Soil structure interaction of bridges for seismic analysis

This report documents the current state-of-practice in soil-structure interaction analyses for seismics desing of bridges. It covers several relevant areas such as ground motion aspects, various issues on soil-structure interaction for typical foundations and methods of substructuring to reduce the number of degrees of freedom for foundation elements. The ground motion topic includes seismic hazard analyses, seismic perfomance criteria, spectrum compatible time history, spatial variation of ground motions, effects of local soil condition, and the recent ATC-32 recommendations on standard ARS (Acceleration Response Spectrum) design criteria. The soil-structure interaction area discusses modeling of foundation stiffness and damping for spread footing, gravity caisson, large diameter drilled shaft, and pile group. The concept of substructuring technique is described fully. The technique offers two approaches: one bases on kinmatic soil-structure interaction and the other based on inertia interaction. Both are intented to reduce the number of degrees of freedom for the foundation to be used in the global bridge analyses. (AU)

Hysteretic models for cyclic behavior of deteriorating inelastic structures

The purpose of this report is to provide a theoretical basis for a range of rule-based piecewise linear hysteretic models (in Section 2), as well as differential equation-based smooth hysteric models (in Section 3). Also to provide a sound and formal reasoning for the basis of the above-mentionedmodels that are founded on the fundamentals of mechanics and the interrelationship between these various types of models (in Section 4). This research fits in with two more of MCEERïs missions: outreach to the professional user community; and to extend the fundamental knowledge base to enable a high level of computational simulation to be conducted

Response modification factors for seismically isolated bridges

The 1997 AASHTO "Guide Specifications for Seismic Isolation Design" specify response modification factors for the substructures of isolated bridges that are lower than those specfied for the substructures of non-isolated bridges. this report presents the rationale behind these specifications and presents reseach results that lead to the establishment of appropriate response modification factors for isolated bridges. The reseach concertrated on dynamic analysis of simple models of seismic-isolated and non-isolated bridges for a range of isolation system and substructure behaviors and for seismic excitation characterized by AASHTO ground motion spectra for a range of soil conditions and acceleration coefficients. the study investigated the displacement ductility demand in the substructure of these bridges and established the appropriate value of the ductility-based portion of the response modification factors.The study concludes that response modification factors should be lower in the substructures of isolated bridges than in the substructures of non-isolated bridges because : A) elastic or nearly elastic substructure behavior is required for proper behavior of the isolation system, and B) isolated bridges exhibit more sensivity in the substructure ineslastic response due to variability the seismic input (AU)

Experimental investigation of the dynamic response of two bridges before and after retrofitting with elastomeric bearings

This study presents an investigation of the lateral dynamic response behavior of two highway bridges before and after the replacement of the original steel bearings. The bearings on the Northbound bridge were replaced with ordinary neoprene elastomeric bearings, while for the Southbound bridge, laminated rubber seismic isolation bearings were used. The study is in two parts: an experimental field investigation under quick release testing and an analytical modeling study to investigate seismic performace of the bridges with different support conditions. The analytical portion of the study focused on the development of no-linear structural models that could reproduce the experimentally observed displacement time histories. Once validated the models were then used to predict the transient-seismic performance of the bridges with different bearing conditions under a variety of strong ground motions. If existing steel bearing are rehabilitated with engineered isolation bearings the seismic performance may improve marginally. Adding supplemental shock absorbing dampers may help to improve performance under these adverse near field affects (AU)

Structural steel and steel / concrete interface details for bridges

This report assesses the seismic performance of details associated with steel bridge towers extending from a massive concrete substructure, as well as the seismic perfomance of other steel sub-and superstructure details for new construction. Issues addressed are: 1-Identifying the most ductile cross-sections 2-investigating the application of eccentrically braced frames. 3-Exploring details to replace buckled plates or sharpes following an earthquake. 4. Investigating anchor bolt performance under lateral and uplift loads. 5- developing economical moment connection details between steeel superstructures and concrete substructures. The most efficient member cross-sections were identified as tubular sections due to their self stiffening nature and optimal resistance both local and overall buckling. Other shapes can provide adequate perfomance, however and may be more economical due to savings in production and fabrication. Moment resisting frames (MRFs) concentrically braced frames (CBFs) and eccentrically braced frames (EBFs) can all supply adequate seismic resistance, providing that they are detailed correctly. Anchor bolts embedded in concrete can be designed with sufficient depth and edge distance or hairpin reinforcement such that failure does not occur in the concrete. Brittle fractures of anchor bolts in the northridge earthquake resulted in a recommmendation that upset threads shoulds be used. shear keys can be used when anchor bolts do not provide sufficient shear resistance, Anchor bolts can either be designed to remain elastic or to yield. (AU)

Capacity design of bridge piers and the analysis of overstrength

The capacity design philosophy has now become the design norm for the seismic design of most structural systems. For bridge systems, this means it is necessary to assess the overstrength capacity of columns prior to proceeding with the design of the foundation and superstructure. This reseach is devoted to developing deterministic procedures to obtain overstrength factors for column elements. For this purpose a moment-curvature appoach is explored. A parametric study is then conducted to investigate the factors that effect overstrength. A simplified design methodology is proposed based on plastic analysis of overstrength moment capacity. A design example shorwing the step by step procedure is also presented (AU)

Extraction of nonlinear hysteretic properties of seismically isolated bridges from quick - release field tests

A time domian system identification method is used to identify the hysteretic properties of lead-rubber bearings installed in seismically isolated bridge systems. The longitudinal or transverse motion of the superstructure is idealized as a single degree of freedom (SDOF) system, where the total damping effect has been divided two parts.

Development and application of a new model for fracture behavior analysis of structures

A new method for fracture analysis of reinforced concrete structures is proposed. The concrete is modeled as an assembly of distinct elements made by dividing the concrete virtually. These elements are connected by distributed springs in both normal and tangential directions. The reinforcement bars are modeled as continuos springs conneting elements together. Local failure of concrete is modeled by failure of springs connecting elements when the stress calculated from forces acting on springs exceed the critical stress. The element formulation and the computer code were developed and the accuracy of the method were verified by comparing many experimental results. In these experiments the results showed good agreement in determining the failure load, the load-deflection relations, the prediction of crack initiation, crack location and crack propagation. The formulations used to ddevelop this method are simple and efficient in modeling the mechanical behavior of RC structures. (AU)

More about the study of floor-plan shape influence on buildings : Response to earthquakes

This paper presents the results pf applying defferent dynamic analysis methods to the study of the influence of floor-plan irregularities in the earthquake-resistant behaviour of buildings. A computer program, SET (Structural Engineering Tool) 2D and 3D, is used. At the end, numerical examples of model's applications are illustrated.(AU)

Algoritmos genéticos : Una nueva vía para la optimización estructural

La mecánica natural Darwiniana de supervivencia de los seres mejor adaptados, puede ser aplicada a la resolución de diferentes problemas de optimización en varios campos de la ingeniería. En este trabajo se discute un algoritmo genético simple para ser utilizado en la optimización de estructuras y modelos de elementos finitos. Mediante la asimilación de estructuras a cromosomas representados por cadenas de bits y partiendo de una población generada aleatoriamente, se efectúan los procesos naturales de selección, cruce y mutación. Se evoluciona así, a través de las generaciones, hacia la estructura óptima. Los A.G., como la propia naturaleza, es discreta y encaja perfectamente en la vida real donde para optimizar estructuras se dispone de unos determinados tipos de elemntos estructurales, existentes en el mercado (AU)

Estudio de configuraciones irregulares de fachadas o de pórticos interiores de una población de edificios ya proyectados

Consiste este trabajo especial de grado, en el estudio de configuraciones irregulares de fachadas o de pórticos interiores de una población de edificios ya proyectados, en la ciudad de Caracas, y que presentan irregularidades tanto en su configuración en planta como en su configuración vertical. De dicha población se escogieron aquellos donde fué posible conseguir los planos estructurales y de éstos, se escogieron un número limitado fachadas para luego hacer un análisis estructural como pórtico bajo una solicitación externa tipo (fuerzas sísmicas), arbitraria pero igual en todos los casos. Al trabajo original se le sumó un catálogo, en donde se muestra toda la población de edificios cansados, describiendo para cada uno, su irregularidad, con el fin de que el lector aprecie, en forma más clara, las características de las diferentes configuraciones (AU)