Showing papers on "OpenSees published in 2015"

Shaking table test and numerical simulation of a 1/2‐scale self‐centering reinforced concrete frame

Abstract: Summary Self-centering reinforced concrete frames are developed as an alternative of traditional seismic force-resisting systems with better seismic performance and re-centering capability. This paper presents an experimental and computational study on the seismic performance of self-centering reinforced concrete frames. A 1/2-scale model of a two-story self-centering reinforced concrete frame model was designed and tested on the shaking table in State Key Laboratory of Disaster Reduction in Civil Engineering at Tongji University to evaluate the seismic behavior of the structure. A structural analysis model, including detailed modeling of beam–column joints, column–base joints, and prestressed tendons, was constructed in the nonlinear dynamic modeling software OpenSEES. Agreements between test results and numerical solutions indicate that the designed reinforced concrete frame has satisfactory seismic performance and self-centering capacity subjected to earthquakes; the self-centering structures can undergo large rocking with minor residual displacement after the earthquake excitations; the proposed analysis procedure can be applied in simulating the seismic performance of self-centering reinforced concrete frames. To achieve a more comprehensive evaluation on the performance of self-centering structures, research on energy dissipation devices in the system is expected. Copyright © 2015 John Wiley & Sons, Ltd.

Modelling of masonry infill walls participation in the seismic behaviour of RC buildings using OpenSees

Abstract: Recent earthquakes show that masonry infill walls should be taken into account during the design and assessment process of structures, since this type of non-structural elements increase the in-plane stiffness of the structure and consequently the natural period. An overview of the past researches conducted on the modelling of masonry infilled frame issues has been done, with discussion of past analytical investigations and different modelling approaches that many authors have proposed, including micro- and macro-modelling strategies. After this, the present work presents an improved numerical model, based on the Rodrigues et al. (J Earthq Eng 14:390–416, 2010) approach, for simulating the masonry infill walls behaviour in the computer program OpenSees. The main results of the in-plane calibration analyses obtained with one experimental test are presented and discussed. For last, two reinforced concrete regular buildings were studied and subjected to several ground motions, with and without infills’ walls.

Seismic response of mega buckling‐restrained braces subjected to fling‐step and forward‐directivity near‐fault ground motions

Abstract: Summary Special characteristics of earthquakes in the near-fault regions caused failures for many modern-engineered structures. Fling-step and forward-directivity are the main consequences of these earthquakes. High-amplitude pulses at the beginning of the seismograph have been obviously presented in forward-directivity sites. These pulses have high amount of seismic energy released in a very short time and caused higher demands for engineering structures. Fling-step is generally characterized by a unidirectional large-amplitude velocity pulse and a monotonic step in the displacement time history. These monotonic steps cause residual ground displacements that are associated with rupture mechanism. In this paper, the seismic performance of steel buckling-restrained braced frames with mega configuration under near-source excitation was investigated. Fourteen near-fault records with forward-directivity and fling-step characteristics and seven far-faults have been selected. Nonlinear time-history analyses of 4-story, 8-story, 12-story and 15-story frames have been performed using OpenSees software. After comparing the results, it is shown that, for all frames subjected to the selected records, the maximum demand occurred in lower floors, and higher modes were not triggered. Near-fault records imposed higher demands on the structures. The results for near-fault records with fling-step were very dispersed, and in some cases, these records were more damaging than others. Copyright © 2014 John Wiley & Sons, Ltd.

Numerical evaluation: AISI S400 steel-sheathed CFS framed shear wall seismic design method

Abstract: Seismic design provisions for steel-sheathed cold-formed steel framed shear walls, specific to Canada, are not provided in the existing AISI S213 Standard “North American Standard for Cold-Formed Steel Framing – Lateral Design”. A multi-phase approach was adopted in order to develop appropriate seismic design provisions for inclusion in the new AISI S400 Standard “North American Standard for Seismic Design of Cold-Formed Steel Structural Systems”. The approach included the extensive displacement based testing and dynamic testing of shear walls, the analysis of the test data results, the development and calibration of dynamic numerical models in OpenSees, and lastly, the dynamic response history analyses of twelve archetype buildings designed for the seismic hazard in three cities; Halifax (low seismic), Montreal (medium seismic) and Vancouver (high seismic). It is this last phase of the study that is documented herein. Fragility curves were developed from the buildings' response to 44 scaled ground motion records following a method adapted from the FEMA P695 analysis methodology. The results showed that the design method is appropriate, including the seismic force modification factors of Rd=2 and Ro=1.3. Improvement to the predicted seismic response of CFS framed buildings could be forthcoming if one were to account for the contribution of the non-structural gypsum wall panels.

Effect of Bracing Systems on Fire-Induced Progressive Collapse of Steel Structures Using OpenSees

Abstract: This paper investigates the effect of various bracing systems on the fire-induced progressive collapse resistance of steel-framed structures using OpenSees. Two types of bracing systems (vertical and hat bracing) and various fire scenarios (single and multi-compartment fires) are considered. Four collapse mechanisms of steel frames in fire are found through parametric studies. General collapse is characterized by the collapse of the heated bay followed by lateral drift of adjacent cool bays. Global collapse of a whole frame is due to the buckling of ground floor columns. Another two lateral collapse modes (local and global) are caused by catenary action developed in the heated beams under large deflections. Vertical bracing systems have positive effects on increasing the lateral restraint of the frame against local or global drift, while when arranged at edge bays of frames they negatively contribute to the spreading of a local damage to global collapse in the form of sequential buckling of adjacent columns through load-transfer mechanisms. For a more realistic arrangement of vertical bracings inside the frame, the bracing acts as a barrier to restrain the spread of local damage to the rest of the frame. Instead, using hat bracing can effectively optimize the load-transfer path through a more uniform redistribution of loads in columns and enhance the resistance of structures against progressive collapse. It is found that the application of vertical bracing systems alone on the steel frames to resist progressive collapse is proved to be unsafe and a combined vertical and hat bracing system is recommended in practical design.

Experimental tests and global modeling of masonry infilled frames

Abstract: The effects of infill panels on the response of r.c. frames subjected to seismic action are widely recognized. Numerous experimental investigations were effected and several analytical models were developed on this subject. This work, which is part of a larger project dealing with specific materials and structures commonly used in Italy, discusses experimental tests on masonry and samples of bare and infilled portals. The experimental activity includes tests on elemental materials, and 12 wall samples. Finally, three one-bay one-story reinforced concrete frames, designed according to the outdated Italian technical code D.M. 1996 without seismic details, were tested (bare and infilled) under constant vertical and cyclic lateral load. The first cracks observed on the framed walls occurred at a drift of about 0.3%, reaching its maximum capacity at a drift of 0.5% while retaining its capacity up to a drift of 0.6%. Infill contributed to both the stiffness and strength of the bare reinforced concrete frame at small drifts thus improving overall system behavior. In addition to the experimental activities, previously mentioned, the recalibration of a model proposed by Comberscue (1996) was evaluated. The accuracy of an OpenSees non linear fiber based model of the prototype tested, including a strut element was verified through a comparison with the final experimental results. This work has been partially supported by research grant DPC-ReLUIS 2014.

Numerical Simulation and Seismic Fragility Analysis of a Self-Centering Steel MRF with Web Friction Devices

Abstract: This article presents the seismic fragility analysis of a self-centering steel moment-resisting frame (SC-MRF) with web friction devices. A detailed numerical model of the SC frame was developed using the Open System for Earthquake Engineering Simulation (OpenSees) and the elastoplastic responses of the SC-MRF were studied, including the strength degradation under cyclic loading, tendon rupture, beam buckling, bolt bearing and friction loss, etc. The proposed simulation approach is validated by comparing the simulated results with those in existing hybrid-simulation tests, quasi-static pushover test and low cyclic tests, where good agreement is observed. In addition to the well-established performance limit states (i.e., immediate occupancy, collapse prevention and global dynamic instability), two unique performance limit states (i.e., the recentering and repairable limit states) are defined for the SC-MRF. Finally, incremental dynamic analyses are conducted to evaluate the seismic fragilities regarding t...

Advanced materials for control of post-earthquake damage in bridges

Abstract: This paper presents analytical modeling to study the seismic response of bridge systems with conventional and advanced details. For validation, a 33 m quarter-scale model of a four-span bridge incorporating innovative materials and details seismically tested on the shake tables at the University of Nevada, Reno was taken. The bridge specimen involved use of advanced materials and details to reduce damage at plastic hinges and minimize residual displacements. A three-dimensional, nonlinear model incorporating the response of the innovative materials was developed to study the bridge response using the finite-element software OpenSees. Existing finite-element formulations were used to capture the response of the advanced materials used in the bridge. The analytical model was found to be able to reproduce comparable bent displacements and bent shear forces within reasonable accuracy. The validated model was further used to study different types of bridges under suite of scaled bi-directional near-fault ground motions. Comparisons were made on behavior of five different bridge types, first conventional reinforced concrete bridge, second post-tensioned column bridge, third bridge with elastomeric rubber elements at the plastic hinge zone, fourth bridge with nickel–titanium superelastic shape memory alloy (SMA) reinforcing bar and fifth bridge with CuAlMn superelastic SMA reinforcing bar. Both the SMA used bridges also utilized engineered cementitious composite element at the plastic hinge zone. The results showed effectiveness of the innovative interventions on the bridges in providing excellent recentering capabilities with minimal damage to the columns.

Nonlinear Thermomechanical Analysis of Structures Using OpenSees

Abstract: The behaviour of heated structures is strongly governed by thermal induced deformation and degradation of material properties. This thesis presents an augmentation of the software framework OpenSees to enable thermomechanical analysis of structures. The developments contributed to OpenSees are tested by series of benchmark cases and experimental results. OpenSees is an object-oriented, open source software framework developed at UC Berekeley for providing an advanced computational tool to simulate non-linear response of structural frames to earthquakes. OpenSees was chosen to be extended to enable the modelling of structures in fire. The development of this capability involved creating new thermal load classes to define the temperature distribution in structural members and modifying existing material classes to include temperature dependent properties according to Eurocodes. New functions were also added into the existing corotational beam/column element (2D and 3D) to apply temperature related loads. A new geometrically nonlinear shell element was created (based on the existing linear MITC4 shell element in OpenSees) using total Lagrangian formulation. Appropriate thermal load, material and section classes were also developed for enabling thermomechanical analysis using the nonlinear shell element. A number of benchmark tests were carried out to verify the performance of the new developments implemented in OpenSees. The benchmark tests involved subjecting beams and plates to a range of through depth temperature gradients with OpenSees results compared against closed form solutions. Further verification was also carried out by comparing OpenSees results with ABAQUS results. The extended OpenSees framework was also used to model experiments such as two plane steel frames at elevated temperatures, the Cardington Restrained Beam Test and the Cardington Corner Test and an earthquake damaged reinforced concrete (RC) frame subjected to a subsequent fire. The existing DruckerPrager material class in OpenSees was used to the model concrete in the composite floor in the Cardington tests and in the RC frame. The pinching material available in OpenSees was used to model the beams and columns in the RC frame to consider the cyclic degradation of strength and stiffness during the increasing cyclic displacements imposed on the RC frame before the fire. In all cases the results from OpenSees show good agreement with test data.

Shear strength analyses of internal diaphragm connections to CFT columns

Abstract: Previous theoretical equations for the shear capacity of steel beam to concrete filled steel tube (CFT) column connections vary in the assumptions for the shear deformation mechanisms and adopt different equations for calculating shear strength of each component (steel tube webs, steel tube flanges, diaphragms, and concrete etc.); thus result in different equations for calculating shear strength of the joint. Besides, shear force-deformation relations of the joint, needed for estimating building drift, are not well developed at the present. This paper compares previously proposed equations for joint shear capacity, discusses the shear deformation mechanism of the joint, and suggests recommendations for obtaining more accurate predictions. Finite element analyses of internal diaphragm connections to CFT columns were carried out in ABAQUS. ABAQUS results and theoretical estimations of the shear capacities were then used to calibrate rotational springs in joint elements in OpenSEES simulating the shear deformation behavior of the joint. The ABAQUS and OpenSEES results were validated with experimental results available. Results show that: (1) shear deformation of the steel tube dominates the deformation of the joint; while the thickness of the diaphragms has a negligible effect; (2) in OpenSEES simulation, the joint behavior is highly dependent on the yielding strength given to the rotational spring; and (3) axial force ratio has a significant effect on the joint deformation of the specimen analyzed. Finally, modified joint shear force-deformation relations are proposed based on previous theory.

Response modification factor of mixed structures

Abstract: Mixed structures consist of two parts: a lower part and an upper part. The lower part is usually made of concrete while the upper part is made of steel. Analyzing these structures is complicated and code-based design of them has many associated problems. In this research, the seismic behavior of mixed structures which have reinforced concrete frames and shear walls in their lower storeys and steel frames with bracing in their upper storeys were studied. For this purpose, seventeen structures in three groups of 5, 9 and 15 storey structures with different numbers of concrete and steel storeys were designed. Static pushover analysis, linear dynamic analysis and incremental dynamic analysis (IDA) using 15 earthquake records were performed by OpenSees software. Seismic parameters such as period, response modification factor and ductility factor were then obtained for the mixed (hybrid) structures using more than 4600 nonlinear dynamic analysis and used in the regression analysis for achieving proper formula. Finally, some formulas, effective in designing such structures, are presented for the mentioned parameters. According to the results obtained from this research, the response modification factor values of mixed structures are lower compared to those of steel or concrete ones with the same heights. This fact might be due to the irregularities of stiffness, mass, etc., at different heights of the structure. It should be mentioned that for the first time, the performance and seismic response of such structures were studied against real earthquake accelerations using nonlinear dynamic analysis, andresponse modification factor was obtained by IDA.

Seismic fragility assessment of a tall reinforced concrete chimney

Abstract: Summary In China, a considerable proportion of reinforced concrete (RC) industrial chimneys in operation was designed and constructed in accordance with less rigorous outdated seismic criteria during the end of 19th and early 20th century. However, few research works have been reported till date on a realistic overall assessment of the seismic performance of these existing aging RC chimney structures. Therefore, in this study, fragilities of existing RC chimney were studied. For this purpose, an existing 240 m tall RC chimney was selected and structurally modeled with a lumped mass beam (stick) model by means of the OpenSees analysis program. In order to capture the uncertainties in ground motion realizations, a series of 21 ground motions are selected from the Next Generation Attenuation database as the input motions. To develop the analytical fragility curves, nonlinear incremental dynamic analysis of the studied RC chimney was then carried out using the selected input motions, which were normalized to different excitation levels. The section curvature ductility ratio was considered as the damage index. Based on material strain and sectional analysis, four limit states (LSs) were defined for five damage state. The seismic responses of the all sections were utilized to evaluate the likelihood of exceeding the LSs. Then the peak ground acceleration (PGA)-based seismic fragility curves of the structure were constructed assuming a lognormal distribution. Finally, under the light of these fragility curves, the damage risks in existing RC chimney were discussed. The analytical results indicated that for design level earthquake of PGA = 0.1 g (g is the gravitational acceleration) and the maximum considered earthquake of PGA = 0.22 g, the probabilities of exceeding the light damage state were around 1.5% and 44%, respectively, while the exceedance probabilities corresponding to moderate, extensive and complete damage states were approximately zero in both cases. On the other hand, fragility analysis revealed that the RC chimney structure had considerable ductility capacity and was capable to withstand a strong earthquake with some structural damages. Copyright © 2014 John Wiley & Sons, Ltd.

Seismic vulnerability of box girder continuous bridges under spatially variable ground motions

Abstract: In seismic design of extended multi-span bridges the question always arises whether using either natural or artificially generated earthquake records that are identical at all bridge supports is valid or not. It is likely that earthquake ground motion remarkably differs at the various support/pier locations in terms of amplitude, frequency content and arrival time, inducing under certain circumstances significant forces and deformations that would not develop if the assumption of synchronous excitation was adopted. This paper hence illustrates the impact of ground motion spatial variability on the seismic performance and vulnerability of extended continuous box girder bridges in both bridge orthogonal directions (longitudinal and transverse). For illustration purposes, a nine-span bridge with a total length of 430 m is adopted. Non-linear time history analyses are carried out using opensees software. The effects of the spatial variability in the ground motions at the different bridge supports are investigated using a set of 20 artificially simulated seismic ground motions generated using sim software developed in the mid-nineties, considering different degrees of loss in coherency and various soil types (i.e., frequency contents). Results of the non-linear time history analyses performed in an incremental dynamic analysis context are hence manipulated through a probabilistic analysis framework to generate fragility curves associated with various performance levels for the case study bridge. Fragility curves giving the conditional probability of exceeding various performance levels are then integrated with generated hazard curves defining the expected seismic hazard in Egypt. The outcome of this integration process results in values of mean annual frequency of exceeding pre-defined performance levels.

Evaluation of wall deflections and ground surface settlements in deep excavations

Abstract: A study to illustrate the ground surface and wall movements due to excavation is undertaken based on two-dimensional nonlinear finite element (FE) modeling and analysis. The FE model and nonlinear solution strategy are built in the open-source software platform, OpenSees, of the Pacific Earthquake Engineering Research Center. A drained analysis was carried out using Drucker–Prager multisurface kinematic plasticity model for sand. The proposed model is verified using a case history in an urban excavation. The ground surface settlement and wall deflection for the various conditions of excavation are calculated. The results indicate that the extent of the settlement zone depends on excavation depth and is predicted as 2.5–3 times excavation depth, and the upper limit of the maximum ground surface settlement is equal to the maximum lateral wall deflection.

Experimental and Analytical Studies on a Reinforced Concrete Frame Retrofitted with Buckling-Restrained Brace and Steel Caging

Abstract: A hybrid seismic retrofit scheme, which utilizes a buckling-restrained brace (BRB) to enhance the lateral strength, stiffness and energy dissipation capacity of a seismically-weak reinforced concrete (RC) frame and steel caging to improve the flexural and shear strength of the deficient RC columns and beam-column joints, is proposed. Cyclic quasi-static tests were performed on two half-scale one-story, and single-bay RC frames to evaluate the efficiency of the hybrid retrofit technique. The performance of the BRB, the steel caging, and the connections of the retrofitted frame were studied and found to be satisfactory. The BRB significantly increased the lateral load resistance and energy dissipation capacity of the frame, and the steel caging could effectively change failure mechanism of the RC frame by preventing column failure. Analytical models were developed with OpenSees to simulate the experimentally observed behaviors. Reasonable agreements were achieved between the numerical and experimental results.

Beam-element-based analysis of locally and/or distortionally buckled members: Application

Abstract: The companion paper [1] presents a theory to account for the effects of local and/or distortional buckling in a seven degrees of freedom beam finite element analysis. The theory considers the effect of local and/or distortional buckling as a reduction in tangent rigidities, and determines the reduced tangent rigidities by an a priori post-local or post-distortional buckling analysis of a short length of section. In this paper, the presented method is incorporated into the open source software package OpenSees which was previously modified to account for warping effects [2] . The accuracy of the method is validated by comparison with shell element results for beams, columns and frames.

Performance of semirigid timber frame with Lagscrewbolt connections: experimental, analytical, and numerical model results

Abstract: This paper presents analytical and numerical models for semirigid timber frame with Lagscrewbolt (LSB) connections. A series of static and reverse cyclic experimental tests were carried out for different beam sizes (400, 500, and 600 mm depth) and column–base connections with different numbers of LSBs (4, 5, 8). For the beam–column connections, with increase in beam depth, moment resistance and stiffness values increased, and ductility factor reduced. For the column–base connection, with increase in the number of LSBs, the strength, stiffness, and ductility values increased. A material model available in OpenSees, Pinching4 hysteretic model, was calibrated for all connection test results. Finally, analytical model of the portal frame was developed and compared with the experimental test results. Overall, there was good agreement with the experimental test results, and the Pinching4 hysteretic model can readily be used for full-scale structural model.