Zhi Wu Yu

Numerical Simulation of Vertical Random Vibration of Train-Slab Track-Bridge Interaction System by PEM

The paper describes the numerical simulation of the vertical random vibration of train-slab track-bridge interaction system by means of finite element method and pseudoexcitation method. Each vehicle is modeled as four-wheelset mass-spring-damper system with two-layer suspension systems. The rail, slab, and bridge girder are modeled by three-layer elastic Bernoulli-Euler beams connected with each other by spring and damper elements. The equations of motion for the entire system are derived according to energy principle. By regarding rail irregularity as a series of multipoint, different-phase random excitations, the random load vectors of the equations of motion are obtained by pseudoexcitation method. Taking a nine-span simply supported beam bridge traveled by a train consisting of 8 vehicles as an example, the vertical random vibration responses of the system are investigated. Firstly, the suitable number of discrete frequencies of rail irregularity is obtained by numerical experimentations. Secondly, the reliability and efficiency of pseudoexcitation method are verified through comparison with Monte Carlo method. Thirdly, the random vibration characteristics of train-slab track-bridge interaction system are analyzed by pseudoexcitation method. Finally, applying the 3σ rule for Gaussian stochastic process, the maximum responses of train-slab track-bridge interaction system with respect to various train speeds are studied.

Possible existing seismic analysis errors of long span structures and bridges while utilizing multi-point earthquake calculation models

A long-span structure is a common type of public building, but its seismic characteristics are distinct from other types of buildings because of its long span. Calculation models considering multi-point excitation are required in the seismic analysis of long-span structures. However, correlative studies have already clearly shown that important but often overlooked errors exist in previously developed multi-point excitation calculation models. The process of establishing displacement and acceleration models for multi-point seismic analysis is reviewed. Error sources and criteria of the two models are explained using rigorous theoretical derivation. Error characteristics and distributions in multiple structural types, such as ordinary structures without dampers and damper-installed structures with concentrated damping, are also described. Modifications for multi-point excitation displacement and acceleration models, for time history and stochastic analysis, respectively, are proposed, and these modified models are used to assess errors in the conventional models. Numerical examples are solved using conventional displacement and acceleration models and two corresponding modified models. The properties, components and distribution of errors in the conventional models are demonstrated. The findings presented in this paper can provide a sound basis for the practical application of multi-point excitation calculation models in seismic analysis.

Efficient and Accurate Method for Calculating the Stochastic Seismic Response of a Nonproportionally Damped Structure

AbstractIn the case of nonproportionally damped structures, the forced decoupling method is often adopted in approximate analysis. However, it generally cannot satisfy the precision requirements for practical engineering. Taking this into account, methods to calculate the stochastic seismic response of nonproportionally damped structures are systematically studied in this paper. Based on the pseudoexcitation method, an efficient and accurate iteration method with the advantages of high computational efficiency and iteration convergence is also proposed. This new method is preferred for practical engineering because of real expressions. Finally, a numerical example is carried out to verify the properties and advantages of the proposed method.

Influences of Temperature and Time on Concrete Compressive Strength after Natural Cooling

The mechanical properties of C40 ordinary concrete after different temperature and time were experimentally researched. A detailed review of experimental phenomena and the influence of the significant parameters were given, and the relationship between the compressive strength and temperature and time were discussed. The experiment showed that with the exposure of temperature and time increasing, the concrete cubic compressive strength tends to decrease generally. According to the experiment results, the formula for strength-temperature-time relationship after the fire was built. According to the coupling effects of temperature and time to the compressive strength, a new fire size classification standard was established.

The Influence of Corrosion Types on the Deterioration of Shear Strength of Corroded RC Beams

The paper examines the effect of the corrosion types of reinforcing steel on the deterioration of shear strength of corroded reinforce concrete (RC) beams. Three possible types of corrosion, i.e., uniform (or general), pitting (or localized) and hybrid corrosion, for RC beams exposed to de-icing salts or sea environments are considered. From the investigation of the present paper, it can be concluded that the corrosion types have great effect on the time-dependent shear strength of a corroded RC beam. Generally, the shear strength degraded fastest for hybrid corrosion among the three corrosion types. When the corrosion rate is relative small, the deterioration of the shear strength for the uniform corrosion is greater than that for pitting corrosion. However, when the corrosion rate becomes larger, the deterioration of the shear strength for the pitting corrosion is greater than that for uniform corrosion as corrosion propagates.

Pseudo Excitation Method Based on Multiple Degree of Freedom Modal Equation

In recent years, on the one hand destructive earthquakes frequently occur, and vibration control devices such as dampers and isolation become increasingly application in actual large-scale structures, on the other hand modern structures with complex styles and diversified materials possess significant non-proportional damping characteristics, and traditional methods based on Rayleigh damping assumption no longer meet seismic calculation requirement. According to this situation, firstly multiple degree of freedom modal equation and pseudo excitation method are both utilized to establish a new efficient calculation method system which is suitable for stochastic analysis of non-proportionally damped structure, and the new method can achieve adjustive calculation efficiency and accuracy. Furthermore, advantages and features of established method are analyzed, and guidance suggestions for appropriate method use are also proposed. Finally, numerical study is carried out to verify conclusions proposed by theoretical derivation, and efficiency and accuracy of established method system used in stochastic response calculation of non-proportionally damped structure is confirmed.

Discussion on Several Problems in the Study of Corrosion Mechanism of Steel - Concrete Composite Beams

It’s of great practical significance to improve the durability of steel-concrete composite beam structures.The deterioration of steel-concrete composite beams includes stud rust, reinforcement corrosion, corrosion of the steel beams and concrete performance degradation, which are caused by a number of factors. The corrosion mechanism of composite beams is discussed, and some basic issues remain to be solved in this area are analyzed in the paper. Besides, some research directions in the future are put forward.

Experimental Study on the Corrosion Law of Chloride in Concrete under Artificial Climate

Corrosion experimental were conducted in order to study concrete samples of two strength grades during different erosion period under the condition of designed artificial chloride erosion.Through drilling powder of concrete surface at various depths,chloride ions’concentration at different depths of corroded samples were determinated,and the diffusion coefficients of chloride ion after 90,180,240 days of corrosion were calculated on the basis of Fick’s second law.It turns out that obvious convection zone exitsin corroded concrete,and the thickness of convection zone increases with the erosing time and the reduction of concrete strength grade.In addtion,fitted datas of chloride ion diffusion coefficient increases with the reduction of concrete strength grade.

Influences of Temperature and Time on Concrete Compressive Strength after Water Cooling

The concrete residual compressive strength after different burning temperature and time by water cooling was experimentally studied. A detailed review of experimental phenomena in fire and the concrete failure characteristics in compressive test were given, and the relationship between the compressive strength and temperature, time were discussed. It is found that with the exposure of temperature and time increasing, the concrete residual compressive strength tends to decrease generally, but when the temperature is low or time is short, the concrete residual compressive strength tends to increase on the contrary. According to analysis of the experiment results, the formula for concrete residual compressive strength and temperature-time relationship after the fire was built.

A Probabilistic-Based Durability Analysis of Concrete Structures Exposed to Chloride Containing Environment

Time dependency of chloride transport has been studied and an improving model for predicting the apparent chloride diffusion coefficient has been proposed. A probabilistic-based durability analysis to estimate failure probability on the chloride ingress into concrete has been carried out. Results of the analysis demonstrate that the improving model in this paper is appropriate for long term service life prediction of concrete structures exposed to chloride containing environment.