Showing papers on "OpenSees published in 2006"

Earthquake performance of steel frames with nitinol braces

Abstract: In this paper, the seismic performance of a three- and a six-storey steel frame equipped with different bracing configurations is assessed. The bracing systems consist of traditional buckling-restrained steel braces and superelastic Nitinol shape-memory alloy (SMA) braces. Background on the behaviour of SMAs is provided and a state-of-the-art review of the applications of such new materials in earthquake engineering is presented. A uniaxial constitutive model for superelastic SMAs is then implemented into the finite element platform OpenSees and nonlinear dynamic analyses are performed. Finally, the seismic performance of the structures under investigation is judged through the evaluation of several response quantities, to determine the efficacy of the new bracing system in reducing earthquake-induced vibrations.

Pre- and post-test mathematical modelling of a plan-asymmetric reinforced concrete frame building

Abstract: Pre- and post-test analyses of the structural response of a three-storey asymmetric reinforced concrete frame building were performed, aimed at supporting test preparation and performance as well as studying mathematical modelling. The building was designed for gravity loads only. Full-scale pseudo-dynamic tests were performed in the ELSA laboratory in Ispra. In the paper the results of initial parametric studies, of the blind pre-test predictions, and of the post-test analysis are summarized. In all studies a simple mathematical model, with one-component member models with concentrated plasticity was employed. The pre-test analyses were performed using the CANNY program. After the test results became available, the mathematical model was improved using an approach based on a displacement-controlled analysis. Basically, the same mathematical model was used as in pre-test analyses, except that the values of some of the parameters were changed. The OpenSees program was employed. Fair agreement between the test and numerical results was obtained. The results prove that relatively simple mathematical models are able to adequately simulate the detailed seismic response of reinforced concrete frame structures to a known ground motion, provided that the input parameters are properly determined.

Validation of a Fast Hybrid Test System with Substructure Tests

Abstract: This paper presents the substructure testing methodology developed for a state-of-the-art fast hybrid test system and the testing of a steel zipper frame. The testing technique is based on the pseudodynamic test concept that combines model-based simulation with physical testing. In the hybrid tests presented here, only the bottom-story braces of a three-story zipper frame were tested, while the rest of the frame was modeled in a computer during a test using a general structural analysis framework OpenSEES. The tests have demonstrated the capability and reliability of the system. The discussion also covers pertinent issues and considerations for carrying out a successful test.

Seismic Vulnerability of Typical Multiple-Span California Highway Bridges

Abstract: Multiple-span reinforced concrete highway overpass bridges constitute a large number of the total inventory of bridges in California, particularly bridges of new design. Performance of these bridges is therefore integral to the evaluation of transportation network performance under high intensity earthquake scenarios. Additionally, probabilistic quantification of bridge response and vulnerability will provide insight into the evaluation of current designs at different levels of seismic hazard. Performance of bridges at the demand, damage, and loss levels can be evaluated using the Pacific Earthquake Engineering Research (PEER) Center’s performance-based earthquake engineering framework. This paper illustrates probabilistic seismic bridge vulnerability evaluation using two typical single column-per-bent, five-span, post-tensioned box girder, reinforced concrete highway bridge types. The first bridge type has a straight deck and 22-foot columns of equal height above grade. The second bridge type has 50-foot high columns. Each bridge type has a variety of column configurations designed for different seismic demands typical for a variety of bridge sites in California. A complex model of the structures is created in OpenSees that accounts for nonlinear behavior of the columns, deck, abutments, and expansion joints at the abutments. This model is developed in a modular fashion to allow incorporation of improved soil models, models for emerging structural components, technologies, and use of new analysis methods. Seismic demand models are then developed using nonlinear time history analysis, including far- and near-field excitation. Damage in the columns is determined from a database of experimental tests and, finally, approximate repair cost ratios are estimated from the ascertained discrete damage states. Four bridge models are implemented for both types of bridges considered. The vulnerability of the base bridge types is presented in this paper as a benchmark with which to compare the use of enhanced performance structural elements and demands due to liquefaction and lateral spreading, when coupled with geotechnical models of the bridge-soil system.

Dynamic tests and modeling of rc and pt slab-column connections

Abstract: Analytical and experimental studies were undertaken to assess and improve modeling techniques for capturing the nonlinear behavior of flat plate systems using results from shake table tests of two, approximately one-third scale, twostory reinforced concrete and post-tensioned concrete slab-column frames. The modeling approach selected accounts for slab flexural yielding, slab flexural yielding within c2+3h due to unbalanced moment transfer, and loss of slab-tocolumn moment transfer capacity due to punching shear failures. For punching shear failures, a limit state model based on gravity shear ratio and lateral interstory drift was implemented into a computational platform (OpenSees). Comparisons of measured and predicted responses indicate that the analytical models were capable of reproducing the experimental results quite well for both test specimens, both prior to and after significant yielding.

Cyclic Softened Membrane Model for Nonlinear Finite Element Analysis of Concrete Structures

Abstract: This paper describes how a Cyclic Softened Membrane Model (CSMM) was developed to rationally predict the cyclic shear responses of reinforced concrete (RC) elements, including the pinching effect in the hysteretic loops, the shear stiffness, the shear ductility and the energy dissipation capacities. This CSMM model was verified by the tests of fifteen RC panels at the University of Houston. The test results confirmed that the orientation of the steel bars and the percentage of steel in a panel are the two most important variables that influence the cyclic response of RC panel elements. Using OpenSees as a framework, the concept of the CSMM was simplified from a 2-D model into a 1-D model and implemented into a finite fiber element program for the prediction of concrete frame structures subjected to cyclic or dynamic loading. The developed program is validated by the reversed cyclic load tests of a reinforced concrete column and by the shake table tests of a prestressed concrete frame. The CSMM has recently been implemented into an OpenSees-based finite element program for a 2-D RC element that will allow structural engineers to predict the monotonic, cyclic and dynamic responses of structures containing walls. This 2-D RC element is validated in this paper by the prediction of the monotonic responses of two RC panels subjected to shear stresses.

Analytical Assessment of a Major Bridge in the New Madrid Seismic Zone

Abstract: A comprehensive study is underway at the Mid-America Earthquake (MAE) Center to assess the seismic response of an existing major bridge, considering soil-structure interaction. The Caruthersville Bridge on I-55 has 59 spans with a total length of 7,100 feet and was built in the early seventies across the Mississippi River between Missouri and Tennessee. The site is in the vicinity of the New Madrid central fault, at a distance of about 5 km from a presumed major fault. The superstructure consists of eleven units supported on a variety of elastomeric and steel bearings. The main river crossing is composed of two-span cantilever steel truss and ten-span steel girders, whilst approach spans are precast prestressed concrete girders. The substructure includes piers on deep caissons and bents on steel friction piles. Detailed three-dimensional dynamic response simulations of the entire bridge including Soil-Structure Interaction (SSI) effects are undertaken using several analytical platforms. The finite element analysis programs SAP2000 and ZEUS-NL (the MAE Center analysis platform) are employed for elastic and inelastic analysis of the structure, respectively. The Pacific Earthquake Engineering Research (PEER) Center analysis platform OpenSees is used for an inelastic simulation of the foundation and the underlying substrata. The assessment methodology is presented, including modeling of the bridge and its foundation system. The SSI analysis is a key element in the current study due to the length of the bridge, the massive and stiff foundation and the relatively soft deep soil underlying the site. The impact of refined modeling and simulations and the pressing need to account for various structural and nonstructural members in vulnerability assessment of major highway bridges are emphasized. The comprehensive hazard study, the realistic modeling approach and the advanced analytical tools employed in the assessment enable the identification of areas of vulnerability in the bridge. They also enable the assessment of its response with a number of different retrofitting schemes. The state-of-the-art simulation methodologies described in the paper enable bringing the most recent research outcomes to support practice and to improve public safety.

Modules in OpenSees for the Next Generation of Performance-Based Engineering

Abstract: This paper outlines and demonstrates structural reliability and other gradient-based applications available for use in OpenSees. The Open System for Earthquake Engineering Simulation is an open-source, object-oriented finite element software framework developed for performance-based earthquake engineering analysis. OpenSees began as the computational platform for seismic simulations of structural and geotechnical systems in the Pacific Earthquake Engineering Research Center (PEER). Parallel computing, database, and hybrid simulation capabilities are included in the OpenSees framework making it an ideal environment for network-based simulations, e.g., in the NSF-sponsored George E. Brown Jr. Network for Earthquake Engineering Simulation (NEES). From this introduction of the framework in OpenSees for gradient-based applications in performance-based engineering, the objective of this paper is to summarize the top-level reliability and sensitivity computations in OpenSees. A listing of specific finite element modules available for gradient computations in OpenSees is provided, followed by representative examples of response sensitivity analysis and probabilistic reliability assessment.

Degradation of Bridge Column Axial Load Capacity with Increasing Lateral Displacement Ductility

Abstract: Current knowledge of the post-earthquake load carrying capacity of reinforced concrete columns is limited. During earthquake excitation, ductile reinforced concrete columns lose strength and stiffness as they accumulate damage. The primary question remaining after an earthquake scenario is what level of residual load carrying capacity exists for columns, both laterally and axially. This residual strength is particularly important for highway bridges where post-earthquake decision-making hinges on functionality of the primary non-redundant load carrying elements, namely the columns. This paper addresses the experimental and analytical program that is currently underway in order to test the residual axial load carrying capacity of California Department of Transportation (Caltrans) bridge columns. Scaled models of typical bridge columns used in California today the tested in two phases. The first phase involves lateral cyclic loading using a prescribed displacement history to a pre-set level of lateral displacement ductility. The second phase involves crushing the specimen axially to determine the residual axial force versus axial deformation relationship. Specimens are tested to varying maximum displacement ductility levels in order to facilitate development of axial loss versus ductility demand curves for design, and to calibrate finite element fiber cross-section beam column elements used in OpenSees software package. Two groups of specimens are described: small-scale square column specimens, and ¼-scale circular column specimens. Test results for the first group show there is a reasonably simple relation between the sustained lateral displacement ductility and the residual axial load carrying capacity. The second group will be tested at the NEES Equipment Site in Berkley in order to validate the findings from the small-scale tests and calibrate the finite element models. Such calibrated models will be used to assess the ability of an entire bridge to sustain gravity and traffic load after an earthquake and, thus, to provide rational decision-making criteria for engineers and inspectors to evaluate the load carrying capacity of a bridge after an earthquake and it’s functionality in a highway network system.