Editor’s Note

The primary themes of the papers appearing in this issue of the Journal are metal structures and structural identification. The remaining papers cover three technical areas: concrete structures, wood structures, and special design issues.

This opening issue of the Journal for 2009 begins with a two-part paper by Lee et al. revisiting the anchoring mechanism believed to be the source of the postbuckling strength of plate girder web panels. The first paper in the series is titled “Further Insights into Postbuckling of Web Panels. I: Review of Flange Anchoring Mechanism.” The study finds that the anchoring mechanism does not develop, regardless of the weight of the flanges, without transverse stiffeners that have sufficient axial stiffness necessary to keep the flanges from moving inwards toward the web panel during the anchoring action. In practical plate girders, the contribution of the flange anchoring mechanism to the postbuckling strength is negligibly small, not because the flanges are too flexible to function as anchors, but because the transverse stiffeners are axially too flexible to support the flanges. Taking advantage of the flange anchoring mechanism in practical design is hence impractical, if not impossible. The follow-on study “Further Insights into Postbuckling of Web Panels. II: Experiments and Verification of New Theory” highlights the source of plastic hinge-like failure modes often observed in tests. It is found that such failure modes, accompanied by severe flange deformations, are not due to the anchoring action of the flanges, but to direct shear force acting on the flange cross sections. The study also examines why the horizontal anchoring mechanism cannot develop in interior panels or in end panels with a heavy end stiffener. Findings from the overall study reveal that the anchoring mechanism is virtually nonexistent in normal plate girders. It is also found that a tension field can develop in the end panel, and therefore the writers recommend that any restriction contrary to this observation found in current design codes be repealed.

Results from 18 large-scale tests of steel bracing members are presented by Fell et al. in “Experimental Investigation of Inelastic Cyclic Buckling and Fracture of Steel Braces.” The brace specimens include square hollow structural shapes (HSS), pipe and wide-flange sections. Among the numerous parameters investigated, loading history, width-thickness ratio, and slenderness ratio are shown to have the largest influence on brace ductility. The test data suggest that for some HSS and pipe specimens, current seismic design provision limits on maximum width-to-thickness ratios may not provide sufficient ductility for seismic design. Measurements of brace stiffness, tensile strength, and compressive strength are shown to compare well with design formulae. In “Seismic Column Demands in Ductile Braced Frames,” Richards investigates seismic demands in buckling restrained braced frames (BRBFs), special concentrically braced frames (SCBFs), and eccentrically braced frames (EBFs) for varying story height and strength levels using nonlinear time-history analyses. For columns at the base of 9- and 18-story BRBFs and EBFs, axial demands were found to be 55 to 70% of demands typically used in design, suggesting potential cost savings on columns, anchor rods, base-plates, and foundations. However, in low-rise SCBFs considered in the study, the column axial demands were significantly greater that commonly used in design. It is also observed that column rotation demands were lower than those reported in previous research.

The analysis of existing data forms the basis of the investigation by Gardner and Cruise for “Modeling of Residual Stresses in Structural Stainless Steel Sections.” The collated residual stress data are used to develop models for predicting the magnitude and distribution of residual stresses in press-braked, cold-rolled, hot-rolled, and fabricated stainless steel structural sections. Press-braked and cold-rolled sections generally show low membrane residual stresses, but high bending residual stresses. For the stainless steel hot rolled angles, both the membrane and bending residual stresses were of relatively low magnitude. The proposed residual stresses patterns based on either mean or characteristic values may be incorporated into finite element or simulation models to allow for their influence on structural response. The membrane residual stresses, although lower in magnitude, are expected to have a stronger effect on the structural response than the bending residual stresses.

Results from on-site tests on a newly built long-span suspension bridge are presented by He et al. in “System Identification of Alfred Zampa Memorial Bridge Using Dynamic Field Test Data.” The dynamic tests were conducted on the bridge just before it opened to traffic. These tests provided a unique opportunity to identify the modal properties of the bridge in its as-built condition with no previous traffic loads or seismic excitation. A benchmark study on modal identification is performed using three different state-of-the-art system identification algorithms based on ambient as well as forced vibration measurements. Overall, the modal parameters identified using these system identification methods are found to be in very good agreement for both ambient and forced vibration tests. However, the modal damping ratios identified from forced vibration test data are in general higher than those estimated from ambient vibration data. The identified dynamic properties are also compared with their analytical counterparts from a three-dimensional finite element model. The measured modal properties can serve as a baseline for future health monitoring studies of the bridge.

“Assessment of Repairs and Strengthening of a Historic Masonry Pagoda Using a Vibration-Based-Method” by Li et al. describes the dynamic characteristics of a historic monument following a series of maintenance and refurbishment projects. Instrumented data following foundation reinstatement, foundation strengthening, and structural reinforcement were used to evaluate the change in stiffness of each story of the pagoda to assess the effects of each phase of work on the condition of the monument. Findings from the proposed vibration-based method are shown to qualitatively agree with observations from external inspection, thereby offering a simple technique to identify structural weakness and to assess the effects of strengthening and repair of an a historic structure.

In “Influence of Longitudinal Reinforcement on One-Way Shear in Slabs and Wide Beams,” Lubell et al. present results from six new experimental tests on slender shear-critical members of varying width, reinforcement ratio, and reinforcement strain at the time of failure and are compared with published data. The data are used to evaluate sectional shear models for slender reinforced concrete members developed by several research groups. It is demonstrated that the one-way shear stress at failure was accurately predicted using a recent modified compression field theory-based (MCFT) capacity model that incorporates a “size effect” factor related to the member depth, and a “strain effect” factor related to the longitudinal reinforcement demands. Design implications for shear associated with the flexural reinforcement design strength are identified.

The addition of adhesives to resist uplift wind forces at the roof framing-to-sheathing is investigated by Turner et al. in “Tests of Adhesives to Augment Nails in Wind Uplift Resistance of Roofs.” Two specimen configurations were used to compare the tensile resistance of typical nail connections to that of connections that also included acrylic foam tape or construction adhesive. Connections were tested using monotonic deflection rates and a cyclic loading protocol that was a modified version of the CUREE protocol. The results demonstrated that the specimens constructed with the addition of adhesives provided an improvement over the ones with only a nail. The addition of construction adhesive resulted in the highest resistances for monotonic tests, whereas the addition of adhesive tape provided the most strength in the cyclic tests.

The treatment of counteracting loads is the subject of Ellingwood and Li’s paper “Counteracting Structural Loads: Treatment in ASCE Standard 7-05.” Beginning with the 1998 edition, the load factor on nominal dead load in the ASD combination was increased to 0.6 to achieve consistency between ASD and LRFD. This paper examines the treatment of counteracting loads in ASCE Standard 7 from a reliability viewpoint, and shows that failure to reduce the gravity load for design may lead to inadequate safety. Findings from the study show that in situations where counteracting loads govern design, the dead load factor must be substantially lower than 0.9 to achieve comparable measures of reliability and performance for ASD and LRFD. To ensure a measure of consistency between strength design (or LRFD) and ASD for those specifications that still allow or require its use, the writers suggest that the factor applied to dead load in situations where its effect is counteracted by the effects of other lateral or uplift forces should not be increased above 0.6.