Editor’s Note

An analytical study to develop “Plastic Axial Load and Moment Interaction Curves for Fire-Exposed Steel Sections with Thermal Gradients” is presented by Garlock and Quiel. The prototype beam-columns used in the study are wide-flange (WF) steel sections that are part of a high-rise moment-resisting steel building. This study evaluates the effects of plate thickness, section depth, and the direction of bending (i.e., strong versus weak axis) on the plastic P-M interaction diagram of WF sections with thermal gradients. Results show that a thermal gradient may have a significant effect on the yield capacity of beam-columns, and evaluations that are made assuming a uniform temperature throughout the section may lead to overestimations or underestimations of the true strength of the section.

“Analytical Performance Simulation of Special Concentrically Braced Frames” by Yoo et al. describes the analytical component of a coordinated analytical and experimental research study undertaken to improve the understanding and design of the seismic performance of SCBF connections. Inelastic deformation of the brace in SCBFs places severe inelastic demands on gusset plate connections. A validated analytical model was used to evaluate the inelastic performance of structural elements and to determine the frame ductility and concentration of plastic strains at potential locations of member or weld tearing or fracture. Detailed comparisons between experimental observations and computed results show that the analyses provided good correlation with actual behavior. The results are used to develop a methodology to predict tearing or cracking of key elements in the frame.

Lehman et al. present results from a research program aimed at “Improved Seismic Performance of Gusset Plate Connections.” A new gusset plate design approach is proposed in which the yield mechanisms of the brace are balanced with the yield mechanisms of the connection and the failure modes of the system to achieve a target yielding hierarchy and suppress unwanted failure modes. Full-scale one-story, one-bay frames are designed and tested to investigate the seismic performance of current and proposed design methods. Variations in balance factors between the brace, gusset plate, and weld are considered in the evaluation to evaluate possible yield mechanisms and failure modes and to obtain the desired yielding hierarchy. Comparison of the observed and measured performance of each specimen is made, and specific design expressions to improve the seismic engineering of SCBF systems are proposed.

“Lateral Torsional Buckling Strength of Tubular Flange Girders” composed of concrete-filled compression flange tubes is investigated by Kim and Sause. A finite-element–based parametric study is conducted to investigate the influence of girder geometry and material strength on the lateral torsional buckling strength. Design flexural strength formulas for construction conditions and for ultimate strength in the final constructed condition are developed.

Hall and Connor investigate “Influence of Base Plate Flexibility on the Fatigue Performance of Welded Socket Connections” commonly used in sign support structures. Results from a three-specimen static load testing program, extensive finite-element analysis, and parametric study show that base plate flexibility, primarily base plate thickness, has a major influence on the stress behavior in the tube wall adjacent to the welded socket connection. Increasing base plate thickness is shown to be a very effective method to improve the fatigue resistance of this detail. Based on this trend, a simple procedure that incorporates the influence of base plate flexibility into the infinite life, nominal stress approach to fatigue design is proposed.

Yang and Ashour evaluate the strut-and-tie model specified by ACI 318–05 and the mechanism analysis based on plasticity theory to predict the “Load Capacity of Reinforced Concrete Continuous Deep Beams.” The influence of such parameters as compressive strength of concrete, shear-span-to-overall-depth ratio, main longitudinal bottom reinforcement, and shear reinforcement on the load capacity is investigated using both methods and compared to experimental results obtained from the literature. Experimental results are found to be closer to the predictions obtained from the mechanism analysis than the strut-and-tie model. The strut-and-tie model is shown to overestimate the load capacity of continuous deep beams having a small amount of longitudinal bottom and shear reinforcements.

Findings from an experimental program designed to examine the “Long-Term Behavior of Prestressed Composite Beams at Service Loads for One Year” are reported by Xue et al. The test program consisted of two prestressed composite beams and one nonprestressed composite beam. Using the age-adjusted effective modulus method and energy principle, the creep stiffness and force matrices of prestressed composite (PSC) beams are developed. A time-dependent analytical model is presented to predict the long-term behavior of PSC beams. The analytical simulations are validated with experimental results.

“Tendon Stress in Unbonded Posttensioned Masonry Walls at Nominal In-Plane Strength” by Wight and Ingham deals with the accurate estimation of stress in unbonded tendons at the nominal strength limit for calculating the in-plane flexural strength of posttensioned masonry walls. A finite-element model validated against large-scale in-plane structural testing of posttensioned concrete masonry walls is used to evaluate the accuracy of existing code equations. A revised equation is then shown to provide a more accurate prediction of tendon stress for walls loaded in-plane and is recommended for inclusion in future masonry code revisions. Drysdale et al. report on findings from an experimental study to examine “Shear Capacity for Flange-Web Intersection of Concrete Block Shear Walls.” The investigation focuses on different specimen configurations to study the shear transfer mechanism between wall flanges and webs. Analytical models based on shear-friction failure mechanisms are proposed and validated against experimental results.

“Effect of Timber Ties on the Behavior of Historic Masonry” is evaluated by Vintzileou through testing stone masonry specimens representing portions of perimeter walls in compression or in diagonal tension. Test results show that the timber ties contribute to enhancement of the ultimate strain and reduction in opening of vertical cracks of masonry subjected to compression. When timber-reinforced masonry is subjected to diagonal compression, the cracking load is several times higher than that of unreinforced masonry. These findings are consistent with observations in historic structures.

An “Implicit Higher-Order Accuracy Method for Numerical Integration in Dynamic Analysis” is proposed by Rezaiee-Pajand and Alamatian. By defining weighting factors, current displacement and velocity are assumed to be functions of the accelerations in several previous time steps. Then, the optimum weighted factors are determined so that displacement and velocity errors in the Taylor series expansion are minimized. The method is validated through comparison with commonly used implicit methods such as Newmark- $\beta$ and Wilson- $\theta$ approaches. Results show that the proposed time integration is stable and exhibits improved accuracy.

In “Nonlinear Seismic Response Analysis of Steel–Concrete Composite Frames,” Zona et al. develop a flexible shear connection that allows development of partial composite action, which influences structural deformation and distribution of stresses. A materially nonlinear finite element formulation with realistic uniaxial cyclic constitutive laws is developed using composite beam elements with the proposed deformable shear connection. The resulting finite-element model for a benchmark problem is validated using experimental test results from the literature for quasi-static cyclic tests and then used in numerical simulations of multistory frames made of steel columns and steel-concrete composite beams. It is found that the inclusion of the deformability of the shear connection in the finite-element model has a significant effect on the global dynamic response of SCC frame structures.

Saffari et al. present a methodology for “Nonlinear Analysis of Space Trusses Using Modified Normal Flow Algorithm.” To attain the equilibrium path of space trusses, Newton-Raphson iterations are used along the flow path perpendicular to the Davidenko curves with modified convergence rate. Contrary to the previous methods, this algorithm uses the Hemotopy approach and is based upon new mathematical concepts with the ability to develop complex load-displacement paths of structures with multidegrees of freedom. Finally, three numerical examples are presented using the proposed algorithm, and the results are compared with existing advanced iterative methods that have been used for nonlinear analysis of structures.

Conte et al. summarize findings from “Dynamic Testing of Alfred Zampa Memorial Bridge,” located in northern California, which is the first suspension bridge in the United States with an orthotropic steel deck. The dynamic field tests were conducted just before the bridge opened to traffic and included ambient vibration tests, mainly wind induced, and forced vibration tests based on controlled traffic loads and vehicle-induced impact loads. These tests establish the dynamic (modal) properties of the bridge in its as-built (baseline) condition with no previous traffic loads or seismic excitation. The ambient vibration test data are used to identify the bridge modal parameters using the data-driven stochastic subspace identification method.

Alicioglu and Lus present an investigation of the performance of subspace techniques for modal identification in their paper “Ambient Vibration Analysis with Subspace Methods and Automated Mode Selection: Case Studies.” Several models and structures characterized by increasing degrees of complexity are studied to assess the potential benefits of stochastic subspace identification algorithms and the difficulties that might be experienced during a modal identification analysis. It is concluded that some preconditioning tools are quite helpful to properly focus on the structural modes of interest and in improving the performance of the subspace methods. In general the approaches investigated in the study are found to perform quite satisfactorily for operational modal analysis of engineering structures.

A new energy-based approach is used by Chenouda and Ayoub to develop “Inelastic Displacement Ratios of Degrading Systems.” A new model that considers degradation effects and also permits collapse prediction of structures under seismic excitations is proposed. Results from extensive statistical dynamic analysis of different structural systems are used to propose approximate methods for estimating maximum inelastic displacements of degrading systems for use in performance-based seismic design.

An experimental study was conducted by Dadhiala et al. to investigate “Quasi-Static Strengths and Failure Modes of Tight-Fitting and Round-End Metal-Plate Wooden Truss Joints.” Four joint configurations were tested with a range of connector-plate sizes. The tight-fitting joints were stronger and stiffer than round-end joints. Both the shear strength of the metal plate connector and the teeth pullout strength were smaller in the round-end joints than in the tight-fitting joints. The line contacts between the round-end webs in compression produced greater crushing of the wood, causing buckling of the connector plates and reducing the ultimate loads on these joints. Design guidelines are suggested for round-end joints.

In “Flexural Behavior of Nonposttensioned and Posttensioned Concrete-Filled Circular Steel Tubes,” Tuan explores a novel concept of posttensioning the concrete core inside circular steel tubes to further enhance the synergistic interactions between concrete and steel. Numerical analyses demonstrate the significant increase in the flexural strength of the proposed posttensioned, concrete-filled tubes (CFT) over nonprestressed CFTs.

A damage detection method based on the change of strain energy in each element is proposed by Sharifi and Banan in “Energy Index Method: Technique for Identification of Structural Damage.” The algorithm requires only the stiffness and mass matrices of the baseline structure and a few measured mode shapes of the damaged structure to find the exact location and severity of damage. The damage detection procedure and the behavior of the algorithm are illustrated by a numerical example. Results show that damaged elements of the structure store higher levels of strain energy, thereby more easily enabling detection by the proposed method.