Wei ZHANG

University of Connecticut, Civil and Environmental Engineering, Faculty Member

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Supervisors: CS Cai
Address: Department of Civil and Environmental Engineering
School of Engineering
261 Glenbrook Rd., Unit 3037
University of Connecticut
Storrs, CT 06269-3037

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Fatigue Reliability Assessment for Long-span Bridges under Combined Dynamic Loads from Winds and Vehicles

During a bridge’s life cycle, the stresses from multiple dynamic loads might be superposed and pr... more During a bridge’s life cycle, the stresses from multiple dynamic loads might be superposed and progressive fatigue damage might accumulate and induce serious fatigue damage issues. This paper presents a comprehensive framework for fatigue reliability analysis of long-span bridges under combined dynamic loads from vehicles and winds. After modeling the complicated structure details with equivalent orthotropic material, dynamic stress ranges of a long-span bridge are obtained via solving the equations of motions for the vehicle-bridge-wind dynamic system with multiple random variables considered, for instance, vehicle speeds, road roughness conditions, and wind velocities and directions. After counting the number of stress cycles at different stress range levels using the rainflow counting method, fatigue damage increments are obtained using the fatigue damage accumulation rule. Probability of failure for the fatigue damage at the end of each block of stress cycles and the cumulative probability of failure can be obtained. As a result, the fatigue life and reliability for the given structure details can be obtained. Based on the results from a case study, the dynamic effects from vehicles are found to be relatively small for long-span bridges and the effects from vehicle speeds and road roughness conditions can be neglected. Nevertheless, even though the stresses from either the vehicle loads or wind loads may not be able to induce serious fatigue problems alone, the superposed dynamic stress ranges cannot be ignored for fatigue reliability assessment of long-span bridges.

Evaluating Instantaneous Wind Forces and Moments on Bridge Decks Using Measured Velocity Fields

The wind loads including forces and moments on bridge decks are evaluated in the present study us... more The wind loads including forces and moments on bridge decks are evaluated in the present study using the velocity fields and their derivatives. Based on the conservation of momentum and momentum moment, the integral over the control volume is converted to the surface integral over the boundary of the control volume using the divergence theorem. The wind forces and moments can be calculated based on the obtained velocity fields and their derivatives around bridge decks and the pressure terms in the equations are canceled out. Velocity fields are obtained from Computational Fluid Dynamics (CFD) simulations and Particle Image Velocimetry (PIV) wind tunnel experiments. Results from different turbulence models are compared with each other and wind tunnel experimental results in the literature. The calculated wind loads from numerical integrations of velocity field from PIV experiments or CFD numerical simulations are compared with the results from a direct integration of the CFD simulated surface pressure and the results from force balance measurements in the wind tunnel. Discussions are made upon the results from several cases with different sizes of control volumes and space resolutions for both the square cylinder section and the twin-box girder section.

Wind-induced internal pressures of buildings with multiple openings

Internal pressure plays an important role to design buildings or to predict the structuraldamage ... more Internal pressure plays an important role to design buildings or to predict the structuraldamage due to strong wind events. The current two public hurricane loss predictionmodels take the averaged external pressure as the mean internal pressure when thebuilding is subjected to multiple openings failures. To verify this approximationapproach, a typical 5:12 slope gable roof residential building with 17 openings wassystematically tested to investigate the multiple openings effect on the internalpressure at the LSU Wind Tunnel Laboratory. The results show in general beingconsistent with the previous research results and indicate that: (1) the mean internalpressure coefficient Cp_est_mean estimated by the two public models is closer to themeasured minimum internal pressure coefficient Cp_mea_min instead of themeasured mean internal pressure coefficient Cp_mea_mean ; (2) ASCE 7-10underestimates the internal pressure for both "Partially Enclosed" and "Enclosed" typeof buildings; (3) Helmholtz resonance occurred up to 5.25 Hz for a single 6% windwardopening in this study.

Reliability Based Dynamic Amplification Factor on Stress Ranges for Fatigue Design of Existing Bridges

"A dynamic amplification factor (DAF) or dynamic load allowance (IM) is typically used in bridge ... more "A dynamic amplification factor (DAF) or dynamic load allowance (IM) is typically used in bridge design specifications to include dynamic effects from vehicles on bridges. The calculated live load stress ranges might not be correct due to varied dynamic amplification effects in different regions along the bridge, different road roughness conditions, and multiple stress range cycles generated for one vehicle passage on the bridge. In the present study, a reliability based dynamic amplification factor on stress ranges (DAFS) for fatigue design is proposed to include the fatigue damages from multiple stress range cycles due to each vehicle passage at varied vehicle speeds under various road conditions in the bridge's life cycle. A numerical simulation toward solving a coupled vehicle‐bridge system, including a 3‐D suspension vehicle model and a 3‐D dynamic bridge model, is used to obtain the revised equivalent stress range. The revised equivalent stress range is defined on an equivalent fatigue damage basis, namely, the fatigue damages from multiple stress ranges with varied amplitudes are equivalent with the fatigue damage from one stress cycle of the revised equivalent stress range. DAFS is then defined as the ratio of the nominal live load stress range and the maximum static stress range. A parametric study on DAFS is carried out to analyze the effect from multiple variables in the bridge's life cycle, for instance, faulting days in each year, vehicle speed limit and its coefficient of variance, vehicle type distribution, and annual traffic increase. In order to appreciate the difference of the proposed DAFS and traditional DAF, the calculated fatigue lives from the six approaches related to DAFS or DAF are compared with each other. Similar to DAF for dynamic response on displacements, DAFS is proposed to obtain dynamic stress ranges for fatigue design. As a result, once the DAFS is available, the dynamic stress ranges for fatigue design can be easily obtained by multiplying the maximum static stress range and the DAFS, which helps preserve both the accuracy and simplicity for bridge fatigue design.

Read More: http://ascelibrary.org/doi/abs/10.1061/%28ASCE%29BE.1943-5592.0000387"

Fatigue Reliability Assessment for Existing Bridges Considering Vehicle Speed and Road Surface Conditions

"During the life cycle of a bridge, dynamic impacts due to random traffic loads and deteriorated ... more "During the life cycle of a bridge, dynamic impacts due to random traffic loads and deteriorated road surface conditions can induce serious fatigue issues for bridge components. It is necessary, and more realistic than the deterministic approach, to use reliability methods and treat the input parameters as random variables for the vehicle-bridge dynamic system. This paper presents a framework of fatigue reliability assessment for existing bridges in lifetime serviceability considering the random effects of vehicle speed and road-roughness condition. Since each truck passage might generate multiple stress ranges, revised equivalent stress-range is introduced to include fatigue damage accumulations for one truck passage. Therefore, the two variables, i.e., the stress-range numbers and equivalent stress ranges per truck passage, are coalesced in the newly defined variable based on equivalent fatigue damage. The revised equivalent stress-range is obtained through a fully-computerized approach toward solving a coupled vehicle-bridge system, including a three-dimensional (3D) suspension vehicle model and a 3D dynamic bridge model. At each truck-pass-bridge analysis, deteriorations of the road-roughness condition are considered, and the vehicle speed and road surface profile are generated randomly. When the stress-range threshold is 3.45 Mpa (0.5 ksi) or below, lognormal distribution is proven a good model to describe the revised equivalent stress-range. In addition to the assumptions of other input random variables being normal or lognormal, fatigue reliability index and fatigue life for a target fatigue reliability index are predicted. The effects of the road surface condition, vehicle speed, and annual traffic increase rate on the fatigue reliability index and fatigue life are also discussed.

Read More: http://ascelibrary.org/doi/abs/10.1061/%28ASCE%29BE.1943-5592.0000272"

Particle image velocimetry study and numerical simulation of turbulent near wake of square cylinder

Evolution of unsteady aerodynamic forces for bodies from flow maps

Flow-map mechanism of wind induced vibrations of H-shape bridge decks

Flow field mechanism of wind induced vibration response of large span bridge influenced by guide vanes

Analysis method of vortex shedding from bridge decks based on the revised frozen turbulence hypothesis

Flow-map mechanism of wind induced vibrations of H-shape bridge decks

Experiments on vortex induced vibration of twin-box bridge sections in high and low Reynolds numbers

Research progress of cross correlation algorithms in particle image velocimetry

Comparison and analysis of vortex induced vibration for twin-box bridge sections based on experiments in different Reynolds numbers

Calculation and analysis of static wind stability of long span concrete-filled steel tube arch bridge

Secondary Stress Analysis of the Main Cable of Suspension Bridges

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