Post-earthquake fires can negatively affect the safety and collapse probability of Reinforced Concrete (RC) structures. At present, there has been no systematic effort to assess the performance of RC structures for combined earthquake and fire effects. Developing appropriate guidelines for this scenario requires simulation tools that can accurately capture material behavior during cyclic loading and at elevated temperatures. Ideally, simulation tools must also be conceptually simple and computationally efficient to allow extensive parametric analyses. The goal of the present study is to enhance a previously established modeling approach so that it can describe the performance of RC structures for both cyclic loading and changes in material behavior due to elevated temperatures. The modeling approach is based on the nonlinear truss analogy and has been extensively validated for cyclic loading of RC shear walls and columns. The constitutive models for concrete and reinforcing steel are enhanced with the capability to account for the effect of elevated temperatures. The enhanced material models are validated using experimental data for concrete and steel at elevated temperatures. The capability of the proposed model to analyze structural-level behavior is verified and compared with experimental testing. The method is also endowed with the capability to describe the time-dependent heat conduction in a fire simulation. The use of the enhanced nonlinear truss model is more advantageous than refined finite element models because of its computational efficiency and conceptual simplicity. / Master of Science

A compatibility-based strut-and-tie model C-STM is proposed for analyzing deep beams and disturbed regions with particular emphasis on reinforced concrete bridge piers. In addition to the normal strut-and-tie force equilibrium requirements the model accounts for non-linear behavior through displacement compatibility using inelastic constitutive laws of cracked reinforced concrete. The model is implemented into widely used commercial structural analysis software and validated against results from previously conducted large scale experiments. A near full-scale experiment on a reinforced concrete sub-assemblage that represents cantilevered and straddle pier bents is conducted to investigate the shear-flexure performance of deep (disturbed) regions. Insights into the development of nonlinear behavior and the final collapse failure mechanism are then evaluated and accurately modeled using the C-STM. It is concluded that the proposed C-STM serves as an advanced method of analysis that can predict with suitable accuracy the force-deformation response of both D- and B- regions, deep beams, and beam-columns. This provides engineers with a supplementary analysis tool that can be used to assess the nonlinear behavior of bridge piers with stocky members and/or large disturbed regions.

Reinforced concrete (RC) moment frame structures make up a notable proportion of buildings in earthquake-prone regions in the United States and throughout the world. The beam-column (BC) joints are the most crucial regions in a RC moment frame structure as any deterioration of strength and/or stiffness in these areas can lead to global collapse of the structure. Thus, accurate simulations of the joint behavior are important for assessment of the local and global performance of both one-way and two-way interior BC joints. Such simulations can be used to study the flexural-shear-bond interaction, the failure modes, and sensitivity of various parameters of structural elements. Most of the existing analytical approaches for interior BC joints have either failed to account for the cyclic bond-slip behavior and the triaxial compressive state of confined concrete in the joint correctly or require so many calibrations on parameters as to render them impractical. The core motivation for this study is the need to develop robust models to test current design recommendations for 3D beam-column-slab subassemblies subjected to large drifts. The present study aims to first evaluate the flexural-shear-bond interactive behavior of two-way beam-column-slab interior connections by both finite element and nonlinear truss methodologies. The local performance such as bond-slip and strain history of reinforcing steel are compared with the experimental results for the first time. The reliability of applied finite element approach is evaluated against a series of one-way interior BC joints and a two-way interior beam-column-slab joint. The accuracy and efficiency of the nonlinear truss methodology is also evaluated by the same series of joints. Results show good agreement for finite element method against both global and local response, including hysteretic curve, local bond-slip development and beam longitudinal bar stress/strain distributions. The nonlinear truss model is also capable in obtaining satisfactory global response, especially in capturing large shear cracks. A parametric study is exhibited for a prototype two-way interior beam-column-slab joint described in an example to ACI 352R-02, to quantify several non-consensus topics in the design of interior BC connections, such as the joint shear force subjected to bidirectional cyclic loading, the development of bond-slip behavior, and the failure modes of two-way interior joints with slab. Results from connections with different levels of joint shear force subjected to unidirectional loading show that meeting the requirements from ACI 352 is essential to maintain the force transfer mechanism and the integrity of the joint. The connections achieved satisfactory performance under unidirectional loading, while the bidirectional monotonic loading decreases the joint shear force calculated by ACI 352 by 10%~26% based on current results. Poorer performance is obtained for wider beams and connections fail by shear in the joint rather than bond-slip behavior when subjected to bidirectional cyclic loading. In general, the study indicates that the ACI352-02 design methodology generally results in satisfactory performance when applied to 2D joints (planar) under monotonic and cyclic loads. Less satisfactory performance was found for cases of 3D joints with slabs. / Doctor of Philosophy / Reinforced concrete (RC) moment frames are one of the most popular structure types because of their economical construction and adaptable spaces. Moment frames consist of grid-like assemblages of vertical columns and horizontal beams joined by cruciform connections commonly labelled as beam-column joints. Because of the regularity of the grid and the ability to have long column spacing, moment frames are easy to form and cast and result in wide open bays that can be adapted and readapted to many uses. In RC structures, steel bars embedded in the concrete are used to take tensile forces, as concrete is relatively weak when loaded in tension. Forces are transferred between the steel and concrete components by so-called “bond” forces at the perimeter of the bars. The proper modeling of the behavior of bond forces inside the beam-column joints of reinforced concrete moment frames is the primary objective of this dissertation. Reinforced concrete moment frames constitute a notable proportion of the existing buildings in earthquake-prone regions in the United States and throughout the world. The beam-column joints are the most crucial elements in a RC moment frame structure as any deterioration of strength and/or stiffness in these areas can lead to global collapse of the structure. Physical experimentation is the most reliable means of studying the performance of beam-column joints. However, experimental tests are expensive and time-consuming. This is why computational simulation must always be used as a supplemental tool. Accurate simulations of the behavior of beam-column joints is important for assessment of the local and global behavior of beam-column joints. However, most of the existing analytical approaches for interior beam-column joints have either failed to account for the bond-slip behavior and the triaxial compressive state of confined concrete in the joint correctly or require so many calibration parameters as to render them impractical. The present study aims to provide reliable numerical methods for evaluating the behavior of two-way beam-column-slab interior joints. Two methods are developed. The v first method is a complex finite element model in which the beam-column joint is subdivided into many small 3D parts with the geometrical and material characteristics of each part carefully defined. Since the number of parts may be in the hundreds of thousands and the geometry and material behavior highly non-linear, setting up the problem and its solution of this problem requires large effort on the part of the structural engineer and long computation times in supercomputers. Finite element models of this type are generally accurate and are used to calibrate simpler models. The second method developed herein is a nonlinear truss analogy model. In this case the structure is modelled as nonlinear truss elements, or elements carrying only axial forces. When properly calibrated, this method can produce excellent results especially in capturing large shear cracks. To evaluate the accuracy and to quantify the current seismic design procedure for beam-column joints, a prototype two-way interior beam-column-slab joint described in an example to ACI 352R-02, the current design guide used for these elements in the USA, is analytically studied by the finite element methodology. The study indicates that the ACI352-02 design methodology generally results in satisfactory performance when applied to one-way (planar) joints under monotonic and cyclic loads. Less satisfactory performance was found for cases of three-dimensional (3D) joints with slabs.

Reinforced concrete (RC) structures constitute a significant portion of the building inventory in earthquake-prone regions of the United States. Accurate analysis tools are necessary to allow the quantitative assessment of the performance and safety offered by RC structures. Currently available analytical approaches are not deemed adequate, because they either rely on overly simplified models or are restricted to monotonic loading. The present study is aimed to establish analytical tools for the accurate simulation of RC structures under earthquake loads. The tools are also applicable to the simulation of reinforced masonry (RM) structures. A new material model is formulated for concrete under multiaxial, cyclic loading conditions. An elastoplastic formulation, with a non-associative flow rule to capture compression-dominated response, is combined with a rotating smeared-crack model to capture the damage associated with tensile cracking. The proposed model resolves issues which characterize existing concrete material laws. Specifically, the newly proposed formulation accurately describes the crack opening/closing behavior and the effect of confinement on the strength and ductility under compressive stress states. The model formulation is validated with analyses both at the material level and at the component level. Parametric analyses on RC columns subjected to quasi-static cyclic loading are presented to demonstrate the need to regularize the softening laws due to the spurious mesh size effect and the importance of accounting for the increased ductility in confined concrete. The impact of the shape of the yield surface on the results is also investigated. Subsequently, a three-dimensional analysis framework, based on the explicit finite element method, is presented for the simulation of RC and RM components under cyclic static and dynamic loading. The triaxial constitutive model for concrete is combined with a material model for reinforcing steel which can account for the material hysteretic response and for rupture due to low-cycle fatigue. The reinforcing steel bars are represented with geometrically nonlinear beam elements to explicitly account for buckling of the reinforcement. The strain penetration effect is also accounted for in the models. The modeling scheme is validated with the results of experimental static and dynamic tests on RC columns and RC/RM walls. The analyses are supplemented with a sensitivity study and with calibration guidelines for the proposed modeling scheme. Given the computational cost and complexity of three-dimensional finite element models in the simulation of shear-dominated structures, the development of a conceptually simpler and computationally more efficient method is also pursued. Specifically, the nonlinear truss analogy is employed to capture the response of shear-dominated RC columns and RM walls subjected to cyclic loading. A step-by-step procedure to establish the truss geometry is described. The uniaxial material laws for the concrete and masonry are calibrated to account for the contribution of aggregate interlock resistance across inclined shear cracks. Validation analyses are presented, for quasi-static and dynamic tests on RC columns and RM walls. / Ph. D.