Introducing a Composite Beam in Precast Moment Frame with Aim of Eliminating Reinforcement Congestion in the Beam Without the Need for In-Suite Concrete

Precasting is a useful approach for building structures in large numbers over a short period of time or for a specific type of them. Precast structures due to the discrete design and construction of the members have different nature than in-situ structures. The constraints on the construction and arrangement of members in these structures have led to beams with simple and poor connection design. In many cases, resistant moment connectionrequires a high reinforcement bar density for getting moment capacity in the beams, and particularly in the joint area. On the other hand, the use of in-situ concrete at the beam to provide the necessary resistance is in conflict with the precastapproach. To solving these problems, the interiorjoint of a structure with an intermediate moment frame system was compared in two precast and in-situ structures by linear dynamic analysis. Considered structure had 4 stories with 3.2 m height and 4 bays in 6 m length. The case study was symmetrical. Applied load cases includes dead, live and earthquake loads. This structure was designed for residential applications and Earthquake design of it was based on Iranian code of practice for seismic resistant design of buildings, Standard 2800. Design of mentioned joints in precast and in-situ cases was based on ACI318-08 and PCI 2004 codes. Moment and shear capacity of beams and columns, splice length of reinforcements, moment axial interaction of columns, joint shear, intermediate moment concrete and steel frame specifications, possible plastic hinge region in beams and design order in connection segments were considered in design process. A finite element based software, ATENA 3D, used for structural analysis of joints. Nonlinear behavior of concrete, longitudinal and transverse reinforcements and strain concentration of concrete hinge were investigated through this software at this research paper. To study the performance of connection joints through nonlinear finite element analysis, the accurate and idealized definition of applied materials is so essential. therefore various material models are used in software. For instance CEB-FIP model is used to investigate the stress-strain curves and also the concrete-reinforcement bonding, Von Mises plasticity theory is used for post yielding behavior of steel connection segments and Interface model is used according to Mohr-Coulomb theory. To study the behavior of connection joints the Force-Drift curves, Crack opening schemeand measured strain of reinforcements and steel segment are investigated. At this paper, we tried to present a moment resistant connection which is simple in production, transportation and assembling. The results show that the introducing beam and its connection, in comparison with equivalent in-situ connection, performs satisfactorily under the efforts of resistance, energy dissipation and ductility. Also plastic hinges formation at proposed beams is due to longitudinal reinforcements yielding just like as in-situ beams, but the position of plastic hinges is in distance with the connection area. In addition to mentioned capabilities, proposed connection can lead to a reduction in reinforcement concentration in beam and also in-situ concrete placing can be eliminated in construction assembling stages. It has been found that, the proposed precast connection can be presented as a good alternative for currently used moment resistant precast connections.