Editor’s Note

An efficient approximate method that uses multi-degrees-of-freedom (MDOF) modal equations of motion is presented by Lin et al. in “Seismic History Analysis of Asymmetric Buildings with Soil–Structure Interaction.” The response of two-way asymmetric shear buildings excited by two-directional seismic ground motions is considered wherein soil–structure-interaction (SSI) forces are simulated using frequency-independent soil springs and dashpots. The MDOF modal response histories are obtained using step-by-step integration and the seismic response of the whole SSI system are determined from the arithmetic summation of the modal response histories. The efficiency of the proposed method was validated using numerical examples of a two-way asymmetric four-story building with large and small SSI effects.

In “Application of Energy Balance Concept in Seismic Evaluation of Structures,” Leelataviwat et al. present a seismic evaluation procedure based on a framework that utilizes energy demand and capacity diagrams. The monotonic force-displacement curve of the structure is converted into an energy capacity plot that is superimposed over the corresponding energy demand plot for the given hazard level to determine the expected peak displacement demand. The proposed analysis procedure is then applied to a sample three-story moment frame structure to estimate the displacement demands and the results compared with the results from nonlinear dynamic analysis as well as those from other well-established nonlinear static procedures. It is shown that the proposed method can be effectively used to estimate the response of the moment frame under a given ground motion.

Findings from tests on full-scale prefabricated steel stair assemblies are reported by Higgins in “Prefabricated Steel Stair Performance under Combined Seismic and Gravity Loads.” The stair assemblies were production-run units of a standard system and were designed for a typical steel-frame building. Two different stair assembly units were tested: one with checker plate and one with concrete-filled pans. Lateral drifts were imposed in both orthogonal stair directions followed by factored live and dead gravity loads. Both stair assemblies completed the testing protocol by demonstrating full design gravity load capability after undergoing lateral displacements in both orthogonal directions, and there were no appreciable differences between the performances of the two different stair assemblies. It is concluded that subassemblage testing that isolates the stair-to-landing connection detail may not be able to adequately characterize performance given the complex applied forces and deformation constraints in full stair assemblies

Three components of the rotational mechanism of reinforced concrete beams subjected to cyclic loads are described by Haskett et al. in “Yield Penetration Hinge Rotation in Reinforced Concrete Beams.” The rotation owing to yield penetration of the reinforcing bars is mathematically quantified using partial-interaction theory. Additionally, a variable hinge length that is specific to the rotation limit owing to fracture of the reinforcing bar is developed. Yield penetration rotation is shown to be highly dependent on the reinforcing bar diameter and not on its yield strength, and the rotation owing to bar slip can be determined from the sum of both the elastic and strain hardening components of reinforcing bar material stiffness. The partial-interaction numerical model is shown to be in good agreement with empirically derived hinge lengths for yield penetration. The writers contend that the model can be applied to any reinforced concrete beam or slab, with any bar diameter, for any bar bond characteristics, and for any bar yield and fracture stresses and strains.

A “Hysteretic Model for Reinforced Concrete Columns Including the Effect of Shear and Axial Load Failure” is developed by Sezen and Chowdhury. Flexural, longitudinal bar slip, and shear deformations are predicted under monotonic lateral loads including the effect of strength decay and stiffness degradation. Using monotonic responses as envelopes, cyclic responses corresponding to the three deformation components are calculated. The column under consideration is classified into one of five categories considering the potential for flexural, shear, or axial load failure. The individual cyclic responses are then combined to obtain the total lateral response of the column based on the identified failure mode. The adequacy of the model is demonstrated by comparing predicted cyclic response with available experimental data.

Kang et al. describe findings from a combined analytical and experimental study in “Nonlinear Modeling of Flat-Plate Systems.” The modeling approach accounts for flexural yielding of the slab, flexural yielding owing to unbalanced moment transfer, and loss of slab-to-column moment transfer capacity owing to punching shear failure. Comparisons of measured and predicted responses indicate that the proposed model was capable of reproducing the experimental results well for an isolated connection test, as well as the two shake table test specimens.

A “Framework for Multihazard Risk Assessment and Mitigation for Wood-Frame Residential Construction” to achieve design strategies and risk levels that are consistent with occupant expectations and social objectives is the focus of the paper by Li and Ellingwood. The framework is expected to permit the main sources of uncertainty that affect building performance to be identified, and provides insight on strategies for effective multihazard mitigation efforts. Two typical one-story wood-frame residences with minimum and enhanced hazard-resistant construction practices are considered. Risks owing to wind and earthquake hazards are determined separately for performance levels associated with immediate occupancy, life safety, and collapse prevention. The study finds that cost-effective risk mitigation efforts for wood-frame residential construction should be targeted on those construction practices that are most likely to reduce severe losses under low-probability design events.

The concept of “Performance-Based Wind Engineering for Wood-Frame Buildings” is described by van de Lindt and Dao through the development and application of fragility functions to form different owner/user performance expectations, namely occupant comfort, continued occupancy, life safety, and structural integrity. An example analysis and design for a simple light-frame wood building for all four performance expectations is presented. The final building design necessary to achieve the specified levels of structural performance is discussed for each performance expectation level.

The “Lateral Performance of Nonsymmetric Diagonal-Braced Wood Shear Walls” used in post-and-beam timber buildings is investigated by Li and Lam. Monotonic and reversed cyclic tests were performed on eight full-scale shear walls, with and without gypsum wallboard sheathing. Significant increases in strength, stiffness, and energy dissipation were observed in the shear walls with the additional gypsum wallboard sheathing. An efficient mechanics-based wood shear wall model is introduced to represent the hysteretic load-drift behavior of these diagonal-braced walls. The predicted seismic response of the model is shown to compare well with shake table test results for a simple one-story post-and-beam structure.

“Testing and Analysis of Steel Pipes under Bending, Tension, and Internal Pressure” by Ozkan and Mohareb describes results from full-scale experiments on six pipe specimens. The moment versus curvature relations, peak moment values, and local buckling behavior of the specimens are documented. A nonlinear shell finite element model is also developed using the finite-element analysis (FEA) simulator ABAQUS to predict the moment capacity and the local buckling behavior. The peak moments obtained are compared to the analytically predicted values, and test results are shown to compare well with FEA results. It is observed that under certain combinations of axial tension and internal pressure, pipes with $\mathrm{D}∕\mathrm{t}=81.2$ are able to attain their plastic moment resistances.

In “Finite-Strip Method for the Analysis of Cracked Plates with Application to Plate-Girder Bridges,” Cheung and Song present a new bending crack strip (BCS) that combines the shape functions of the spline finite-strip with the eigenfunction solutions of the differential equations that govern the deflection around a crack. Extra nodal points are used around the cracked area and a substructuring technique is employed to determine the corresponding degrees of freedom of the boundary nodes. Examples are provided to illustrate the accuracy and convergence rate of BCS as well as to demonstrate applications of the proposed model in the analysis of plates and plate girder bridges.

This issue also includes a discussion by Ahmet Turer of the paper “Structural Monitoring and Integrity Assessment of Medieval Towers” by Carpinteri and Lacidogna, which appeared in November 2006. The discusser contends that the role of analytical modeling in structural monitoring and integrity assessment studies was disregarded in this study. The discusser notes that the masonry wall is the thickest at the foundation level, and closely spaced measurements at this location should have similar stresses whereas the values obtained from the flat-jack tests are significantly different from each other. In addition to the accuracy in stress and Young’s modulus measurements, the discusser questions the nondestructive nature of flat-jack testing. The discusser then proceeds to present results from a preliminary finite element (FE) model of the tower to suggest that a calibrated FE model should accompany structural monitoring and measurement-based assessment of structures.

In their closure, the original writers argue that some of Turer’s conclusions are based on analysis of an oversimplified geometry of the structure. Regarding the reported vertical stress in the tower walls, they point out that only in situ flat-jack test results were presented. The flat-jack technique is obviously not the only possible option in each case; however, it was considered the most convenient in the proposed case study, given that a proper application procedure is provided. The writers attribute the scatter in the measured Young’s modulus to the material heterogeneities in the masonry walls. Finally, the writers present their own three-dimensional FE model of the towers, which also accounts for the presence of openings and the variation of the wall thickness at different levels.