Papers in This Issue

The May 2012 issue of the Journal features 16 technical papers. The first six papers focus on fatigue and fracture issues in bridges. The paper “New Method for Concurrent Dynamic Analysis and Fatigue Damage Prognosis of Bridges” by He et al. proposes a new methodology for concurrent dynamic analysis and structural fatigue prognosis. The proposed methodology is based on a novel small time scale formulation of material fatigue-crack growth, which calculates the incremental crack growth at any arbitrary time within a loading cycle. In this approach, the fatigue-crack kinetics is defined on the basis of the geometric relationship between the crack-tip opening displacement and the instantaneous crack growth rate. The proposed crack-growth model can be expressed as a set of first-order differential equations. The structural dynamics analysis and fatigue-crack growth model can be expressed as a coupled hierarchical state-space model. To solve both the dynamic response (structural level) and the fatigue-crack growth (material level) simultaneously, several numerical problems with single-degree-of-freedom and multiple-degree-of-freedom cases are used to show the proposed methodology. Model predictions are validated using coupon testing data from open literature. Next, the methodology is demonstrated using a steel girder bridge. The proposed methodology shows how the concurrent structural dynamics and material fatigue-crack growth analysis, instead of the cycle-counting method in the conventional fatigue analysis, can be achieved. Comparisons with experimental data for structural steels and aluminum alloy show a satisfactory accuracy using the proposed coupled state-space model. The paper “Fatigue Testing and Analysis of Aluminum Welds under In-Service Highway Bridge Loading Conditions,” by Coughlin and Walbridge examines the fatigue behavior of aluminum welds under in-service highway bridge loading conditions. Specifically, calculations performed to establish damage equivalence factors for aluminum for use with the AASHTO and CAN/CSA-S6 codes are first reviewed. Next, small-scale fatigue tests of aluminum welds under simulated highway bridge loading conditions are described. A fracture mechanics model is then validated by comparison with the test results and is used to perform simulations encompassing a wider range of loading conditions. On the basis of this work, the adequacy of the current design provisions is discussed and possibilities for further extending the employed methodology are identified. The paper “Use of CFRP Overlays to Strengthen Welded Connections under Fatigue Loading” by Alemdar et al. evaluates the performance of various methods for preventing and repairing fatigue damage in welded connections. Experimental tests and analytical simulations were carried out to investigate the fatigue performance of cover plate specimens in which the welded connections were reinforced with carbon fiber reinforced polymer (CFRP) overlays. Specimens were loaded in three-point bending induced by a cyclic load to evaluate the change in fatigue-crack initiation life of the welded connections caused by the attachment of the CFRP overlays. Test results showed that when the bond between the CFRP overlays and the steel was maintained, the reduction in stress demand was sufficient to extend the fatigue life of the welded connections from AASHTO fatigue category E in the unreinforced configuration to the infinite fatigue life range. Test results also showed that the fatigue strength of the bond layer was drastically improved by introducing breather cloth material within the bond layer. The paper “Advanced Numerical Modeling of Cracked Tubular K-Joints: BEM and FEM Comparison” by Borges et al. investigates a critical aspect in the design of tubular bridges: the fatigue performance of the structural joints. The estimation of a fatigue-crack life using the linear elastic fracture mechanics (LEFM) theory involves the calculation of stress intensity factors (SIF) at a number of discrete crack depths. The most direct way is to carry out modeling by either the finite-element method (FEM) or the boundary-element method (BEM). For tubular joints commonly found in tubular bridges and off-shore structures, because of the complicated geometry resulting from the tubes’ intersection and welding, the construction of the numerical model often becomes a complex process. This paper presents two different model-construction techniques that have been developed independently at the Swiss Federal Institute of Technology (EPFL) and the Nanyang Technological University (NTU), Singapore, which are based on the BEM and the FEM, respectively. The SIF values obtained by these two methods are compared. It is found that so long as consistent geometrical models are employed, compatible SIF values could be obtained by both approaches. The best and the most consistent values are obtained for the deepest point along the crack front. These values should be used for fatigue-life computations. In the paper “Fatigue Reliability Assessment for Existing Bridges Considering Vehicle Speed and Road Surface Conditions,” Zhang and Cai present a framework of fatigue reliability assessment for existing bridges in lifetime serviceability considering the random effects of vehicle speed and road roughness conditions. Because 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 the equivalent stress ranges per truck passage) are coalesced in the new defined variable on the basis of equivalent fatigue damage. The revised equivalent stress range is obtained through a fully computerized approach for solving a coupled vehicle-bridge system including a three-dimensional (3D) suspension vehicle model and a 3D dynamic bridge model. In each truck-pass-bridge analysis, deteriorations of the road roughness condition are considered and the vehicle speed and road surface profiles are generated randomly. When the stress range threshold is 3.45 MPa (0.5 ksi) or lower, lognormal distribution is proven to be a good model to describe revised equivalent stress range. In addition to the assumptions of other input random variables being normal or lognormal, the 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. In the paper “Fatigue Performance of High-Strength Reinforcing Steel,” Soltani et al. perform an evaluation of concrete reinforcing bar fatigue provisions of AASHTO LRFD Bridge Design Specifications as they pertain to the use of high-strength reinforcing steel. Two large-scale proof tests and a review of available published data demonstrate that presently accepted values for the fatigue or endurance limit for reinforcing steel are applicable and likely conservative when applied to higher strength bars. A minor revision to the AASHTO specification is proposed to eliminate the fatigue penalty resulting from the use of higher strength reinforcing steel. Fatigue considerations are also shown to rarely affect the design of typical reinforced concrete members having specified yield strength, $fy\le 690\mathrm{MPa}$.

Next, three papers are related to uncertainties and reliability issues in bridge engineering. The paper “Statistical Distribution of Bridge Resistance Using Updated Material Parameters” by Orton et al. investigates statistical distribution of resistance (load-carrying capacities) of bridge girders on the basis of the latest material properties available in the literature through Monte Carlo simulation. The results of the analysis show an increase in bias factor and decrease in coefficient of variation (COV) for all types of bridges in comparison with those used in previous calibration studies. The changes in bias factor and COV are the result of higher bias and lower COV in material properties owing to better quality control in concrete and steel manufacturing. The most significant change in resistance distribution has been observed in the case of steel and concrete bridges. Little change was observed in the case of prestressed bridges because the material properties of prestressing steel, which is the most sensitive parameter in the prestressed bridges, did not change significantly following the previous calibration study. With these resistance distributions, the calibration of the load factor in the AASHTO Specification is expected to lead to a lower live load factor, thereby possibly reducing the material costs. In addition, the ratio of actual to required (design) resistances of representative bridges in Missouri was determined. The analysis showed that almost all representative bridges had a capacity-to-demand ratio greater than 1 according to current AASHTO standards. The paper “Simplified Method for Evaluating the Redundancy of Twin Steel Box-Girder Bridges” by Samaras et al. investigates redundancy issues in damaged fracture-critical bridges (FCBs). Multiple cases of FCBs have experienced a failure in one of their fracture-critical elements without collapsing, which suggests that the current AASHTO provisions may not accurately account for the inherent redundancy that exists in various FCB structural systems. To improve the understanding of how twin steel box-girder bridges behave after suffering a full-depth fracture in one of their girders, simplified analytical methods have been developed and are presented in this paper. The proposed methodology has been validated against data from full-scale tests and provides a convenient means for predicting response. The paper “Structural Redundancy Evaluation of Steel Tub Girder Bridges” by Hunley and Harik presents results from an analytical investigation of the redundancy of twin steel tub girder bridges. Parametric nonlinear finite-element analyses are used to determine the role of different bridge components in developing load transfer from a damaged girder to an undamaged girder. The key parameters of span length, bridge continuity, curvature, location of girder damage, and type and spacing of external bracing are investigated. The results of this study indicate that twin steel tub girder bridges can be classified as redundant if the bridge is designed in accordance with the AASHTO LRFD Design Code and additional design and proportioning criteria are incorporated. Minimum design criteria are proposed to allow for a redundant classification, thus reducing fabrication and maintenance/inspection costs for this increasingly popular bridge type.

The next three papers in this issue are related to dynamic loads on bridges. The paper “Procedure for Predicting Blast Loads Acting on Bridge Columns” by Williams and Williamson presents research that advances the understanding of blast loads acting on bridge columns. Unlike large wall panels for which much of the existing knowledge about blast effects against structures has been established, the research presented in this paper focuses on slender structural components in which the effects of cross-sectional geometry, engulfment of blast pressures, and clearing effects strongly influence loading history. On the basis of the findings obtained from this study, a simplified procedure for predicting blast loads acting against bridge columns is proposed. The paper “Flutter, Galloping, and Vortex-Induced Vibrations of H-Section Hangers” by Chen et al. presents an in-depth investigation on the hangers’ aerodynamic performances in forms of flutter, galloping, and vortex shedding through a series of wind tunnel tests. First, using a sectional model and aeroelastic model test of the longest hanger in the bridge, observed field vibration is confirmed to be torsional flutter under large attack angles from 15 to 25°. The experimental onset velocity coincides well with the field observation. The flutter derivative $A{2}^{*}$ becomes positive at a low reduced wind velocity, which further implies that the H-section is prone to flutter instability. The web perforation may also increase the galloping critical velocity to some extent but has no obvious effects on flutter instability and the Strouhal numbers, at least for the shallow H-section with $D/B=0.416$. A total of 16 H-section models with different $D/B$, web perforation ratio, and flange perforation ratio have been tested to investigate their effects on the aerodynamic behaviors of hangers. In the paper “Treatment of $P\mathrm{\text{-}}\Delta$ Effects in Displacement-Based Seismic Design for SDOF Systems,” Wei et al. present an investigation on $P\mathrm{\text{-}}\Delta$ effects during displacement-based seismic design methods. To achieve a practical and general-purpose solution considering the $P\mathrm{\text{-}}\Delta$ effects in various DSD methods for SDOF systems, the existing approaches of including $P\mathrm{\text{-}}\Delta$ effects in current seismic analysis and design have been evaluated by carrying out a large set of nonlinear time-history analyses. On the basis of statistical data developed through this analysis, new design formulas and recommendations on the threshold of neglecting $P\mathrm{\text{-}}\Delta$ effects and allowable design thresholds have been developed.

The last four papers pertain to different areas of bridge engineering. The paper “Moment and Shear Load Distribution Factors for Multigirder Bridges Subjected to Overloads” by Bae and Oliva presents modified moment and shear load distribution factor equations for vehicles to quickly determine their effects on multigirder bridges. Finite-element analyses of 118 multigirder bridges and 16 load cases of overload vehicles for each multigirder bridge have been performed, and the load distribution factor equations for the multigirder bridges have been proposed on the basis of the results of the analyses. Various configurations of the vehicles, number of bridge spans, skew angles of the bridge, and diaphragms have been considered in developing the equations. The developed equations have been found to be capable of replacing a time-consuming 3D finite-element analysis rationally and conservatively. The paper “Numerical Simulation of Partial-Depth Precast Concrete Bridge Deck Spalling” by You et al. presents results of numerical simulations performed to investigate the spalling mechanism observed in several partial-depth precast prestressed concrete (PPC) bridge decks. Corrosion-induced cracking of prestressed steel reinforcement and panel butting were modeled using two-dimensional (2D) finite-element analysis to examine the nature of crack propagation that triggers the observed spalling effect. A parametric study was carried out on the basis of field observations from several bridges. FEM results showed that spalling is sensitive to side and bottom cover, and spacing of reinforcement resulting from to bridging cracks. Findings indicate that the spalling mechanism is triggered by the presence of a critical bridging crack. In the paper “Effect of Temporary Shoring Location on Horizontally Curved Steel I-Girder Bridges during Construction,” Sharafbayani and Linzell examine the effects of shoring tower positioning on curved bridge behavior at different stages of construction. Sequential analyses of multiple idealized double-span curved bridges with varying radii were conducted using nonlinear finite-element models; and vertical deformations, rotations of the girders, and shoring tower reactions were compared for different shoring support locations and different erection sequences. On the basis of the results, optimal shoring locations were obtained for the curved girders at different construction stages. The paper “Investigation of Extreme Environmental Conditions and Design Thermal Gradients during Construction for Prestressed Concrete Bridge Girders” by Lee has carried out experimental and analytical studies to determine the transverse temperature gradient that is needed to predict the lateral thermal behavior of the girders, especially during construction before the placement of the bridge decks. Experimental tests were conducted on a prestressed BT-63 concrete girder segment. The analytical results were found to be in good agreement with the experimental measurements. The analytical model was then used to determine the seasonal temperature gradients in four standard PCI girder sections at selected cities in the United States. On the basis of these findings, vertical and transverse temperature gradients were developed to aid engineers in predicting the thermal behavior of prestressed girders during construction.