Andreas Stavridis

Finite-Element Modeling of Nonlinear Behavior of Masonry-Infilled RC Frames

The evaluation of the seismic performance of masonry-infilled reinforced concrete (RC) frames has been a major challenge for structural engineers. This paper addresses pertinent issues on the development and calibration of nonlinear finite-element models for assessing the seismic performance of these structures. The modeling scheme considered here combines the smeared and discrete crack approaches to capture the different failure modes of infilled frames, including the mixed-mode fracture of mortar joints and the shear failure of RC members. A systematic approach is presented here to calibrate the material parameters, and the accuracy of the nonlinear finite-element models has been evaluated with experimental data. The comparison of the numerical and experimental results indicates that the models can successfully capture the highly nonlinear behavior of the physical specimens and accurately predict their strength and failure mechanisms. The validated models have been used to assess the sensitivity of the numerical results to the modeling parameters and to identify the critical material parameters through a parametric study.

Finite-Element Model Updating for Assessment of Progressive Damage in a 3-Story Infilled RC Frame

AbstractThis paper presents a study on the identification of progressive damage, using an equivalent linear finite-element model updating strategy, in a masonry infilled RC frame that was tested on a shake table. A two-thirds-scale, 3-story, 2-bay, infilled RC frame was tested on the UCSD–NEES shake table to investigate the seismic performance of this type of construction. The shake table tests induced damage in the structure progressively through scaled historical earthquake records of increasing intensity. Between the earthquake tests and at various levels of damage, low-amplitude white-noise base excitations were applied to the infilled RC frame. In this study, the effective modal parameters of the damaged structure have been identified from the white-noise test data with the assumption that it responded in a quasi-linear manner. Modal identification has been performed using a deterministic-stochastic subspace identification method based on the measured input–output data. A sensitivity-based finite-ele...

Shake-Table Tests of a 3-Story Masonry-Infilled RC Frame Retrofitted with Composite Materials

AbstractThis paper presents a study that investigated the effectiveness of retrofitting unreinforced masonry infill walls with composite materials to enhance the seismic performance of infilled nonductile RC frames. The primary retrofit scheme considered was the use of engineered cementitious composite overlays. Shake-table tests were conducted on a 2/3-scale, 3-story, 2-bay, masonry infilled RC frame that had one bottom-story wall retrofitted with engineered cementitious composites. The influence of this retrofit on the performance of the structure was investigated using the experimental observations and results of nonlinear finite element analyses. Furthermore, after walls in the second story of the structure were damaged, they were repaired by injecting epoxy into cracked mortar joints, and strengthened with a glass-fiber reinforced polymer overlay. It has been shown that both retrofit schemes are effective in enhancing the seismic performance of the structure and preventing diagonal shear failures of ...

Seismic performance of non-ductile RC frames with brick infill

This paper summarizes some of the findings of a research project that investigates the seismic performance of masonry-infilled, non-ductile, RC frames, including the development of reliable analytical methods for performance assessment and effective retrofit techniques. Quasi-static tests were conducted on small and large-scale, single-story, single-bay, RC frames infilled with brick masonry walls with and without openings. Some of the infill walls were strengthened with an engineered cementitious composite material. Furthermore, two 2/3-scale, three-story, two-bay, masonry-infilled, RC frames were tested on a shake table. One was tested with no retrofit measures, and the other had infill walls strengthened with the engineered cementitious composite and fiber reinforced polymeric material in the first and second stories, respectively. The tests have demonstrated the effectiveness of the retrofit measures. Computation models that combine smeared and discrete cracks have been developed and validated by the experimental data. Some of the experimental and numerical results are presented in this paper.

Validation of a fast hybrid test system with substructure tests

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.

Bond strength and cyclic bond deterioration of large-diameter bars

A study on the bond strength and cyclic bond deterioration of large diameter reinforcing bars embedded in well-confined concrete is presented. The study included monotonic pullout tests and cyclic pull-pull tests conducted on No. 11, No. 14, and No. 18 (36, 43, and 57 mm) reinforcing bars. The bond stress-slip relations obtained from the tests are presented, and the effects of the compressive strength of concrete, bar size, pull direction (for a vertically cast bar), and slip history on the bond strength are examined. Moreover, a phenomenological bond stress-slip law for monotonic and cyclic loading is proposed for bars embedded in well-confined concrete. This law has been validated with experimental results obtained in this study and in previous research.

Numerical Investigation of the In-Plane Performance of Masonry-Infilled RC Frames with Sliding Subpanels

AbstractA number of construction techniques have been proposed to improve the seismic performance of infilled RC frames by increasing the strength and stiffness of the infill and/or the frame. The increase of the seismic capacity of infilled frames with these techniques can improve substantially their seismic performance as long as the demand does not exceed the capacity, but it can eventually lead to brittle failures once the capacity is exceeded. This study assesses numerically a new construction technique that introduces flexibility to the system to ensure its ductile behavior and minimal damage by splitting the infill in subportions and allowing the sliding along the horizontal joints connecting these subportions. A numerical model, validated with data from tests on the components of the proposed structural system and able to capture the interaction between them, has been developed to provide insight into the load-transfer mechanism that develops, and to optimize the proposed detailing. A parametric s...

Structural Identification of an 18-Story RC Building in Nepal Using Post-Earthquake Ambient Vibration and Lidar Data

Few studies have been conducted to assess post-earthquake performance of structures using vibration measurements. This paper presents system identification and finite element modeling of an 18-story apartment building that was damaged during the 2015 Gorkha earthquake in Nepal. In July 2016, a few months after the earthquake, the authors visited the building and collected its ambient acceleration response. The recorded data are analyzed and the modal parameters of the structure are identified using an output-only system identification method. A linear finite element model of the building is also developed based on the geometry of the building and its material properties to estimate numerically its dynamic characteristics. The identified modal parameters are compared to those of the model to identify possible shortcomings of the modeling and identification approaches. The identified natural frequencies and mode shapes of the first two vibration modes are in good agreement with the model.

Shake-Table Tests of a Full-Scale Three-Story Reinforced Masonry Shear Wall Structure

AbstractThis paper presents the shake-table tests of a full-scale, 3-story, reinforced concrete masonry structure. The structure was a special reinforced masonry shear wall system designed according to current code provisions for an area of high seismicity. It consisted of three lines of walls arranged in an H-shape in plan view, with one lineal and two T-walls aligned in the direction of the table motion and four lineal walls oriented perpendicular to the table motion. The seven walls were separated by door openings and were connected with lintels and precast slabs with cast-in-place concrete topping. The structure was subjected to a series of dynamic tests including nine seismic excitations with intensities exceeding the maximum considered earthquake used in the design. The structure had a capacity that exceeded considerably the design base shear, and it withstood all but the last two excitations with little or no damage. The paper presents the design of the test structure, its dynamic response to the s...

Effects of prediction error bias on model calibration and response prediction of a 10-story building

This paper investigates the application of Hierarchical Bayesian model updating to be used for probabilistic model calibration and response prediction of civil structures. In this updating framework the misfit between the identified modal parameters and the corresponding parameters of the finite element (FE) model is considered as a Gaussian distribution with unknown parameters. For response prediction, both the structural parameters of the FE model and the parameters of the misfit error functions are considered. The focus of this paper is to (1) evaluate the performance of the proposed framework in predicting the structural modal parameters at a state that the FE model is not calibrated (extrapolation from the model), and (2) study the effects of prediction error bias on the accuracy of the predicted values. The test structure considered here is a ten-story concrete building located in Utica, NY. The modal parameters of the building at its reference state were identified from ambient vibration data using the NExT-ERA system identification method. The identified modal parameters are used to calibrate parameters of the initial FE model as well as the misfit error functions. Before demolishing the building, six of its exterior walls were removed and ambient vibration measurements were also collected from the structure after wall removal. These data are not used to calibrate the model; they are only used to validate the predicted results. The model updating framework of this paper is applied to estimate the modal parameters of the building after removal of the six walls. Good agreement is observed between the model-predicted modal parameters and those identified from vibration tests.