Editor’s Note

The November 2007 issue of the Journal of Bridge Engineering begins with two papers on the subject of pedestrian induced lateral vibrations of bridges. In “Autoparametric Resonance in a Pedestrian Steel Arch Bridge: Solferino Bridge, Paris,” Blekherman presents the use of the perturbation method of multiple time scales to construct first order nonlinear differential equations to determine the steady state solutions along with their stability. Critical load values are determined through bifurcation analysis. The autoparametric resonance is possible if an excited torsional mode is near a primary resonance and an external load parameter caused by the pedestrians is greater than or equal to this value. The paper explains how to determine the autoparametric resonance.

The second paper on this subject is by Ricciardelli and Pizzimenti titled, “Lateral Walking-Induced Forces on Footbridges.” This paper presents the results of an experimental investigation of the lateral forces exerted by one walker on a fixed floor. The results are then used to calibrate deterministic and stochastic lateral loading models. The authors intend for this work to provide the background for more sophisticated dynamic models allowing for bridge-crowd interaction.

The next two papers deal with seismic issues. Pan, Agrawal, and Ghosn in “Seismic Fragility of Continuous Steel Highway Bridges in New York State” present the results of an analytical seismic fragility analysis of a typical steel highway bridge in New York state. The uncertainties associated with the estimation of material strength, bridge mass, friction coefficient of expansion bearings, expansion gap size, bridge structural properties, earthquake characteristics, and capacity versus demand were included in their work. The results indicated that the bridges have a low likelihood of collapse during expected earthquakes.

“Comparison between Shape Memory Alloy Seismic Restrainers and Other Bridge Retrofit Devices” is the second seismic paper. Andrawes and DesRoches present their findings on a sensitivity study and a case study that used computer simulations to compare the effectiveness of shape memory alloys versus other seismic retrofit devices. Their results revealed that the effectiveness of the devices is a function of the characteristics of the bridge frames and the ground motion characteristics.

The fifth paper in this issue of the Journal is “Implications of Design Assumptions on Capacity Estimates and Demand Predictions of Multispan Curved Bridges” by Mwafy, Elnashai, and Yen. The paper presents detailed seismic performance assessment of a complex bridge designed as a reference application of modern design codes for the Federal Highway Administration (FHWA). The impact of the design assumptions on the capacity estimates and demand predictions of a multispan curved bridge was investigated. The work looked at the “as-designed” and “as-built” bridge and found that a comparison of seismic demand to available capacity for seemingly conservative design assumptions may lead to erroneous and potentially nonconservative response expectations.

The next two papers also pertain to horizontally curved bridges. Kim, Laman, and Linzell in “Live Load Radial Moment Distribution for Horizontally Curved Bridges” present their work determining the effect of major parameters on maximum total bending moments of curved girders, establishing the relationship between key parameters and girder distribution factors, and developing new approximate distribution factor equations. Their results demonstrated that the most significant parameters affecting girder distribution factors are radius, span length, cross frame spacing, and girder spacing. The authors conclude with girder distribution factors that provide more refined live load analysis for preliminary design.

Zhao and Roddis in “Fatigue Behavior and Retrofit Investigation of Distortion-Induced Web Gap Cracking” studied a welded plate girder bridge that developed fatigue cracks at small web gaps close to the girder top flange. A previous repair had used a slot retrofit to soften the connection plate end, but cracks were found to have reinitiated at some of the repairs and were again propagating. Using a comprehensive finite element analysis, the results found that the stresses developed at the top flange web gaps could exceed yielding under the fatigue truck loading and that the slot repair induced higher magnitude fatigue stresses in the web gap. For a permanent repair, it was recommended that a welded connection-plate-to-flange attachment be used for future bridge retrofits.

“Challenges in the Application of Stochastic Modal Identification Methods to a Cable-Stayed Bridge” is the first of two articles on cable-stayed bridges. Magalhães, Caetano, and Cunha present the analysis of data collected in the ambient vibration test of the International Guadiana cable-stayed bridge between Portugal and Spain using different output-only identification techniques. The purpose of the investigation was to compare the performance of the various techniques and evaluate their efficiency in modal identification, specifically the existence of closely spaced modes, the perturbation produced by local vibration of stay-cables and the variation of modal damping coefficients with wind velocity. The identified natural frequencies and mode shapes were compared with the corresponding modal parameters provided by a numerical model. In addition, the variability of some modal damping coefficients were related with the variation of wind characteristics and associated with a component of aerodynamic damping.

The second cable-stayed bridge paper is by Chen and Cai titled, “Equivalent Wheel Load Approach for Slender Cable-Stayed Bridge Fatigue Assessment under Traffic and Wind: Feasibility Study.” Little research on the fatigue performance of long-span bridges subjected to both wind and traffic has been reported. The question of whether fatigue could become a dominant issue for long-span bridges is still under discussion. The authors suggest that the current AASHTO LRFD specifications may be adequate for short and medium span bridges, but for long span bridges carrying many heavy trucks simultaneously, they may not be sufficient. This paper describes their work investigating the traffic effects on bridge performance including fatigue, as the “one truck per bridge” normally used for fatigue design does not represent actual traffic conditions.

“AASHTO-LRFD Live Load Distribution for Beam-and-Slab Bridges: Limitations and Applicability” by Yousif and Hindi, presents a comparison between the live load distribution factors of simple span slab-on-girder concrete bridges based on AASHTO-LRFD and finite element analysis. A total of 886 bridge superstructure models were analyzed and the finite element analysis results found that the AASHTO-LRFD code may significantly overestimate the live load distribution factors.

Vigh and Kollár in “Routing and Permitting Techniques of Overweight Vehicles” present a new, faster, and robust method for verification of bridges subjected to overweight permit vehicles. Deck-girder bridges and for a box girder are used to illustrate numerical examples.

The twelfth paper in this issue is by LeBeau and Wadia-Fascetti and is titled “Comparative Probabilistic Initial Bridge Load Rating Model.” They offer a model that applies to initial bridge load ratings based on allowable stress, load factor, and LRFD. Bridge load ratings are calculated probabilistically considering random variables representative of load and resistance effects. The paper concludes by applying the model to a beam in a prestressed concrete bridge at the beginning of its service life.

“Structural Behavior of Short Span Precast Channel Beam Bridges without Shear Reinforcement” by Heymsfield, Durham, and Jones is the last technical in this issue. It presents the findings of a study that investigated precast non-prestressed concrete channel beam sections without shear reinforcement. While the study’s original purpose was to establish a correlation for inspection purposes between the beam’s visual deteriorated state and its structural capacity, the authors found during testing that the majority of the beams exhibited load capacities greater than the initially required design strength. It was determined that the member strength was a function of the concrete compressive strength.

There are two technical notes in this issue. The first by Roy and Thiagarajan titled “Nonlinear Finite-Element Analysis of Reinforced Concrete Bridge Approach Slab” numerically studied the extent and location of cracks in approach slabs using nonlinear finite-element analysis. The authors found that the use of a thicker approach slab was a more viable solution than improving the soil strength under a thinner one.

The second technical note “Application of Orthotropic Thin Plate Theory to Filled Steel Grid Decks for Bridges” is by Huang, Kaliakin, Chajes, Mertz, and Shenton. This paper reviews orthotropic thin plate theory, discusses the determination of the flexural rigidities using Huber’s assumption, and applies the theory to concrete-filled grid decks. The accuracy of the orthotropic plate analysis is assessed by comparing it to results from an earlier finite-element analysis.

There are three discussions in this issue. The first two are on the paper “Consistent Approach to Calculating Stresses for Fatigue Design of Welded Rib-to-Web Connections in Steel Orthotropic Bridge Decks.” The first discussion by Wolchuk, notes that many decks begin service already cracked or highly stressed before carrying the first live, as a result of fabrication and welding factors that affect fatigue resistance.

The second discussion on this paper is by Camo and he notes that the authors discussed three analytical approaches for the evaluation of details. Comments and the application of these approaches are presented.

The last discussion on “Damage Localization and Finite-Element Model Updating Using Multiresponse NDT Data,” by Q. Qin raises some questions regarding the paper.

Finally, a closure for the paper “Damage Localization and Finite-Element Model Updating Using Multiresponse NDT Data” by W. Qin presents responses to Q. Qin’s questions.