Moment-Rotation Behavior of “Pinned” Column Base-Plate Connections in Low-Rise Metal Buildings

In the past few decades, the popularity of metal buildings has grown immensely and today, they are extensively used for low-rise constructions in United States. They account for approximately fifty percent of the total low-rise business construction market and are widely used in many sectors of the North American economy including house manufacturing facilities and warehouses, retail stores, shopping centers, schools, libraries and medical or athletic facilities (MBMA, 2015). Low-rise metal buildings are used in all geographic locations, including high seismic regions. In the design of low-rise metal building systems, column base connections are commonly modeled as pinned supports with no rotational stiffness for both serviceability and strength limit states. However, past studies have indicated that base connections, which are designed as pinned supports, exhibit a non-negligible level of rotational stiffness. Neglecting the rotational stiffness of the base connection may result in a significant overestimation of the lateral displacement of the frames. This additional displacement is addressed by increasing the flexural stiffness of the frame members thereby unnecessarily increasing the cost of low-rise metal buildings. However, there is a distinct lack of design guidelines and experimental data to support the use of rotational stiffness or moment capacity at the so-called pinned column bases. To bridge this gap, this study investigated the moment-rotation behavior of nineteen “pinned” base-plate connections through an experimental program consisting of two phases and an analytical parametric study that included the simulation of over two hundred different base-plate connection configurations. The tests were performed on full-scale specimens subjected to horizontal cyclic displacements with increasing amplitude and a constant axial loading. The first phase of the experimental research involves testing of columns stubs from eight typical low-rise metal building designs in the United States on a steel foundation. In the second phase of the experimental research, eleven column stubs were tested on concrete foundations and a more systematic investigation was performed to study the influence of various parameters, including base-plate dimensions, number of anchor rods, anchor rod diameter and gage distance on the connection behavior. All the tested specimens showed high deformation capacity and a considerable rotational stiffness. The findings from the first phase of the experimental program were used for analysis of typical gabled frames, where the base connections were modeled as rotational springs with the same stiffness as measured in the tests. The results indicated that including the rotational stiffness of the pinned base connections reduces the frame deflections and the frame weight. The outcomes from the second phase of the experimental program indicated that the geometrical characteristics of the column base-plate connections have a significant impact on the overall behavior, rotational stiffness and moment capacity of the connections. For this reason, a parametric study was performed to evaluate the most influential parameters of the column base-plate connections on the rotational stiffness and moment capacity of the connections. Prior to the parametric study, the numerical models were validated using the experimental data from the second phase of testing. It was observed that the most influential parameters on the rotational stiffness and moment capacity of the connections are the base-plate thickness, anchor rod number and diameter and flange thicknesses. Finally, rotational stiffness and the moment capacity measured during the experimental study were compared with the calculated rotational stiffness and moment capacity based on the provisions of the American and European design codes. It was observed that the moment capacity of the connections calculated based on the American design codes was closer to the ones recorded during the experiments, compared with the ones calculated according to Eurocodes. It is expected that the results from this investigation adds knowledge that could later be used for revision of the metal building design codes and standards.