Editor’s Note

There are two main themes in this month’s issue: steel structures and concrete and masonry structures. With 10 of the 14 technical papers and a discussion addressing steel structures, the issue is skewed more toward the former than the latter. Another technical discussion and closure paper on reliability round off the issue. Six of the papers on steel are collected in a special section on steel I-girders, while the other four address system response and constitutive modeling. The papers on concrete and masonry address a variety of issues through modeling and experimentation.

Within the special section are five related papers by Professor D. W. White (Georgia Tech) and his coworkers, plus a sixth by Yura et al. on the same general topic. The five companion papers represent a distillation of the results of an ASCE/SEI special project that was conducted to develop an experimental basis for decisions made in the development of the AASHTO 2004 and AISC 2005 provisions for stability design of all types of steel I-section members. The product of this effort was reviewed by the ASCE-SEI Technical Administrative Committee (TAC) on Metals, which suggested that the report be published in the Journal pending successful peer review. The report was rewritten as a series of five papers as described next, each of which underwent the normal rigorous review process implemented by the Journal’s editorial board.

The first paper in the special section, “Unified Flexural Resistance Equations for Stability Design of Steel I-Section Members,” by White, presents the fundamental logic and calculations behind the AASHTO 2004 and AISC 2005 provisions. The provisions in these documents for flexural design of steel I-section members have been revised in their entirety relative to previous specifications so as to simplify their logic, organization, and application while also improving their accuracy and generality. While both sets of provisions for flexural resistances are fundamentally the same, the organization in AASHTO emphasizes design of typical welded I-girders, while AISC emphasizes design of noncomposite nonhybrid compact doubly symmetric I-section members. The combined AISC-AASHTO rules are presented as a single set of flowcharts applicable for all types of steel I-section members.

The second paper, “Shear Resistance of Transversely Stiffened Steel I-Girders” by White and Barker, evaluates the accuracy and ease of use of what the authors consider to be the 12 most promising models for computing the shear resistance of transversely stiffened steel I-girders. Statistical analyses are conducted on the predictions by the various models using an updated data set from an extensive experimental database. The authors assert that the models adopted by AASHTO 2004 and AISC 2005 give the best combination of accuracy and simplicity for calculation of the shear resistance of transversely stiffened I-girders.

The third paper, “Shear Strength and Moment-Shear Interaction in Transversely Stiffened Steel I-Girders” by White et al., presents the results from the collection and data analysis of a total of 186 high-shear low-moment, high-shear high-moment, and high-moment high-shear experimental I-girder tests. The authors placed particular emphasis on the extent to which web shear postbuckling (tension-field action) strength is developed in hybrid I-girders, as well as on the interaction between the flexural and shear resistances in hybrid and nonhybrid I-section members. The authors suggest that within specific constraints that address the influence of small flange size, several existing models can be used without the need for consideration of M-V strength interaction for both nonhybrid and hybrid I-girder designs.

In “Unified Flexural Resistance Equations for Stability Design of Steel I-Section Members—Uniform Bending Tests,” White and Jung evaluate the lateral torsional and flange local buckling (LTB and FLB) resistance predictions from the AASHTO 2004 and AISC 2005 and previous specifications versus uniform bending experimental test results. The authors indicate that the notional reliability for LTB is found to be reasonably constant and consistent with the targeted level in the first LRFD specification of 1986. The authors show that the unified resistance equations, combined with a simple design-oriented procedure for calculation of elastic LTB K factors, capture the test results accurately throughout the inelastic and elastic LTB ranges. They also suggest that the reliability of FLB is slightly higher than that of LTB.

In the last of the five companion papers, “Unified Flexural Resistance Equations for Stability Design of Steel I-Section Members—Moment Gradient Tests,” White and Kim evaluate the LTB and FLB resistance predictions from the updated specifications versus moment gradient experimental test results. The authors considered two types of moment gradient tests: (1) tests in which the moment varies linearly within the critical unbraced length; and (2) tests in which the member is subjected to concentrated transverse load at a specified height relative to the depth of the cross section, resulting in a multilinear moment diagram within the critical unbraced length. By using an extensive database of test results, reliability indices are estimated for The LRFD of buildings based on the statistics from these tests combined with established statistics for material and fabrication bias factors and the ASCE 7 load model. The authors suggest that, in certain cases, the reliability index is substantially larger for moment gradient loading than that for uniform bending.

Wrapping up the special section on I-girders is a paper by Yura et al., “Global Lateral Buckling of I-Shaped Girder Systems,” in which the authors derive a closed-form solution for global buckling of twin girder systems interconnected via cross-frames. The proposed solution, which is suitable for implementation in design specifications, was developed for a uniform moment-loading condition. Using finite-element analyses (FEA) to verify the closed-form solution and extend it to more practical loading conditions, the authors show that the load height condition had only a minor effect on twin girders compared with the published effects on single girders. Both singly and doubly symmetric sections are studied, along with a variety of influential parameters. The authors also propose a method for improving the global buckling capacity through the use of a partial top-flange lateral bracing system.

Returning to regular papers, Choi and Park, in “Ductility and Energy Dissipation Capacity of Shear-Dominated Steel Plate Walls,” report on an experimental study that was performed to investigate the potential maximum ductility and energy dissipation capacity of steel plate walls with thin infill plates. The authors tested three specimens of a three-story steel plate wall along with a concentrically braced frame (CBF) and a moment-resisting frame (MRF) for the purpose of comparison. The test results showed that the steel plate walls exhibited much better ductility and energy dissipation capacity than to the CBF and MRF. On the basis of these test results, the authors concluded that shear-dominated steel plate walls with thin infill plates possess excellent ductility capacity, as well as high strength and stiffness. In “Direct Analysis and Design of Steel Frames Accounting for Partially Restrained Column Base Conditions,” Eroz et al. discuss the effect of partial column base fixity on system response using the direct analysis method (DM) promoted by the AISC 2005 specifications. They also proposed an extension of DM for frames continuing web-tapered members and provided examples to illustrate their point.

In “Three-Stage Full-Range Stress-Strain Model for Stainless Steels,” Quach et al. present a stress-strain model for stainless steels that is capable of accurate predictions of response over the full ranges of both tensile and compressive strains. The new model derives from the Ramberg-Osgood model and is calibrated to existing experimental data. The authors demonstrate the accuracy of the proposed model by comparing its predictions with experimental results.

The last paper on steel structures in this issue is “Validation of Cyclic Void Growth Model for Fracture Initiation in Blunt Notch and Dogbone Steel Specimens,” by Kanvinde and Deierlein. The authors conducted tests and finite-element analyses of blunt notch and dogbone specimens to demonstrate the application and validation of the cyclic void growth model (CVGM), which was previous by proposed to evaluate the initiation of ductile fracture under cyclic loading in steel structures. The authors assert that the test specimens they used reflect stress-and-strain conditions encountered in structural steel components and provide sufficiently strong stress-and-strain gradients to validate the characteristic length assumptions in the model. They show that models employing the CVGM criterion predict fracture with good accuracy across the specimen geometries, steel types, and loading histories considered.

The first paper dealing with concrete and masonry structures is “Uniaxial Shear-Flexure Model for Reinforced Concrete Elements” by Mostafaei and Vecchio. In it, the authors propose a simple performance-based analysis of reinforced concrete columns subjected to shear, flexure, and axial loads. The presented model, which the authors indicate is simpler than existing models, is shown to simulate experimental load-deformation responses well. Wallace and Elwood, in “Investigation of the Axial Load Capacity for Lightly Reinforced Wall Piers,” used a shear-friction model to establish the ability of wall piers to support vertical loads after substantial loss of pier lateral load capacity. The authors suggest that typical wall piers are capable of sustaining relatively large lateral drift ratios prior to loss of vertical load-carrying capacity, which is consistent with postearthquake observations. However, they also assert that the drift capacity may be substantially less for poorly detailed walls.

Also in the vein of earthquake loading, Puntel and Saouma in “Experimental Behavior of Concrete Joint Interfaces under Reversed Cyclic Loading” report on a large-scale test that they conducted on concrete joints subjected to constant compressive confinement and reverse cyclic shear load. They discuss shear strength, dilatancy degradation, and overall response; they also describe roughness as being composed of two orders of asperities with different sizes. The last paper dealing with concrete and masonry structures is “Analytical Prediction of the Seismic Performance of Masonry Infilled Reinforced Concrete Frames Subjected to Near-Field Earthquakes” by Madan and Hashmi. In their paper, the authors focus on the effect of near-field earthquakes on performance of RC frames with masonry infills. Using nonlinear static and dynamic analysis, they investigate the effect on seismic response of distribution of masonry infill panels in the elevation.

This month’s issue has two discussions. In the first, Gamble discusses “Experiments on Distortional Buckling of I-Beams,” Journal of Structural Engineering, Vol. 133, No. 7, July 2007, pp. 1009-1017, by T. Zirakian and H. Showkati. Gamble questions the relationship between ultimate and yield stresses presented by Zirakian and Showkati. He indicates that the listed ultimate strength values are between 3.2 and 4.5 times the yield, whereas he expects them to be in the range of 1.5 to 1.7 times yield. Gamble also indicates that there is no description of the criteria used to define yield stress and that no specific standard is cited for the steel. Zirakian and Showkati did not provide closure to the challenges posed by Gamble.

The second discussion article is by Chopra, who discusses “Observations on the Reliability of Alternative Multiple-Mode Pushover Analysis Methods,” Tjhin et al., Journal of Structural Engineering, Vol. 132, No. 3, March 2006, pp. 471-477. In their original paper, Tjhin et al. suggested that the robustness of the multimode pushover procedures that they evaluated was open to question. They reached this conclusion by comparing seismic demands estimated by a modified modal pushover analysis (MPA) procedure proposed by Chopra, results from a new MPA method that they had proposed, and demands determined by a nonlinear response history analysis. They also suggested that the modified MPA is hampered by the occurrence of reversals in higher-mode pushover.

Chopra agrees that reversals are a potential impediment to MPA but notes that their occurrence is rare. He discusses ways by which the effects or reversals can be eliminated and contends that the modified MPA procedure chosen by the authors estimates the response contributions of higher modes by assuming the building to be linearly elastic, which implies an additional approximation beyond those in the original MPA procedure that he developed with Goel. He also contends that the number of case studies used by the authors is too limited for them to draw their conclusions. In their closure, Tjhin et al. responded to the points raised by Chopra and agreed with him about the importance of identifying conditions under which simplified analysis procedures such as MPA can be used with confidence.