Editor’s Note

The primary themes of the papers appearing in this issue of the Journal are analysis and computation, wind effects, and seismic effects. The remaining papers cover four technical areas: concrete structures, metal structures, wood structures, and structural identification. Also included are two technical notes and a series of discussions on a previously published paper dealing with tension stiffening in concrete slabs.

A two-part paper on the progressive collapse analysis of multistory frames opens this issue of the Journal. In both papers, the alternate path method is used whereby two-dimensional (2D) models of the frames are loaded with gravity effects and then one or more first-floor columns are suddenly removed and the resulting large displacement inelastic response of each frame is investigated. In the first paper, Khandelwal et al. utilize computationally efficient macro models that are calibrated by using detailed finite-element models of beam-column subassemblages to investigate the progressive collapse resistance of seismically designed steel moment frame buildings. The simulation results show that the frame designed for high seismic risk has somewhat better resistance to progressive collapse than the system designed for moderate seismic risk. The better performance is attributed to layout and system strength rather than to the influence of improved ductile detailing.

The second paper, by Bao et al., extends the same approach to reinforced concrete frame structures. A simplified analytical model of a beam-column joint is used to represent essential and critical actions in the floor beams and the transfer of these forces through the joint region to the vertical elements. Two prototype buildings designed for lateral load requirements in a nonseismic and a seismic region are then considered in progressive collapse studies. As in the case of steel frames, the study also finds that special reinforced concrete (RC) moment frames detailed and designed in zones of high seismicity perform better and are less vulnerable to progressive collapse than RC frame structures designed for low to moderate seismic risk. In general, the alternate path method is shown to be useful for judging the ability of a system to absorb the loss of a critical member, though it does not provide information about the reserve capacity of the system.

An approach to predicting the strengths of concrete slabs including the case of combined in-plane and transverse loading is proposed by Liu and Teng in “Nonlinear Analysis of Reinforced Concrete Slabs Using Nonlayered Shell Element.” The nonlayered form is derived from the layered Mindlin type of shell element, and the material matrix is derived from the assumed simple stress-strain curves of the materials. Numerical examples are presented to show that the approach is capable of estimating the deflection as well as the strength of RC slabs under both in-plane and out-of-plane loads with acceptable accuracy. A “Fiber-Element Model of Posttensioned Hollow Block Masonry Shear Walls under Reversed Cyclic Lateral Loading” is presented by Madan et al. to predict the nonlinear in-plane flexural behavior of a masonry wall. The proposed approach accounts for the postcrack rocking type of motion encountered in these walls under cyclic loading. The model also attempts to incorporate the effect of the absence of bond between the longitudinal reinforcing steel and the surrounding masonry. Theoretical predictions are shown to reasonably compare with experimental results.

A new description of “Equivalent Static Wind Loads on Long-Span Roof Structures” based on the load-response-correlation (LRC) approach is proposed by Fu et al. The equivalent static wind load (ESWL) for a given peak displacement response is expressed in terms of the mean and dynamic components, and the wind loading inputs are determined based on the wind tunnel test results of multiple point pressure measurements on rigid structural models. The world’s longest spatial lattice structure is considered to illustrate the determination of the ESWLs by the proposed approach and to demonstrate its effectiveness in the design and analysis of long-span roof structures. It is shown through the example that the proposed approach can be used in conjunction with wind tunnel tests in predicting the response components not directly measured from the model tests and providing the design loads accurately.

The effects of 10 different opening configurations on the internal pressures in a typical two-story North American house were examined by using volume-scaled wind tunnel experiments by Kopp et al. in “Wind-Induced Internal Pressures in Houses.” Findings from the study show that the ASCE 7–05 standard significantly underestimated the peak internal pressure coefficients for all configurations with dominant openings, regardless of whether there was significant Helmholtz resonance or not. Peak external roof pressures were observed to be highly correlated in time with the internal pressures. It is shown that sealing the attic space from the main (living) space of the house has a significant benefit in not allowing the internal pressure to act on the underside of roof sheathing.

Kwon et al. in “e-Analysis of High-Rise Buildings Subjected to Wind Loads” presents features of the NatHaz Aerodynamic Loads Database (NALD), which integrates the latest advances in data management and mining for interactive queries of aerodynamic load data and contains an integrated online analysis framework for determining the resulting base moments and equivalent static wind loads for survivability and accelerations for serviceability. Examples are presented to illustrate the capabilities of NALD, including comparisons of response estimates to demonstrate the flexibility of the analysis engine to provide a platform that can be readily expanded and supplemented to yield a comprehensive, simplified, and efficient avenue for e-analysis of high-rise buildings.

“Effect of Soil–Structure Interaction on Seismic Isolated Bridges” is studied by Ucak and Tsopelas. A generic bilinear hysteretic model is used to model the isolation system. The behavior of the pier is assumed to be linear, and the foundation system is modeled with frequency-dependent springs and dashpots. Two bridge systems were considered, one representative of short, stiff highway overpass systems and another representative of tall, flexible multispan highway bridges. The results from comprehensive nonlinear time history analyses with far-field and near-fault accelerograms show that soil-structure interaction causes higher-isolation system drifts, as well as, in many cases, higher pier shears when compared to the fixed-pier bridges.

“Three-Dimensional Seismic Response of Humboldt Bay Bridge-Foundation-Ground System” by Elgamal et al. presents results from a simulation study that indicate that permanent ground deformation may induce settlement and longitudinal/transversal displacements of the abutments and foundations. The relatively massive approach ramps are also likely to contribute to this damage condition, which imposes large stresses on the bridge foundations, supporting piers, and superstructure.

“Analytical Model of Ground Motion Pulses for the Design and Assessment of Seismic Protective Systems” by He and Agrawal utilizes pulse period, decay factor, and shape parameters to account for both buildup and decaying phases of pulses. For a base-isolated building with 2.5-s natural period and equipped with supplemental viscous dampers, it is demonstrated that supplemental dampers are most effective when the structural period is close to the pulse period. Displacement reduction using supplemental passive dampers is smaller when the pulse period is longer or shorter than the structural period, although the displacement demand is quite significant.

“Dynamic Shear and Axial-Load Failure of Reinforced Concrete Columns” by Elwood and Moehle discusses the global response of frames and shear and axial load response of shear-critical interior columns. An experimental program examined the behavior of two half-scale, one-story frames with axial loads representative of those expected for the lower story of a multistory building. A comparison of the results from the two specimens indicates that the behavior of the frame is dependent on the initial axial stress on the center column. The specimen with lower axial load failed in shear but maintained most of its initial axial load. For the specimen with higher axial load, shear failure of the center column occurred at lower drifts and earlier in the ground motion record and was followed by axial failure.

In their paper “Two-Dimensional Grid Strut-Tie Model Approach for Structural Concrete,” Yun and Kim propose an improved approach employing a single type of grid strut-tie model whereby various load combinations can be considered. In addition, the approach performs an automatic selection of an optimal strut-tie model by evaluating the capacities of struts and ties using a simple optimization algorithm. The validity and effectiveness of the presented approach is verified by conducting the analyses of the four reinforced concrete deep beams tested to failure and the design of the shear-wall with two openings.

A simple yet efficient “Physical Theory Hysteretic Model for Steel Braces” is developed by Dicleli and Calik to simulate the inelastic cyclic axial force–axial deformation and axial force–transverse deformation relationships of steel braces. The model combines analytical formulations based on the nonlinear behavior of the brace with some semiempirical normalized formulae developed from available experimental data. The model accounts for the growth effect and degradation of buckling capacity due to the Baushinger effect and residual kink present within the brace. The analytically obtained axial force versus axial displacement as well as axial force versus transverse displacement hysteresis loops are shown to compare reasonably well with the experimental observations.

A mechanics-based method for “Predicting Strength of Wood I-Joist with a Circular Web Hole” is proposed by Pirzada et al. Peak tensile stresses obtained from curved beam analysis is shown to agree well with those predicted using the finite-element method. Failure load is attained by the application of a fracture mechanics-based finite-area method. The developed method is used to predict failure loads of wood I-joists of different depths with different web-hole sizes. Simplification of the method leads to the development of a simple expression that can be used for engineering design. The predicted strengths are found to be in good agreement with the corresponding test results.

Findings from an experimental study to identify component stiffness degradation are presented by Chen et al. in “Large-Scale Shake Table Test Verification of Bridge Condition Assessment Methods.” Two large-scale shake table progressive damage experiments, one on a two-column reinforced concrete bridge bent specimen and the other on a two-span, three-bent specimen, were performed, in which the specimens were subjected to earthquake ground motions with increasing amplitude. In each of the damaged stages between two strong motions, low-amplitude vibrations of the specimens were used to identify the postevent component stiffness coefficients by optimizing the parameters in a linear time-invariant model. The identified stiffness degradation is shown to be consistent with the experimental hysteresis and could be quantitatively related to the capacity residual of the components.

“Control of Civil Structures Using a Semiactive Stiffness System Based on Variable Amplification” is investigated by Walsh et al. The system consists of a novel variable amplification device (VAD) connected to a simple spring. When integrated with other semiactive components, the stiffness of the VAD-spring system can be adjusted based on feedback from the structure’s response. The proposed system is simulated for an eight-story building subject to multiple seismic excitations, and results indicate that the VAD-spring system is an effective means of controlling vibrations in seismically excited buildings. In “Gusset Plate Connections to Circular Hollow Section Braces under Inelastic Cyclic Loading,” Martinez-Saucedo et al. attempt to resolve issues related to slotted hollow structural section connections that are typically reinforced with steel cover plates to increase the net area at the critical location to avoid premature fracture under tension-loading cycles. The potential for an innovative detail, wherein both the tube and the plate are slotted in such a manner that the weld terminates at the tube gross cross section, is demonstrated by means of three large-scale specimen tests under pseudodynamic loading.

The issue concludes with three discussions of a paper by Gilbert on “Tension Stiffening in Lightly Reinforced Slabs,” which appeared in June 2007. In his discussion, Bischoff provides additional background on the tension stiffening and efforts by others to incorporate this effect by modifying the effective moment of inertia. He also points to the need to consider important issues such as shrinkage restraint, preloading from construction loads, and long-term effects. Kaklauskas et al. share the view that shrinkage and creep need to be considered in deformation calculations, and they provide information and results using their own model, which incorporates shrinkage effects. Finally, Burns et al. note that the quality of an EC2 deflection prediction is highly dependent on the chosen calculation accuracy and, especially if the simplified method is used, on the load and support situation.

In his closure, Gilbert supports Bischoff’s recommendation to replace Branson’s equation with Bischoff’s equation for the effective moment of inertia in future editions of the American Concrete Institute Building Code requirements. Gilbert concurs that restraint of concrete shrinkage has a profound effect on cracking in reinforced concrete members. He is not convinced that the approach proposed by Burns et al. can be easily extended to account for long-term effects using a constant bond stress.