Editor’s Note

The September 2010 issue of the ASCE Journal of Bridge Engineering has 13 technical papers, 1 case study, and 2 technical notes. The issue begins with three papers on the seismic behavior of bridges and their components. Companion papers titled “Seismic Fragility of Multispan Simply Supported Steel Highway Bridges in New York State. I: Bridge Modeling, Parametric Analysis, and Retrofit Design” and “Seismic Fragility of Multispan Simply Supported Steel Highway Bridges in New York State. II: Fragility Analysis, Fragility Curves, and Fragility Surfaces” by Pan et al. study dynamic seismic behavior and fragility of a typical highway bridge in New York State through nonlinear three-dimensional (3D) finite-element (FE) models of the bridge. These papers also investigate two seismic retrofit strategies for multispan simply supported steel bridges: (1) steel bearing replacement by elastomeric bearings and (2) deck/girder splicing (continuity) of simply supported spans including the replacement of steel bearings by elastomeric bearings. The detailed finite-element analysis shows that the second retrofit option is the most effective. The results of the seismic fragility analysis show that typical multispan simply supported steel bridges in New York State have more than 50% probability of exhibiting slight damage when subjected to earthquakes with peak ground accelerations (PGAs) of $0.51\mathrm{g}$ . A 50% probability of incurring moderate damage is observed for earthquakes with $\mathrm{PGA}=0.63\mathrm{g}$ , and 50% probabilities of extensive damage and collapse are obtained for earthquakes with PGAs equal to $1.02\mathrm{g}$ and $1.50\mathrm{g}$ , respectively. The detailed fragility analysis of the as-built bridge shows that the fixed steel bearings in the bridge are the most vulnerable components. For the bridge retrofitted by considering (1) steel bearing replacement by elastomeric bearings and (2) deck/girder splicing (continuity) of simply supported spans including the replacement of steel bearings by elastomeric bearings, it is observed that although both retrofit strategies reduce the fragility of bridge piers drastically as compared to the as-built condition, the second retrofit strategy (i.e., the combination of steel bearing replacement and superstructure continuity) is overall more effective in reducing the seismic fragility of both piers and bearings. The manuscript titled “Investigation on the Impact of Seasonally Frozen Soil on Seismic Response of Bridge Columns” by Wotherspoon et al. investigates the seismic response of a simple two-span prototype bridge system during warm and frozen temperatures. Models from both temperature conditions were subjected to a range of seismic intensities to examine the effect of seasonal freezing on the response of the system. Stiffness characteristics were defined using cyclic models of a bridge pier that were previously developed and validated using results from an experimental program on identical full-scale column-foundation units tested during the summer and winter months. Frozen conditions increased the maximum bending moment and shear force demands for all seismic intensities, with nonlinearity in the column/foundation reducing the difference between the peak responses at higher intensities. At the depth of maximum foundation shear for the frozen model, demand was three times higher than for the unfrozen for the $500\text{-year}$ return period and twice during the $\mathrm{2,500}\text{-year}$ event. This is significant as one will assume shear is not critical at this location if the effects of frozen conditions are ignored. Apart from the smallest intensity event, increased peak lateral displacements were developed by the warm model down the length of the column and foundation. However, the displacement demand to capacity ratio was higher at the column top for the frozen model, exceeding the capacity during the $\mathrm{2,500}\text{-year}$ return period event.

The next four papers in this issue focus on concrete decks and girders. “Relaxing the Stud Spacing Limit for Full-Depth Precast Concrete Deck Panels Supported on Steel Girders (Phase I)” by Badie et al. investigates the issue of relaxing AASHTO Load Resistance and Factor (LRFD) Bridge Design Specifications requirement that the spacing between the shear connectors for steel girders should not exceed $610\mathrm{mm}$ $\left(24\mathrm{in.}\right)$ . The authors investigate the possibility of extending this limit to $\mathrm{1,220}\mathrm{mm}$ $\left(48\mathrm{in.}\right)$ for stud clusters used with full-depth precast concrete deck panels installed on steel girders based on push-off specimens. They also recommend testing of full-scale composite beams to investigate the ultimate capacity of studs grouped in clusters spaced at $\mathrm{1,220}\mathrm{mm}$ $\left(48\mathrm{in.}\right)$ . In “Interface Shear Connection Analysis of Ultrahigh Performance Fiber-Reinforced Concrete Composite Girders,” authors Wu and Han analyze the interface shear connection behavior for ultrahigh performance fiber-reinforced concrete (UHPFRC) and normal concrete (NC) composite girders. The shape and dimension of the shear stud in the conducted tests are referenced from the traditional interface connection design and engineering experiences. The interface shear connection parameters, i.e., initial stiffness and slippage capacity of a single shear stud, are measured from three groups of lateral direct push test specimens with different numbers of studs. Based on their research, the authors recommend that the interface shear connection degree may be used as a minimum design standard for UHPFRC-NC composite interface shear connection design. The paper titled “Experimental Performance of Full-Depth Precast, Prestressed Concrete Overhang, Bridge Deck Panels” by Mander et al. presents an experimental investigation of the performance of a new full-depth precast overhang panel system for concrete bridge decks. In contrast to conventional cast-in-place deck overhangs, the proposed full-depth precast overhang system has the potential to speed up construction, reduce costs, and improve safety. When compared to the conventional cast-in-place overhang behavior, the experimental results show that the precast full-depth overhang introduces different behavior modes, largely owing to the influence of the partial depth panel-to-panel connection. This reduces the capacity by 13 percent, yet the failure load is still within AASHTO load requirements. In “Improved Longitudinal Joint Details in Decked Bulb Tees for Accelerated Bridge Construction: Fatigue Evaluation,” the authors Li et al. investigate improved continuous longitudinal joint details for decked precast prestressed concrete girder bridge systems. Precast concrete girders with an integral deck, which are cast and prestressed with the girder, provide the benefits of rapid construction, improved structural performance, and durability. Despite these advantages, use of this type of construction has been limited to isolated regions of the United States because of a perceived problem with durability of longitudinal joints used to connect adjacent girders. Based on experimental and finite-element results, the authors show that an improved longitudinal joint detail is a viable connection system that transfers the forces between the adjacent decked bulb tee (DBT) girders.

The remaining five technical papers in this issue focus on various areas of bridge engineering. “Equivalent Barge and Flotilla Impact Forces on Bridge Piers” by Yuan and Harik proposes a hand-calculation method for determining barge or flotilla impact forces on bridge piers by incorporating pier geometry, interaction between barges, and impact duration. The proposed method is derived on the basis of finite-element dynamic simulations of Jumbo Hopper (JH) barges and flotillas made up of JH barges impacting bridge piers. “Design and Construction of Modern Bamboo Bridges” by Xiao et al. presents the design, construction, and testing of modern bamboo bridges. The authors demonstrate that the laminated bamboo girders have satisfactory stiffness and load-carrying capacity. The use of carbon fiber-reinforced plastics (CFRP) can further enhance the stiffness and capacity of the bamboo girders. Based on the test results and analysis, a $10\mathrm{m}$ long single-lane roadway bridge was designed and constructed. Field tests using an overloaded two-axle truck with a total weight of $8.6\mathrm{t}$ , which exceeded the design truckload of $8.0\mathrm{t}$ , shows that the midspan deflection corresponding to the critical service loading condition is much smaller than the limiting displacement prescribed by the code. “Identifying Critical Sources of Bridge Deterioration in Cold Regions through the Constructed Bridges in North Dakota” by Kim and Yoon investigates the performance of constructed bridges in cold regions through a unique approach of a combined multiple regression and geographic information system (GIS) technology. The study identifies critical sources affecting deterioration of 5,289 bridges sampled from the National Bridge Inventory database inspected between 2006 and 2007. Typical parameters examined include the physical, material, and environmental factors associated with the existing bridges. It is observed that traffic volume significantly influences the level of deterioration of the bridge decks. The year built is the most significant contribution to the structural deficiency of the bridges, followed by the structural characteristics and traffic volumes. The presence of water particularly influences the deterioration. Concrete bridges are more durable than steel bridges. Truss systems may not be recommended for cold regions. “Bridge Model Updating Using Response Surface Method and Genetic Algorithm” by Deng and Cai proposes a new practical and user-friendly FE model updating method. The proposed approach utilizes response surface method (RSM) for the best experimental design of parameters to be updated, based on which numerical analysis can be performed to obtain explicit relationships between the structural responses and parameters from the simulation results. The parameters are then updated using the genetic algorithm (GA) by minimizing an objective function. This method has also been applied to model updating of an existing bridge. Results show that this method works well and achieves reasonable physical explanations for the updated parameters. “Charles Macdonald” by Griggs highlights the contributions of Charles Macdonald, who was one of the leading bridge engineers from 1867 to 1912. His bridges crossed major rivers of the United States and Australia. He was chief engineer for several major bridge companies and finished his career as a member of the board appointed to select the replacement design for the Quebec Bridge, which had collapsed under construction in 1907. “Rapid Ranking Procedures for Gusset Plate Connections in Existing Steel Truss Bridges” by Higgins et al. presents a screening process and a simplified rapid screening process for ranking gusset plate connections in steel truss bridges to help bridge engineers identify possible vulnerable connections and to aid field inspections. The procedures consider member demands relative to the connection geometric proportions for four different parameters: fasteners, plate tension, plate compression, and overall horizontal shear. The methods are demonstrated for two bridges, including the collapsed I-35W Bridge, and clearly identify connections U10 and L11 as vulnerable for three of the four parameter types (fasteners were not identified as vulnerable for these connections). The ranking approach is not proposed as a substitute for thorough, detailed, and expert assessment of the connections but rather allows rating engineers to more quickly prioritize detailed evaluations in an ordered systematic way from the most likely vulnerable connections to the least likely vulnerable connections.

A case study titled “In-Service Diagnostics of a Highway Bridge from a Progressive Damage Case Study” by Whelan and Janoyan documents a field test of an end-of-service bridge span with prescribed, progressive damage to a bearing as well as several diaphragm connections. Thirty dual-axis accelerometers were distributed across the bridge span with data acquisition and transmission facilitated by a real-time lossless wireless sensor network. A highway department service truck applied traffic excitation to the structure through routine passes on a consistent lane of traffic. Output-only system identification was applied to the baseline time history response to develop a state-space model of the bridge dynamics used for forward prediction in the form of a Kalman filter. Simple statistical evaluation of the prediction error in the model demonstrates that variance can be used to localize and generally quantify the degree of damage in the structure. The case study additionally illustrates the potential importance of monitoring lateral acceleration along the girders to permit identification of damage to elements, such as the diaphragms, that contribute primarily to the lateral and torsional response of primary structural members.

There are two technical notes in this issue. “I-35W Bridge Collapse” by Hao presents an interesting study on the I-35W Bridge collapse on August 1, 2007, based on original design drawings, investigation of material evidence provided by the National Transportation Safety Board (NTSB), and a full-scale computational load rating of the bridge superstructure. This study found that the thickness of gusset and the thickness of the side wall of the upper chords were designed proportional to the bending moment solution of a one-dimensional influence line analysis. This fact reveals that the NTSB-disclosed undersized gusset plates are the consequence of a bias toward a “one-dimensional model” in the original design, which did not give sufficient consideration to the effects of the forces from diagonal truss members. Although the bridge’s truss-cell structure was appropriately designed, the design of the node that connected the floor members to the main truss-frame was inadequate to effectively distribute live and dead loads. Consequently, the local redundancy provided by the truss-cells was significantly reduced. A 3D, nonlinear, finite-element, computation-based load rating indicates that some of the gusset plates had almost reached their yield limit when the bridge experienced the design load condition. The bridge was sustained by the additional safety margin provided by the ultimate strength of the ductile steel that composed the gusset plates. “Shear Strength of a Lightweight Self-Consolidating Concrete Bridge Girder” by Dymond et al. investigates the use of lightweight self-consolidating concrete (LWSCC) for bridge components because of its significantly lower self-weight and low viscosity in its fresh state, which eliminates the need for vibration during fabrication. Their experimental investigation on a composite section fabricated with a single precast bulb tee LWSCC simply supported beam and a lightweight concrete cast-in-place deck with two point loads to quantify the web-shear strength of the girder shows that the theoretical predictions for the web-shear strength were all conservative when compared to the experimentally measured failure strength. They have recommended further research on the use of LWSCC girders in the bridge industry to better understand the material properties, structural properties, and cost advantages.