Journal of Structural Engineering

This paper examines efficacy of tuned mass dampers (TMD) in controlling self-excited motion resulting from negative damping. New optimal TMD parameters are suggested which provide better performance than those suggested in the literature. The dependence of TMD performance on structural damping is highlighted. The equations of motion of a combined system comprised of multiple TMDs attached to a bridge deck are presented where the bridge motion is described in terms of reduced-order modal coordinates. Details concerning the multimode coupled flutter of long-span bridges with auxiliary TMDs are provided. The effectiveness and limitations of TMDs for controlling multimode bridge flutter are examined, emphasizing the dependence of TMD performance on the bridge dynamic and aerodynamic characteristics. This study shows that the effectiveness of TMDs is rather limited in controlling a hard-type flutter characterized by negative aerodynamic damping that grows rapidly with increasing wind speed beyond the onset of flutter. However, it is relatively effective in controlling a soft-type flutter in which the negative damping builds up slowly with increasing wind speed. Robust TMD design issues are also discussed in light of their sensitivity to design parameters in the vicinity of optimal values.

In this paper the results of a test program devoted to the assessment of the cyclic behavior of beam-to-column welded connections are presented. The specimens are representative of the common European practice, which differs from both U.S. and Japan practice, due to the size of the connected members and to the welding technique and details. Three specimen groups, characterized by different values of the relative column-beam-panel zone strengths, are designed and tested under different loading histories. In particular six tests (five cyclic and one monotonic) have been carried out on each group of specimens, encompassing a total of 18 tests. The tests have evidenced the effect of column-to-beam strength ratio and of panel zone design on the cyclic behavior and failure modes of the connections. Furthermore the dependence of the absorbed energy and cumulative plastic rotation of the joint upon the applied loading history is shown. Finally design implications are suggested on the basis of the test results.

This paper delineates the development of a prototype hybrid knowledge-based system for the optimum design of liquid retaining structures by coupling the blackboard architecture, an expert system shell VISUAL RULE STUDIO and genetic algorithm (GA). Through custom-built interactive graphical user interfaces under a user-friendly environment, the user is directed throughout the design process, which includes preliminary design, load specification, model generation, finite element analysis, code compliance checking, and member sizing optimization. For structural optimization, GA is applied to the minimum cost design of structural systems with discrete reinforced concrete sections. The design of a typical example of the liquid retaining structure is illustrated. The results demonstrate extraordinarily converging speed as near-optimal solutions are acquired after merely exploration of a small portion of the search space. This system can act as a consultant to assist novice designers in the design of liquid retaining structures.

Proper material constitutive models for concrete-filled tube (CFT) columns are proposed and verified by the nonlinear finite element program ABAQUS against experimental data. The cross sections of the CFT columns in the numerical analysis are categorized into three groups, i.e., circular section, square section, and square section stiffened by reinforcing ties. Via the numerical analyses, it is shown that for circular CFT columns, the tubes can provide a good confining effect to the concrete especially when the width-to-thickness ratio $D/t$ is small (say $D/t<40)\text{.}$ For square CFT columns, the tubes do not provide a large confining effect to the concrete especially when the width-to-thickness ratio $B/t$ is large (say $B/t>30)\text{.}$ The confining effect of square CFT columns with reinforcing ties is enhanced by the use of reinforcing ties especially when the tie spacing is small and the tie number (or tie diameter) is large.

Improved damage spectra are proposed to quantify the damage potential of recorded earthquake ground motion. The damage spectra are based on a combination of normalized hysteretic energy and deformation ductility of a series of inelastic single-degree-of-freedom systems. The damage spectra proposed will be zero if the structure remains elastic, i.e., no significant damage is expected, and will be unity if there is a potential of collapse. By varying a coefficient in their formulations, improved damage spectra can be reduced to commonly used normalized hysteretic energy or displacement ductility spectra. The damage spectra are computed for hundreds of horizontal ground motions recorded during the Landers and Northridge earthquakes. Source-to-site attenuation of the damage spectra in the Northridge earthquake is examined. Calibration of the damage spectra for an instrumented building damaged during the Northridge earthquake is also carried out. The improved damage spectra are promising for assessment of the performance-based seismic vulnerability of existing structures. For example, following an earthquake, near real-time contour maps of damage spectral ordinates at selected periods provide useful information on the spatial distribution of the damage potential of recorded ground motion for specific types of structures. The concept of damage spectra is also promising for carrying out performance-based design of new structures.

The computational assessment of system reliability of structures has remained a challenge in the field of reliability engineering. Calculation of the failure probability for a system is generally difficult even if the potential failure modes are known or can be identified, because available analytical methods require determination of the sensitivity of performance functions, information on mutual correlations among potential failure modes, and determination of design points. In the present paper, a method based on moment approximations is proposed for structural system reliability assessment that is applicable to both series and nonseries systems. The point estimate method is applied to evaluate the first few moments of the system performance function of a structure from which the moment-based reliability index and failure probability can be evaluated without Monte Carlo simulations. The procedure does not require the computation of derivatives, nor determination of the design point and computation of mutual correlations among failure modes; thus, it should be computationally effective for structural assessment of system reliability.

Plastic interaction relationships for pipes subjected to biaxial bending, biaxial shear, torsion, axial force, and internal/external pressure were recently developed. This paper reports a full-scale experimental program that consists of six tests aimed at verifying the validity of the interaction relationships under loads that induce shear force, bending, and twisting moments. The pipe configurations in the postbuckling range of deformation are observed and documented. The peak loads obtained in the experiments agree very well with analytical predictions. Based on the experimental results obtained in this study and on other complementary experiments previously conducted, it is proposed that the interaction relationships be adopted in the design of elevated stocky steel pipes. An example is provided in order to illustrate the applicability of interaction relationships in a practical design problem.

A nondestructive crack detection (NCD) scheme to locate a crack and estimate the crack size by using changes in the modal parameters of a class of bridges is presented in this paper. First, the theory underlying a nondestructive crack localization and crack-size estimation algorithm for beam-type structures is outlined. Next, a field experiment to verify the feasibility and applicability of the NCD scheme is detailed. In that experiment, predamage and postdamage modal data extracted from a full-scale plate–girder bridge are described. The modal data are fed into the NCD scheme that localizes crack and estimates crack size in the test structure. The result of the analysis indicates that the proposed NCD algorithm correctly locates the crack and accurately estimates the size of the crack.

This paper presents a general method for the limit analysis of structures made of rigid blocks, which can be used in the assessment of ancient masonry structures. This method takes into account nonassociated flow rules and limited compressive stresses for masonry. The mathematical programming problem resulting from the formulation is a nonlinear optimization problem. The feasible initial solution is obtained by solving a mixed complementarity problem while, in a second step, the complementarity constraint is temporarily relaxed to allow for optimizing the load factor. A tie element added to the rigid block formulation permits the analysis and design of strengthening. Several results, obtained by a computer implementation of the method, are presented and discussed. The results show the validity of the method by comparison with experimental data, and also show the benefit of the approach in providing a better insight into the behavior of ancient masonry structures.

The effect of loss of bond for longitudinal reinforcement on the structural behavior of RC beams is investigated in a test series of six beams where nominal length without bond is varied. Loss of bond is created with plastic tubes surrounding the longitudinal reinforcement leaving short bonded lengths over support and at positions where stirrups are crossing the longitudinal reinforcement. Using a bond-stress slip relationship made it possible to see that the total bond force could be calculated as the sum of local bond forces. Maximum bond force occurs at maximum load and when the available bond force decreases the load carrying capacity is also reduced. Even for significant loss of bond, the reduction in shear capacity was moderate, implying that short bond lengths are sufficient to create high bond forces. Local areas with bond contributes efficiently to create anchorage for the longitudinal bars. It was also found that the loss of bond was compensated by increased utilization of stirrups, and that loss of bond does not lead to more brittle types of failure.

Standard construction $2.4m\times 2.4m$ woodframe shearwalls were tested using different loading protocols. The CUREE-Caltech protocols, recently developed by Krawinkler in 2001, were compared with other previously used protocols. The following protocols were considered in the study: monotonic, CUREE-Caltech standard (CUREE), CUREE-Caltech near-fault, sequential phased displacement (SPD), and international standards organization. Findings from the research indicate that loading sequence has a significant influence on shearwall performance. Protocols with a large number of cycles (SPD) tended to produce nail fatigue fractures, which were not nearly as prevalent in protocols with lower numbers of cycles (CUREE). The research clearly showed that the nail fatigue fractures were associated with a large energy demand related to the number of cycles, which resulted in a reduced ultimate strength and deformation capacity. The initial stiffness of the specimens was relatively unaffected by the loading protocol used; however, there was significant variation in the stiffness values obtained depending on the stiffness definition used.

A study on the effects of gypsum wallboard and stucco nonstructural finish materials on the seismic capacity of woodframe shearwalls was carried out. The focus was on the performance of shearwalls with these finish materials combined with a wood structural sheathing (plywood or oriented strand board). Also investigated were the effects of dynamic loading with rates on the order of a woodframe structural response to a seismic event. All testing was performed using standard construction 2.4 m square woodframe shearwalls loaded at the top plate. Findings from the study indicate that nonstructural finish materials have a substantial influence on the performance of woodframe shearwalls, causing increases in strength and stiffness, and reductions in deformation capacity. There was also a noticeable change in failure mode created by the application of finish materials. The loading rate study, comparing specimens under quasi-static loading with specimens loaded dynamically, showed only modest differences caused by dynamic loading. Small increases in strength and stiffness were observed in some specimens; however, in general, the effect was negligible.

The paper describes a series of tests on cold-formed channels with inclined simple edge stiffeners compressed between fixed ends. The simple edge stiffeners of the channel sections consist of simple lips. The new North American Specification and the Australian/New Zealand Standard for cold-formed steel structures provide design rules for channels with inclined edge stiffeners. However, the design rules are mainly based on tests of sections having edge stiffeners perpendicular to the flanges. The provisions for inclined edge stiffeners are determined based on intuition without rigorous experimental evidence. Hence, it is important to obtain test data for sections with edge stiffeners not perpendicular to the flanges. A series of compression tests was performed on cold-formed channels with inclined edge stiffeners. The edge stiffeners of the channels were inclined at different angles for both outwards and inwards. The distortional buckling mode, where the flange and lip rotate about the flange-web junction restrained by the web, was observed in all tests. The test strengths are compared with the design strengths predicted by the North American Specification, the American Iron and Steel Institute Specification, and the Australian/New Zealand Standard for cold-formed steel structures. For the North American Specification and the American Iron and Steel Institute Specification, the distortional buckling mode is not explicitly accounted for in design. However, the Australian/New Zealand Standard has explicit design rules for distortional buckling in addition to the edge stiffened design rules which are the same as the American Iron and Steel Institute Specification.

Finite element analysis (FEA), combined with on-site and laboratory tests, might be an effective and versatile tool for studying existing structures, providing useful insights on structural resisting resources and failure mechanisms. When adopted in structural rehabilitation studies, FEA might help the engineer in defining the retrofitting strategies, and in testing their effectiveness. This paper presents an application of nonlinear FEA to the diagnostic study of the wooden dome of Palazzo della Loggia (Brescia, Italy, 1914), in order to assess the feasibility of its preservation, and to aid in planning its strengthening. The FEA demonstrated the influence of local mechanisms, such as clearances and splitting failures at joints, on the large existing crown deflection. Small scale tests were then carried out to study local phenomena and to assess the efficiency of some retrofitting techniques aimed at reinforcing the joints. The numerical model was then used to simulate the strengthening works. The results proved that the wooden roof could be preserved by repairing a few critical joints only, thus considerably limiting the impact of the retrofitting works.

A method is suggested to reduce the structural seismic response using viscous dampers with their effect magnified with mechanical levers (lever arms). The lever arms are used to magnify the drift and the drift velocities transferred from the structure to the dampers thus producing larger energy dissipation in smaller devices. The increase of energy dissipation in each unit results in a reduction of the number of units necessary to achieve the same reduction in structural response, or it results in a reduction of the size of dampers required for the same purpose. The use of the proposed technique permits one to reduce the number of frame bays obstructed by diagonal or Chevron bracing elements. An optimal design procedure, based on the linear quadratic regulator (LQR) technique, is proposed in this study to obtain the initial properties of the viscous dampers. The study presents the amplification and the contribution of the construction details to such amplification. A seven-story structure model with the proposed system was simulated numerically in order to check the damping efficiency. The numerical example shows that using the lever arms for dampers’ connection, it yields significant reduction in the structural response, very close to that designed according to the LQR strategy.