Editor’s Note

This March 2011 issue of the Journal features 15 technical papers. The issue begins with two papers on various aspects of timber bridges. In the manuscript “Development of Live-Load Distribution Factors for Glued-Laminated Timber Girder Bridges” by Fanous et al., the authors present simple relationships for calculating live-load distribution factors for glued-laminated timber girder bridges with glued-laminated timber deck panels. A comparison between live-load distribution factors obtained from field tests and analytical models in ANSYS with those obtained using the American Association of State Highway and Transportation Officials Load and Resistance Factor Design (AASHTO-LRFD) live-load distribution relations shows that the live-load distribution factors obtained using the AASHTO-LRFD are conservative. The authors propose accurate relationships based on statistical analysis that can be utilized to calculate the live-load distribution factors in the design of glued-laminated girder bridges. In the manuscript “Slip between Glulam Beams in Stress-Laminated Timber Bridges: Finite Element Model and Full-Scale Destructive Test,” Ekevad et al. investigate horizontal shear stresses in stress-laminated timber bridge decks consisting of several sawn timber beams or glulam beams held together with prestressed steel bars. A full-scale test and corresponding finite-element simulations for a specific load case by the authors shows the occurrence of horizontal slip between beams. The finite-element model handled both vertical and horizontal frictional slip using an elastic-plastic material model. The results show that the finite-element model gives reliable results and that slip in general leads to permanent deformations that may increase with load cycling. Horizontal slip between beams over a large area of the bridge deck begins at a low load, resulting in a redistribution of load between beams and does not load to immediate failure. Vertical slip between beams starts at a high load close to the load application point and leads to failure.

The next three papers in this issue relate to steel bridges. The paper entitled “Ultimate Capacity Destructive Testing and Finite-Element Analysis of Steel I-Girder Bridges” by Bechtel et al. investigates system-level design and rating of steel I-girder bridges through testing of a $1/5$-scale slab-on-steel girder bridge to ultimate capacity. The authors have also developed an analytical model of the bridge. The test results demonstrate a significant reserve capacity of the steel girders. The response of the specimen is governed by the degradation of the reinforced concrete deck. In order to accurately capture the response of the specimen in an analytical model, the degradation of the deck and other key features of the specimen have been modeled using a dynamic analysis algorithm in the commercially available finite-element program ABAQUS. In “Skewed Slab-on-Girder Steel Bridge Superstructures with Bidirectional-Ductile End Diaphragms,” authors Celik and Bruneau investigate the implementation of ductile diaphragms in skewed bridges. In particular, they propose and numerically investigate two bidirectional end diaphragm configurations (namely, EDS-1 and EDS-2) with buckling restrained braces (BRBs). Bidirectional end diaphragm is a new concept and can be implemented both in straight and skewed steel bridge superstructures to resist bidirectional earthquake effects. To assess the relative effectiveness of the proposed systems and to investigate how various parameters relate to seismic response, closed-form solutions are developed using nondimensional bridge geometric ratios. Numerical results indicate that skewness more severely impacts on-end diaphragm behavior when $\phi \ge 30$. Comparisons further reveal that although both end diaphragm systems can be used with confidence as ductile seismic fuses, each of the two systems considered has advantages that may favor its implementation, depending on project specific constraints.

The next three papers in this issue focus on behavior of reinforced concrete decks. In the paper on “Behavior of Reinforced Concrete Bridge Decks on Skewed Steel Superstructure under Truck Wheel Loads,” authors Fu et al. focus on the behavior of skewed concrete bridge decks on steel superstructure subjected to truck wheel loads. They performed finite-element analysis for typical skewed concrete decks. The numerical modeling was verified using in situ deck strain measurement during load testing of a bridge skewed at 49.1°. The analysis results show that service truck loads induce low strains/stresses in the decks, unlikely to solely initiate concrete cracking. The local effect of wheel load significantly contributes to the total strain/stress response. Nevertheless, repeated truck wheel load application may cause cracks to become wider, longer, and more visible. The global effect of the deck as part of the composite cross section may be negligible or significant depending on the location. The current design approach conservatively estimates the local effect but ignores the global effect. It is also observed that an increase in skew angle doesn’t significantly increase total strain/stress effect attributable to the truck wheel load. The paper on “Field Study of Overload Behavior of an Existing Reinforced Concrete Bridge under Simulated Vehicle Loads” by Zhang et al. presents field experiments on a simply supported concrete girder bridge located at Province Road 209 of Hunan Province, China. Test loads were applied to the bridge through a group of hydraulic jacks to simulate the heavy vehicle loads of 196 kN and 294 kN on single-lane loaded and two-lane loaded cases, respectively. The measured results showed that at the deflection limit, the measured load-carrying capacity of the bridge was significantly higher than the designed value. During the process of experiments, the transverse connection stiffness of the bridge varied insignificantly. Cumulated damage to the structure was observed when the simulated cyclic loads were increased to three times the weight of the 294 kN truck on a two-lane loaded case. In the paper on “Experimental Evaluation of Axial Behavior of Strengthened Circular Reinforced-Concrete Columns,” authors Sezen and Miller investigate various approaches to strengthen circular columns. In addition to commonly used steel jacketing and wrapping by fiber-reinforced polymer (FRP) composites approaches, concrete jackets reinforced with spiral rebar, welded wire fabric (WWF), and a new steel reinforcement called prefabricated cage system (PCS) have been investigated experimentally under different axial load applications. Fifteen identical specimens were constructed, strengthened, and tested: one column with no strengthening; three columns strengthened with FRP; two with steel jacketing; and nine with concrete jacketing (two with WWF, three with spiral rebar, and four with the new reinforcement). Bare or unretrofitted specimens had a 152 mm (6 in.) diameter, whereas the outside diameter of concrete jacketed specimens was 254 mm (10 in.). Effectiveness of each strengthening method in increasing the stiffness, axial capacity, and displacement ductility has been investigated using the experimental data. The paper on “Uncertainty Analysis of Creep and Shrinkage Effects in Long-Span Continuous Rigid Frame of Sutong Bridge” by Pan et al. presents modified prediction models for creep and shrinkage for high-strength concrete used in the continuous rigid frame of Sutong Bridge, China. Results indicate that the accuracy of prediction of creep and shrinkage can be enhanced greatly by carrying out short-term creep and shrinkage measurements on concrete and modifying the prediction model parameters accordingly. They also investigated probabilistic analysis methods of structural creep and shrinkage effects. The paper also presents two approaches of mitigating deflections used in the continuous rigid frame of Sutong Bridge.

The next seven papers in this issue are on various aspects of bridge engineering. In paper on “Output-Only System Identification of Posttensioned Segmental Concrete Highway Bridges,” authors Altunişik et al. investigate system identification of a highway bridge using finite-element methods and ambient vibration testing on the posttensioned Gülburnu Highway Bridge located on the Giresun-Espiye state highway. Finite-element modeling of the bridge was done using SAP2000 software, and dynamic characteristics were obtained analytically. During the test, ambient excitations were provided from the traffic effects over the bridge. Ambient vibration tests were applied to the bridge to identify dynamic characteristics. The selection of measurement time, frequency span, and effective mode number were considered from the similar studies in the literature. Two output-only system identification methods, named Enhanced Frequency Domain Decomposition and Stochastic Subspace Identification, estimated dynamic characteristics of the bridge experimentally. The accuracy and efficiency of both methods have been investigated and compared with finite-element results. Results suggest that the ambient vibration measurements are enough to identify structural modes with a low range of natural frequencies. In addition, the dynamic characteristics obtained from the finite-element model of the bridge have a good correlation with experimental frequencies and mode shapes. In the paper “Cracking Mechanism and Simplified Design Method for Bottom Flange in Prestressed Concrete Box Girder Bridge,” authors Xiang et al. investigate cracking mechanisms in the bottom flange of box girder during construction and countermeasures. The stress field in the bottom flange associated with bottom continuity tendon is presented, and the propagation of cracks during tensioning is simulated by nonlinear analysis according to the actual construction sequence. A cracking mode that is not easy to detect in field investigation is illustrated through numerical and theoretical study. It is caused by the deficient shear strength of the bottom flange due to the voids in tendon ducts. Based on numerical results and field investigation, four types of cracking in the bottom flange are identified and discussed. A simplified design method is recommended for cracking control. The paper on “Long-Term Behavior of Prestressed Old-New Concrete Composite Beams” by Wang et al. presents both theoretical and experimental studies of the long-term behavior of prestressed old-new concrete composite beams under sustained loads. General differential equations governing the relationship between the incremental deflection and incremental internal forces of the composite beams have been derived. Closed-form solutions for simply supported composite beams have been obtained and validated using test results already reported in a previous literature on steel-concrete composite beams. The experimental investigation consisted of static long-term load tests carried out on four prestressed old-new concrete composite beams. The behavior of the old-to-new concrete interface, time-dependent deflections, concrete strains, and prestress losses were carefully observed over 260 days. The long-term test program showed that the midspan deflections and concrete strains increased with time because of creep and shrinkage of the new prestressed concrete. The slip strains at the old-to-new concrete interface were found to be relatively small, indicating that the interface bond was sound enough to prevent slip and that the prestressing loads were effectively transferred to the old concrete. The proposed theoretical models predicted the long-term behavior of the prestressed old-new concrete composite beams with an acceptable degree of accuracy. The paper on “Experimental Testing of Pile-to-Cap Connections for Embedded Pipe Piles” by Richards et al. investigates embedded pile-to-cap connections for concrete-filled pipe piles. Four full-scale specimens, each consisting of a cap with two piles, were investigated in the field under cyclic loading. The specimens had minimal reinforcement and varying amounts of pile embedment. Results show that the moment resistance of embedded piles can be significantly greater than what is typically calculated based on the flexural reinforcement and embedment bearing. Excess moment capacity may be explained by friction between the pile and cap at the connection. This friction mechanism is described and discussed in the context of experimental results from other studies. The paper on “Influence of Foundation Scour on the Dynamic Response of an Existing Bridge” by Foti and Sabia proposes dynamic tests as a tool for assessing and monitoring scoured foundations. Two different approaches have been applied as potential tools for monitoring scour of foundations on the basis of measurements of traffic-induced vibrations. One approach is based on modal identification for bridge spans, while the other is based on the observation of the dynamic response of pier foundations. With respect to the latter, numerical simulation on a reference finite-element model have been used to choose the parameters for scour monitoring. Both approaches are applied on experimental data collected on the structure before and after retrofitting, showing their effectiveness. In the paper “Overview of Potential and Existing Applications of Shape Memory Alloys in Bridges,” Dong et al. systematically review and summarize the applications of shape memory alloys (SMAs) in bridge structures. The unique properties of SMAs are first presented, and then several simplified one-dimensional constitutive material models of superelastic SMAs are introduced. Finally, applications of SMAs in bridge structures are discussed from five different aspects. The paper on “Equivalent Modal Damping of Short-Span Bridges Subjected to Strong Motion” by Lee et al. investigates four different methods for estimating the equivalent modal damping ratios of a short-span bridge under strong ground motion by considering the energy dissipation at the boundary. The Painter Street Overcrossing (PSO) is investigated because of seismic data availability. Computed responses using the response spectrum method with the equivalent damping ratios estimated by the methods are compared with the recorded responses. The results show that the four methods provide reasonable estimation of equivalent modal damping ratios and the neglecting off-diagonal elements in the damping matrix is the most efficient and practical method. The equivalent damping ratio of the PSO was found to be nearly 25% under an earthquake with peak ground acceleration of $0.55g$, which is much higher than the conventional assumption of 5%.