# Your search: "author:"Panagiotou, A""

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## Scholarly Works (146 results)

In recent decades, improvement in construction and design practices and better estimation in seismic demands has led to an increasing number of reinforced concrete special moment resisting frame (SMRF) buildings with height and member sizes exceeding those typically built in the past. While current codes improved greatly over the years, many design specifications introduced around the prevailing practices from decades ago remain in effect. The aim of this dissertation is to address some potentially problematic areas in current design standards and propose ways to improve them. Specifically, the focal points of the work presented concern with two separate areas in the design of reinforced concrete SMRF buildings.

The first topic is the investigation of the transverse steel spacing requirements in the plastic hinge zones of reinforced concrete SMRF beams. Two large reinforced concrete SMRF beams were built and subjected to earthquake-like damage in the laboratory test with the goal: (a) to demonstrate that the maximum hoop spacing limits specified in the concurrent 2008 ACI 318 Code could produce a beam with performance inferior to the implied expectations at design level ground shaking intensity, and (b) to evaluate the effect of reducing this hoop spacing limit and recommend code changes for the 2011 ACI 318 Code. The experiments included two 30 in. x 48 in. beams with identical size, material properties, and longitudinal reinforcement ratio, but different transverse hoop spacing, which were subjected to reverse cyclic displacement history to simulate the earthquake-induced deformations expected at the design earthquake (DE) hazard level. The first specimen, Beam 1, was designed with the 2008 ACI 318 hoop spacing requirement and exhibited limited ductility before experiencing sudden and significant loss of load bearing capacity at a displacement ductility of 3.4. The second specimen, Beam 2, built with reduced hoop spacing, showed notable improvement in response and was capable of sustaining 90% of its load bearing capacity up to a displacement ductility level of 6.5. Of the two specimens, only Beam 2 sustained the deformation levels compatible with the DE shaking intensity without significant loss of strength. Both beams, however, failed due to longitudinal bar buckling, which pointed to potential vulnerability in the current transverse reinforcement detailing using multiple piece hoops consisting of stirrups with vertical and horizontal crossties and bracing only alternate longitudinal bars with vertical crossties. Further experimental research in this area is strongly recommended.

The second topic concerns with the global nonlinear response of reinforced concrete SMRFs under strong ground motion, with emphasis placed on seismic shear demand in SMRF columns. Current ACI 318 specifications offer two different approaches in calculating the seismic shear demand, however with some ambiguity and much room for free interpretation that can vastly impact the shear capacity of the column and potentially result in unconservative design. Total of eight numerical models of buildings with perimeter SMRFs of varying configurations were analyzed in two separate studies (four buildings are presented in Chapter 5 and the other four in Chapter 6) under multiple ground acceleration records to find the mean shear envelopes in the columns. Depending on the interpretation of the ACI 318 code, various levels of conservatism in estimating column shears were achieved. A common design approach to estimate seismic column shear from the joint equilibrium with beams having reached the probable moment strengths, while the unbalanced moment is distributed evenly between the columns above and below, was shown to lead to unconservative seismic shear estimate, in some cases resulting in half of the actual demand computed in the nonlinear dynamic analyses. It is demonstrated that the seismic shear demand on columns is better estimated with a method based on amplifying the seismic shear calculated with the elastic code-prescribed modal response spectrum analysis with the system overstrength and dynamic amplification factors.

The objectives of this dissertation is to investigate the use of rocking foundations in bridges for enhanced seismic design and performance and the reduction of post-earthquake damage. The seismic response of bridge systems was studied numerically using three-dimensional nonlinear models, whereas bridge columns with rocking foundations and superstructure mass were studied both numerically and experimentally. The experimental part consisted of the shake-table testing of large scale bridge columns with shallow rocking foundations using physical modeling of the underlying soil. Using the data from these tests, a three-dimensional model with Winkler springs was modified and validated for rocking shallow foundations designed with high factors of safety against vertical loads. The proposed model was then used on a parametric study to investigate the seismic demand on a large variety of bridge piers with rocking shallow foundations.

This dissertation pursues three main objectives: (1) to investigate the seismic response of tall reinforced concrete core wall buildings, designed following current building codes, subjected to pulse type near-fault ground motion, with special focus on the relation between the characteristics of the ground motion and the higher-modes of response; (2) to determine the characteristics of a base isolation system that results in nominally elastic response of the superstructure of a tall reinforced concrete core wall building at the maximum considered earthquake level of shaking; and (3) to demonstrate that the seismic performance, cost, and constructability of a base-isolated tall reinforced concrete core wall building can be significantly improved by incorporating a rocking core-wall in the design.

First, this dissertation investigates the seismic response of tall cantilever wall buildings subjected to pulse type ground motion, with special focus on the relation between the characteristics of ground motion and the higher-modes of response. Buildings 10, 20, and 40 stories high were designed such that inelastic deformation was concentrated at a single flexural plastic hinge at their base. Using nonlinear response history analysis, the buildings were subjected to near-fault seismic ground motions as well as simple close-form pulses, which represented distinct pulses within the ground motions. Euler-Bernoulli beam models with lumped mass and lumped plasticity were used to model the buildings. The response of the buildings to the close-form pulses fairly matched that of the near-fault records. Subsequently, a parametric study was conducted for the buildings subjected to three types of close-form pulses with a broad range of periods and amplitudes. The results of the parametric study demonstrate the importance of the ratio of the fundamental period of the structure to the period of the pulse to the excitation of higher modes. The study shows that if the modal response spectrum analysis approach is used--considering the first four modes with a uniform yield reduction factor for all modes and with the square root of sum of squares modal combination rule--it significantly underestimates bending moment and shear force responses. A response spectrum analysis method that uses different yield reduction factors for the first and the higher modes is presented.

Next, this dissertation investigates numerically the seismic response of six seismically base-isolated (BI) 20-story reinforced concrete buildings and compares their response to that of a fixed-base (FB) building with a similar structural system above ground. Located in Berkeley, California, 2 km from the Hayward fault, the buildings are designed with a core wall that provides most of the lateral force resistance above ground. For the BI buildings, the following are investigated: two isolation systems (both implemented below a three-story basement), isolation periods equal to 4, 5, and 6 s, and two levels of flexural strength of the wall. The first isolation system combines tension-resistant friction pendulum bearings and nonlinear fluid viscous dampers (NFVDs); the second combines low-friction tension-resistant cross-linear bearings, lead-rubber bearings, and NFVDs. The designs of all buildings satisfy ASCE 7-10 requirements, except that one component of horizontal excitation is used in the two-dimensional nonlinear response history analysis. Analysis is performed for a set of ground motions scaled to the design earthquake (DE) and to the maximum considered earthquake (MCE). At both the DE and the MCE, the FB building develops large inelastic deformations and shear forces in the wall and large floor accelerations. At the MCE, four of the BI buildings experience nominally elastic response of the wall, with floor accelerations and shear forces being 0.25 to 0.55 times those experienced by the FB building. The response of the FB and four of the BI buildings to four unscaled historical pulse-like near-fault ground motions is also studied.

Finally, this dissertation investigates the seismic response of four 20-story buildings hypothetically located in the San Francisco Bay Area, 0.5 km from the San Andreas fault. One of the four studied buildings is fixed-base (FB), two are base-isolated (BI), and one uses a combination of base isolation and a rocking core wall (BIRW). Above the ground level, a reinforced concrete core wall provides the majority of the lateral force resistance in all four buildings. The FB and BI buildings satisfy requirements of ASCE 7-10. The BI and BIRW buildings use the same isolation system, which combines tension-resistant friction pendulum bearings and nonlinear fluid viscous dampers. The rocking core-wall includes post-tensioning steel, buckling-restrained devices, and at its base is encased in a steel shell to maximize confinement of the concrete core. The total amount of longitudinal steel in the wall of the BIRW building is 0.71 to 0.87 times that used in the BI buildings. Response history two-dimensional analysis is performed, including the vertical components of excitation, for a set of ground motions scaled to the design earthquake and to the maximum considered earthquake (MCE). While the FB building at MCE level of shaking develops inelastic deformations and shear stresses in the wall that may correspond to irreparable damage, the BI and the BIRW buildings experience nominally elastic response of the wall, with floor accelerations and shear forces which are 0.36 to 0.55 times those experienced by the FB building. The response of the four buildings to two historical and two simulated near-fault ground motions is also studied, demonstrating that the BIRW building has the largest deformation capacity at the onset of structural damage.

This dissertation approaches the subject of three-dimensional (3D) seismic analysis of reinforced concrete (RC) wall buildings at near-fault sites by first studying two main problems separately: (1) the characterization of base excitation for buildings located at near-fault sites, and (2) modeling the behavior of RC buildings accurately including inelastic behavior and the failure mode. The dissertation culminates with the 3D response history analysis of two 20-story RC core wall buildings models, including the slabs and columns, subject to a strong near-fault ground motion record.

First, the presence and characteristics of multiple pulses [with dominant period TP between 0.5 and 12 s] in historical near-fault ground motion records is studied. An iterative method for extracting multiple strong pulses imbedded in a ground motion is presented. The method is used to extract multiple strong velocity pulses from the fault-normal horizontal component of 40 pulse-like ground motion records from 17 historical earthquakes, with magnitudes ranging from MW6.3 to MW7.9, recorded at a distance less than 10 km from the fault rupture with a peak ground velocity greater than 0.6 m / s. The relationships between the dominant period of the extracted pulses, associated amplitudes, and earthquake magnitude are presented, indicating that the amplitude of the strongest pulses with 1.5 s ≤ TP ≤ 5 s, does not depend significantly on the earthquake magnitude.

Next, the effect of soil-foundation-structure interaction (SFSI) for a 20-story core wall building with a caisson foundation subject to single pulse motions is investigated using two-dimensional (2D) nonlinear finite-elements and fiber beam-column elements; nonlinear site effects on the free-field motion and structural response is discussed. The nonlinear site effects for free-field motions result in a de-amplification of peak surface acceleration due to soil yielding, and a maximum of 64% amplification of peak acceleration and velocity of at specific pulse periods for deep soils. SFSI, after removing the nonlinear site effect, has a negligible effect on the maximum value of peak roof acceleration and peak roof drift ratio over the pulse periods considered; however, the effect of the increased flexibility due to SFSI is observed in the peak drift ratio and peak base shear response.

A couple of chapters of this dissertation are dedicated to the development and verification of a three-dimensional nonlinear cyclic modelling method for non-planar reinforced concrete walls and slabs. This modeling approached - called the beam-truss model (BTM) - consists of (i) nonlinear Euler-Bernoulli fiber-section beam elements representing the steel and concrete in the vertical and horizontal direction, and (ii) nonlinear trusses representing the concrete in the diagonal directions. The model represents the effects of flexure-shear interaction (FSI) by computing the stress and strains in the horizontal and vertical directions and by considering biaxial effects on the behavior of concrete diagonals. In addition, the BTM explicitly models diagonal compression and tension failures (shear failures) under cyclic or dynamic loading. The BTM is first validated by comparing the experimentally measured and numerically computed response of eight RC walls subjected to static cyclic loading, including two non-planar RC walls under biaxial cyclic loading. Then, the BTM is extended to modeling slabs and validated with a two-bay slab-column specimen. Finally, the BTM is validated by comparing the experimentally measured and numerically computed response and failure mode of a 5-story coupled wall RC building under seismic base excitation.

The final chapter presents the 3D response history analysis of two 20-story RC core wall buildings subject to a strong near-fault ground motion record. The 20-story building model includes the RC core wall, post-tensioned slabs, and columns; the core wall and slabs are modeled using the developed BTM while the columns are modeled with fiber-section beam-column elements. The two 20-story RC core wall buildings considered have similar geometry: one is conventionally designed to develop plastic hinging at the base of the core-wall, and the second is designed with a damage resistant structural system that combines two seismic isolation planes. Analysis is conducted using the two horizontal components of the historical TCU52 ground motion recorded 0.7 km from the fault plane of the MW7.6 1999 Chi-chi, Taiwan earthquake. The seismic response and damage of the two buildings is discussed.

Modern reinforced concrete bridges are designed to avoid collapse and to prevent loss of life during earthquakes. To meet these objectives, bridge columns are typically detailed to form ductile plastic hinges when large displacements occur. California seismic design criteria acknowledges that damage such as concrete cover spalling and reinforcing bar yielding may occur in columns during a design-level earthquake.

The seismic resilience of bridge columns can be improved through the use of a damage resistant hybrid fiber reinforced concrete (HyFRC). Fibers delay crack propagation and prevent spalling under extreme loading conditions, and the material resists many typical concrete deterioration mechanisms through multi-scale crack control.

Little is known about the response of the material when combined with conventional reinforcing bars. Therefore, experimental testing was conducted to evaluate such behaviors. One area of focus was the compression response of HyFRC when confined by steel spirals. A second focus was the tensile response of rebar embedded in HyFRC. Bridge columns built with HyFRC would be expected to experience both of these loading conditions during earthquakes.

The third focus of this dissertation was the design, modeling, and testing of an innovative damage resistant HyFRC bridge column. The column was designed to rock about its foundation during earthquakes and to return to its original position thereafter. In addition to HyFRC, it was designed with unbonded post-tensioning, unbonded rebar, and headed rebar which terminated at the rocking plane. Because of these novel details, the column was not expected to incur damage or residual displacements under earthquake demands exceeding the design level for ordinary California bridges. A sequence of scaled, three dimensional ground motion records was applied to the damage resistant column on a shaking table. An equal scale reinforced concrete reference column with conventional design details was subjected to the same motions for direct comparison.

Compression tests showed that the ductility of HyFRC is superior to concrete in the post-peak softening branch of the response. HyFRC achieved a stable softening response and had significant residual load capacity even without spiral confinement. Concrete required the highest tested levels of confinement to achieved comparable post-peak ductility. Tension tests showed that HyFRC provides a substantial strength enhancement to rebar well beyond their yield point. Interesting crack localization behavior was observed in HyFRC specimens and appeared to be dependent on the volumetric ratio of rebar.

The damage resistant HyFRC bridge column attained its design objectives during experimental testing. It exhibited pronounced reentering behavior with only light damage under earthquake demands 1.5 to 2.0 times the design level. It accumulated only 0.4% residual drift ratio after seven successive ground motions which caused a peak drift ratio of 8.0%. The conventional reinforced concrete column experienced flexural plastic hinging with extensive spalling during the same seven motions. It accumulated 6.8% residual drift ratio after enduring a peak drift ratio of 10.8%.