Editor’s Note

Metal, concrete, masonry, and wood structures constitute the primary themes of the papers appearing in this issue of the Journal. Also included are a selection of papers on dynamic effects and structural control. The final three papers include a system identification study, a combined analytical-experimental investigation of an innovative crosstie track-work system, and a reliability-based evaluation of steel frames.

The effect of base plate thickness on various sizes and geometries of gusset stiffener details for through-plate socket connections is investigated by Azzam and Menzemer in “Numerical Study of Stiffened Socket Connections for Highway Signs, Traffic Signals, and Luminaire Structures.” Fatigue test results revealed an improved fatigue resistance for socket connections compared to stiffened details. Parametric finite element studies revealed behavior that contradicts current AASHTO specifications. The study also highlights the need for fatigue investigations and experiments to validate current fatigue categories for such structures.

The “Assessment of Subzero Fracture of Welded Tubular K-Joint” fabricated from cold-formed rectangular hollow sections is investigated numerically and experimentally by Björk et al. An extensive experimental program of welded K-joints tested to failure at temperatures between $23°\mathrm{C}$ and $-60°\mathrm{C}$ revealed that the primary failure mode was ductile tearing of the chord flange at the toe of the tension brace-to-chord weld. Finite element simulations revealed that crack advance, once initiated, would be expected to continue at nearly constant load. Results from tests and advanced finite elements analyses are presented by Ye and Rasmussen in “Compression Strength of Unstiffened Elements in Cold-Reduced High Strength Steel.” Analytical studies demonstrate that the effect of gradual yielding may be the main reason for the relatively low strength of G550 steels in the intermediate slenderness range, rather than the lack of strain-hardening. Based on tests carried out on short single-angle and double-angle stub columns, design recommendations specific to G550 steels are made suggesting that a reduction factor of 0.9 is appropriate for unstiffened plate elements.

Findings from a study on the structural behavior of an innovative moment-resisting connection to concrete-filled circular hollow sections are described by Yao et al. in “Experimental and Numerical Investigation of the Tensile Behavior of Blind-Bolted T-Stub Connections to Concrete-Filled Circular Columns.” The favorable strength and stiffness characteristics of this type of connection suggest that it could be an alternative to the conventional welded moment-resisting connections. The writers also develop a three dimensional finite-element (FE) model to simulate the behavior of blind-bolted T-stub connections, and a stiffness model composed of various nonlinear springs to predict the pullout behavior of cogged bars within concrete-filled steel tubes. The cogged extensions to the blind bolts were found to effectively relieve the stress concentration on the thin tube wall.

“Continuum Model for In-Plane Anisotropic Inelastic Behavior of Masonry” by Calderini and Lagomarsino deals with anisotropic damage progression in masonry structures under static incremental and dynamic loads. Masonry is considered as a composite material made up of blocks, mortar bed joints, and mortar head joints. Mortar bed joints are represented as interfaces characterized by cohesion, tensile strength, and friction, while mortar head joints are considered as geometrical discontinuities. The damage processes are described by means of an energy approach, while hysteretic behavior is described by considering a Coulomb-type friction law on the mortar bed joints. The model is implemented in a general purpose finite-element code (ANSYS) and a simple example of a cyclic load history is presented to demonstrate the effectiveness of the model.

Bhargava et al. propose “Suggested Empirical Models for Corrosion-Induced Bond Degradation in Reinforced Concrete” using a wide range of published experimental studies. The models are then validated against the referred experimental data and it is demonstrated that the proposed models provide reasonable estimates of the reduction in bond strength for corroded reinforcement. The writers also propose a method for evaluating the flexural strength of reinforced concrete (RC) beams with corroded reinforcement failing in bond.

The “Design of Optimally Reinforced RC Beam, Column, and Wall Sections” using a conjugate gradient search method is proposed by Aschheim et al. The required reinforcement to provide resistance ideally equal to the required strength from factored load effects is computed. The total reinforcement is determined optimally, either to obtain a global minimum or to obtain a minimum subject to constraints, such as equal reinforcement in all faces.

In “Performance-Based Seismic Design of Wood Frame Buildings Using a Probabilistic System Identification Concept,” van de Lindt et al. validate a new approach through its application in the design of a two-story wood shear wall and a two-story wood frame building. An existing ten-parameter hysteresis model commonly used for seismic analysis of wood frame structures was used in both examples to represent the nonlinear system to be identified. The “design” is then checked by applying a suite of ordinary ground motion records and comparing the multi-record incremental dynamic analysis of the peak deformation response to the target (performance) curve. The approach is found to work well for the example structures investigated.

Load tests to examine the effect of fiber reinforced polymer (FRP) reinforcing rods (Z-spikes) on the flexural stiffness of deteriorated railroad crossties are reported by Gutkowski et al. in the paper “FRP Z-Spike Repairing of Wood Railroad Crossties.” Ten rows of Z-spikes were inserted sequentially in pairs starting at each end of the crosstie specimen perpendicular to the primary bending axis to provide horizontal shear reinforcement and improve flexural stiffness. Load tests were performed after each successive row installation. Results from load tests show an average increase in effective flexural stiffness (EI) of 58% with all ten rows of shear spikes inserted into the crossties.

Results of an experimental investigation examining several lightweight engineered timber floors (ETF) configurations are reported by Bernard in “Dynamic Serviceability in Lightweight Engineered Timber Floors.” The tests established that most ETFs satisfy existing design criteria intended to preclude bodily oscillations under foot traffic. However, these same floors failed to display satisfactory performance with respect to vibrational disturbances. Common variations in design such as, changes in fasteners, or the inclusion of blocking have little effect on vibrational disturbances. However, minimum levels of structural damping may control such phenomena.

Fujino and Hoang have developed “Design Formulas for Damping of a Stay Cable with a Damper.” Accurate asymptotic formulas are analytically derived for the modal damping ratio of a general cable. The formulation is suitable for dampers with finite support stiffness, and includes parameters such as, amount of sag, flexural rigidity of the cable, and stiffness of the damper support. The formulation is also extended to high-damping rubbers that are currently used in engineering practice.

Herrera et al. discuss findings from an experiment carried out on a moment resisting frame (MRF) comprised of concrete filled tubes (CFTs) and wide flange beams in “Seismic Performance Evaluation of a Large-Scale Composite MRF Using Pseudodynamic Testing.” The hybrid pseudodynamic test method is used to subject the test structure to various seismic input levels. P-delta effects associated with the gravity frames in the prototype building were included analytically in the tests. Results from the tests indicate that effective seismic performance of composite MRFs with CFT columns can be achieved. Results of experiments and analyses on fully restrained steel moment connections comprising deep W-shape beams and deep and square built-up box columns are presented by Kim et al. in “Seismic Performance of Pre-Northridge Welded Steel Moment Connections to Built-Up Box Columns.” Two full-size specimens were tested using a standard connection pre-qualification test protocol and each reportedly failed by brittle fracture in the CJP welds of the beam flanges at connection drift angles less than 0.8% radian. ABAQUS finite element models of the specimens were validated using the test results. The locations of the cracks in the beam flange CJP welds coincide with the region of high axial stresses, which varies depending on the column type, presence of horizontal diaphragms, and thickness of box-column plates.

A methodology for “Vibration Controller Design for Confined Masonry Walls by Distributed Genetic Algorithms” is investigated by Joghataie and Asbmarz. An active tendon control mechanism, comprised of prestressed tendons and an actuator, is used and a nonlinear control algorithm is developed based on Wilson’s- $\theta$ instantaneous linear optimal control method. To achieve the best performance, weights in the performance index corresponding to displacement, velocity, acceleration, and control force have been optimized by distributed genetic algorithm (DGA). The control strategy is analytically demonstrated to be an effective solution even for earthquakes of high intensity.

“Bang-Bang and Semiactive Control with Variable Stiffness TMDs” by Collins et al. employs a semiactive control strategy for the vibration control of structures using a stiffness varying tuned mass damper (TMD). In addition to controlling the response of structure using bang-bang control with a control force applied to the tuned mass damper (TMD), the methodology attempts to minimize the maximum value of the frequency response function (FRF) for the coupled structure-damper system. The performance of the control strategy is examined under harmonic excitations and is compared to previous results on bang-bang control and minimax shaping of the FRF. To examine the robustness of the controller, a wind excitation obtained from the Davenport spectrum is applied to a structure with low natural frequency. The control strategy is shown to achieve substantial response reduction with relatively low control force requirements.

A “Nonlinear Model-Based System Identification of Lead-Rubber Bearings” is proposed by Ahn and Chen and applied to a three-span continuous base-isolated bridge. To compare two experiment data sets based on identified parameters, force responses are calculated under predefined displacement functions. Hypothesis testing is used to conclude the closeness of force responses between two data sets. The proposed method is applied to quick-release tests of the bridge to investigate aging and temperature dependent effects of lead-rubber bearings.

The “Behavior of an Integrated Crosstie Trackwork System” is investigated by MacDougall et al. An innovative crosstie trackwork system is developed to address competing requirements for high stiffness to ensure efficient operation of the Linear Induction Motors (LIM) required in advanced rail transit systems, and low track stiffness for acceptable ride quality. A finite element model is developed to predict the response of a crosstie to loads that simulate the passage of a steel-wheeled transit vehicle. Using observed data from fatigue experiments, the model is shown to conservatively predict crosstie deflections and stresses. The Hot Spot Stress approach is used to provide a conservative fatigue life prediction of the crosstie.

The “Strength and Reliability of Steel Frames with Random Properties” is studied by Buonopane. Monte Carlo simulation is used to analyze the behavior of two structures: a low-rise industrial frame and a grain storage bin. The analyses consider randomness in yield strength, elastic modulus, residual stresses, out-of-plumb and out-of-straightness imperfections, and the effects of spatial correlation of these properties. The results provide probabilistic relationships between material and geometric properties of steel frames and the overall strength of the frames. The results confirm the importance of including geometric imperfections for proper assessment of frame stability.