Analytical Investigation of Proposed Semi-Rigid Beam to Column Frame Connections

Precast concrete structures have been widely used since the last century. Fast production, quick erection, higher quality, economical aspects, lower labor costs etc. are of noticeable advantages of such structures compared to that of in-situ concrete structures. Considering frame structures, connections play a vital rule in local and global behavior of precast concrete structures. Catastrophic failures and losses are incurred globally due to failure in connection regions, so connections are considered to be the weak spots in precast concrete structures. Consequently, a great amount of attention and care is required in designing and forming connections, especially in precast concrete structures. In addition, compared to monolithic structures, it is relatively more difficult and more time consuming to achieve rigidity in connections due to the nature of precasting. Plus, difficulties arising from construction and structural details will neutralize inherent characteristics of precasting. Thus, obtaining a connection with details that are simple enough to be constructed easily on site, which, of course, satisfies demanding mechanical characteristic, can be of great importance. In this paper, two new types of beam to column connections are proposed. These connections are designed, modeled and analyzed numerically using nonlinear finite element software, ABAQUS. Main goal of the research was to achieve constructible and easily erectable connection detail which can provide satisfactory lateral strength, stiffness, ductility and energy absorption.
Embedded steel corbels are used as members which transmit tension due to imposed positive moment and shear in negative moment in addition to their role as seating in initial stages of construction. Continuity is provided with bolting or welding of bottom bars to the corbel and then connection area is filled completely with expansive grout. Eccentricity of transmitted forces is a decisive factor especially in dynamic loadings, thus, in design, it is minimized by adjusting bar and corbel size and position and welding locations, size and shapes. Top bars are passed through holes, previously cast into the precast concrete column and are embedded in in-situ concrete of slabs. T shaped assemblies of the connections are modeled and laterally loaded until ultimate concrete strain is reached. In terms of strength, both connections were capable of achieving 95 percent of equivalent monolithic assembly. Considering lateral stiffness, proposed connections were able to provide initial stiffness of more than 80 percent of equivalent monolithic connection. Precast connections were 20 to 30 percent less ductile than their monolithic counterpart. Noticing relative geometric complexity and difference in force transmission mechanisms of connections, lower ductile behavior of connections is justifiable. Effects of axial column load are studied on response of the assemblies. Compressive axial load relatively improves lateral stiffness and energy absorption of the connections. By imposing axial tension on column, lateral stiffness and strength is significantly reduced.
Comparing before mentioned mechanical characteristics of proposed connections with their equivalent monolithic assembly, satisfactory response under lateral monotonic loading is observed. Based upon results derived from this study, proposed connections may be used as semi rigid beam to column connections in precast concrete frames, instead of fully rigid connections.