Repair of Steel Composite Beams with Carbon Fiber-Reinforced Polymer Plates

One significant cause of deterioration of steel bridge structures is the corrosion due to extensive use of deicing salts in winter weather. The investigation presented in this paper focused on the behavior of steel composite beams damaged intentionally at their tension flange to simulate corrosion and then repaired with carbon fiber-reinforced polymer (CFRP) plates attached to their tension areas side. Damage to the beams was induced by removing part of the bottom flange, which was varied between no damage and loss of 75% of the bottom flange. All beams were tested to failure to observe their behavior in the elastic, inelastic, and ultimate states. To help implement this strengthening technique, a nonlinear analytical procedure was also developed to predict the behavior of the section/member in the elastic, inelastic, and ultimate states. The test results showed a significant increase in the strength and stiffness of the repaired beams. Through the use of CFRP plates, all damaged beams were fully restored to their original (undamaged state) strength.     

Strength of Stiffened Flanges in Horizontally Curved Box Girders

Longitudinal stiffeners are often attached to increase the buckling strength of thin-walled box girder flanges. The minimum required rigidity for longitudinal stiffeners for curved box girder flanges is given by the AASHTO “Guide specifications for horizontally curved steel girder highway bridges.” However, this requirement is simply adopted from the current AASHTO specifications for straight stiffened flanges. The validity of this requirement has been questioned in a series of recent studies. The effect of important design parameters on the minimum required stiffener rigidity is investigated numerically in this study by examining the prebuckling stress distribution and elastic and inelastic buckling stresses of horizontally curved stiffened flanges. In order to characterize and quantify the analytically collected data, a series of parametric studies were performed. A new equation for the minimum required rigidity for the longitudinal stiffeners is derived from regression analyses. Through the evaluation of a few selected case studies and a design example, the validity and reliability of the proposed new equation is demonstrated.     

Behavior of Pultruded Fiber-Reinforced Polymer Beams Subjected to Concentrated Loads in the Plane of the Web

Results of the behavior of pultruded fiber-reinforced polymer (FRP) I-shaped beams subjected to concentrated loads in the plane of the web are presented. Twenty beams with nominal depths from 152.4 to 304.8?mm were tested in three-point bending with a span-to-depth ratio of four. Load was applied to the top flange directly above the web—12 without bearing plates and 8 with bearing plates of varying width and thickness. All test specimens failed with a wedgelike shear failure at the upper web-flange junction. Finite-element results support experimental findings from strain gauge and digital image correlation data. Bearing plates increased beam capacity by 35% or more as a function of bearing plate width and thickness. Bearing plates increased average shear stress in the web at failure from 17.4 to 27.2?MPa—below the accepted value of in-plane shear strength (69?MPa). A design equation is presented, and predicted capacities are compared with experimental results. The average value of experimental capacity to predicted capacity is 1.12 with a standard deviation of 0.11 and coefficient of variation (COV) of 0.10 for sections up to 304.8?mm deep.     

Effect of Longitudinal Stress Gradients on Elastic Buckling of Thin Plates

This paper analyzes the effect of longitudinal stress gradients on the elastic buckling of thin isotropic plates. Two types of thin plates are considered: (1) a plate simply supported on all four edges and rotationally restrained on two longitudinal edges; and (2) a plate simply supported on three edges with one longitudinal edge free and the opposite longitudinal edge rotationally restrained. These two cases illustrate the influence of longitudinal stress gradient on stiffened and unstiffened elements, respectively. A semianalytical method is derived and presented herein to calculate the elastic-buckling stress of both types of rectangular thin plates subjected to nonuniform applied longitudinal stresses. Finite-element analysis using ABAQUS is employed to validate the semianalytical model for plates with fixed and/or simple supports. Empirical formulas are produced to calculate the buckling coefficients of plates with fixed and/or simple supports under longitudinal stress gradients. The results help establish a better understanding of the effect of longitudinal stress gradients on the elastic buckling of thin plates and are intended to aid in the development of design provisions to include these effects in the strength prediction of thin-walled beams under moment gradients.     

Effects of Longitudinal Stiffeners on Curved I-Girder Webs

Two previous papers by the writers described the buckling and finite-displacement behavior of curved I-girder web panels without longitudinal stiffeners subjected to pure bending, presented a theoretically pure analytical model, and presented equations that describe the reduction in strength due to curvature. This paper describes the optimum location and strength effects of one and two longitudinal stiffeners attached to curved I-shaped plate girders. Using lateral load and lateral pressure analogies similar to that described in the earlier papers, strength reduction equations are formulated for curved plate girders with longitudinal stiffeners. A comprehensive comparison is made between the equations developed in this investigation and design equations used in Japanese and American design guides, and the applicability and superiority of the method is demonstrated.     

Shear Strength of Normal Strength Concrete Beams Reinforced with Deformed GFRP Bars

This paper evaluates the shear strength, Vc, of intermediate length (2.5 < a∕d < 6) simply supported concrete beams subjected to four-point monotonic loading and reinforced with deformed, glass fiber-reinforced polymer (GFRP) reinforcement bars. Six different overreinforced GFRP designs, ρ > ρb, were tested with three replicate beams per design. All samples failed as a result of diagonal-tension shear. Measured shear strengths at failure are compared with theoretical predictions calculated according to traditional steel-reinforced concrete procedures and recently published expressions intended for beams reinforced with GFRP. Recommendations are made regarding the adequacy of shear strength prediction equations for GFRP-reinforced members. The study concludes that shear capacity is significantly overestimated by the “Building Code Requirements for Structural Concrete and Commentary” (ACI 318-99) expression for, Vc, as a result of the large crack widths, small compression block, and reduced dowel action in GFRP-reinforced flexural members. Shear strength was found to be independent of the amount of longitudinal GFRP reinforcement. A simplified empirical equation for predicting the ultimate shear strength of concrete beams reinforced with GFRP is endorsed.     

Parametric Study on the Dynamic Instability Behavior of Laminated Composite Stiffened Plate

This paper deals with the study of dynamic or parametric instability behavior of laminated composite stiffened plates with step-uniform and concentrated in-plane harmonic edge loading. The eight-noded isoparametric degenerated shell element and a compatible three-noded curved beam element are used to model the plate and the stiffeners, respectively. The method of Hill’s infinite determinant is applied to analyze the dynamic instability regions. Numerical results are presented through convergence and comparison with the published results from the literature. The effects of parameters like loading type, stiffening scheme, lamination scheme, dynamic load factor, and boundary conditions are considered in the dynamic instability analysis of laminated composite stiffened plate. It has been shown that the type of loading and the width of loading have remarkable effect on the dynamic instability characteristics of the stiffened plate.     

Externally Reinforced Welded I-Beam-to-Box-Column Seismic Connection

This paper presents an externally reinforced I-beam-to-box-column seismic connection. An inclined rib-plated collar-plated configuration with web plates is used to ensure planar continuity between I-beam and box-column webs; the rib plates, inclined in plan between the beam web and the two column web planes, along with collar-plates encircling the box-column at beam flange levels and web plates in plane with the rib plates at the beam web level constitute the new configuration. This connection configuration relieves stresses on box-column flanges and helps in force transfer to the box-column webs. Performance evaluation of the proposed connection configuration shows that sufficient inelasticity is mobilized in the beam away from the column face with connection elements and welds remaining elastic. The seismic performance of the proposed connection is also found to be better than two state-of-the-art connection schemes in terms of higher strength, stiffness, and higher reserve strength of the welds under cyclic displacement loading.     

基于分形理论无腹筋混凝土梁的受剪性能

基于裂缝的发展及分布形态，探究无腹筋混凝土梁在不同剪跨比和纵筋配筋率作用下的剪切性能，采用剪跨比分别为1.5、2、2.5和纵筋配筋率分别为1.28%、1.62%、1.99%的9组无腹筋混凝土梁进行四点加载受剪试验，通过应用分形几何理论对试验梁表面的裂缝进行分析，使用盒计数法计算得到分级荷载及极限荷载作用下梁表面裂缝的分形维数，探讨了梁表面分形维数与极限荷载、分级荷载及跨中挠度之间的关系。结果表明:剪跨比与极限荷载及开裂荷载成反比，而纵筋配筋率与极限荷载成正比，但其对于开裂荷载的影响较小。无腹筋混凝土梁不论在分级加载作用下还是极限荷载作用下都具备明显的分形特征，在分级荷载作用下的分形维数在0.964～1.449，在极限荷载作用下的分形维数在1.33附近。分级荷载、跨中挠度与分形维数之间呈现较好的对数关系，分级荷载与分形维数的变化曲线受剪跨比及梁纵筋配筋率的影响具有一定的规律性，而跨中挠度受剪跨比的影响较小，在纵筋配筋率作用下，其曲线的曲率呈现出先增大后减小的趋势，但极限荷载与分形维数之间的关系具有一定的差异性，极限荷载会随着剪跨比的增大呈现出先增大后减小的趋势，随着纵筋配筋率的增大呈现出的差异性较大。      

Pushover Analysis of Inelastic Seismic Behavior of Greveniotikos Bridge

Nonlinear pushover analysis is a powerful tool for evaluating the inelastic seismic behavior of structures. The paper deals with the nonlinear analysis process followed during the independent design check of the Greveniotikos Bridge in Greece. The nonlinear response of the bridge was investigated from the first pier hinging to the inelastic equilibrium condition during the design-level earthquake and then up to the ultimate limit state. The effects on the seismic demand of period lengthening and damping increase produced by structural deterioration were evaluated. Onset and progression of plastic hinges were determined along with the pier stiffness distribution and the force reduction factors to be used in spectral analysis. The nonlinear loading conditions of plastic hinges were analyzed to assess their rotation capacity and shear strength. Finally, the safety factor from progressive collapse condition was evaluated. The parametric approach followed in this work permitted evaluation of the effects of several parameters on the inelastic structure response, thus enhancing confidence with result evaluation.     

Experimental Study on Moment–Plastic Rotation Capacity of Hybrid Beams

The moment–inelastic rotation behavior of hybrid steel girder bridges is experimentally investigated. Six welded girders having compact flanges and webs are statically loaded under three-point bending condition in order to simulate the interior pier section of a two-span continuous girder. Six girders, three with hybrid sections and three with homogeneous sections, are designed with three types of web slenderness ratios, resulting in three pairs of hybrid and homogeneous girders. The inelastic rotation capacities obtained from the experimental tests are then compared between hybrid and homogeneous girders. In addition, the results are compared with the prediction moment–inelastic rotation curve proposed in 1998 by White and Barth. It is concluded that, under the condition in this study, hybrid girders have more deformation capacity than homogeneous girders, and that the prediction curve is more conservative for a specimen with higher web slenderness ratio.     

Ductility Evaluation of Concrete-Encased Steel Bridge Piers Subjected to Lateral Cyclic Loading

An experimental and analytical study was conducted to investigate the ductility of concrete-encased steel piers, referred to as “steel-reinforced concrete (SRC) construction.” Based on the cyclic lateral loading tests of SRC column specimens, the restorable and ultimate limit states are defined as the point when concrete cover spalling occurs (equivalent to longitudinal bar buckling) and the point when flange buckling of the H-shaped steel occurs, respectively. To estimate the lateral displacement capacity at both the restorable and ultimate limit states, the curvature distribution of the column was calculated based on the buckling analysis of the longitudinal bar, which was restrained by a concrete cover and transverse reinforcement, and of the steel flange encased in concrete. The lateral displacement was obtained by integrating the curvature distribution. Comparison of the computed results with experimental results, including other writers’ reports, confirmed that the proposed method can appropriately estimate the lateral displacement at the restorable and ultimate limit states, and it can accurately evaluate the buckling characteristics of the longitudinal bar and steel flange components of SRC column specimens.     

Fatigue Performance of Concrete Beams Strengthened with CFRP Plates

Recent increases in bridge design loading requirements have highlighted the need for fast, efficient, and durable strengthening methods. External steel plate bonding provides a satisfactory solution, but carbon fiber reinforced plastic (CFRP) offers the added advantages of resistance to corrosion, low weight, and high mechanical strength. This paper examines the fatigue performance of CFRP-strengthened concrete beams as part of a project investigating the use of CFRP as an alternative to steel. Five reinforced concrete beams were tested in fatigue; two control beams and three strengthened with externally bonded CFRP plates. Three loading options were used: (1) apply the same loads to both plated and unplated beams, (2) apply loads to give the same stress range in the rebar in both beams, and (3) apply the same percentage of the ultimate load capacity to each beam. Fatigue fracture of the internal reinforcement steel would appear to be the dominant factor governing failure, and it would appear reasonable to expect the same fatigue life for plated and unplated beams with comparable values of stress range in the steel bar.     

抗弯刚度比对加劲板屈曲性能的影响

以一大型薄壁钢结构的加劲板为研究对象,采用有限元方法,考虑了13种不同的刚度比、多种不同的加劲肋布置方式以及边界条件等因素,分析了加劲板线性屈曲和非线性屈曲性能.抗弯刚度比对加劲板的屈曲性能影响显著,加劲板最佳抗弯刚度比将其线性屈曲模态划分为整体屈曲和局部屈曲,其值为10~20.加劲板非线性屈曲荷载随抗弯刚度比增大而提高.另外,在加载方向增加加劲肋布置可以提高加劲板局部屈曲荷载,在非加载方向增加加劲肋布置对加劲板的局部屈曲性能影响较小.      

Response of RC T-Beams Strengthened for Flexure with Staggered CFRP Plates

The results of an analytical and experimental study on the behavior of reinforced concrete T-beams retrofitted with carbon-fiber-reinforced polymer (CFRP) plates are discussed in this paper. CFRP plates were bonded to the underside of the beams with the main objective of increasing the service life load capacity. A test series comprising a prototype beam and six 5-m-long simply supported beams were tested under repeated cyclic and monotonic load conditions to failure. Particular emphasis was given to the development of the CFRP plates and to the behavior of the service and ultimate load ranges. This paper examines variables that have not previously been considered such as the use of staggered plates and the use of plates on beam with curtailed longitudinal steel reinforcement. The effect of diagonal tension cracking is also considered in this study by adapting a simple version of the modified compression field theory into the discrete element method. An important conclusion in this paper is that staggered CFRP plates can be used in lieu of full-length plates when considering flexural strengthening of beams.     

Three-Dimensional Nonlinear Finite-Element Analysis of Gasketed Flange Joint under Combined Internal Pressure and Variable Temperatures

Performance of a flange joint is characterized mainly due to its “strength” and “sealing capabilit.” A number of analytical and experimental studies have been conducted to study these characteristics only under internal pressure loading. The effect of steady state thermal loading is a well recognized problem and makes the problem more complex under combined application of internal pressure and temperature. To investigate fundamental joint characteristics, joint strength and sealing capability under combined internal pressure, and variable steady state thermal loading, a three-dimensional nonlinear finite-element analysis of gasketed flange joint is carried out using ANSYS commercial software. Reduced joint strength is concluded due to the flange rotation resulting in flange yielding under bolt up and applied operating conditions. Reduced joint sealing is concluded due to the variation in gasket seating or contact pressure due to bolt load variation. Flange rotation, bolt bending, and joint relaxation concluded fatigue and dynamic behavior in the gasketed joint. These effects are observed to be more pronounced at higher temperatures. Verification of finite-element model with available classical theories is also performed and discussed.     

Fiber Reinforced Polymer Shear Strengthening of Reinforced Concrete Beams with Transverse Steel Reinforcement

The paper aims to contribute to a better understanding and modeling of the shear behavior of reinforced-concrete (RC) beams strengthened with carbon fiber reinforced polymer (FRP) sheets. The study is based on an experimental program carried out on 11 beams with and without transverse steel reinforcement, and with different amounts of FRP shear strengthening. The test results provide some new insights into the complex failure mechanisms that characterize the ultimate shear capacity of RC members with transverse steel reinforcement and FRP sheets. After the discussion of the above topics, a new upper bound of the shear strength is introduced. It should be capable of taking into account how the cracking pattern in the web failing under shear is modified by the presence of FRP sheets, and how such a modified cracking pattern actually modifies the anchorage conditions of the sheets and their effective contribution to the ultimate shear strength of the beams.     

Performance of a Nongasketed Flange Joint under Combined Internal Pressure and Bending Loading

Performance of a bolted flange joint is characterized mainly due to its “strength” and “sealing capability.” A number of analytical and experimental studies have been conducted to study these characteristics only under internal pressure loading. A very limited amount of work is found in the literature under combined internal pressure and bending loading. Due to the ignorance of this external loading, i.e., bending loading, the optimized performance of the bolted flange joint cannot be achieved. The present design codes do not address the effects of bending loading on the structural integrity and sealing ability. To investigate joint strength and sealing capability under combined loading, an extensive comparative experimental and numerical study of a nongasketed flange joint is carried out and overall joint performance and behavior is discussed. Actual joint load capacity is determined under both the design and proof test pressure with maximum additional external bending loading that can be applied for safe joint performance. In addition, as experimentally it is impossible to test different flange joint sizes under combined loading application, hence finite element model developed and verified with the experimental results, presented in this paper can be used as base to study the behavior for different nongasketed flange joint sizes for different classes under combined pressure and bending loading.     

Damage Assessment and Ductility Evaluation of Post Tensioned Beams with Hybrid FRP Tendons

The study presented in this article concentrated on investigating the ductility and characterization of damage in concrete beams post tensioned with hybrid carbon-glass fiber-reinforced polymer (HFRP) composites. The investigation included an approach for design of flexural members with HFRP tendons and characterization of damage, load deformation response, ultimate strength, and failure modes. Direct tensile tests of hybrid FRP rods in a previous study had indicated elastoplastic response, enhanced ductility, and increased strain capacity. In this context, the current study focused on design and fabrication of post tensioned beams using glass or steel rebars for partial prestressing. All the beams were tested in flexure under four-point bending configuration. Results of the study are presented in terms of ductility index and enhanced load-deflection response in comparison with the conventional FRP materials. Damage characterization involved evaluating the specific features of the acoustic emissions for detecting the elastoplastic transition in the hybrid tendons. The method involved use of a high-resolution fiber-optic interferometer for detection and separation of acoustic emissions. By using the time domain response, it was possible to spatially localize the damage at various stages of the loading. Spectral energy of the acoustic emissions facilitated separation of carbon and glass fiber fractures.     

Seismic Behavior of Hollow Stiffened Steel Bridge Columns

Rectangular columns constructed from steel plates are widely used to support highway bridges in Japan. Columns of this type, designed without special consideration for ductility, sustained damage during the 1995 Hyogo-ken Nanbu earthquake. This paper describes tests of 24 large-scale models of hollow and concrete-filled stiffened rectangular columns in order to investigate their seismic performance. Testing under constant axial loading and cyclic bending as well as on the shaking table was carried out. It was found that columns partially filled with concrete had a larger strength than did hollow columns, but their displacement capacity was sometimes smaller. Bridge column models tested on the shaking table tended to sustain increasing displacements in only one direction, and columns tested by reverse cyclic loading possessed member displacement ductility capacities between 2.6 and 6.1, even though the columns had not been designed specifically for ductility. A rational and simple empirical method for estimating the deformation capacity of hollow columns subjected to reverse cyclic loading that considers the different modes of buckling is proposed and a design example is provided.