Journal of Structural Engineering

The energy dissipated per cycle and the characteristic strength of lead-rubber bearings reduce with increasing number of cycles as a result of heating of the lead core. The reduction is substantial depending on the geometric characteristics of the bearing and the speed of motion. This paper presents a theory that is capable of predicting the dependency on time of the characteristic strength and energy dissipated of lead-rubber bearings. The theory is based on predicting the temperature of the lead core and relating the temperature to the strength. A much simpler theory is also presented based on neglect of heat conduction through the steel shim and end plates of the bearing. The theory is useful in predicting the cyclic behavior of lead-rubber bearings for simplified analysis, in extrapolating experimental data from one scale to another and in developing models for dynamic response history analysis of seismically isolated structures that account for the time-dependent mechanical properties of the bearings. A companion paper presents results of eight tests of six lead-rubber bearings that provide verification of the theoretical solution.

The energy dissipated per cycle and the characteristic strength of lead-rubber bearings reduce with increasing number of cycles as a result of heating of the lead core. A theory which is capable of predicting the histories of temperature rise in the core, of the energy dissipated and of the characteristic strength of lead-rubber bearings has been presented in a companion paper. This paper presents a comparison of predictions of this theory to experimental results obtained in the testing of six lead-rubber bearings. The analytical predictions are found to be sufficiently accurate. Moreover, finite-element analyses results are presented that provide insight into the distribution of temperature in the lead core and end and shim plates of the bearings. These distributions confirm the assumptions made in the development of the theoretical solution.

Under certain conditions isolation bearings can sustain undesirable tensile forces or uplift, the occurrence of which potentially entail detrimental effects on the isolation-system and the superstructure. This study reports on the effect of uplift-restraint, provided by the $XY$ -friction-pendulum (FP) sliding isolation-system, on the seismic response of base-isolated structures. Belonging to the new generation of isolation hardware, the uplift-restraining FP isolator $\left(XY\text{-FP}\right)$ is capable of providing uplift-restraint to the superstructure by virtue of its inherent property of developing tension. To better understand the phenomena of uplift or tension of isolators and their impact on the performance of structural and isolation-systems, nonlinear response-history analysis of a seismically-isolated real building subjected to bidirectional horizontal seismic excitation is conducted. The analysis was performed with an enhanced version of program 3D-BASIS-ME, which was modified to include an element representative of the mechanical behavior of the tension-capable $XY$ -FP isolator. The results demonstrate that tension in individual $XY$ -FP isolators, and by extension the increase of isolation-system friction force, did not have any appreciable effect on either the total isolation-system response or the superstructure response.

A building may suffer damage during an earthquake as a result of inelastic deformations developed in the members or connections. It is important that the structural integrity of the building be assessed to ensure the safety of the occupants. This assessment includes evaluating the ability of the structure to resist the demand from subsequent aftershocks and a major earthquake. In this paper a practical methodology to determine the low-cycle fatigue life of welded structural steel connections subject to inelastic cyclic loading is presented. The methodology is based on concepts of low-cycle fatigue and micromechanics, where an accumulated crack length based on a time history of strain and the corresponding triaxiality stress condition that develops in the structural component is calculated and used to establish the fatigue life. The methodology was used to predict the fatigue life of welded beam-to-column connection test specimens subjected to inelastic loading. A comparison with test results indicates that the methodology predicts reasonably well the relationship between number of cycles to fracture and the plastic rotation range observed in the test specimens.

The effects of coupling of two horizontal seismic acceleration components are investigated for the ultimate states of thin-walled circular steel columns used as bridge piers. Ultimate states of these columns are mainly governed by the instability caused by the local buckling at the lower part of the columns. First, an accurate bidirectional pseudodynamic experiment is carried out to examine their ultimate seismic behaviors as well as to confirm the accuracy of our nonlinear dynamic FEM shell analysis. The experiment showed that the columns under bidirectional seismic excitations suffer larger local buckling deformations than those under unidirectional excitations. Furthermore, the experimental results agree well with those obtained by the FEM analysis. Based on the so-called pushover analysis under monotonically increasing unidirectional horizontal displacement, two types of circular ultimate interaction curves for columns are derived in terms of two horizontal restoring force components and two sway displacement components. The validity of these two interaction curves is examined by the nonlinear dynamic shell analyses under various bidirectional seismic excitations. In this analysis, the ultimate states of columns are identified by the elastic-plastic stability theory. The numerical analysis shows that the ultimate interaction curves expressed by the horizontal restoring force components give a good prediction of the ultimate states of columns. For the sake of practical applications, a formula is derived to provide the ultimate interaction curves for arbitrary thin-walled circular columns.

This paper presents an analytical study of local instability behavior in a reduced beam section (RBS) beam subjected to cyclic loading. A general finite-element method is used for numerical experiments here. Even though lateral instability in the RBS beam is prevented, strength deterioration may occur due to local buckling of the RBS portion. The writers suggest that stiffeners arranged in the RBS portion can delay the occurrence of local buckling. To address the possibility that reinforcement of the RBS portion might offset the advantage of RBS, the stress distribution of beam end of the RBS with stiffeners was compared to that of an RBS beam without stiffeners. Stiffening the RBS portion did not cause an excessive stress concentration. These analyses suggest that an RBS beam can be sufficiently strengthened by placing two stiffeners at the boundaries that divide the RBS portion into three equal regions and the stiffener thickness required is the same thickness as the web.

Buckling restrained braces are intended to yield in both axial tension and compression. The gusset plates connecting them to the adjacent beams and columns are thick and often stiffened to prevent buckling, and as a result increase the stiffness of the beam-to-column connection substantially. The increased stiffness of the beam-to-column connection negatively impacts the seismic performance of the system by: increasing the portion of the base shear force that is resisted by the frame relative to that resisted by the braces; increasing the maximum base shear force that the system is subjected to for a given earthquake motion; and transferring moment to the braces for which they were not designed. Further, failure of gusset welds partly due to the opening and closing of the beam-to-column joint has been observed in recent experiments. To mitigate these effects and increase the efficiency of buckling restrained braced frames, a novel connection where the gusset is only connected to the beam and is offset from the column face is proposed and tested in a three-story frame under quasistatic loading. The connection is shown to accommodate frame drifts as large as 3% and satisfy the buckling restrained brace performance requirements of the 2005 AISC Seismic Provisions for Steel Buildings. Additionally, the portion of base shear force resisted by the braces relative to that resisted by the surrounding frame is shown to be consistent with design assumptions. This connection configuration could be particularly useful for retrofit of seismically deficient moment resisting frames.

Conventional design of steel plate shear walls (SPSWs) assumes that 100% of the story shear is resisted by each infill panel. Following this approach, strength provided by the boundary frame moment resisting action, which provides the SPSW with overstrength, is neglected. While this design assumption has a positive impact on seismic performance of SPSWs, no analytical work has been done to quantify the magnitude of this overstrength in general terms. Such preliminary work is conducted in this paper. Based on plastic analysis of SPSWs, this paper investigates the relative and respective contributions of boundary frame moment resisting action and infill panel tension field action to the overall plastic strength of SPSWs, followed by a proposed procedure to make use of the strength provided by the boundary frame moment resisting action. Procedures for design of SPSWs having weak infill panels are also developed in this paper. Then, results from a series of time history analyses using validated models are presented to compare the seismic performances of SPSWs designed using different design assumptions. Future work needed to provide greater insight on SPSW designs is also identified.

The flexural behavior of rectangular masonry sections is investigated by considering nonlinear stress-strain in compression and limited-tension material. A preliminary analysis shows that, with variation in the parameters specializing with the adopted conventional constitutive law, the experimentally observed behavior of different kinds of masonry materials can be approximated. Subsequently, moment-curvature dimensionless curves are derived numerically by satisfying the equilibrium equations of the section. In the case of negligible tensile strength these curves exhibit a typical shape that can be approximated by analytical expressions; consequently, suitable functions are proposed that are governed by only a few parameters which depend on the compression level of the section and the available ductility of the material. The reliability of the proposed modeling procedure is also verified by using the experimental results of a previous study, where masonry elements subjected to an increasing eccentric normal load were considered. Therefore, the analytically defined flexural behavior is transferred to the characteristic sections of a discrete model of slender masonry column, in order to reproduce the experimental results of full-scale tests on bearing masonry walls subjected to increasing lateral deflections for stability analyses. Comments on the results obtained with this model would be useful for further investigations.

The need to identify the physical properties of a structure given its force-response (input-output) relationship is driven primarily by the need to validate the approximate solution models, such as finite-element models. This paper proposes a method, which combines the structural system identification and model updating techniques, for the damage detection of a steel frame structure and a RC frame. The damage detection procedure consists of two steps: (1) identifying the system dynamic characteristics using the subspace identification (SI) technique from input/output measurements and (2) developing a damage assessment method for structural members (including joints) based on a progressive finite-element model updating and a large-scale optimization using a nonlinear least-square technique. The proposed method was verified through a shaking table experimental study using: (1) a 1/4-scale six-story steel frame structure by loosening the connection bolts for damage simulations and (2) a two-story RC frame subject to different levels of ground excitations back to back. As demonstrated by experimental results, the proposed damage detection method, based on the combination of SI technique and the model updating approach, is very effective for the damage assessment of frame structures. The method not only can detect the damage locations but also can quantify the damage severities.