Reliability of composite steel bridge beams designed following AASHTO''s LFD and LRFD specifications

Composite steel beam bridges designed following AASHTO's Load Factor Design (LFD) method and the newly developed Load and Resistance Factor Design method (LRFD) are proportioned for different loading conditions. They need to satisfy these conditions at maximum design load, an overloading condition, and fatigue load. The ultimate strength of compact composite steel beams is based on the fully plastic stress distribution. The capacity of non-compact beams is based on the yield or partially-plastic moment, depending on whether the beam violates all or few of the compactness or ductility requirements. In this paper, the reliability for the ultimate flexural capacity limit state of composite steel beams is investigated. The structural reliability is measured in terms of the reliability index. The statistical data on strength are generated starting from statistics on material properties and using the Monte Carlo simulation method. Statistical data on load components are compiled from the available literature. The scope of the study covers a wide range of rolled beams, span lengths, beam spacings, and two yield strengths. The deterministic study showed that the design of composite steel beams is usually not governed by the maximum design load combination, but rather by the overloading condition. The results of the reliability study indicate non-uniformity in the safety of steel bridges that are designed by the AASHTO LFD code. This is also true for AASHTO's LRFD code since it is not calibrated for the overloading condition. The value of the reliability index is a function of the compactness classification, method of design, beam spacing, span length, and section size.     

Behavior of Corrugated Web I-Girders under In-Plane Loads

A theoretical formulation of the linear elastic in-plane and torsional behavior of corrugated web I-girders under in-plane loads is presented. A typical corrugated web steel I-girder consists of two steel flanges welded to a corrugated steel web. Under a set of simplifying assumptions, the equilibrium of an infinitesimal length of a corrugated web I-girder is studied, and the cross-sectional stresses and stress resultants due to primary bending moment and shear are deduced. The analysis shows that a corrugated web I-girder will twist out-of-plane simultaneously as it deflects in-plane under the action of in-plane loads. In the paper, the in-plane bending behavior is analyzed using conventional beam theory, whereas the out-of-plane torsional behavior is analyzed as a flange transverse bending problem. The results for a simply supported span subjected to a uniformly distributed load are presented. Finally, finite element analysis results are presented and compared to the theoretical results for validation.     

Reliability-based condition assessment of deteriorating concrete bridges considering load redistribution

A wide variety of models have been proposed for estimating the reliability of highway bridges. For reinforced concrete bridges subjected to environmental attack, time-variant reliability methods have to be used. In this study, the condition of reinforced concrete girder bridges is assessed using a time-variant system reliability approach in which both load and resistance are time-variant quantities. Several system models are considered, including failure of any girder (series system) and failure of a specified number of adjacent girders (series-parallel system). Adaptive importance sampling is used to determine the cumulative-time system failure probability. An existing reinforced concrete T-beam bridge located near Pueblo, Colorado, is investigated. The influence of resistance degradation and post-failure load redistribution is included. A comparison of reliability estimates for several system models is given, including the influence of correlation among initial girder strengths. The results can be used as a guide for the selection of system models for bridge reliability analysis, identification of critical girders in a bridge system, and for the development of optimal reliability-based maintenance strategies for reinforced concrete highway bridges.     

Girder Differential Deflection and Distortion-Induced Fatigue in Skewed Steel Bridges

The prevalence of fatigue cracking in steel bridge girders due to out-of-plane web distortion motivates development of procedures to evaluate the effects of distortional fatigue. In a previous study sponsored by the Minnesota Department of Transportation (Mn/DOT) the frequency and magnitude of distortional stresses on a typical skewed, steel bridge with staggered, bent-plate diaphragms were assessed. The results revealed a diaphragm deformation mechanism that causes distortional fatigue in the girder web gap, leading to simple, accurate estimates of fatigue stress if bridge properties and differential vertical deflection between girders are known. In the present study, linear finite element models are used to represent composite steel bridges and identify bridge parameters that influence relative deflection of adjacent girders. Parameters found to have a significant effect on differential deflection include girder spacing, angle of skew, span length, and deck thickness. These results are incorporated in a simple procedure that is intended for use in management schemes for skewed, steel-girder bridges, with staggered, bent-plate diaphragms, susceptible to web gap distortional fatigue.     

箱梁预制短线匹配施工关键技术

本文以厦漳跨海大桥预应力混凝土箱梁施工为例就连续箱梁短线匹配预制工艺、关键工序及控制方法进行了介绍,与传统的现浇方法相比,短线匹配法具有施工速度快、周期短、质量容易控制,成桥后混凝土收缩徐变小等优点,具有广泛的应用前景.     

Live Load Distribution Formulas for Single-Span Prestressed Concrete Integral Abutment Bridge Girders

In this study, live load distribution formulas for the girders of single-span integral abutment bridges (IABs) are developed. For this purpose, two and three dimensional finite-element models (FEMs) of several IABs are built and analyzed. In the analyses, the effects of various superstructure properties such as span length, number of design lanes, prestressed concrete girder size, and spacing as well as slab thickness are considered. The results from the analyses of two and three dimensional FEMs are then used to calculate the live load distribution factors (LLDFs) for the girders of IABs as a function of the above mentioned parameters. The LLDFs for the girders are also calculated using the AASHTO formulas developed for simply supported bridges (SSBs). The comparison of the analyses results revealed that LLDFs for girder moments and exterior girder shear of IABs are generally smaller than those calculated for SSBs using AASHTO formulas especially for short spans. However, AASHTO LLDFs for interior girder shear are found to be in good agreement with those obtained for IABs. Consequently, direct live load distribution formulas and correction factors to the current AASHTO live load distribution equations are developed to estimate the girder live load moments and exterior girder live load shear for IABs with prestressed concrete girders. It is observed that the developed formulas yield a reasonably good estimate of live load effects in prestressed concrete IAB girders.     

A new formulation for longitudinally stiffened webs subjected to patch loading

This study proposes a new formulation for patch loading longitudinally stiffened webs using genetic programming (GP) for the first time in the literature. The database for the GP formulation is based on extensive experimental results from the literature. The results of the GP based formulation are compared with existing models and design codes. The results indicate that the proposed GP formulation performs quite well compared to existing models and design codes.     

Live load factors for parking garage members

This paper proposes a simplified, but effective, algorithm to obtain critical design moments for clamped and/or elastically supported parking garage girders, beams and slabs. Maintaining the uniformly distributed load concept, the equivalent vehicle load factors which can simulate vehicle load effects without conducting additional sophisticated numerical analyses for the concentrated loads are introduced. After determining a standard design vehicle of 2.4 tons through an investigation of small to medium-sized vehicles made in Korea, finite element analyses for concentrated wheel loads were carried out by referring to the influence lines for the girders and beams and influence surfaces for the slabs. Based on the obtained member forces, we determined the equivalent vehicle load factors for the girders, beams and slabs which represent the ratios of member forces under vehicle loads to those under uniformly distributed loads. In addition, the relationships between the equivalent vehicle load factors and sectional dimensions were established by regression and used to obtain the design moments by vehicle loads. The member forces calculated by the proposed method are compared with the results of different approaches mentioned in current design codes with the objective of establishing the relative efficiencies of the proposed method.     

船厂龙门起重机塔架顶升与大梁提升技术

本文以介绍青岛北海船厂200吨龙门起重机安装施工为例，总结液压提升技术在重型设备安装中的应用，进一步完善了液压提升施工工艺。     

Shear Buckling Resistance of GFRP Plate Girders

Thin webs of glass-fiber-reinforced polymer (GFRP) girders are sensitive to shear buckling, which can be considered an in-plane biaxial compression-tension buckling problem, according to the rotated stress field theory. An extensive experimental study was performed, which shows that an increasing transverse tension load significantly increases the buckling and ultimate loads caused by a decrease in the initial imperfections and additional stabilizing effects. The stacking sequence also greatly influenced the buckling behavior. Higher bending stiffness in the compression direction increased the buckling and ultimate loads, while higher bending stiffness in the tension direction changed the buckling mode shape. The general solution obtained using the Fok model accurately modeled the experimental results, while the simplified solution (modified Southwell method) provided accurate results only at higher tension loads.