Ioannis Koutromanos

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.

Triaxial Constitutive Model for Concrete under Cyclic Loading

AbstractThis paper presents a material model for the analysis of concrete under multiaxial, cyclic loading conditions. An elastoplastic formulation, having a nonassociative flow rule to capture compression-dominated behavior, is combined with a rotating smeared-crack model to capture the tension-dominated behavior. The proposed formulation resolves the issues that exist in many available concrete material models related to properly capturing the crack opening and closing behavior and accounting for the effect of confinement on the strength and ductility under compression-dominated stress states. The accuracy of the model is validated through analyses for reinforced concrete components. A parametric study demonstrates the importance of accounting for the increase in ductility due to the confinement effect. Additional analyses elucidate the impact of using different yield surfaces for the elastoplastic model on the simulation results.

Constitutive Model for Reinforcing Steel under Cyclic Loading

AbstractThe hysteretic stress-strain response of steel reinforcement can significantly affect the performance of reinforced concrete (RC) structures. Additionally, the possibility of inelastic buckling and fracture of longitudinal bars can have a negative impact on the ductility of RC flexural members. This paper presents a uniaxial material model to describe the inelastic behavior of reinforcing steel under cyclic loading. As a starting point, an existing hysteretic law is employed, which is capable of capturing the material behavior in the absence of buckling and rupture. One issue with this existing law is the need for an iterative stress calculation. A series of enhancements to the hysteretic law are established, including the formulation and implementation of a noniterative stress update procedure and the capability to efficiently account for local inelastic buckling and rupture due to low-cycle fatigue. The proposed material model is validated using the results of experimental tests.

Numerical Study of Masonry-Infilled RC Frames Retrofitted with ECC Overlays

AbstractThis paper presents a nonlinear finite-element modeling method to evaluate the seismic performance of reinforced concrete (RC) frames that have unreinforced masonry infill walls retrofitted with engineered cementitious composite (ECC) overlays. The method has been validated by results of shake-table tests conducted on a 2/3-scale, three-story, two-bay, infilled frame. Results of a numerical study conducted with the validated model have provided further insight into the performance of a structure retrofitted with this technique. A simplified analytical method to assess the shear capacity of an ECC overlay and determine the number and size of shear dowels required to prevent dowel failure has been proposed to ensure a more ductile behavior of the retrofitted structure. Results have shown that the performance of a retrofitted system can be greatly improved by preventing dowel failure so that the benefit of the ductile inelastic behavior of the ECC can be fully exploited. Furthermore, methods to estim...

Nonlinear Beam-Based Modeling of RC Columns Including the Effect of Reinforcing Bar Buckling and Rupture

This study presents a beam-based modeling approach for the analysis of reinforced concrete (RC) frame members under cyclic loads that can capture the effect of inelastic buckling and rupture of reinforcing steel bars. The approach uses force-based elements with a fiber-section model and a corotational formulation to account for the geometric nonlinearity effect on the response of columns. A recently proposed phenomenological uniaxial model for steel reinforcement, capable of simulating inelastic buckling and rupture due to low-cycle fatigue, is used for the reinforcing steel fibers. Numerical simulation models also account for strain penetration effects in the analyses. The modeling approach is validated with the results of experimental tests on RC columns under cyclic loads. A sensitivity study is also pursued to elucidate the impact of bar buckling and strain penetration on the analytical results.

SEISMIC PERFORMANCE OF MASONRY-INFILLED RC FRAMES WITH AND WITHOUT RETROFIT

This paper presents the findings of a research that focused on the seismic performance of masonry-infilled, nonductile, RC frames. This research has resulted in improved analytical methods and effective retrofit techniques to assess and enhance the performance of these structures. The methods were validated by a series of quasi-static tests conducted on one-story frame specimens as well as shake-table tests conducted on two 2/3-scale, three-story, two-bay, masonry-infilled, RC frames. This paper focuses on the observations from the shake-table tests and the further insight gained from a numerical study conducted with finite element models. The first shake-table test specimen had no retrofit measures, and the second had infill walls in the first and second stories strengthened with Engineered Cementitious Composite (ECC) and Fiber Reinforced Polymeric (FRP) overlays, respectively. The tests demonstrated the effectiveness of the retrofit measures. Finite element models that combine smeared and discrete cracks have been used in a numerical study to examine the benefits of the ECC retrofit and the influence of the capacity of the shear dowels that connect an ECC overlay to the RC beams on structural performance. It has been shown that these shear dowels play a critical role in enhancing both the strength and ductility of a retrofitted structure.