Editor’s Note

A “Truss Model for Nonlinear Analysis of RC Members Subject to Cyclic Loading” is developed by Park and Eom. A reinforced concrete (RC) member is idealized by longitudinal, transverse, and diagonal truss elements. Each truss element is modeled as a composite element composed of concrete and reinforcing steel. The model is validated through comparison of analytically simulated responses to experimental observations. The theoretical basis for “Analytical Modeling of the Pre- and Postyield Behavior of Bond in Reinforced Concrete” is described by Ruiz, Muttoni, and Gambarova. Using reasonable assumptions to simplify the differential equation that governs the phenomenon, simple expressions are derived for interface slip, stress, and strain in the reinforcement and the bond stress at the bar-concrete interface. Satisfactory comparison with test results is achieved for applications ranging from pull-out tests and tension ties to beams in bending. The modeling of composite flexural action of strengthened RC members is presented by Thermou, Pantazopoulou, and Elnashai in “Flexural Behavior of Brittle RC Members Rehabilitated with Concrete Jacketing.” The model considers relative slip at the member-jacket interface and establishes mechanisms that are mobilized to resist this action. The model is verified through comparison with published experimental data on RC jacketed members.

Attard, Nappi, and Tin-Loi extend a finite element procedure developed for studying fracture in concrete to “Modeling Fracture in Masonry.” Triangular units are grouped into rectangular zones to represent brick units with surrounding mortar joints and fracture is simulated through a constitutive softening-fracture law at the boundary interface nodes. The formulation is validated by examples comprising direct tension, micro-shear, and three-point bending of masonry panels.

The development and validation of a “Cast Modular Panel Zone Node for Steel Special Moment Frames” is presented in a two-part paper by Fleischman et al. The panel zone dissipator modular node (PZ-MN) dissipates energy through stable yielding of the panel zone and is configured for optimal seismic performance through a casting process. The first paper describes the analytical development including investigation of trial configurations and key parameters leading to the prototype design. The experimental verification of the full-scale PZ-MN prototype is presented in the companion paper. The specimen, which was subjected to FEMA-350 qualification testing, exhibited remarkable performance exceeding qualifying drift angles and possessing stable hysteretic behavior. Guidelines for frame design are also proposed based on the analytical and experimental findings.

Weggel, Zapata, and Kiefer investigate the serviceability of an economical glass curtain wall system that provides a low level of blast resistance for appropriate applications in “Properties and Dynamic Behavior of Glass Curtain Walls with Split Screw Spline Mullions.” A finite element model is calibrated to low-amplitude static test results and then used to perform modal and transient analyses. The analytical simulations are shown to compare well with measured responses. A new type of structure to dissipate impact energy directly into the slab or into fuse supports is proposed by Delhomme et al. in “Damage Mechanisms of a Reinforced Concrete Rock-Shed Slab Impacted by Blocks.” The dynamic phenomena resulting from the impact is evaluated by means of experiments on one-third scale models which reveal that the slab is damaged by three primary mechanisms: punching, bending, and surface level breaking below the impacted zone.

Vargas and Bruneau investigate the “Effect of Supplemental Viscous Damping on the Seismic Response of Structural Systems with Metallic Dampers.” Parametric studies are carried out to examine the effectiveness of adding various levels of viscous damping on the equivalent hysteretic damping and on spectral floor accelerations for short, intermediate, and long period structures. In “Seismic Strengthening of Rocking-Critical Masonry Piers,” Rai and Goel suggest the elimination of the undesirable compressive mode of failure of stabilized rocking piers at large drifts through the use of energy dissipating devices to limit the forces in the verticals. A simple mechanics-based nonlinear load-deformation relationship is developed for the stabilized pier response which can be used in a displacement-based procedure to design the energy dissipation devices. A simplified procedure for “Damage Analysis of Structures on Elastic Foundation” is proposed by Avilés and Pérez-Rocha. To simplify the consideration of soil-structure interaction effects, an equivalent fixed-base oscillator with the same yield strength and energy-dissipation capacity as the actual flexible-base structure is applied. The significance of soil-structure interaction on structural performance is documented.

“Multiobjective Optimization for Performance-Based Design of Reinforced Concrete Frames” by Zou et al. addresses the minimization of life-cycle cost of a building frame subject to multiple levels of seismic performance criteria. The initial material cost is expressed in terms of the design variables and the expected damage loss is expressed as a function of seismic performance levels and their associated failure probability. The multi-objective optimization problem is solved by the $ϵ$ -constraint method to produce a Pareto optimal set from which the best compromise solution is selected. The methodology is demonstrated on a ten-story frame.

Power spectral density functions coherent with the elastic response spectrum are used in conjuction with statistical linearization techniques by Di Paola, La Mendola, and Navarra in “Stochastic Seismic Analysis of Structures with Nonlinear Viscous Dampers” to determine equivalent linear damping. Comparisons with Monte Carlo simulations confirm that the linearization produces satisfactory results in terms of second order moments and absolute peak response measures for the kind of nonlinearity induced by fluid dampers. Dym and Williams discuss the consequences of using two models for “Estimating Fundamental Frequencies of Tall Buildings.” The study finds that the model based on the Timoshenko beam is appropriate for shear wall buildings while the coupled shear-flexure beam model is appropriate for shear wall-frame buildings.

The issue concludes with a discussion by Emil Simiu on the paper “Peak Wind Load Comparison: Theoretical Estimates and ASCE 7” by Tieleman, Elsayed, and Hajj that appeared in July 2006. The discussers point out that to ensure that the safety margins against wind effects are adequate, it is necessary to consistently account for the contributions of individual uncertainties to the overall uncertainty. They proceed to compliment the writers for initiating a debate on issues that could lead to improvement of the pressure specifications in future codes and standards. The writers acknowledge the significance of associating a probabilistic measure with design wind loads as noted by the discussers, but contend that the effects of uncertainties in wind speed on estimates of wind loads should be introduced through variations in the incident flow, in numerical or wind tunnel simulations.