Laura N. Lowes

Update to ASCE/SEI 41 Concrete Provisions

A proposed supplement to ASCE/SEI 41 Seismic Rehabilitation of Existing Buildings has been developed for the purpose of updating provisions related to existing reinforced concrete buildings. Based on experimental evidence and empirical models, the proposed supplement includes revisions to modeling parameters and acceptance criteria for reinforced concrete beams, columns, structural walls, beam-column joints, and slab-column frames. The revisions are expected to result in substantially more accurate and, in most cases, more liberal assessments of the structural capacity of concrete components in seismic retrofit projects.

Concrete-Steel Bond Model for Use in Finite Element Modeling of Reinforced Concrete Structures

Reinforced concrete requires bond between plain concrete and the reinforcing steel. Accurate numerical modeling of structures that exhibit severe bond-stress demand requires exact representation of bond-zone response. A bond element is presented for use in high-resolution finite element modeling of reinforced concrete structures subjected to general loading. The model is defined by a normalized bond stress versus slip relationship and a relationship between maximum bond strength and the concrete and steel stress-strain state. A finite element implementation of the model is proposed that enables a 1- or 2-D representation of bond-zone action. Nonlocal modeling is used to incorporate the dependence of bond strength on the concrete and steel material state. Comparisons of simulated and observed response for systems with uniform and variable bond-zone conditions are presented.

Lumped-Plasticity Models for Performance Simulation of Bridge Columns

In bridge seismic design, a lumped-plasticity model based on a specified plastic-hinge length expression is used to estimate the ultimate displacement capacity. In contrast to current design guidelines, performance-based earthquake engineering (PBEE) requires assessment of damage for multiple demand levels. Implementation of PBEE of bridges therefore requires analytical methods capable of predicting bridge damage for multiple levels of earthquake demand. This paper evaluates models for PBEE of bridge columns, including new expressions for effective elastic stiffness, plastic-hinge length, and strain at onset of bar buckling. Data from 37 tests of large-scale circular bridge columns with post-1980 design details were used for model evaluation and development. The models were used to compute displacements and strains associated with various damage states and the resulting mean values and standard deviations. Findings indicate that the new model provides more accurate and precise predictions of damage in bridge columns than do existing models.

Fragility Functions for Older Reinforced Concrete Beam-Column Joints

Fragility functions are developed to predict the method of repair required for older reinforced concrete beam-column joints damaged due to earthquake loading. The results of previous experimental studies are used to develop empirical relationships between damage states and engineering demand parameters, such as interstory drift, joint deformation, and number of load cycles. Damage states are proposed and linked deterministically with commonly employed methods of repair; these damage states are characterized by parameters such as concrete crack width, extent of concrete spalling, and yielding and buckling of reinforcement. Probability distributions are fit to the empirical data and evaluated using standard statistical methods. The results of this effort are families of fragility functions that predict the required method of repair for a damaged joint.

Seismic Design and Performance of Self-Centering Steel Plate Shear Walls

The self-centering steel plate shear wall (SC-SPSW) is a new seismic load-resisting system that combines the strength and stiffness properties of the SPSW with the recentering capabilities of posttensioned (PT) beam-to-column connections. This paper outlines a proposed seismic design procedure aimed at achieving specified structural performance targets and analytical methods for modeling such a system. A series of 3-and 9-story prototype buildings located in a high seismic zone in California was designed using the procedure, and nonlinear, dynamic analyses were performed for these prototype buildings using ground motions representing three seismic hazard levels. The analysis results show that the systems achieved the desired performance objectives, including recentering of the lateral system.

Machine Vision-Enhanced Postearthquake Inspection

AbstractCurrent postearthquake inspection of structures relies on certified inspectors to make an assessment of the existing safety of the structure based primarily on qualitative measures. Completing the required inspection takes weeks to complete, which has adverse economic and societal impacts on the affected population. This paper proposes an automated framework for rapid postearthquake building evaluation. Under the framework, the visible damage (cracks and spalling) inflicted on RC members (columns) is detected using machine vision. The damage properties are then measured in relationship to the column’s dimensions and orientation, so that the existing state of the column can be approximated as a damage index. The column damage index is then used to query fragility curves of similar buildings, constructed from the analyses of existing and ongoing experimental data. The framework is expected to automate the collection of building damage data, to provide a quantitative assessment of the building damage...

Experimental Investigation of Self-Centering Steel Plate Shear Walls

AbstractA series of subassembly tests were conducted to investigate the behavior of the self-centering steel plate shear wall (SC-SPSW) system under cyclic loading. The SC-SPSW system utilizes thin steel web plates to provide energy dissipation and the primary strength and stiffness of the system, whereas posttensioned (PT) beam-to-column connections provide recentering capabilities. In this new system, the web plate is intended to yield under cyclic loading, whereas the boundary elements and PT connection elements remain undamaged. The web plate can then be replaced relatively easily following significant inelastic cycles. This experimental program studies the effects of various design parameters on the system and connection response and compares the response with approximate analytical formulas. The experimental results show that the SC-SPSW system has high ductility, high initial stiffness, recentering capabilities, an overall system response as anticipated, and more energy dissipation than expected.

Fragility Functions for Modern Reinforced-Concrete Beam-Column Joints

Fragility functions are developed to predict the method of repair required for modern reinforced-concrete beam-column building joints subjected to earthquake loading. These fragility functions, in combination with similar fragility functions developed previously for older joints, are used to compare damage progression in older versus modern joints. To develop fragility functions for modern joints, the results of previous experimental investigations are used to generate empirical relationships between damage and earthquake demand, damage states are linked deterministically with commonly employed methods of repair, and the empirical data are modeled using a standard probability distribution. The demand parameters, damage states, methods of repair, and probability distribution used in the current study are chosen to facilitate comparison with results from the previous study. The results of this study are a family of fragility functions that can be used to predict the method of repair required for a modern joint damaged due to earthquake loading and an improved understanding of the relative vulnerability of older versus modern components.

Evaluation of Retrofit of Beam-Column T-Joints in Older Reinforced Concrete Bridge Structures

Experimental tests and analyses investigated the seismic behavior of reinforced concrete beam-column T-joints. The test specimens were representative of interior beam-column joints in multicolumn bridge frames constructed in California in the 1950s and 1960s. Three one-third scale models were tested, one representing the as-built joint and two representing retrofit joints. The as-built specimen had relatively little ductility capacity, whereas the retrofitted specimens exhibited improved performance. The nominal joint stress state, joint force-transfer mechanisms, and bond stress characteristics were investigated as a means of assessing joint behavior. This study clarifies the seismic behavior of lightly reinforced bridge T-joints and verifies a design procedure for retrofitting these joints.

Experimental Testing to Determine Concrete Fracture Energy Using Simple Laboratory Test Setup

Insight into reinforced concrete structural behavior can be provided and experimental investigation can be complemented by nonlinear finite element analysis. Specification of material parameters, including post-peak tensile response curve shape, fracture energy, and concrete tensile strength is typically required in a concrete structure finite element models development. A number of different tests for fracture energy and post-cracking response determination has resulted from previous research. A very stiff, closed-loop test machine typically needs to be used in these methods so load application can be performed under displacement control since extremely brittle response is exhibited by test specimens. A recommended fracture energy test method was employed in a recently computed University of Washington study, with an open-loop testing machine and test specimens modified to include counterweights. Test-generated post-cracking response data and fracture energy data fall within the typically observed recommended test range. Additionally, the laboratory-observed load-displacement response was reproduced, with acceptable accuracy through using these data for concrete constitutive model calibration for nonlinear finite element analyses.