Editor’s Note

Sixteen technical papers and one technical note selected for this issue of the Journal cover the following subject areas: structural monitoring and damage detection, analysis and computation, seismic effects, wind effects, concrete and fiber reinforced concrete structures, wood structures and structural reliability.

Carpinteri and Lacidogna utilize NDT methods to determine the extent and evolution of damage in two medieval buildings in Italy in their paper, “Structural Monitoring and Integrity Assessment of Medieval Towers.” The damage process in some portions of the masonry were monitored using acoustic emission techniques while an ad hoc theory based on fractal concepts is proposed for assessing the stability of masonry structures. The change in modal parameters of a gradually damaged component is investigated by Unger, Teughels and De Roeck in “System Identification and Damage Detection of a Prestressed Concrete Beam.” The results show that damage detection in the case of prestressed concrete is difficult particularly in the early damage state. However, a damage pattern that corresponds to the locations of final failure could be identified.

The “Use of Modal Flexibility for Damage Detection and Condition Assessment: Case Studies and Demonstrations on Large Structures” is presented by Catbas, Brown, and Aktan through case studies and demonstrations on large structures. Frequency response functions are used to approximate modal flexibilities which are then utilized to compute deflection profiles. Among the challenges in directly using dynamic test data is the fact that it may not be possible to test the entire structure, hence the test grid is truncated and the modal flexibility obtained from measurements will be incomplete. Uncoupling environmental effects when examining the data is another source of complexity. Approaches to mitigate these obstacles are discussed.

A method for “Shape Selection for Lattice Structures” is proposed by Hearn and Adams. The method uses joint equilibrium equations to determine a minimum set of adjustments to the lattice geometry to achieve equilibrium reference states. The process consists of a force computation that indicates the number and identity of variable joint coordinates, if any, followed by a shape computation using variable joint coordinates. The method does not require initial estimates of member forces, stiffnesses or inertial properties.

Kalkan and Kunnath propose a new “Adaptive Modal Combination Procedure for Nonlinear Static Analysis of Building Structures” that integrates concepts from the capacity spectrum method described in ATC-40, the adaptive pushover method original proposed by Gupta and Kunnath and the modal pushover analysis advocated by Chopra and Goel. A novel feature of the procedure is that the target displacement is estimated and updated dynamically during the analysis. This is achieved by incorporating energy-based modal capacity curves in conjunction with constant-ductility capacity spectra. The effectiveness of the method is demonstrated by predicting critical demand parameters such as inter-story drifts in moment-frame buildings subjected to both far-fault and near-fault records. Li et al. report findings from a “Shaking Table Test of a 1:20 Scale High-Rise Building with a Transfer Plate System.” Results from the test indicate that the building will not collapse under a major earthquake. Notwithstanding the 76% reduction in lateral stiffness at the transfer plate, much of the observed damage occurred above the transfer plate. In order to minimize damage, the authors recommend strengthening the walls between the fourth and fifteenth story levels, as well as reducing any change in stiffness at the transfer plate zone. The “Cyclic Lateral Load Response of Bridge Column-Foundation-Soil Systems in Freezing Conditions” is obtained experimentally by Suleiman, Sritharan and White through outdoor testing of three large-scale units installed in low-plasticity glacial till soil. When compared to testing the specimen at an ambient temperature of $23°\mathrm{C}$ , an identical unit tested at $-10°\mathrm{C}$ showed an 170% increase in effective elastic stiffness, a significant upward shift in the region of maximum moment and a 64% reduction in the length of the plastic region. The use of an oversized foundation was shown to minimize the effects of freezing conditions on the column response.

Traditional power-law exposure coefficient curves used in North American codes are compared with curves derived from modern models of the planetary boundary by Peter Irwin in “Exposure Categories and Transitions for Design Wind Loads.” It is concluded that traditional exposure coefficients are reasonably consistent with modern boundary layer theory for heights below $300\mathrm{m}$ . The paper also examines the relationship between the dimensions and density of ground roughness obstacles and the exposure coefficient. Simple expressions are proposed for calculating the effect of an upwind change in roughness on the exposure coefficient.

A consistent and general approach to assess the “Strength of Two-Dimensional Nodal Zones in Strut–Tie Models” is proposed by Young Yun. The validity of the approach is examined by evaluating the ultimate strength of several nodal zones which were tested to failure. The author does however indicate that the method also has several limitations since it requires that the struts and ties and their corresponding forces must be predetermined, followed by a finite element analysis of the nodal zone that accounts for material nonlinearity and incorporates the effects of confinement. An analytical study to determine the effects of bearing stress distributions on concrete bearing strength and bearing plate flexural demand is summarized by Escobar-Sandoval, Whittaker, and Dargush in “Concentrically Loaded Circular Steel Plates Bearing on Plain Concrete.” A finite element parametric study is performed to derive formulas for the concrete bearing stress distribution, and a procedure for design of concentrically loaded circular steel bearing plates on unreinforced concrete is proposed. The “Structural Response of Elements Combining Ultrahigh-Performance Fiber-Reinforced Concretes and Reinforced Concrete” is investigated by Habel, Denarié, and Brühwiler. Based on a parametric study, the authors conclude that composite UHPFRC-concrete elements with a minimum strain-hardening magnitude of 0.2% provide higher stiffness, increased resistance, and delay the formation of localized microcracks under service conditions.

Results from “Structural Analysis of the Wooden Structure of the Historical Building of Ali Qapu” are presented by Mehrdad Hejazi. The study finds that most of the main structural members of this 350-year old structure are adequately designed. The only inadequacies revealed by the analysis are four columns and the two beams connecting these columns. Recommendation is also made for stiffening the secondary beams and adding a system of lateral braces. The “Racking Performance of Structural Insulated Panels” is examined by Kermani and Hairstans. A SIP is formed from polystyrene foam and is typically sandwiched between two layers of oriented strand boards. Results from a comprehensive experimental and parametric study indicate that SIPs perform as an effective composite material possessing the strength and stiffness necessary to sustain required design loads.

Meløysund et al. present results from a study examining the safety against the collapse of 20 existing buildings in Norway in “Increased Snow Loads and Wind Actions on Existing Buildings: Reliability of the Norwegian Building Stock.” The investigation indicates a low reliability for a considerable number of buildings according to current building regulations. Scenarios for future climate changes suggest increased winter precipitation and an increase in the frequency of strong winds, thereby further decreasing the reliability of the buildings. The performance of deteriorating bridges is analyzed by Neves, Frangopol, and Petcherdchoo in “Probabilistic Lifetime-Oriented Multiobjective Optimization of Bridge Maintenance: Combination of Maintenance Types” to determine the best combinations of condition, safety and cost which satisfy all performance constraints. The benefits of the combination of preventive and essential maintenance actions over single maintenance type strategies are discussed. A framework for “Optimizing Bridge Network Maintenance Management under Uncertainty with Conflicting Criteria: Life-Cycle Maintenance, Failure, and User Costs” is developed by Liu and Frangopol. The framework integrates time-dependent structural reliability prediction, highway network performance assessment, and life-cycle cost analysis.

Neuenhofer’s paper investigates the accuracy of approximate methods recommended in several design guidelines for practicing engineers for calculating the “Lateral Stiffness of Shear Walls with Openings.” Comparing results obtained from these recommended methods to those obtained from finite element analyses, it is found that the so-called hand calculation method, despite its complexity, is highly approximate and often enormously overestimates the stiffness of shear walls with openings.

Michael Scott and Tanarat Potisuk argue that the paper by Fafitis on “Nonlinear Truss Analysis by One Matrix Inversion” which appeared in the May 2005 issue of the Journal is not significantly different from the modified Newton–Raphson algorithm, also known as the chord method, where the stiffness matrix is held constant over the course of a load increment. The author’s proposal to hold the “inexact” stiffness matrix constant during the equilibrium iteration can actually lead to an increase in the number of iterations. Furthermore, the selection of the inexact stiffness matrix for the equilibrium iterations is not arbitrary, as the author suggests.

Ayoub, A. (2006). “Nonlinear analysis of RC beam-columns with bond-slip.” J. Eng. Mech., 132(11).

Bakker, M. C. M. (2006). “Shear-flexural buckling of cantilever columns under uniformly distributed load.” J. Eng. Mech., 132(11).

Dusenberry, D. O. and Hamburger, R. O. (2006). “On practical means for energy-based analyses of disproportionate collapse potential.” J. Perform. Constr. Facil., 20(4).

Ellingwood, B. R. (2006). “Mitigating risk from abnormal loads and progressive collapse.” J. Perform. Constr. Facil., 20(5).

Kaewkulchai, G. and Williamson, E. B. (2006). “Modeling the impact of failed members for progressive collapse analysis of frame structures.” J. Perform. Constr. Facil., 20(4).

Marjanishvili, S. and Agnew, E. (2006). “Comparison of various procedures for progressive collapse analysis.” J. Perform. Constr. Facil., 20(4).

Puntel, E., Bolzon, G. and Saouma, V. E. (2006). “A fracture mechanics based model for joints under cyclic loading.” J. Eng. Mech., 132(11).

Ruth, P., Marchand, K. A. and Williamson, E. B. (2006). “Static equivalency in progressive collapse alternate path analysis; reducing conservatism while retaining structural integrity.” J. Perform. Constr. Facil., 20(4).

The November/December issue of the Journal of Bridge Engineering is devoted to the subject of bridge monitoring. All papers in this issue will appeal to readers who are interested in this topic.