Mid-America Earthquake Center
D. P. Abrams*
National Science Foundation, CMS-9701785

The headquarters of one of three national earthquake engineering research centers is headed at the University of Illinois at Urbana-Champaign. The Mid-America Earthquake Center concentrates on reducing potential earthquake losses in the central and eastern United States by focusing on identifying local seismic hazards and developing strategies for retrofit of essential facilities and transportation systems. The center includes partners at the University of Memphis, Washington University, St. Louis University, Georgia Institute of Technology, Massachusettes Institute of Technology, and Texas A&M University. Numerous research projects will be funded through the center over a five-year period in the areas of structural and geotechnical engineering, seismology, social science, urban planning, and economics.

Performance of Rehabilitated URM Components
D. P. Abrams*
NSF Mid-America Earthquake Center

The project investigates strength, stiffness, and deformation capacities of unreinforced masonry walls retrofitted with different rehabilitation techniques. Static load reversal tests on large-scale wall specimens are being done to define nonlinear force-deflection behavior and to relate lateral story drifts with specific performance limit states. The effectiveness of rehabilitation techniques applicable to masonry construction in the eastern and central United States will be explored, including reinforcing, post-tensioning, surface coatings, and infilled openings. Computational models will be developed to simulate measured behavior and to help extrapolate test results to a wider range of wall types and configurations.

Yield Point Spectra for Seismic Design Considering P-Delta Effects
M. Aschheim,* E. Black
University of Illinois

A new spectral representation of inelastic seismic demands is being developed. This representation, known as a yield point spectrum, is particularly useful for seismic design of new structures and for the evaluation and rehabilitation of existing structures. Admissible design regions, stated in terms of combinations of strength and stiffness, can be determined graphically to satisfy damage and drift considerations in a performance-based design. This representation also may be used to show the influence of second-order (P-Delta) effects on inelastic demands. Methods for considering P-Delta effects on multistory structures are being developed and will be validated by example case studies.

Utility Software for Dissemination of Mid-America Earthquake Center Data
M. Aschheim,* D. Abrams, M. Inel, E. Black
NSF Mid-America Earthquake Center

The project develops a graphic-user interface to link current and anticipated data and software products to users of the center's research data. Catalogs of ground motions, recorded and synthetic ground motions, soil and structural materials properties, data obtained in structural and geotechnical studies, socio-economic data, inventory data, and societal response data will be accessed. The central engine of the software will be a single-degree-of-freedom nonlinear dynamic analysis program that contains menus of recorded and synthetic ground motions and libraries of measured and simulated hysteresis relations. Interfaces to permit real-time ground motion synthesis, response computation, and searching of inventories will be developed.

Dynamic Tests of Low-Rise Building Systems
M. Aschheim,* D. Abrams
NSF Mid-America Earthquake Center

This project investigates nonlinear dynamic response of structural systems typically used for low-rise essential facilities using reduced-scale idealized structures subjected to simulated earthquake motions using a shake table. Tests will emphasize the dynamic response of flexible diaphragms and the interaction of in-plane and out-of-plane response of masonry walls. The project is coordinated with other projects of the Mid-America Earthquake Center. Test data will be used to confirm or improve current computational methods for estimating response and will be correlated with results from analytical methods prescribed in FEMA 273 to suggest updated guidelines.

Inventories of Transportation Networks
M. Aschheim,* C. Wissawapaisal, S. French* (Georgia Tech), M. Lipinski* (Memphis Univ.)
NSF Mid-America Earthquake Center

The project assembles GIS-based inventories of critical transportation facilities for the Mid-America Earthquake Center study area. These facilities include highways, bridges, railways, natural gas pipelines, waterways, port facilities, and airports. The GIS format will allow these transportation data to be integrated with essential facilities and seismic hazard information being developed by other projects. Basic inventory information will be developed on the number, type, and location of transportation facilities. Structural characteristics will be identified in a detail necessary to guide future research efforts and to support subsequent damage modeling and loss estimation.

Waveform Independence of R-Factors
M. Aschheim,* I. Cuesta
University of Illinois

An explanation has been lacking for the observation that R-factors are reasonably consistent from one ground motion to another. For many records, only a short interval of motion contributes to most of the peak inelastic demand over a relatively broad range of oscillator stiffnesses. This interval of motion is not sufficient in itself to characterize the inelastic demand of the record. However, it is found that inelastic demands may be characterized using the elastic spectrum obtained from the record in conjunction with R-factors obtained from relatively simple pulses. This work may provide a basis for improved estimation of site-specific inelastic spectra.

Improving the Seismic Response of Highway Bridges Using Conventional Elastomeric Bearings
M. Aschheim,* N. Hawkins, C. Wissawapaisal
University of Illinois

Steel-reinforced elastomeric bearings are frequently used to accommodate thermal movements in highway bridges. However, their mechanical properties make them desirable for isolating the superstructure from seismic motions. Their use in regions characterized by infrequent seismicity will be explored. Analytical studies will assess their utility for new design and for rehabilitation of existing bridges, considering changes in stiffness and damping caused by seasonal temperature variations and aging. Experimental tests may be used to determine mechanical characteristics.

Strategies for Displacement-based Design of Reinforced Concrete Bridges
M. Aschheim,* M. Inel
University of Illinois

Traditional seismic design practice has been concerned with providing ductility to members deforming inelastically. Often, this has resulted in columns having relatively large diameters and an increased vulnerability to shear failure. Ductility capacity, however, is only important in its relation to ductility demand, and this can be expressed equivalently in terms of displacement capacity and demand. This project explores design strategies having the objective of achieving sufficient displacement capacity relative to the corresponding displacement demands.

Micromechanics Correlation of CVN Energy to Fracture Parameters
R. H. Dodds, Jr.,* X. Gao
U.S. Nuclear Regulatory Commission, N00167-97-K-0029

The Charpy V-notch specimen is an industry standard quality assurance test for material toughness. Empirical relationships between energy measured in this test and material fracture toughness (Klc and J) are currently employed in practice. This work employs detailed numerical modeling of precracked CVN specimens coupled with micromechanics models of material response to develop a mechanistic relationship between CVN energy and fracture toughness. The 3-D nonlinear analyses of precracked CVN specimens include both inertia and strain-rate effects. Initial efforts are focused on a cleavage mechanism of fracture using material properties typical of an A533B pressure vessel steel.

Fracture Mechanics in the Ductile-to-Brittle Transition Region
R. H. Dodds, Jr.,* X. Gao
NASA Ames Research Center, NCC2-5022; U.S. Nuclear Regulatory Commission, N00167-97-K-0029

Large-scale numerical computations are employed to couple a micromechanics model for initiation of cleavage fracture with inelastic deformation at the structural level. Previous efforts along these lines have successfully resolved the specimen size and deformation dependence of cleavage fracture toughness, Jc, to the lower-to-mid region of the ductile-to-brittle transition of ferritic materials. Experimentally verified models to scale cleavage fracture toughness with specimen size, relative crack size, strain hardening and loading mode (tension vs. bending) are now available. Current efforts focus on extending the methodology to accommodate small amounts of stable, ductile tearing prior to fracture by cleavage commonly observed in the mid-to-upper transition region.

Models for Ductile Crack Growth in Thin Aluminum Structures
R. H. Dodds, Jr.,* A. Gullerud, A. Roy
NASA Ames and Langley Research Centers, NAG 2-1126

Models to predict extensive amounts of ductile crack growth in thin, 2024-T3 aluminum sheets are being developed. Multisite damage at rivet holes in aging aircraft represents a key application of this work, where predictions of remaining strength play a major role in repair decisions. The 3-D numerical models employ the crack tip opening angle as a fracture parameter and are implemented in a code for large-scale analyses running on parallel computers using MPI.

Fracture of Welded Steel Joints
R. H. Dodds, Jr.,* C. Matos
University of Illinois

Welded steel joints in moment-resisting frames have exhibited unexpected brittle fractures during recent earthquakes in California and in Japan. This study applies micromechanical models for cleavage fracture to assess the significance of residual stresses, material properties, and geometric details of the design on the fracture behavior under both static and dynamic loading. Comparisons with small-scale experiments performed on welded plates validate the fracture mechanics models.

Computational Cell Modeling of Ductile Crack Growth
R. H. Dodds, Jr.,* A. Gullerud, X. Gao
U.S. Nuclear Regulatory Commission, N00167-97-K-0029; NASA Ames Research Center, NAG 2-1031; U.S. Office of Naval Research, Graduate Fellowship Directorate

In this work, we adopt a phenomenological constitutive relation for dilatant plastic materials to model ductile tearing in solids. Fixed-size computational cells (elements) are arranged on the plane ahead of the crack, where macroscopic strain softening effects are produced by growing cavities (voids) within the cells. The fixed-cell size introduces an explicitly specified length scale into a model that otherwise has a very weak length scale. Upon attainment of a failure criterion, cells are removed from the model and the crack advanced. As implemented in our 3-D, nonlinear, finite-element code, this model enables realistic prediction of tearing resistance (J-Da) for fracture test specimens and flawed structural components.

Software for Large-Scale, Nonlinear 3-D Analysis of Solids
R. H. Dodds, Jr.,* A. Gullerud, C. Matos
NASA Ames and Langley Research Centers, NAG 2-1126; U.S. Office of Naval Research, Graduate Fellowship Directorate

WARP3D is a research code for the solution of 3-D solid models subjected to static and impact loads. Specific features in the code oriented toward the investigation of ductile fracture in metals include a robust finite strain formulation, a general J-integral computation facility (with inertia, face loading), an element extinction facility to model crack growth, nonlinear material models including viscoplastic effects, and a dilatant plasticity model for void growth. Central features of WARP3D involve a linear-preconditioned conjugate gradient (LPCG) solver implemented in a blocked, element-by-element format and a modern sparse matrix solver.

Field Tests and Analysis of Railway Bridges
D. A. Foutch,* W. L. Gamble, D. H. Tobias
National Science Foundation, CES 87-17706, CMS 94-02224; Association of American Railroads

Wheel loads and bridge response measurements are being recorded for several railway bridges across the United States. One objective is to measure wheel loads under the current operating conditions. Another objective is to determine how older steel riveted bridges respond to these loads. A model for predicting the remaining life of an existing bridge is being developed. Full-scale tests of longitudinal forces in railway bridges are also being done.

Torsional Seismic Response of Structures
D. A. Foutch,* S. P. Schneider
U.S. Army Construction Engineering Research Laboratories; University of Illinois

One of the greatest uncertainties concerning calculating the inelastic response of a building for seismic loads is the torsional response resulting from nonsymmetric stiffness, strength, and/or mass. A series of tests of one-story structures will be conducted on the earthquake simulator at USACERL. Each structure will have a different nonsymmetry and will be shaken by bi-axial earthquake motions. This is possible as a result of the recent up-grade of the USACERL earthquake simulator.

Seismic Design and Behavior of Low-Ductility Steel Moment Frames
D. A. Foutch,* S. Yun
Federal Emergency Management Agency

The Northridge earthquake demonstrated several problems for the seismic behavior of steel moment frames. This project is aimed at developing seismic design and evaluation procedures for steel frames with low ductility. Parameters that will be investigated are local buckling, weak columns, and welded and bolted connections. This work is part of the SAC Phase 2 project administered as a joint venture among the Structural Engineers Association of California, the Applied Technology Council, and the California Universities for Research in Earthquake Engineering.

Performance-based Design of Steel Frames for Seismic Loads
D. A. Foutch,* K. Lee
Federal Emergency Management Agency

The Loma Prieta, Northridge, and Kobe earthquakes demonstrated that current seismic design codes are inadequate. Over 200 steel buildings in the Los Angeles area experienced fractured connections leaving them at risk of collapse. The purpose of this research is to develop new design and evaluation procedures aimed at specific performance goals. The procedure will be based on reliability concepts and uncertainties in loads, member behavior, and accuracy of the analysis procedures. This work is part of the SAC Phase 2 project administered jointly by the Structural Engineers Association of California, the Applied Technology Council, and the California Universities for Research in Earthquake Engineering.

Railroad Bridge Assessment Methods
D. Foutch,* K. Hjelmstad*
NSF Mid-America Earthquake Center

The seismic vulnerability of railway bridges in the central United States will be investigated. Major river crossings will be the focus of the investigation. Particular attention will be given to stone piers which are characteristic of these turn-of-the-century bridges.

Torsional Effects on Building Structures
D. Foutch,* S. Schneider
U.S. Army Construction Engineering Research Laboratories

This is an experimental study to investigate the 3-dimensional seismic performance of building structures. Test specimens will investigate building mass, stiffness, and strength eccentricities, as well as bi-directional seismic excitations.

Structural Control Using Neural Networks
J. Ghaboussi,* S. P. Schneider,* Y. K. Wen,* S. L. Paul,* K. Bani-Hani
National Science Foundation, CMS 95-03209

Neural networks are ideal adaptive controllers for nonlinear problems. Nonlinear adaptive structural control methods are being developed in which the main controller is a trained neural network. Methods are being developed for training of the neural network controller with the measured response of the structure. Another important advantage of using a neural network as the controller is that it can also learn to compensate for the inherent delays in the control system. Various neural network controllers are being developed and tested in computer simulation. The next stage of this research will consist of laboratory tests on scale-model structures.

Structural Control Using Gravity Actuators
J. Ghaboussi,* S. P. Schneider,* S. L. Paul,* Y. J. Kim
National Science Foundation, CMS 95-03209

Most of the proposed seismic structural control methods are highly energy intensive; during earthquakes, the structural control systems require very large energy inputs, precisely when all the energy sources and the distribution systems are disrupted. Our innovative design uses the reserve kinetic energy stored in suspended large weights attached to a cable-pulley system and anchored by a mechanism to drive, release, and catch the weights. During earthquakes, when the energy sources are disrupted, the required horizontal forces are generated by successively dropping the weights and catching them. This operation requires very little energy, enough to operate the release and catch mechanism and the computer controlling it. This energy can be provided by a small dedicated generator.

Development of Structural Design Methods Using Genetic Algorithms
J. Ghaboussi,* S. Shrestha, A. Raich
University of Illinois

A genetic algorithm is a computational model that imitates the process of Darwinian evolution. Genetic algorithms are emerging as a powerful tool in optimization and design problems. They use the elements of evolution, such as competition, natural selection, genetic recombination and mutation, and apply them to a population of design individuals. A number of generations are simulated and very fit individuals (according to the design criteria) emerge in the population. The application of this method to the process of structural design is being explored. Probably, the most difficult and creative aspect of the structural design is the design of the form and the shape of the structure and its topology. A second step is the proportioning of the members.

Seismic Behavior of Cable-stayed Bridges
J. Ghaboussi,* N. M. Hawkins,* P. K. Reddy
University of Illinois

Cable-stayed bridges are highly redundant structures. The stiffness of the structural members is dependent on the internal forces under the dead load. The member forces are generated during the construction and are dependent on the specifics of the sequence of construction. A method is being developed for simulation of the construction of the cable-stayed bridges, using incremental, nonlinear, finite-element method. The internal forces determined from this analysis are then used as the initial condition for nonlinear, 3-D, dynamic analysis of the structure under earthquake loads. In these analyses, the effect of the subsurface condition and the foundation-superstructure interaction are taken into account.

Computational Intelligence Methods in Earthquake Ground Motion Modeling
J. Ghaboussi,* Y. K. Wen,* X. Wu, J. Lin
University of Illinois

In the seismic design of structures, the earthquake ground motion is often represented by a response spectrum and, increasingly, by the time history of ground acceleration. In this project, we are exploring the application of soft computing tools, such as neural networks, genetic algorithms, and fuzzy logic, to develop tools for generating site-specific ground motion time histories, which take into account the nature and proximity of the sources of the seismic hazard and the local ground conditions, and at the same time are compatible with the accepted engineering design practices. The methodology being developed will also be capable of utilizing the increasing number of the actual recorded earthquake ground accelerations time histories.

Bridge Evaluation and Fault Detection
J. Ghaboussi,* J.-H. Chou
Association of American Railroads

Computational intelligence methods are being used to develop methods for inspection, evaluation, and fault detection of railway bridges. The objective is to instrument and remotely monitor the bridges. Passing of a train over the bridge is assumed to constitute a test. The measurements are used to evaluate the state of the bridge and to determine some key damage indices. Currently, a genetic algorithm is being used to determine the parameters of a model of the bridge by minimizing the differences between the model predictions and measurement. Neural networks also will be used in later phases of the project.

Detection of Hidden Faults in Rails
J. Ghaboussi,* X. Wu, J.-H. Chou
Association of American Railroads; Sperry Rail Service

Hidden cracks in rails, if not detected, can grow and lead to track failure and possible costly derailments. The conventional method of rail inspection is periodically run detector cars which may use ultrasonic detection techniques. The objective of this project is to use neural networks to aid in the process of detection. Using field measurements, neural networks are trained to identify possible hidden flaws from the traces generated by the ultrasonic detectors.

Nonlinear Structural Control Using Genetic Algorithms
J. Ghaboussi,* Y. J. Kim
National Science Foundation, CMS 95-03209

In optimal control algorithms, the control gains are determined by minimizing a quadratic cost function. We have developed a new control algorithm by using genetic algorithms to directly determine the optimal control gains. This genetic algorithm-based methodology gives far superior performance when compared with the conventional optimal control method. The methodology for GA-based control has been developed and tested in numerical simulations. It is being tested in a series of structural control experiments on a shake table. A GA-based control algorithm is being applied to nonlinear structural control problems.

Structural Control as a Secondary Protective System
J. Ghaboussi,* M. Walters
National Science Foundation, CMS 95-03209

Current seismic design code requires no structural damage for moderate earthquakes and no collapse for severe earthquakes. In this project, we are investigating the use of structural control as a secondary protective system for reduction of structural damage, while the primary protective system, which addresses the life safety issues, continues to be the code-based design. The aim is that the secondary protective system with active control will enable the structure to survive severe earthquakes with no structural damage. The research is being conducted by performing a number of designs on benchmark structures with and without the active control system and evaluating their relative life cycle costs.

Analysis of Seismic Retrofit Measures for Major Bridges
J. Ghaboussi,* D. Foutch,* M. Aschheim,* S. Nam, W. Kornkasem
NSF Mid-America Earthquake Center; University of Illinois

Methods of analysis are being developed for seismic analysis of major river-crossing bridges in the central United States. Evaluation of seismic behavior of existing bridges, as well as seismic evaluation of any proposed retrofit measure require nonlinear dynamic finite element analysis. It is specially important for these long-span structures to include the effects of soil-structure interaction and multiple support excitation caused by seismic waves traveling in the ground. Several methods of soil-structure interaction modeling and analysis will be developed and their performance will be evaluated. A fully evaluated simplified method of analysis for practical applications will be developed.

Active Control of Structures Using Neural Networks and Fuzzy Logic
J. Ghaboussi,* S. Schneider, Y. Wen
National Science Foundation, CMS 95-03209

This is a study on the earthquake simulator in Newmark Civil Engineering Laboratory to determine the effectiveness of a neural network control algorithm to mitigate building deformations during a large seismic event. Minimizing story drift and accelerations for a 3-story active tendon system is the primary consideration of this study.

Behavior of Beams/Slabs and Other Structural Elements with In-Plane Forces under Impulsive Loading
W. J. Hall,* R. W. Welch
University of Illinois; U.S. Army

The purpose of this study is to better define the structural behavior of beams and slabs subject to in-plane forces or restraints while resisting intense high-explosive loadings. Current analysis techniques for shallow-buried protective structures neglect this extremely important behavioral action. Complete behavioral knowledge, to include the effect of lateral restraint, is necessary for properly defining design criteria, margins of strength, and strengthening procedures for protective construction.

High-Explosive Shock Transmission and Shock Spectra
W. J. Hall,* G. W. McMahon
U.S. Army Waterways Experiment Station; University of Illinois

The objective of this research is to investigate the ground motion effects associated with accidental detonation of munitions stored in underground rock cavities, with emphasis on defining source and wave transmission parameters. In addition, the response of simple systems (shock spectra studies) will be undertaken and will include consideration of nonlinear effects. The goal is to arrive at techniques for estimating explosive "effects" on civilian and military systems and for improving design and analysis techniques.

Shear Strength of Precast Concrete Inverted Tee Beams
N. M. Hawkins,* S. L. Wood,* R. A. Victor
Precast/Prestressed Concrete Institute

After the January 17, 1994, Northridge (Calif.) earthquake, it was observed that the ends of several inverted tee and double tee beams were cracked in shear at locations in which shear cracking was not anticipated. The partial collapse of some of the precast garages in the earthquake has been attributed to the propagation of those cracks. The implications of such failures for the precast concrete industry are being explored through the conduct of a survey of producer practices, a detailed finite-element analysis of conditions in the end of inverted tee beams, and laboratory testing of such beams.

Precast Concrete Slabs for Airfield Pavements
N. M. Hawkins,* E. J. Barenberg,* S. Wattar
Precast/Prestressed Concrete Institute

Concrete is the preferred material for most airfield pavements. However, rehabilitation of existing concrete pavements at busy airports causes schedule disruptions felt nationwide. In this project, the feasibility of using precast concrete slabs for pavement replacement is being examined through: (1) the development of a design for a typical precast slab, (2) examination of optimum construction procedures for supporting and post-tensioning such slabs, and (3) characterizing appropriate details for joints between construction segments.

Use of Carbon Composites and Robotic Winding Technology to Upgrade Weather-damaged Bridge Columns
N. M. Hawkins,* W. L. Gamble
Federal Highway Administration; Illinois Department of Transportation; XXsys Technologies

A combined field and laboratory examination is being made of the use of carbon composites and robotic winding technology to upgrade corrosion-damaged bridge columns in regions where subsequent freeze-thaw resistance and enhanced seismic performance are simultaneous considerations. Corrosion-damaged columns, simulating columns as repaired in the field, are being reproduced in the laboratory. The corrosion-damaged areas of those columns will be removed, the concrete repaired, the columns wrapped with carbon fiber jackets of varying forms, the wrapped columns subjected to multiple freeze thaw cycles, and the seismic performance of the inadequate length lap splices at the column bases examined. The performance of the laboratory columns will be correlated with that of the field columns at the end of the two-year study period.

Assessment of Remaining Capacity of Deteriorated Pretensioned Deck Beam Bridges
N. M. Hawkins,* D. A. Lange,* J. Fuentes
Illinois Department of Transportation

There are over 1,200 pretensioned deck beam bridges on the IDOT inventory of Illinois state highways and an additional 6,000 such bridges on county inventories. Several of these bridges in the northern part of the state have shown unacceptable levels of corrosion deterioration after only about 20 years of service. Procedures that can be used to determine the degree of corrosion in existing bridges, the rate at which that corrosion is proceeding, and the likely remaining service life of those bridges are being explored.

Seismic Strengthening of Bridge Columns
N. M. Hawkins,* Q. Zhong
NSF Mid-America Earthquake Center

Many of the bridge columns built in mid-America in the 1960s have inadequate strength lap splices at connections between those columns and their foundation beams. During an earthquake, those lap splices are likely to be the first, or one of the first, elements of the bridge to fail. This project is exploring cost-effective methods for assessing the vulnerability of such bridge columns and for seismically strengthening their lap splices in order to reduce the economic losses likely due to disruption of mid-American transportation networks during a future earthquake.

Parameter Estimation in Complex Linear Structures
K. D. Hjelmstad,* T. Pothisiri
University of Illinois

Numerical algorithms are being developed to estimate parameters in complex linear structures subjected to static and dynamic loads. The current work is focused on the estimation of constitutive parameters of elements in complex structures with known topology and geometry, and includes the development of the estimation algorithms as well as explicit formulas for estimating the influence of errors in the data. Novel methods for efficiently estimating parameters with incomplete informations are being developed and evaluated. Subsequent research will treat nonlinear estimation problems, including geometry estimation with known topology and estimation of parameters in systems with nonlinear response.

Nondestructive Evaluation of Larger Structures
K. D. Hjelmstad,* T. Pothisiri
University of Illinois

Numerical algorithms are being developed to locate and evaluate damage in complex linear structures subjected to static and dynamic loads. The methods are based upon a finite-element representation of the structure. The concerns in damage detection problems include the adequacy of information, the spatial distribution of information, the type of excitation, and the effects of noise in the measurements. Damage inference is based on changes in estimated parameters from baseline values.

Dynamic Response of Low-Rise Buildings
K. D. Hjelmstad,* J. Lewis
NSF Mid-America Earthquake Center

Low-rise structures are peculiar by virtue of the importance of the flexibility of the floor diaphragms to the dynamic response of the structure under earthquake excitation. In practice, one of the first steps engineers often take in retrofitting low-rise structures to improve EQ resistance is to stiffen the floor diaphragms, despite some evidence from research that this action may be inappropriate. In this research, we are using simple dynamical models to try to determine the key design and response parameters that govern the design decisions for this class of structure.

Nonlinear Dynamics of Hysteretic Systems
K. D. Hjelmstad,* Y. Araki
Japan Society for the Promotion of Science

There exists a gap between the notion of structural stability typically used in the design of structures and and the notion of structural stability used in the context of dynamical systems. This research examines the gap between theory and practice for certain important classes of problems in dynamical instability (e.g, parametric resonance). The goal of the research is to identify problems involving structural response to dynamic loads where assurance of sufficient static strength is inadequate as a design philosophy.

Computer Tools for Design Using the Strut and Tie Model
D. Kuchma,* T. Tjhin
University of Illinois

All portions of structural concrete structures may be classified as either B-beam regions or D-discontinuity regions. Current design procedures for D-regions are empirical. One emerging and rational procedure for the design of D-regions is to imagine that an internal truss (strut and tie model) supports the applied load. The design involves providing reinforcement to serve as the tension members of the truss. A Windows-based computer program is being developed that enables the designer to quickly create a supporting truss, select reinforcement, and modify the geometry to satisfy stress limits.

Computation of Bifurcation and Instabilities in Complex Structures
I. D. Parsons,* A. Ronnau
National Science Foundation, ECS 91-57304

This project will produce efficient computational models and methods for studying the collapse of complex structures. Attention is primarily directed toward developing multigrid algorithms that can track the nonlinear response of a structure, detect critical points (i.e., limit and bifurcation points), and switch from one equilibrium path to another. These methods will be implemented on a variety of high-performance computers, such as workstations and vector-parallel supercomputers. The new tools will be applied to problems of practical importance, such as the buckling of stiffened, possibly delaminated, composite shells, and should provide new insights into the behavior of various complex structures.

Parallel Adaptive Solutions of Structural Mechanics Problems
I. D. Parsons,* A. Namazifard
National Science Foundation, ECS 91-57304

Distributed memory parallel computers are now available that promise rapid computational rates measured using standard industry benchmarks. The challenge is to translate these figures into fast turnaround times for practical problems, so that structural engineers can make full use of these machines. To this end, iterative solution algorithms are being combined with adaptive mesh refinement techniques to solve finite-element problems on the Connection Machine (which is a state-of-the-art parallel computer). The combination of optimum algorithms with innovative computer architectures will allow the solution of currently intractable problems.

Structural Analysis of Solid Rocket Motors
I. D. Parsons,* K. D. Hjelmstad,* A. Namazifard, E. Taciroglu, J. Hales, C. Schranz
DOE Center for Simulation of Advanced Rockets

Simulation of the normal and abnormal burns of solid rocket motors represents a considerable computational challenge. The coupled nature of the relevant physics, the length scales involved, and the computational resources required all demand careful thought. Our group is developing structural models that permit a full understanding of the deformations experienced by a solid rocket motor. Individual research topics include: adaptive multigrid algorithms, moving front schemes, parallel implementation on distributed memory machines, material modeling, and multiphysics coupling.

Estimation of Tank Car Inspection Interval
D. A. Pecknold,* F. V. Lawrence, Jr.,* O-C. Lee
Union Tank Car, T99-101

Railway tank cars with stub sills are subject to fatigue cracking in weldments in the area where the tank is supported on the sill, which may eventually lead to component failure. In this project, a damage tolerance analysis (DTA) methodology is being developed, in cooperation with other tank car builders/owners and Southwest Research Institute, with the objective of establishing required inspection intervals for stub sill-type tank cars. The DTA involves the use of finite-element stress analyses of the tank cars in conjunction with fracture mechanics-based fatigue crack growth calculations.

Strength of Simple Joints
D. A. Pecknold,* J. B. Park, C. C. Ha
Offshore Tubular Joint Research Center; Edison Welding Institute for the American Petroleum Institute, EDISON WELD 97-219

Design formulas for the ultimate static strength of steel tubular joints in offshore structures have, since the early 1970s, been based primarily on data from large-scale testing programs. Over the last decade, there has been a dramatic increase in the use of nonlinear finite-element analysis as a more economical means of addressing specific static strength issues, particularly in geometrically complex joints. The objective of this phase of the project is to establish appropriate guidelines for modeling and numerical analysis of tubular joints and to carry out a parametric study on the strength of simple uniplanar K-joints under balanced in-plane brace loading.

Strength of Complex Joints
D. A. Pecknold,* J. B. Park, C. C. Ha
Offshore Tubular Joint Research Center; Edison Welding Institute for the American Petroleum Institute, EDISON WELD 97-220

Design formulas for the ultimate static strength of steel tubular joints in offshore structures have, since the early 1970s, been based primarily on data from large-scale testing programs. Over the last decade, there has been a dramatic increase in the use of nonlinear finite-element analysis as a more economical means of addressing specific static strength issues, particularly in geometrically complex joints. The objective of this phase of the project is to establish appropriate guidelines for modeling and numerical analysis of tubular joints with internal ring-stiffeners and to carry out a parametric study on the strength of ring-stiffened double tee (DT) joints under brace compression loading.

Improved Numerical Methods for Structural Dynamics Calculations
A. R. Robinson,* S.-Y. Chang
University of Illinois

Numerical methods are being examined that result in highly accurate representations of the low-frequency motions of a structure and damp the higher modes considerably. A numerical process has been developed that is highly efficient for linear and nonlinear structures. Unlike most competing methods, it permits the use of large time intervals without giving rise to instabilities or gross deviations that are caused by the accumulation of the linearization error.

Seismic Rehabilitation of Nonductile Reinforced Concrete Frames Using Viscoelastic Dampers
S. L. Wood,* D. A. Foutch,* P. A. Brady, J. R. Hayes, Jr.
National Center for Earthquake Engineering Research

A large percentage of the damage in past earthquakes has resulted from the collapse or partial collapse of reinforced concrete buildings that were designed with reinforcement details that do not satisfy current building code requirements. This project investigates the feasibility of using viscoelastic dampers to improve the seismic response of existing, lightly reinforced frames and flat plate buildings. Small-scale models of critical connections and of a complete three-story building were tested on an earthquake simulator. The viscoelastic dampers were incorporated into steel braces attached to the reinforced concrete frames. The combination of braces and dampers proved to be effective in reducing the displacements that lead to earthquake damage.

Evaluation of Steel Frames with Open Brick Infills
S. Schneider,* D. P. Abrams
National Center for Earthquake Engineering Research

The purpose of the research is to examine strength, stiffness, and ductility of existing steel frames with open brick infills through a series of laboratory tests and a parallel analytical investigation. Large-scale experiments are done on frame-infill specimens subjected to static, equivalent seismic loadings. Computational models are developed based on a composite masonry-steel concept and verified with the test data so that estimates of seismic response for actual buildings can be made. The end-product of the research will be a set of proposed guidelines for evaluating lateral seismic resistance of existing steel buildings with brick masonry infills or cladding.

Alternative Details for Moment-resisting Steel Connections
S. P. Schneider*
National Science Foundation, CMS 95-01449

More than 100 steel frames buildings were significantly damaged during the 1994 Northridge (Calif.) earthquake and subsequent aftershocks. Joints once thought to possess large inelastic deformation capacity exhibited severe crack growth. Cracking of the weld at the lower beam flange was typical of the damage noted in steel frames. The damage was wide-spread primarily because the fully welded steel connection is a standard detail in many design offices. Considering the damage observed after the Northridge earthquake, structural engineers are in dire need of practical details for steel beam-to-column connections. The object of this research is to investigate analytically and experimentally practical and economical alternative details for the fully welded moment-resisting connection.

Inelastic Flexural Capacity of Pressurized Pipe
S. P. Schneider*
Edison Welding Institute

Buried pressurized pipe is often repaired using fully welded sleeves. However, because of uneven soil settlement, frost heaves, and other conditions, the welded sleeve can be subjected to bending. To investigate this behavior, two full-scale pressurized pipes were tested monotonically, in four-point bending, until clear failure was apparent. Tests showed that wall bulging, and not distress of the sleeve or weldment, was the failure mode of the pipe. Analytical work indicates a modest amount of ductility can be achieved before the on-set of local buckling.

Bolted Flange Plate Connections with and without Energy-dissipating Dampers
S. Schneider*
Structural Engineering Association of California/SAC Joint Venture

This is an experimental program to test eight full-scale steel connections with and without energy-dissipating dampers. This experimental study will compare behavior of traditional bolted flange plate connections to the inelastic cyclic behavior of connections with friction-based damping devices.

Performance of Rehabilitated Steel Connections
S. Schneider, D. Foutch, R. Leon* (Georgia Tech)
NSF Mid-America Earthquake Center

This project focuses on the strength, stiffness, and deformation capacity of existing and rehabilitated steel-frame structures. Structural detailing was not considered for typical steel frames constructed in the central U.S. This is an analytical and experimental program to investigate the inelastic behavior of existing steel-frame connections and investigate the repairs to such connections.