Editors' Note

The lead-off paper by Medina and Krawinkler presents an “Evaluation of drift demands for the seismic performance assessment of frames.” Utilizing a probabilistic performance-based framework, relationships between drift demand and a seismic intensity measure are established for nondeteriorating frames subjected to ordinary ground motions without near-fault effects. One of the findings from the study indicate that for frames with periods in excess of 0.3 seconds, the maximum interstory demands migrate from the upper to lower levels as the inelasticity increases. The next paper by Akkar et al. proposes a simple procedure to establish “Drift estimates in frame buildings subjected to near-fault ground motions.” The procedure estimates the peak interstory drift in low- to medium-rise buildings across the building height given spectral displacement and beam-to-column stiffness ratio provided the ratio of the system fundamental period to the pulse period is less than 1.5.

The effectiveness of variable damping is investigated by Sahasrabudhe and Nagarajaiah in “Experimental study of sliding base isolated buildings with MR dampers in near-fault earthquakes.” It is demonstrated that the variable damping reduces base displacements and superstructure responses further than passive low/high damping cases. A configuration in which the dampers are directly installed at beam-column joints is evaluated by Hwang et al. in “Analytical and experimental study of toggle-brace-damper systems.” It is shown that the toggle brace system is more efficient than diagonal brace damper systems in controlling the seismic response of a stiff structural system. However, since gaps exist in the hinge connections of the toggle-brace-damper system that influence the overall damping ratio, care must be taken in installation of such dampers.

Sritharan et al. summarize findings from an experimental study on the “Seismic behavior of a concrete steel integral bridge pier system.” Anchorage length requirements of the column longitudinal bars were found to govern the depths of the cap beam and girders. The provision of mechanical anchorages at the ends of the longitudinal reinforcing steel reduced the anchorage requirements permitting shallower and lighter cap beams and girders. A kinematic model relating displacement ductility factor to local curvature ductility demand is proposed by Song et al. in “Analytical model for ductility assessment of fixed-head concrete piles.” The curvature ductility demand is shown to depend on the ratio of first yield to ultimate lateral force, the postyield stiffness ratio, and the depth to the plastic hinge below the pile head. “Practical performance model for bar buckling” is developed by Berry and Eberhard from observations of bar buckling in cyclic tests of 62 rectangular and 42 spirally reinforced concrete columns. A fragility function expressing probability of bar buckling as a function of drift demand is derived from a statistical analysis of key variables that are determined to control bar buckling.

“Coupled dynamic analysis and equivalent static wind loads on buildings with 3D modes” is presented by Chen and Kareem. The proposed framework takes into account the correlation between wind loads in principal directions and coupling of modal response components. Results from a systematic experimental investigation are described in “Evaluation of vibration damping of GRP-reinforced glulam composite beams” by Naghipour et al. A modification of the Half Band Power method is proposed and shown to improve the accuracy of the original method when considering damping of composite materials with relatively high damping ratios. Two- and three-dimensional finite-element analyses of prismatic specimens and curved beams are utilized by Aicher and Dill-Langer to study the “Effect of lamination anisotropy and lay-up in glue-laminated timbers.” The study showed a strong influence of the lamination sawing pattern and of grouping within the glulam section and provides a theoretical basis for optimizing lay-ups. Dean and Shenton discuss results of static tests on wood frame shear walls with and without hold-down anchors in “Experimental investigation of the effect of vertical load on the capacity of wood shear walls.” The study finds that such a wall has additional reserve capacity than is reflected in current code requirements for allowable shear.

The inelastic behavior of concentrically braced frames with and without cold-formed steel studs (CFSS) is investigated by Celik et al. in “Cyclic testing of braces laterally restrained by steel studs.” Experimental results indicate that the cumulative energy dissipation capacity, at similar ductility levels, can be significantly increased when CFSS members are used to laterally restrain the braces against buckling. May and Menzemer report on an experimental investigation in “Strength of bolted aluminum alloy tension members” in which 72 aluminum alloy tension members consisting of angles, channels, and structural tees were fabricated and tested to failure. The test results are complemented with finite-element analyses to propose a design rule for calculation of net section rupture strength. The final technical paper in this issue by Trahair is concerned with “Nonlinear elastic nonuniform torsion.” Expressions are derived for nonlinear “Wagner” section constants for narrow rectangular, doubly symmetric I-sections and mon-symmetric equal-angle sections, and a finite-element method for analyzing nonlinear torsion is described.

This issue also includes two technical notes. The first is titled “Wind tunnel testing and the sector-by-sector approach to wind directivity effects” and is authored by Simiu and Filliben. The writers conclude that the current approach used by wind tunnel operators to specify extreme wind effects may lead to nonconservative estimates. The second technical note by Kim et al. proposes a method for “Determination of natural frequencies and mode shapes of structures using subspace iteration method with accelerated starting vectors.” The issue concludes with a discussion by Niall on the paper “Model for lateral excitation of footbridges by synchronous walking” by Nakamura, which appeared in January 2004. The discusser notes that the formulation proposed in the original paper has no analytical solution and the numerical solution may be daunting to a design engineer. A solution for estimating the amplitude envelope is proposed by the discusser. The author agrees that the proposed solution is useful in applicable cases and cautions the reader on the sensitivity of the results to the model variables.

The editor’s note in the January 2005 issue of JSE summarized the lead-off paper by Omika et al. by stating that “as many as 32 core columns in the second tower were destroyed upon impact while only nine core columns were damaged by aircraft impact in the first tower.” In fact, 32 core columns were destroyed in the first tower (WTC1) and only 9 were destroyed upon impact in the second tower (WTC2). Additionally, this statement could have been interpreted as the main reason that the second tower collapsed in a shorter duration after impact than the first tower. Findings from the study by Omika et al. indicate that WTC2 collapsed sooner than WTC1 because the floor locations of the 32 columns destroyed in WTC1 were almost symmetrical, while in WTC2, the 9 damaged columns were located eccentrically to the east side, and the heat capacity and the supporting capability of the remaining perimeter columns in WTC2 were less than those of WTC1.

Areiza-Hurtado, M., Vega-Posada, C., and Aristizabal-Oacha, J. (2005). “Second-order stiffness matrix and loading vector of a beam-column with semirigid connections on an elastic foundation.” J. Eng. Mech., 131(7).

Bolton, R., Sikorsky, C., Park, S., Choi, S., and Stubbs, N. (2005). “Modal property changes of a seismically damaged concrete bridge.” J. Bridge Eng., 10(4).

Calçada, R., Cunha, A., and Delgado, R. (2005). “Analysis of traffic-induced vibrations in a cable-stayed bridge.” J. Bridge Eng., 10(4).

Dicleli, M., Mansour, M. Y., and Constantinou, M. (2005). “Efficiency of seismic isolation for seismic retrofitting of heavy substructured bridges.” J. Bridge Eng., 10(4).

Ito, M., Nozaka, K., Shirosaki, T., and Yamasaki, K. (2005). “Experimental study on moment-plastic rotation capacity of hybrid beams.” J. Bridge Eng., 10(4).

Lin, C. C., Wang, J. F., and Chen, B. L. (2005). “Train-induced vibration control of high-speed railway bridges equipped with multiple tuned mass dampers.” J. Bridge Eng., 10(4).

Maleki, S. (2005). “Seismic modeling of skewed bridges with elastomeric bearings and side restrainers.” J. Bridge Eng., 10(4).

Wang, X., et al. (2005). “Overview of a modal-based condition assessment procedure.” J. Bridge Eng., 10(4).