Editor’s note

This final issue for 2006 contains 18 technical papers and one discussion item. The primary themes of the papers appearing in the issue focus on structural optimization and metal and composite structures. Also included are papers on fiber-reinforced concrete and masonry structures, seismic effects, wind effects, structural identification, and safety and reliability.

A two-part paper by Kargahi, Anderson, and Dessouky examining “Structural Weight Optimization of Frames Using Tabu Search” opens this issue of the journal. The optimization procedure is presented in the first paper, wherein the search method is tested to determine the minima of a function in a nonlinear nonconvex optimization problem. The procedure is applied to minimize the weight of three skeletal moment-resisting frames. The analytical study is extended in the second paper to evaluate the seismic performance of the optimized structures. It is shown that the tabu search (TS) algorithm produces a structure with lower weight than the optimization module of a commercially available program. Additionally the TS-designed frames resulted in seismic performance that is comparable with that of the original nonoptimized frame.

Two formulations based on the concept of simultaneous analysis and design are presented by Wang and Arora in “Alternative Formulations for Structural Optimization: An Evaluation Using Frames.” Response variables such as nodal displacements and member forces, in addition to design variables, are treated as optimization variables. Equilibrium equations become equality constraints in the optimization process, and the objective function and constraints become explicit functions of the optimization variables. Hence, their derivatives can be obtained more readily than in conventional optimization approaches. The effectiveness of the proposed methodology is demonstrated for framed structures.

High-performance “Distributed Genetic Algorithms (GA) for Structural Optimization on a PC Cluster” are proposed by Park et al. The algorithm consists of a $\mu$ -GA running on a master computer and multiple simple GAs running on slave computers. The algorithm is applied to minimum-weight design of steel structures, and results indicate that the process is computationally efficient and that the problem-dependent parameter-tuning process can be avoided. “Reliability-Based Optimization of Fiber-Reinforced Polymer (FRP) Composite Bridge Deck Panels” is developed by Thompson, Eamon, and Rais-Rohani and applied to minimize the weight of FRP composite bridge–deck panel configurations. The weight minimization problem comprised a deterministic strength constraint and two probabilistic deflection constraints. It was found that deflection constraints governed the design and that the optimization algorithm yielded little improvement for shallow panels but significant weight savings for deeper panels.

Material tests and analyses are presented by Kanvinde and Deierlein to investigate the accuracy of “The Void Growth Model and the Stress Modified Critical Strain Model to Predict Ductile Fracture in Structural Steels.” Although both models are shown to predict fracture accurately for all samples considered in the study, the application of the model to situations with high stress and strain gradients is shown to be sensitive to the characteristic length of the models. The “Fatigue Behavior of Welded Aluminum Light Pole Support Details” is investigated by Azzam and Menzemer through experimental and analytical studies. Nineteen specimens with cast shoe-base socket connections and 10 specimens with through-plate socket connection details were tested. An increase in the base-plate thickness for the through-socket connections decreased stress concentration adjacent to the weld toe, whereas a similar increase for the shoe-base connections did not appreciably change the peak normal stresses in the tube.

The long-term behavior of composite bridge beams subjected to sustained loads is examined by Giussani and Mola in “Service-Stage Analysis of Curved Composite Steel-Concrete Bridge Beams.” The coupling between vertical displacement and torsional rotation results in a system of sixth-order integro-differential equations that is solved by application of the Fourier series expansion and subsequent transformation into Volterra integral equations. The study finds that the effects of concrete creep contribute more significantly to long-term deflections than the effects of stress redistribution do. A new finite-element method that accounts for the effects of volume change caused by corrosion is presented by Goto and Kawanishi in “Change of Weld Residual Stresses due to Loss of Material.” If the initial distribution of residual stresses and deflection, as well as the corrosion profile, is known, the proposed method can evaluate the additional stress and deflection changes attributable to loss of material. The methodology is validated by an experiment in which a groove is mechanically formed on the surface of a steel box column to simulate local corrosion loss.

Chen, Young, and Uy present findings from an experimental program designed to study the “Behavior of High Strength Structural Steel at Elevated Temperatures.” It is shown that the reduction factors of yield strength and elastic modulus of high strength and mild steel are similar for the temperature range from 22 to $540°\mathrm{C}$ . An “Experimental Investigation of Aluminum Alloy Thin-Walled Tubular Members in Combined Compression and Bending” is reported by Zhu and Young. On the basis of observations from the testing that included 27 beam–columns and 4 pure bending tests, the writers conclude that the strengths predicted by the American, Australian/New Zealand, and European specifications are generally conservative. On the basis of tests of specimens under end-one-flange and interior-one-flange loading conditions, Ren, Fang, and Young summarize findings from nonlinear parametric analyses in “Finite Element Simulation and Design of Cold-Formed Steel Channels Subjected to Web Crippling.” It is concluded that design strengths calculated with U.S. specifications are generally nonconservative for channel sections with unstiffened flanges having web slenderness ranging from 7.8 to 108.5 subjected to web crippling.

“The Influence of Toughness on the Apparent Cracking Load of Fiber Reinforced Concrete Slabs” is experimentally examined by Bernard. Test results indicate that plasticity in the immediate postcrack range can influence the peak load associated with first crack even if the modulus of rupture is unchanged. Plastic methods of analysis that are based on yield line theory are recommended for predicting load resistance associated with cracking in heavily reinforced slabs with significant plasticity in the immediate postcrack range. A macromodel to predict the in-plane behavior of concrete masonry is proposed by El-Dakhakhni, Drysdale, and Khattab in “Multilaminate Macro-Model for Concrete Masonry: Formulation and Verification.” The masonry assemblage is replaced by an equivalent material that consists of a homogeneous medium intersected by two sets of planes of weakness along the head and bed joints. The behavior of the equivalent material is determined by smearing the influence of these planes and the reinforcement sets normal and parallel to the bed joints (when present in the case of reinforced masonry).

“New Measure of Severity of Near-Source Seismic Ground Motion” is proposed by Sasani. It is shown that an equivalent rectangular acceleration pulse can be used to characterize a near-fault ground motion and that inelastic responses of structures in the period range $0.2\text{–}1.0\mathrm{s}$ subjected to pulse-type ground motions correlate well with predictions that are based on the equivalent pulse.

A unified analysis framework that integrates both flutter and buffeting analysis is put forth by Xinzhong Chen in “Analysis of Long Span Bridge Response to Winds: Building Nexus between Flutter and Buffeting.” Buffeting response is explicitly expressed in terms of bridge modal properties at varying wind velocities and predicted through flutter analysis. The proposed framework provides physical insight into the aeroelastic response of the bridge and is also computationally more efficient. The study points to the potentially important role of complex aerodynamic admittance functions for an accurate prediction of the coupled buffeting response.

A new methodology for identifying structural parameters of linear dynamic systems using incomplete measurements is discussed by Koh, Tee, and Quek in “Condensed Model Identification and Recovery for Structural Damage Assessment.” Static condensation, dynamic condensation, and system-equivalent reduction expansion processes are adopted to recover the stiffness parameters of the entire system. The validity of the approach is demonstrated both experimentally and numerically for structures modeled as shear buildings.

A framework for “Optimal Seismic Design Considering Risk Attitude, Societal Tolerable Risk Level, and Life Quality Criterion” utilizing stochastic dominance decision rules is proposed by Goda and Hong. The framework is illustrated by numerical examples representing steel frame structures and considering detailed seismicity and cost information. Results from the study indicate that the optimal design obtained by minimizing expected life-cycle cost represents the optimal seismic design for a risk-neutral decision maker and defines the upper and lower bounds on the efficient or optimal seismic designs for risk-seeking and risk-averse decision makers, respectively.

Also included in this issue is a discussion by Shiming Chen of the paper “Finite-Element Model for Externally Prestressed Composite Beams with Deformable Connection” by Dall’Asta and Zona. The discusser seeks clarification on several issues: the modeling of concrete in tension in the hogging moment region of the beam, the capability of the section at the internal support to develop its full rotational capacity, and the strength of the shear connections. The writers clarify that the tension in concrete was ignored throughout the beam, resulting in tensile forces being carried only by the reinforcement at the interior support because the neutral axis shifts into the steel beam. It is further clarified that buckling failure mechanisms are excluded in the analysis, thereby assuming that the beam is restrained against such action, whereas rotation capacity is implicitly included in the model through the specified ultimate strains for the materials.

Ozbakkaloglu, T., and Saatcioglu, M. (2006). “Seismic behavior of high-strength concrete columns confined by fiber reinforced polymer tubes.” J. Compos. Constr., 10(6).

Barnes, R., and Fidell, J. (2006). “The performance in fire of small-scale CFRP strengthened concrete beams.” J. Compos. Constr., 10(6).

Zhou, C. E., and Vecchio, F. J. (2006). “Closed-form stiffness matrix for the four-node quadrilateral element with a fully populated material stiffness.” J. Eng. Mech., 132(12).

MacDougall, C., and Bartlett, F. M. (2006). “Mechanical model for unbonded 7-wire tendon with single broken wire.” J. Eng. Mech., 132(12).

Bu, J. Q., Law, S. S., and Zhu, X. Q. (2006). “Innovative bridge condition assessment from dynamic response of a passing vehicle.” J. Eng. Mech., 132(12).

Noumowe, A., Carre, H., Daoud, A., and Toutanji, H. (2006). “High-strength self-compacting concrete exposed to fire test.” J. Mater. Civ. Eng., 18(6).

Xu, S., Zhao, Y., and Wu, Z. (2006). “Study on the average fracture energy for crack propagation in concrete.” J. Mater. Civ. Eng., 18(6).

Nayal, R., and Rasheed, H. A. (2006). “Tension stiffening model for concrete beams reinforced with steel and FRP bars.” J. Mater. Civ. Eng., 18(6).