DOI: 10.14457/tu.the.2018.139
title: Experimental and numerical studies of precast wide U beam-column system under lateral cyclic load

description: Wide beam-column system is widely used as an efficient gravity load resisting system. Due to many advantages of the wide beam-column system over the conventional beam-column system, the construction of such system is increasing in the region of low to medium level of seismicity. However, depending on the various parameters such as beam width, ratio of beam width to column width, reinforcement detailing etc., the seismic performance of wide-beam column system varies. In this study, the seismic behavior of a typical composite wide beam column system, consisting of precast wide U beam, precast column and cast-in-situ concrete over the precast beam and in the joint region with post-tensioning, was investigated with numerical and experimental approaches. The obtained seismic response was interpreted with displacement-based assessment to directly link the ground motion with structural performance. In addition, in order to minimize the computation time and effort in modeling, an equivalent finite element frame analysis system was developed to represent the full 3D FE analysis of the wide U beam-column joint system under seismic time-history.In the numerical simulation, selection of appropriate constitutive material laws and boundary conditions hold a paramount importance. Therefore, prior to the simulation of seismic response of the post-tensioned precast wide U beam-column system, an intensive investigation on material models which are capable of capturing cyclic behavior was carried out. In this investigation, the most widely used concrete material models (KCC, Winfrith and CSCM) and steel models (Piecewise linear plasticity and Kinematic plasticity) available in LS-DYNA FE code were scrutinized for their potential to simulate the seismic behavior. Based on single element analysis, multi-element analysis with experimental verification and overview of theoretical background, the combination of CSCM and kinematic plasticity model was found to be the excellent combination to capture seismic behavior of reinforced concrete (RC) structures subjected to lateral cyclic load. Adopting the results from constitutive material model investigation, a documented post-tensioned beam-column joint subjected to lateral cyclic load was simulated to validate the adopted methodology and application of constitutive material laws in beam-column joint structures. The results from the numerical analysis showed a good agreement with the experimental results in the literatures with respect to force-displacement behavior and damage pattern. Then, full-scale 3D finite element models of interior and exterior post-tensioned precast wide U beam-column joints were developed. The beam-column joints were subjected to displacement controlled lateral cyclic load. Both the interior and exterior wide U beam-column joints exhibited the drift capacity of 2.2%. However, a significant difference in the load capacities of the joints was observed. The load capacity of the interior joint was found to be 576kN. On the other hand, the exterior joint exhibited a much lower load capacity of 225kN. In addition, the exterior joint showed asymmetric hysteresis having a remarkable difference in load capacity in positive and negative directions. Regarding the failure pattern, a strong-column weak-beam mode of failure was exhibited by the beam-column assemblages. As the precast wide U beam-column joint possesses the unique detailing in the joint region, parametric studies were carried out to elucidate the contribution of each parameter in the joint strength and drift capacity. The parametric study reveals that post-tensioning and middle layer of longitudinal reinforcements enhanced the load capacity of the joint. However, the drift capacity of the precast wide U beam-column joints was found to be unaffected by post-tensioning and middle layer longitudinal reinforcement. On the other hand, the transverse reinforcements in the joint region were found to enhance both drift and load capacities. In addition, the load transfer mechanism in the wide U beam-column connections during lateral cyclic load was elucidated by visualizing vectors of principal stresses. In order to predict the suitability of this wide U beam-column assemblage in various seismicity of Thailand, a displacement-based assessment was carried out.  The ground motion parameters for the displacement-based assessment were determined from the documented sources. Based on the displacement-based assessment, the investigated beam-column joint assemblages were found to partially fulfill the drift demand from various seismic zones of Thailand. A 3D finite element modeling requires high computation time and resources. As a result, the comprehensive 3D finite element analysis (FEA) using solid (brick) elements is not preferable in professional design and analysis. Therefore, in this study, an equivalent finite element (FE) frame analysis system was developed to represent the analysis of a full 3D FEA for the purpose of facilitating the design process in the future. In the developed equivalent system, the concrete sections were defined with Hughes-Liu beam with shear factor of 5/6 obtained from the degeneration of eight node solid elements. The reinforcements were modeled using a smeared approach. A two-storey two-bay frame of precast wide U beam-column system was analyzed under a recorded ground motion having a peak ground acceleration of 0.2g. The effective seismic input method with perfectly matched layers (PMLs) as absorbing boundaries was implemented to define the soil-structure interface (SSI). The proposed equivalent system was able to capture the key characteristics of structures during the ground excitation. In addition to the numerical analyses, a half-scaled interior joint was prepared for a lateral cyclic load test. The test specimen was scaled based on the principal of dimensional analysis and then, verified the adopted dimensions with key structural indices. In the experiment, force-displacement behavior, damage and failure pattern of the specimen, strain development in the rebars, interfacial movement between precast and cast-in-situ concrete will be measured.