Hazim Dwairi

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.

Implementation of Self-Consolidating Concrete for Prestressed Concrete Girders

This report documents the first experience of using self-consolidating concrete for pretressed concrete bridge girders in North Carolina. Under construction in eastern North Carolina was a multi-span bridge which used one hundred thirty AASHTO Type III girders, each 54.8 ft (16.7 m) long. To demonstrate the full-scale field production of self-consolidating concrete, and for comparative purposes, three girders from one production line of five girders were selected for the experimentation. Two of the girders were cast with self-consolidating concrete and one with normal concrete as control. The plastic and hardened properties of both the self-consolidating concrete and the normal concrete were monitored and measured. The plastic properties of self-consolidating concrete included unit weight, air content, slump flow, visual stability index (VSI), and passing ability measured by J-ring and L-box. Hardened properties of the two concretes included temperature development during curing, compressive strength, elastic modulus, and flexural tensile strength, creep and shrinkage. The prestressing force was monitored by load cells . The transfer lengths of prestressing strands were determined by embedded strain gauges , and from the measured strand end-slips. Finally, the three girders were tested in flexure up to the design service load to determine and compare their load-deformation characteristics. Based on the satisfactory results of this study, the two prestressed SCC girders were installed in the bridge for service as other normal concrete girders.

Implementation of Inelastic Displacement Patterns in Direct Displacement- Based Design of Continuous Bridge Structures

Through the use of nonlinear time-history analysis, the displacement patterns of bridges subjected to transverse seismic attack are investigated. The variables considered in the study consist of bridge geometry, superstructure stiffness, substructure strength and stiffness, abutment support conditions, and earthquake ground motion. A series of three inelastic displacement pattern scenarios were identified: (1) rigid body translation (2) rigid body translation with rotation, and (3) flexible pattern. A relative stiffness index that is a function of the superstructure and substructure stiffness was shown to be a key variable in determining the type of displacement pattern a bridge is likely to follow. The results described in this paper have significant implications for performance-based seismic design procedures such as direct displacement-based design (DDBD). If the displacement pattern for a bridge can be identified with significant confidence at the start of the design process, application of approaches such as DDBD can be simplified. However, if the characteristics of the bridge are such that prescribing a pattern at the start of the process is not feasible, then an alternative approach must be employed for DDBD to proceed. Of the three displacement pattern scenarios defined in this paper, the first two require minimal effort in the design. For the third scenario, an iterative algorithm is proposed. Lastly, as a means for verification and demonstration, a series of bridges with various configurations was designed using DDBD for rigid body translation and flexible pattern scenarios. The designs for the flexible scenario showed good agreement with selected target profiles for bridges with up to five spans.

Shear Behavior of Large Concrete Beams Reinforced with High-Strength Steel

This study seeks to quantify the benefits of using high-strength steel for concrete reinforcement and provides experimental evidence of its high strength capabilities. Test results are presented of six large-size concrete beams reinforced with either conventional- or high-strength steel and tested up to failure. The beams were constructed without web reinforcement to evaluate the nominal shear strength provided by the concrete. The shear behavior, ultimate load-carrying capacity, and mode of failure are presented. The applicability of the current ACI design code to large-size concrete beams constructed without web reinforcement is discussed. The influence of the shear span-depth ratio, concrete compressive strength, as well as the type and the amount of longitudinal steel reinforcement is investigated. Findings indicate that using high-strength steel alters the mode of failure from diagonal tension to shear compression failure and results in higher shear strength compared with using conventional steel. Findings also suggest that the current ACI shear design provisions are unconservative for large-size concrete beams without web reinforcement. The expression needs to account for the size effect and reinforcement characteristics.

Review of Parameters Influencing the Seismic Design of Lightweight Concrete Structures

Four aspects related to the seismic behavior of structures built of lightweight concrete are discussed in this paper. The four parameters of interest are: shear strength, flexural strength, ductility, and energy dissipation. After discussing the merits of lightweight concrete, the paper examines the past seismic performance of the material. The second part of the paper focusses on earlier research conducted on the seismic behavior of the lightweight concrete structures and also research on parameters that influence the seismic behavior of the examined concrete. Future research needs are identified.