Jerome F. Hajjar

A distributed plasticity model for concrete-filled steel tube beam-columns with interlayer slip

A fiber-based distributed plasticity finite element formulation is presented to perform three-dimensional monotonic analysis of square or rectangular concrete-filled steel tube (CFT) beam-columns. This stiffness-based beam-column element fomulation accounts comprehensively for all significant geometric nonlinearity exhibited by CFT beam-columns as part of composite frame structures, and the steel and concrete constitutive models account for the significant inelastic phenomena which are seen in CFT experiments. In addition, the finite element formulation accounts for slip between the steel and concrete components of the CFT by incorporation of a nonlinear slip interface. This formulation is able to capture behavior ranging from perfect bond to immediate slip. The calibration and verification of the slip formulation are presented, and the finite element model is verified against experiments of CFT beam-columns subjected to monotonic loading. Results from a preliminary study are presented on the effect of slip on CFT beam-column and composite frame behavior. A related paper extends this formulation to cyclic analysis of composite CFT frames, and provides details of the steel and concrete constitutive models.

Composite steel and concrete structural systems for seismic engineering

Abstract Research within the United States on seismic engineering of buildings using composite steel/concrete structural systems has increased dramatically in the past decade. This paper summarizes recent research on a number of these composite lateral resistance systems, including unbraced moment frames consisting of steel girders with concrete-filled steel tube (CFT) or steel reinforced concrete (SRC) columns; braced frames having concrete-filled steel tube columns; and a variety of composite and hybrid wall systems. The benefits of these structural systems relative to more common systems include their performance characteristics when subjected to service or ultimate loads, and their economy with respect both to material and construction. In addition, more in-depth research results will be presented on one of these composite systems, consisting of partially-restrained steel frames with composite reinforced concrete infill walls. The paper concludes with a summary of probable future design provisions for these composite systems.

A distributed plasticity model for cyclic analysis of concrete-filled steel tube beam-columns and composite frames

Abstract This paper presents the constitutive formulation and cyclic analysis capability of a three-dimensional fiber-based distributed plasticity finite element for square or rectangular concrete-filled steel tube (CFT) beam-columns. A related paper outlines the geometrically nonlinear formulation and the elements formulation for interlayer slip between the steel tube and concrete core. The present paper discusses the steel and concrete cyclic material models, which are each based on stress-space bounding-surface plasticity formulations. The paper details the calibration of these models to steel coupon tests and to experiments of short CFTs in flexure which yield moment-curvature-thrust results. Verification of the model is provided by comparing to several experiments of CFT beam-columns loaded monotonically and cyclically, proportionally and nonproportionally, and in single and reverse curvature. A final example presents a comparison of the analysis to the experimental results of a composite subassemblage consisting of three steel I-girders framing rigidly into a CFT beam-column, which is thus subjected to axial force plus cyclic biaxial bending.

Residual Drifts of Self-Centering Systems Including Effects of Ambient Building Resistance

There has been widespread interest in the development and use of self-centering (SC) lateral resisting systems that eliminate residual drifts after large earthquakes. SC systems often include a restoring force component and a component that dissipates seismic energy. Typically, it is assumed that the criterion for self-centering is satisfied if the restoring force is proportioned to be greater than the force required to yield the energy dissipating component. A parametric SDOF study was conducted using time-history analyses on several prototype buildings to quantify the effect of varying system parameters on structural response including residual drifts. The ambient resistance of the rest of the building was considered, as well as proportioning the system with less restoring force than the yield capacity of the dissipative component. In addition, the probabilistic mechanism that creates a propensity for reducing residual drifts in systems with little or no restoring force is explored and quantified. It was found that a restoring force that is at least one-half of the force required to yield the dissipative component will still reliably eliminate residual drifts in a non-softening system.

Data Processing of Point Clouds for Object Detection for Structural Engineering Applications

This research investigates the use of highresolution three-dimensional terrestrial laser scanners as tools to capture geometric range data of complex scenes for structural engineering applications. Laser scanning technology is continuously improving, with commonly available scanners now able to capture over 1,000,000 points per second with an accuracy of ∼0.1 mm. This research focuses on developing the foundation toward the use of laser scanning to structural engineering applications, including structural health monitoring, collapse assessment, and post-hazard response assessment. One of the keys to this work is to establish a process for extracting important information from raw laser-scanned data sets such as the location, orientation, and size of objects in a scene, and location of damaged regions on a structure. A methodology for processing range data to identify objects in the scene is presented. Previous work in this area has created an initial foundation of basic data processing steps. Existing algorithms, including sharp feature detection and segmentation are implemented and extended in this work. Additional steps to remove extraneous and outlying points are added. Object detection based on a predefined library is developed allowing generic description of objects. The algorithms are demonstrated on synthetic scenes as well as validated on range data collected from an experimental test specimen and a collapsed bridge. The accuracy of the object detection is presented, demonstrating the applicability of the methodology. These additional steps and modifications to existing algorithms are presented to advance the performance of data processing on laser scan range data sets for future application in structural engineering appli∗To whom correspondence should be addressed. E-mail: Jf.hajjar@neu.edu. cations such as robust determination of damage location and finite element modeling.

TRANSVERSE CRACKING IN CONCRETE BRIDGE DECKS

The dominant parameters that lead to premature transverse cracking in bridge decks are determined, and recommendations to reduce cracking tendency in bridge decks are developed. The project is divided into two parts: a field study and a parametric study. The objective of the field study is to determine the correlation between the observed cracking and available design-, material-, and construction-related data. Seventy-two bridges in the Minneapolis/St. Paul metropolitan area are included in the field study. According to the results of the study and correlation with other research, restrained concrete deck shrinkage is the leading cause of cracking. The dominant factors affecting transverse cracking are the longitudinal end restraint, girder stiffness, cross-frame location, splice location, deck thickness, cutoff length of the deck supplemental reinforcing bar, size of the top transverse bar, concrete shrinkage, deck concrete modulus of elasticity, cement content, aggregate type and quantity, air content, and ambient air temperature at deck placement. Recommended practical improvements to bridge deck construction include reducing the shrinkage of the deck concrete through mix design or better curing practices and minimizing deck restraint.

Design Concepts for Controlled Rocking of Self-Centering Steel-Braced Frames

AbstractThe self-centering rocking steel-braced frame is a high-performance system that can prevent major structural damage and minimize residual drifts during large earthquakes. It consists of braced steel frames that are designed to remain elastic and allowed to rock off their foundation. Overturning resistance is provided by elastic post-tensioning, which provides a reliable self-centering restoring force, and replaceable structural fuses that dissipate energy. The design concepts of this system are examined and contrasted with other conventional and self-centering seismic force resisting systems. Equations to predict the load-deformation behavior of the rocking system are developed. Key limit states are investigated including desired sequence of limit states and methods to help ensure reliable performance. Generalized design methods for controlling the limit states are developed. The design concepts are then applied to a six-story prototype structure to illustrate application of the rocking steel fram...

Shear and Friction Response of Nonseismic Laminated Elastomeric Bridge Bearings Subject to Seismic Demands

AbstractLaminated elastomeric bridge bearings are commonly used in areas with low-to-moderate seismicity, although the applications are typically intended for service-level considerations such as thermal movements of the bridge superstructure. These components provide a potential source of displacement capacity frequently neglected in seismic design. An experimental program was carried out to evaluate the behavioral characteristics and performance of steel-reinforced, laminated elastomeric bearings, which had not been designed for seismic demands, as the primary quasi-isolation components for seismic events by permitting slip at the interface of the bearing and substructure. The rubber at the top of the bearing is vulcanized to a steel plate, which is bolted to the test frame to simulate a connection to the superstructure. At the base of the bearing, the elastomer directly contacts concrete representing the substructure, with no restraint of horizontal motion other than friction. The elastomeric bearings ...

Buckling models and stability design of steel frames: a unified approach

Abstract This paper presents a unified approach to elastic analysis and frame stability design within the context of the AISC LRFD Specification. The advantages, proper usage, and limitations of isolated subassembly, story, and system-based buckling models for calculation of column design strengths are addressed. Based on an understanding of the fundamental characteristics of these elastic/inelastic buckling (i.e. effective length) models, significant simplifications are suggested in the calculations necessary for design. This is followed by a discussion of certain anomalies that can occur in the calculation of column strengths, and an explanation of how these anomalies should be avoided. The paper closes with a study of specific equations and fundamental assumptions behind two story-buckling models suggested in the current AISC LRFD Commentary. Example calculations and detailed comparisons to alternative approaches are provided in a companion paper.

Parallel processing for transient nonlinear structural dynamics of three-dimensional framed structures using domain decomposition

Abstract A strategy is presented for the solution of the fully nonlinear transient structural dynamics problem in a coarse-grained parallel processing environment. Emphasis is placed on the analysis of three-dimensional framed structures subjected to arbitrary dynamic loading and, in particular, steel building frames subject to earthquake loading. Concerns include long-duration dynamic loading, geometric and material nonlinearity, and the wide distribution of vibrational frequencies found in frame models. The implicit domain decomposition method described employs substructuring techniques and then a preconditioned conjugate gradient algorithm for the iterative solution of the reduced set of unknowns along the substructure interfaces. Substructuring is shown to provide a natural preconditioner for effective parallel iterative solution.