Editor’s Note

A “Noniterative Equivalent Linear Method for Evaluation of Existing Structures” is proposed by Lin and Miranda. The equivalent period and damping of structures are defined by the ductility ratio; however, for existing structures, it is the strength ratio (elastic lateral strength/yield lateral strength) rather than the ductility ratio that is generally known. Therefore, the maximum inelastic displacement demand of existing structures has to be determined through an iterative procedure until the computed displacement is within an allowable tolerance to the assumed displacement. To avoid iteration and improve accuracy, the authors present results of a comprehensive statistical investigation of the equivalent linearization in which both the equivalent period and damping are defined by strength ratios and periods of vibration. The recommended equivalent linear system is shown to give good predictions of the mean maximum inelastic displacement for systems in all period ranges.

Warn and Whittaker investigate the influence of “Vertical Earthquake Loads on Seismic Isolation Systems in Bridges.” In particular, the response of a bridge isolated with low-damping rubber and lead-rubber bearings is evaluated through earthquake simulation testing. Response data from the experimental testing are used to determine the vertical load on the isolation system due to the vertical component of excitation. A comparison of the normalized vertical load data to the vertical base acceleration showed significant amplification of the vertical response for each simulation and configuration. Disaggregation of the axial load history showed that the summation of maximum values from the vertical earthquake load and overturning moment overestimates the maximum axial load because these maximum values are unlikely to occur simultaneously. Additionally, a spectral analysis procedure using the unreduced vertical stiffness of the bearings was shown to provide a reasonable estimate of the vertical earthquake load.

In “Seismic Performance of a Transfer Plate Structure,” Li et al. report on findings from pseudodynamic tests conducted on a quarter-scale test specimen representing the first 2 stories of an 18-story high-rise building with transfer plate. The columns of the test specimen were strengthened to prevent premature failure under the applied loads. Three types of time-history records were applied, including triangular waves and a series of El Centro earthquake records. Based on experimental observations, the authors conclude that the bottom of the transfer plate will resist forces resulting from El Centro accelerations scaled to $0.16g$ , but that it will be severely damaged when subjected to the same record scaled to $0.64g$ .

In “Robustness of Composite Floor Systems with Shear Connections: Modeling, Simulation, and Evaluation,” Sadek et al. present a computational investigation of the response of a typical concrete deck and steel beam composite floor system with simple shear connections in the event that an interior column is removed. Analyses of a connection subassembly indicate that loads are primarily resisted by cable action after column loss, resulting in increasing tensile forces in the beams and connections that could eventually precipitate failure. Simulation results show that the floor deck contributes significantly to the floor system response through diaphragm action to prevent the exterior column from being pulled inward, and membrane action primarily through the reinforcement mesh and metal deck. The analyses also indicate that the capacity of the analyzed floor system under the column removal scenario is significantly less than the load specified by the General Services Administration’s current progressive collapse guidelines.

Gengshu et al. derive closed-form solutions for the lateral flexural and shear displacements, as well as for the bending moments and shear forces, in “Buckling and Second-Order Effects in Dual Shear-Flexural Systems.” It is found that the total buckling load is a simple summation of the buckling loads of the two component structures acting independently and is independent of the distribution of the vertical loads among the two structures. A simple formula for the amplification factors is proposed for use in routine design. The second-order theory developed by the authors is used to investigate the amplification factors for the lateral displacements and bending moments in a typical shear-flexural structure. It is shown that the amplification factors determined from the proposed formula are in good agreement with those obtained by the more rigorous second-order analysis in most cases.

In “Bidirectional Cyclic Loading Experiment on a 3D Beam-Column Joint Designed for Damage Avoidance,” Li et al. report on the performance of a special joint subassembly under unidirectional loading along both orthogonal directions, as well as under concurrent bidirectional loading. The specimen is shown to perform well up to 4% column drift with only minor flexural cracking in the precast beams and no cracking in the precast column. This superior performance is attributed to steel armoring of the beam ends to mitigate the potential for concrete crushing. A tapered shear-key layout is used to effectively protect the beams against adverse torsional movements. A three-phase force-displacement relationship is proposed that gives due consideration to the prerocking flexural deformation of the beam, the rigid body kinematics during the rocking phase, and the yielding of the external dissipaters and posttensioning tendons. Good agreement between the proposed theoretical model and experimental observation is demonstrated.

An analytical model to simulate the “Nonlinear Dynamic Behavior of Unreinforced Masonry Walls Subjected to Out-of-Plane Loads” is developed by Hamed and Rabinovitch. The model focuses on one-way action across the height of the wall, which is modeled as a structural assembly of masonry units connected by mortar joints. The masonry units and the mortar joints are modeled as first-order shear-deformable flexural members with large displacements, moderate rotations, and small strains. The cracking and nonlinear behavior of the mortar material, the opening and closing of cracks, the rocking phenomenon, and the development of a time-dependent arching force are also considered. It is shown that the dynamic behavior of the wall is nonperiodic or chaotic and is influenced by the various physical and geometric nonlinear effects.

Results from testing of six full-scale walls under reversed cyclic load are presented by Shedid et al. in “Behavior of Fully Grouted Reinforced Concrete Masonry Shear Walls Failing in Flexure: Experimental Results.” The effects of the amount and distribution of vertical reinforcement and the level of axial compressive stress on the inelastic behavior and ductility were investigated. It is observed that yielding of the outermost vertical bars extended to a height equivalent to half the wall length. Also, the top wall displacement (drift) at the onset of yielding of the vertical reinforcement was highly dependent on the amount of reinforcement but only minimally affected by the level of axial compressive load. However, at maximum loads, the displacements were less sensitive to either the amount of vertical reinforcement or the level of axial compression.

Kim and Christopoulos propose a new connection to provide self-centering capacity along with friction mechanisms to dissipate energy in their paper “Friction Damped Posttensioned Self-Centering Steel Moment-Resisting Frames.” A bolt-prestressed friction mechanism with a frictional interface consisting of stainless steel and nonasbestos organic break-lining pads dissipates seismic input energy as the system undergoes lateral deformations. Cyclic tests conducted on the proposed system indicate that the frictional behavior is stable, repeatable, and predictable, although its friction coefficient is relatively low. In addition to developing stiffness and strength characteristics similar to those of welded connections, the connections are also capable of undergoing large deformations without introducing inelastic deformations in the beams or the columns and without residual story drifts.

The issue concludes with a technical note by Ryan and Polanco discussing “Problems with Rayleigh Damping in Base-Isolated Buildings.” Energy dissipation in a base-isolated building is typically accounted for by allocating damping properties independently to the superstructure and to the isolation system. Rayleigh damping applied to the superstructure component alone is often used to represent the superstructure energy dissipation. It is shown that Rayleigh damping results in undesirable suppression of the first mode response even when used as recommended for combining subsystems with disparate damping properties. The authors recommend that stiffness-proportional damping be used in lieu of Rayleigh damping to correct this behavior.