical for pedestrian bridges which will be discussed in the following section. Table 1 – Provisions considering vertical loads exerted by a moving vehicle in different codes.

Code

DLA (IM) or DAF (L: span length in m)

Supplements

EUROCODE [7]

DAF=φ=1.4 - L/500

φ ≥1

AASHTO (1996) [Standard Specifications]

IM=15.24/(L+38.1)

IM ≤ 30%

AASHTO (1998)

Deck joints: all limit states, 75% Independent of span

[LRFD Specifications]

All other components: fatigue and fracture limit states, 15%; all other limit states, 33%

China (CJJ77-98)

IM=0.67-0.30logL, no more than 0.4

2.2

for vehicle load

Dynamic Load Considerations in Pedestrian Bridge Design

The consideration of dynamic load effects on pedestrian bridges normally concentrates on vibrations in SLS. Generally, there are two concepts in guiding the consideration of footbridge vibration. The first concept requires the calculation of the actual dynamic response of the bridge; then one checks if the response is within the acceptable limits for the bridge users. The second approach is based on the request to avoid footbridge natural frequencies, which are in ranges coinciding with typical frequencies for human-induced dynamic excitation. Similar to the DLA of different countries, the provisions for pedestrian bridge vibration are different from different national codes. As shown in Table 2, some codes recommend avoiding the resonant frequency range typical for the first or second force harmonic, the others give a more or less complex design procedure to calculate the response of the bridge and check if it is acceptable. 2.3

Bridge Dynamical Analysis and Experimental Characterization

In theoretical analysis, a bridge is usually modeled as a simple-supported or continuous beam or plate. If shear and torsion deformation can not be neglected, shear deformable beam and plate theories should be applied. When the bridge model is available, it is necessary to build a dynamic force model. For footbridge analysis, the measured dynamic forces can be applied. Every step by a pedestrian can be treated as one impulse and series of steps as impulses along the way and shifted in time. Therefore, load induced by walking can be assumed as sum of loads caused by continual steps, which further can be simulate with moving pulsating point load. One can get the dynamic load factor from the analysis of response in the frequency domain, isolating the contribution of each harmonic [8, 11]. For highway bridges, it is more difficult to measure the dynamic load induced by moving vehicles [12-15]. Criteria to assess whether or not interaction is important were developed. It was concluded that interaction can be ignored if: (1) γ