Editor's Note

This issue starts with a group of four papers on the behavior and design of steel structures. Other papers in the issue address a variety of topics, including composite steel-concrete columns, seismic effects, probabilistic structural design, structural analysis, reinforced concrete analysis, damage detection, wind loading on structures, and timber structures.

The issue opens with “Seismic Performance of Posttensioned Steel MRFS with Friction Devices” by Rojas et al. The authors introduce a posttensioned friction-damped connection (PFDC) suitable for earthquake-resistant steel moment-resisting frames. Inelastic analyses show that steel frames with PFDCs have good strength and energy-dissipation capacity as well as self-centering capability. Based on the analysis results, the authors conclude that frames with PFDCs can deliver better seismic performance than frames with traditional moment-resisting connections. The next paper on steel structures is “Structural Responses of Axially Restrained Steel Beams with Semirigid Moment Connections in Fire” by Tan and Huang. The authors use numerical models to explore the behavior of semirigid connections under fire conditions. Connections are represented by a set of elastic and inelastic springs with temperature-dependant properties. The paper provides numerical predictions of the upper- and lower-bound beam critical temperatures for design purposes. In “Energy Dissipation of Compression Members in Concentrically Braced Frames: Review of Experimental Data,” Lee and Bruneau survey existing experimental data about the inelastic response of concentric steel braces. They quantify the extent of hysteretic energy achieved by bracing members in compression and the extent of degradation of the compression force under cyclic loading to gauge the effectiveness of such members under seismic loading. They conclude that the normalized energy dissipation of braces with moderate slenderness is comparable to that for members with substantially greater slenderness. In “Out-of-Plane Strength Design of Fixed Steel I-Section Arches,” Pi and Bradford use finite-element analysis to investigate the out-of-plane elastic and inelastic flexural-torsional buckling behavior and strength of fixed steel arches. The parameters they consider in their study include: initial imperfections, residual stresses, yielding, and inelastic flexural-torsional buckling. They note that both elastic and inelastic responses of fixed arches differ substantially from those of pin-ended arches and that strength-design equations that utilize the effective length concept for pin-ended arches cannot be directly extended to fixed arches. Based on their findings, they propose design equations for fixed steel arches under uniform compression or uniform bending and an interaction equation to check strength against out-of-plane failure of steel I-section arches.

In “Equivalent Uniform Moment Diagram Factor for Composite Columns in Major Axis Bending,” Tikka and Mirza revisit the equivalent uniform bending moment factor, $C_m$ , that is currently used in the design of concrete encased steel columns with unequal end moments. They present the results of extensive simulations of square, tied, composite columns to investigate the effect of different variables on existing expressions for $C_m$ . The columns studied were subjected to short-term ultimate loads and unequal end moments causing both single and double curvature about the major axis of the member. They propose two alternative expressions for the factor based on their simulation studies.

In the next paper, “Predication of Peak Acceleration of 1-DOF Structures by Scaling Law,” Jiang and Shu discuss the acceleration response of single-degree-of-freedom structures under shock loading based upon Buckingham’s $\pi$ -theorm of nondimension. They establish a scaling law based on the equation of motion and use it to model the behavior of a single-degree-of-freedom system. They describe tests on a model to validate the numerical solution scheme they developed and note that the scaling law is applicable for frequencies other than the resonance frequencies. In “Direct Estimation of the Seismic Demand and Capacity of MDOF Systems through Incremental Dynamic Analysis of an SDOF Approximation,” Vamvatsikos and Cornell exploit the connection between the static pushover and incremental dynamic analysis procedures to develop a method to estimate seismic demand and capacity of first-mode-dominated systems. The method is applicable for behavior ranging from near-elastic to global instability and enables users to obtain accurate estimates of seismic demands and capacities for important limit-states such as immediate occupancy and global dynamic instability. They compare the results of their methodology to data from more refined methods for several multistory buildings and observe that good agreement is achieved.

The only paper on optimization in the issue is “Probabilistic Optimization of Aging Structures Considering Maintenance and Failure Costs” by Kong and Frangopol. In it, the authors discuss the prioritization of maintenance interventions on aging structures. They describe a computer program for probabilistic maintenance optimization using the reliability index as a quantitative measure of performance in the presence of uncertainties and the present value of expected cumulative costs as the objective to be minimized. In “Structural Finite-Element Model Updating Using Ambient Vibration Tests Results” Jaishi and Ren describe a technique that uses ambient vibration test results to update finite-element models of bridge structures. Two cases studies are used to explore various objective functions and demonstrate the proposed method. One of the case studies involves a real concrete-filled tubular arch bridge. The authors show that the updated finite-element model of the bridge is able to produce natural frequencies that are in close agreement with experimental data. They also note that the cost of calculation is relatively low making the technique practical for everyday use. In “Robust Damage Location in Structures Using Taguchi Method,” Kwon and Lin present a damage-detection technique that can estimate the location and severity of damage in the presence of random errors in measured data as well as systematic errors in the analytical model. The authors use simulations of truss and cantilever beam behavior to demonstrate the technique.

In “Nonlinear Finite Element for Reinforced Concrete Slabs,” Phuvoravan and Sotelino present a new finite-element formulation suitable for nonlinear analysis of RC slabs. The element is comprised of a four-node Kirchhoff shell to represent concrete, and a two-node Euler beam to represent bar reinforcement. The beam nodes are eliminated from the structure mesh for computational expediency. The element is validated using experimental data. The issue contains one paper on wind loading: “Wind Tunnel and Uniform Pressure Testing of a Standing Seam Metal Roof Model” by Farquhar et al. In it, the authors describe a test in which a simplified model of a standing seam metal roof was tested to failure under both uniform loading and wind tunnel pressures. The goal of the study is to quantify the relationship between uniform uplift pressures used in standard test procedures and the dynamic pressures that occur during real wind loading. The authors compute an external pressure coefficient as a function of uniform pressure and the mean roof height∕wind speed needed to fail the model. They compare this coefficient to values in ASCE-7-02 and note that the ASCE specified value was larger than the measured value for their test. The authors indicate that the uniform pressure∕wind-speed relationship can be predicted analytically by integrating appropriate influence functions with nonuniform external pressures measured on a static model in a wind tunnel. In “Analytical Simulation of Snow Drift Loading,” O’Rourke et al. present an analytical procedure for simulating roof snow drift loads. The procedure is based on the physics of drift formation including the transport rate, which quantifies snow flux from the snow source area, and trapping efficiency, which quantifies the percentage of transported snow from the source area captured at the drift. Given wind speed and snowfall information, the procedure can be used to compute the maximum drift for a given snow source area. The authors exercise the proposed procedure and compare its results to ASCE-7 predictions. They argue that ASCE 7 procedures are appropriate for leeward roof step and gable roof drift.

The last three papers in the issue are related to timber construction. In the first paper “Damage-Based Seismic Reliability Concept for Woodframe Structures,” van de Lindt presents the results of a study on the development of a damage-based reliability model for light-frame wood structures subjected to earthquake loading. Model development necessitated a small experimental study, which is also described in the paper. After model calibration, a damage-based limit state for reliability analysis is developed to enable calculation of the structural reliability index provided against a prescribed damage level. In the companion papers “Three-Dimensional Model of Light Frame Wood Buildings. I: Model Description,” and “II: Experimental Investigation and Validation of the Analytical Model” Collins et al. present a detailed nonlinear, three-dimensional, finite-element model of light frame wood buildings. The model can be used for both static and transient analysis and accommodate various material and structural configurations. The model is validated through comparisons to experimental data pertaining to a full-scale asymmetric light frame building. The authors suggest that the model could be used to examine the assumptions and responses of light frame buildings and the accuracy of less refined, but more practical analysis tools.

The issue concludes with two technical discussions or previously published papers and the closures provided by the authors. The first article is by Sabouri-Ghomi, who discusses “Plastic Analysis and Design of Steel Plate Shear Walls” by Berman and Bruneau, published in vol. 129(11), November 2003. The discusser points out that some of the equations derived by Berman and Brueau are similar to equations that he previously derived using another methodology. The authors concur with the discussers’ comments and provide further motivation for the methods they used. The second discussion is by Folz, who comments on “Load-Sharing and Redistribution in a One-Story Woodframe Building” by Paevreet et al., published in vol. 129(9), September 2003. The discusser raises a number of questions about the tests presented by the authors. The authors’ rebuttal addresses the key issues brought up by the discusser.