James M. Nau

Equivalent Damping in Support of Direct Displacement-Based Design

The concept of equivalent linearization of nonlinear system response as applied to direct displacement-based design is evaluated. Until now, Jacobsens equivalent damping approach combined with the secant stiffness method has been adopted for the linearization process in direct displacement-based design. Four types of hysteretic models and a catalog of 100 ground motion records were considered. The evaluation process revealed significant errors in approximating maximum inelastic displacements due to overestimation of the equivalent damping values in the intermediate to long period range. Conversely, underestimation of the equivalent damping led to overestimation of displacements in the short period range, in particular for effective periods less than 0.4 seconds. The scatter in the results ranged between 20% and 40% as a function of ductility. New equivalent damping relations for four structural systems, based upon nonlinear system ductility and maximum displacement, are proposed. The accuracy of the new equivalent damping relations is assessed, yielding a significant reduction of the error in predicting inelastic displacements. Minimal improvement in the scatter of the results was achieved, however. While many significant studies have been conducted on equivalent damping over the last 40 years, this study has the following specific aims: (1) identify the scatter associated with Jacobsens equivalent damping combined with the secant stiffness as utilized in Direct Displacement-Based Design; and (2) improve the accuracy of the Direct Displacement-Based Design approach by providing alternative equivalent damping expressions.

Seismic design aspects of vertically irregular reinforced concrete buildings

Seismic building codes such as the Uniform Building Code (UBC) do not allow the equivalent lateral force (ELF) procedure to be used for structures with vertical irregularities. The purpose of this study is to investigate the definition of irregular structures for different vertical irregularities: stiffness, strength, mass, and that due to the presence of nonstructural masonry infills. An ensemble of 78 buildings with various interstory stiffness, strength, and mass ratios is considered for a detailed parametric study. The lateral force-resisting systems (LFRS) considered are special moment-resisting frames (SMRF). These LFRS are designed based on the forces obtained from the ELF procedure. The results from linear and nonlinear dynamic analyses of these engineered buildings exhibit that most structures considered in this study performed well when subjected to the design earthquake. Hence, the restrictions on the applicability of the equivalent lateral force procedure are unnecessarily conservative for certain types of vertical irregularities considered.

Effect of Load History on Performance Limit States of Circular Bridge Columns

In this paper, the importance of displacement history and its effects on performance limit states, the relationship between strain and displacement, and the spread of plasticity in RC structures is explored. An experimental study is underway to assess the performance of 30 circular, well-confined, bridge columns with varying lateral displacement history, transverse reinforcement detailing, axial load, aspect ratio, and longitudinal steel content. Eight of these columns, with similar geometry and detailing, were subjected to various unidirectional displacement histories including standardized laboratory reversed cyclic loading and re-creations of the displacement responses obtained from a nonlinear time-history analysis of multiple earthquakes with distinct characteristics. Longitudinal reinforcing bars were instrumented to obtain strain hysteresis, vertical strain profiles, cross section curvatures, curvature distributions, and fixed-end rotations attributable to strain penetration. Results have shown that the limit state of reinforcement bar buckling was influenced by load history, but the relationship between strain and displacement along the envelope curve was not. The main impact of load history on bar buckling is its influence on accumulated strains within the longitudinal reinforcement and transverse steel.

Repair of Reinforced Concrete Bridge Columns Containing Buckled and Fractured Reinforcement by Plastic Hinge Relocation

AbstractThis paper describes a new repair technique that involves the use of plastic hinge relocation to restore strength and deformation capacity of RC bridge columns. Summarized is the overall repair concept and experimental results that include the reversed cyclic testing of three large-scale bridge columns that were previously damaged, repaired using the proposed methodology, and then subsequently retested. To date, two different repair alternatives were executed using unidirectional carbon fiber sheets in the hoop and longitudinal directions, the latter anchored into the RC footing with 30-mm-diameter carbon fiber anchors. A method for predicting the force-displacement responses of columns repaired in this manner was also developed and found to give reasonable results. Also included in this paper are design considerations, which are carried out in the steps needed to design a repair system to relocate the plastic hinge in a column containing buckled longitudinal reinforcement. The responses show that...

Finite-Element Method to Predict Reinforcing Bar Buckling in RC Structures

Buckling of longitudinal bars is a common form of damage in reinforced concrete (RC) structures subjected to earthquakes. Previous research has illustrated the impact of cyclic loading on bar buckling which often occurs upon the reversal from a tensile loading cycle. This paper presents a finite-element method to predict reinforcement buckling under seismic loading that also captures the details of the buckling mechanism. This method combines a fiber-based model to simulate the reinforced concrete member itself and an independent finite-element model of the local plastic hinge region. The strain demands in the plastic hinge region are determined from the fiber-based model of the overall structure subjected to the ground motion. The strain history is then imposed on the finite element bar buckling model to predict the localized behavior. Comparisons between the model performance and experimental observations are shown to assess the accuracy of the proposed method.

Modified Plastic-Hinge Method for Circular RC Bridge Columns

AbstractThis paper discusses a research program aimed at defining accurate limit-state displacements that relate to specific levels of damage in reinforced concrete bridge columns subjected to seismic hazards. In design, concrete compressive and steel tensile strain limits are related to column deformations through the use of an equivalent curvature distribution. An experimental study was carried out to assess the performance of 30 circular well-confined bridge columns. Material strains, cross-section curvatures, and fixed-end rotations attributed to strain penetration of reinforcement into the adjoining member were quantified by using a three-dimesional (3D) position monitoring system. An equivalent curvature distribution was created that reflects the measured spread of plasticity and components of deformation. When compared with the current approach, the proposed modified plastic-hinge method improved the accuracy of both tensile and compressive strain-displacement predictions, while maintaining similar...

Effect of Seismic Load History on Deformation Limit States for Longitudinal Bar Buckling in RC Circular Columns

This paper investigates the impact of seismic load history on longitudinal bar buckling in reinforced concrete (RC) bridge columns. Previous research has shown that reinforcing bars are prone to buckling upon reversal from tensile strain. To quantify this effect, a hybrid analysis method using both fiber and solid elements is developed and implemented to assess the impact of seismic load history on reinforcing bar buckling. Forty earthquake ground motions are utilized to conduct nonlinear time history analysis of bridge columns using a fiber-based model. The longitudinal bar strain history from the fiber-based model is then utilized as the input to the finite element model. A parametric study is conducted for the purpose of developing design equations that provide strain limits prior to the onset of bar buckling. Simple design approaches are proposed based on the design equations.

Fiber-Based Modeling of Circular Reinforced Concrete Bridge Columns

This article presents the application of fiber-based analysis to predict the nonlinear response of reinforced concrete bridge columns. Specifically considered are predictions of overall force-deformation hysteretic response and strain gradients in plastic hinge regions. This article discusses the relative merits of force-based and displacement-based fiber elements, and proposes a technique for prediction of nonlinear strain distribution based on the modified compression field theory. The models are compared with static and dynamic test data and recommendations are made for fiber-based modeling of RC bridge columns.

Reversed Cyclic Flexural Behavior of Spiral DSAW and Single Seam ERW Steel Pipe Piles

This paper presents the findings of an investigation on the flexural performance of hollow steel pipe piles subjected to reversed cyclic loading. The testing evaluated both spirally double submerged arc welded (DSAW) and traditional longitudinal single seam electric resistance welded (ERW) pipe piles to determine the effects of the spiral welding manufacturing process on the structural performance of the pile. Some of the tests were conducted on previously driven piles to study the effects of driving stresses. The experimental results and observations indicated that the undesirable failure mode of spiral weld cracking did not control the ultimate limit state in any of the spirally welded specimens considered. Although weld fracture did occur in each spirally welded specimen, it did not develop until the specimen was subjected to large inelastic deformations and was ultimately the result of locally increased strains caused by local buckling. Each traditional single seam specimen failed in a similar manner with pile wall local buckling developing at inelastic deformation levels comparable to those of the spirally welded specimens.

Equivalent Viscous Damping Model for Short-Period Reinforced Concrete Bridges

Abstract This paper investigates the effect of spectral shape (intensity and width of the constant acceleration region) and postyield stiffness ratio on equivalent viscous damping for short-period RC bridge columns ( effective period<1s). The modified Takeda degrading stiffness hysteretic model, with parameters appropriate to bridge columns (often termed thin Takeda in the literature), is used for analysis. Insight regarding the importance of these parameters is provided, and a new equivalent viscous damping model is proposed that includes the effect of spectral shape and postyield stiffness ratio, as well as effective period and ductility. The proposed damping model is compared with two existing models. The results indicate that significant improvement is achieved in predicting the peak displacement using the proposed damping model when compared with existing models.