Enhancement of Steel Moment Connections through Non-Traditional Sections and Materials

Abstract

Seismic moment frames have been widely used in earthquake prone areas due to their lightweight and ability to resist lateral forces, while providing high strength and reasonable stiffness. Typically, steel moment frames utilize wide flange beams and columns which leading to alternative structural shapes being overlooked, such as hollow structural sections (HSS). HSS based seismic moment connections have shown potential for good performance during an earthquake, but their detailing requirements require further exploration due to a lack of understanding of their cyclic behavior. Meanwhile, the excessive inelastic deformation concentrated in the HSS beam observed during cyclic tests can lead to the onset of local buckling and early initiation of cracking at the corners of the beam. To mitigate local buckling and increase energy dissipation capacity, a lightweight, non-traditional civil engineering material is explored as a void fill material to increase the resiliency of steel moment frame systems.

An experimental program is undertaken to characterize the cyclic behavior of two reinforced HSS based moment connections whose behavior shows the feasibility of these connections for seismic application. A parametric study of an innovative connection configuration (i.e. welded HSS based collar connection) is conducted to explore detailing requirements for seismic application and the effects of different parameters on the cyclic behavior of the collar connections. These connections provide more feasible field welding requirements and can potential be used in rapid construction. A design approach is derived for the HSS based collar connection based on the assumption that beam plastic hinging will occur prior to weld failure. Experimental testing of two HSS based collar connections is carried out to further characterize their cyclic behavior and explore their potential failure modes and load transfer mechanisms. These experimental test results are used to calibrate finite element models leading to a better understanding of the influences of different parameters on connection performance under cyclic loads and improved design details.

Both experimental and analytical work is conducted toward selection of the most suitable fill material and evaluation of its effects on enhancement of the seismic performance of filled HSS beams. Monotonic bending tests are performed to evaluate the feasibility of four different types of fill materials. Sixteen lb/ft3 polyurethane foam is found to best mitigate local buckling and increase the energy dissipation capacity of the HSS member. Monotonic and cyclic compression tests of the fill materials leads to an understanding of its mechanical properties and provides useful data for development of finite element material models. A parametric study of the empty and filled HSS beams considering different beam width-thickness and depth-thickness ratios is carried out to evaluate the effectiveness of the fill material in terms of postponing local buckling and increasing energy dissipation. Linear regression analysis of the results with respect to width-thickness and depth-thickness ratio limits considering the percent degradation of the moment capacity at 0.04 rad. of rotation, which is the requirement for special moment frames, shows that an expanded range of HSS beams can be used in seismic applications if they are filled with the polyurethane foam. The findings from this work lay the ground work for use of non-traditional civil engineering fill materials more pervasively in structures in order to enhance their damping capabilities and optimize their performance under extreme loads.