Journal of Structural Engineering

Isolation bearings experience tensile forces or uplift under certain conditions during seismic excitation. In general, this phenomenon is undesirable. This paper discusses an uplift prevention mechanism that uses prestress to develop sufficient compressive force on the isolator. A simplified theory of prestressed isolators is presented for three types of isolators, namely flat sliders, spherically shaped sliding bearings friction pendulum system (FPS), and elastomeric bearings. The behavior of prestressed isolators has been experimentally studied and presented. It has been observed that, consistent with the theory, the prestress has significant effects on the behavior of the prestressed isolators. The primary effect of prestress to increase the axial load on the isolators and prevent the uplift phenomenon was clearly observed in all three types of bearings tested in the study. The secondary effect of prestress to introduce additional lateral stiffness was also observed. While the additional axial load affects the behavior of elastomeric bearings, the effects were observed to be minor for the tested elastomeric bearings. In all cases of tested bearings, the experimental response could be predicted with sufficient accuracy by analytical means.

An experimental study of a base-isolated, 1:4 scale, three-story steel frame is presented. The isolation system incorporates laminated rubber (elastomeric) bearings combined with variable-orifice fluid dampers. The dynamic behavior of the variable-orifice dampers is modulated in accordance with an $H_{\infty }$ optimal feedback control algorithm. An experimental shaking table test program was used to evaluate the effectiveness of the variable-orifice dampers in reducing the dynamic response of the isolated test structure when subjected to both near-field and far-field earthquake records. The experimental test results demonstrate that the adaptive isolation system can significantly reduce the superstructure response compared to a conventional passive isolation system. In addition, analytical models of the isolated test structure were developed and calibrated via experimental system identification testing. Numerical simulations of the experimental tests demonstrate that the analytical models are well suited for describing the dynamic behavior of the isolated structure.

During the last 3 decades interest has grown globally in the application of ductile steel plate walls (DSPWs) (or steel plate shear walls) for building lateral load resistance. The supporting theory has evolved from both analytical and experimental research conducted in several countries around the world. The advantages of using DSPWs as the lateral force resisting system in buildings include stable hysteretic characteristics, high plastic energy absorption capacity, and enhanced stiffness, strength and ductility. A significant number of experimental and analytical studies have been carried out to establish analysis and design methods for such lateral resisting systems. Despite these efforts there is still a need for a general analysis and design methodology that not only accounts for the interaction of the plates and the framing system but also can be used to better understand the linear and nonlinear behavior of different DSPW configurations. These configurations include DSPWs with thin or thick steel plates, and with or without stiffeners and openings. In this paper an analytical model of the DSPW that responds to this need and characterizes the structural behavior of these configurations is presented and discussed. The proposed model provides a good understanding of how the different components of the system interact, and is able to properly represent the system’s overall hysteretic characteristics, which can be readily incorporated in practical nonlinear analyses of buildings with DSPWs. The effectiveness of the method is demonstrated by comparing the predicted response with results from experimental studies performed by different researchers.

A multistory building with a large podium structure under earthquake excitation may suffer from whipping effect due to the sudden change of building lateral stiffness and mass at the top of the podium structure. An experimental investigation was carried out in this paper to explore the possibility of using magnetorheological (MR) dampers to connect the podium structure to the multistory building to prevent the whipping effect. The multistory building was constructed as a slender 12-story building model, whereas the podium structure was built as a relatively stiff three-story building model. A MR damper together with a current controller was used to link the three-story building to the 12-story building. The dynamic characteristics of the two buildings without any connection and with a rigid connection were first identified. The two building models without any connection and with the rigid connection were then tested under the scaled El Centro 1940 north–south ground motion. Finally, the two building models connected by the MR damper manipulated by a multilevel logic control algorithm were tested under the specified ground motion. The experimental results show that the MR damper with the multilevel logic control algorithm could significantly mitigate the seismic whipping effect and reduce the seismic responses of both the multistory building and podium structure.

A semiactive tuned mass damper (STMD) system with variable damping is used to control the flutter instability of long-span suspension bridges. For this purpose, a combined vertical and torsional model of a TMD system, which is placed at the middle of the center span, is used. This system has two degrees of freedom, which are tuned close to the frequencies corresponding to vertical and torsional symmetric modes of the bridge, that get coupled during flutter. The variable damping of the system is chosen through a fuzzy logic controller using the displacement and velocity at the center of the bridge as the inputs. The transient response of the bridge due to a given initial condition in presence of the wind self-excited forces is controlled so that critical flutter wind speed for the bridge is enhanced. The methodology is applied to increase the flutter wind speed of the Thomas Suspension Bridge. Also, a numerical study is conducted to investigate the effectiveness of the semiactive control scheme.

In this study, a series of tests were conducted to investigate the behaviors of low-cycle bending-fatigue on A36 square and rectangular steel bars. It is observed that using the test results of constant cyclic loading to predict the strength of low-cycle bending fatigue under random loadings may introduce large error. In this study, the microstructure of the specimens is further examined after the tests and a physical model is established describing the failure mechanism of the steel bar. It is concluded that the strength of low-cycle bending fatigue of A36 structural steel highly depends on the orientation angles of the grains.

In this paper, the low-cycle bending-fatigue strength of square and rectangular steel bars under random loading histories is quantified through an experimental program. Based on the experimental data, and considering the fatigue failure mechanisms, a damage accumulation index formula is established. This formula serves as a foundation for further investigation on structural members with other geometrical shapes, with local buckling effects, and/or with welding residual stress effects, etc. After these effects are considered, design principles for low-cycle bending-fatigue strength of steel members can be developed for earthquake engineering applications.

Experimental investigations are carried out on intersecting attachments to investigate their fatigue strengths. Two types of intersecting attachments are studied, i.e., the T type with one longitudinal attachment intersecting a transverse stiffener at one end, and the H type with a longitudinal attachment intersecting two transverse stiffeners at both ends. The length of the longitudinal attachment in the T type is changed to study its effects on fatigue strength. The scalloped version of the specimens, free of welds at the intersection of the transverse and longitudinal attachments, is also investigated. Test results show that the fatigue strength of the T type attachment is comparable to that of the single longitudinal attachment. The fatigue strength of the H type attachment is in between those of the single longitudinal and transverse attachments. The introduction of the scallops does not influence fatigue strength significantly, but results in a change in the locations of fatigue cracks in the H type attachments. The method suggested by the writers for fatigue strength evaluation of weld toe failures is demonstrated as suitable for evaluating fatigue strength of intersecting attachments.

Establishing the strength requirements for stability bracing generally requires large displacement analyses on imperfect systems. In computational studies on stability bracing, choosing the critical shape of the imperfection can be a difficult decision, particularly for beam bracing systems. There are a number of factors that affect the stability brace forces including the shape and magnitude of the imperfection, the distribution of the imperfection along the length, and the value of the moment at the location of the brace. To develop suitable bracing design provisions, it is necessary to determine the maximum brace forces that are likely to occur in typical applications. However, determining the shape and distribution of the imperfection that produces the largest brace forces can be complicated. This paper studies the effect of different imperfection schemes on the magnitude of the stability brace forces. Results are presented that demonstrate the impact of several imperfection parameters on the bracing behavior and recommendations are made for selecting the critical imperfection shape that maximizes the stability brace forces for beam bracing systems.

An exact formula is derived for the tangent geometric stiffness of inclined cables under self-weight. Comparisons are made with the well-known approximate formula by Ernst. It is found that the accuracy of Ernst’s formula deteriorates with increasing cable slackness and inclination. The error can be significant for certain configurations of the cable that are commonly used in such structures as cable-stayed bridges, guyed towers, electric transmission lines, and conductor cables connecting electrical substation equipment. An accurate and simple approximate formula is developed, which is applicable to a wide range of parameter values.

Performance-based design of structures is becoming the preferred seismic design method. Its use requires special numerical programs capable of predicting the performance of structures during a seismic event well into the nonlinear range. Seismic analysis results obtained from these programs depend on the types of elements and constitutive material laws used. For one type of element used, the results can be sensitive to the size of the elements. This paper presents a simplified finite-element analysis program based on multilayer elements and damage mechanics modeling of concrete behavior. A method to identify the various parameters required to define the behavior of the different materials is presented and some guidance on structural modeling using this type of program is provided. This program is used to predict the behavior of three different structures: overreinforced normal-strength concrete and high-strength concrete (HSC) beams tested monotonically, HSC columns tested under constant axial load and cyclic flexure, and bridge piers subjected to earthquake-type loading by the pseudodynamic test method. It is shown that predictions are in excellent agreement with experimental results.

A finite-element-based procedure is proposed to identify defects in existing structures and evaluate their extent at the local element level for the purpose of health assessment. The procedure is a time-domain system identification technique where input excitation is not required to identify a structure. It estimates the dynamic properties of a structure in terms of stiffness and damping at the element level in a finite-element representation. The method can be used to precisely locate a defective spot in an element. Although input excitation information is not required, examples are used to show that the algorithm is robust enough to identify a structure excited by different types of loading. Structures can be excited simultaneously by multiple loadings, and the response information can be noise-free or noise-contaminated. Defects can be small or relatively large. In all cases, the algorithm identified the structures correctly. The error in the identification is considerably smaller than that of other available methods where input excitation information is used to identify a structure. With the help of examples, it is shown that the algorithm can potentially be used as a nondestructive evaluation technique for health assessment of existing structures with minimum disruption of operations. Since the procedure is very simple and requires only a few seconds of response information, it is expected to be very economical and efficient.

The performance of reinforced concrete (RC) columns plays a crucial role in the overall performance of framed structures and RC bridges. Therefore, the deformation limits associated with the performance of RC columns constitute an important part in the performance criteria for structural systems. On the other hand, it is possible to analytically correlate qualitative performance descriptions to quantitative design parameters such as curvature and drift at the column level. In this paper, probabilistic drift limits of RC columns at three distinctive performance levels are investigated by means of Monte Carlo simulation of the sectional curvature in conjunction with a statistical model of the plastic hinge length. A simple methodology is then introduced to evaluate the performance reliability of different column designs by constructing the fragility curves. With a generic single degree of freedom system consisting of a particular column, the probabilistic drift demands are derived from the displacement response spectra (DRS). The probability distributions of the DRS are established from a large set of ground motion records. Results of fragility curves demonstrate that increasing the flexural strength of a column does not significantly enhance its performance reliability during a probable earthquake; as a result, a design with higher flexural strength and lower ductility could be less reliable than a comparable lower strength and higher ductility design.

Seismic protection systems include the use of special passive devices to introduce supplemental damping into the structure. In building applications, passive devices are normally used in brace configurations and activated by a component of the structural drift. However, in applications where small elastic interstory displacements are expected, the conventional usage of such devices may not be suitable. Lever mechanisms, which amplify the displacements and velocities affecting the damping device, can be employed to overcome this deficiency. This work describes a displacement amplification device (DAD) based on a gear-type mechanism and connected in series with a fluid viscous damper (FVD). Laboratory test results are given here for a small-scale version of the DAD-FVD system. Potential implementations of this amplification system in civil structures are briefly described.

High frequency force balance (HFFB) measurements have recently been utilized to identify the distribution of spatiotemporally varying fluctuating wind loads on buildings. These developments, predicated on their ability to compute any response component of interest, based on actual building characteristics, attempt to offer a framework that eliminates the need for mode shape corrections generally necessary in the traditional HFFB technique. To examine the effectiveness of these schemes with significant practical implications to wind tunnel modeling technology, this technical note utilizes a recent approach to identify the alongwind loading on buildings. The predictions are compared to a widely utilized analytical loading model. It is noted that, akin to the traditional HFFB technique, the accuracy of these identification schemes clearly depends on the assumed wind loading model.