Improving the Seismic Performance of Steel Moment Resisting Frames through Advanced Approaches

Abstract

After the 1994 Northridge earthquake in the US caused significant structural damage to steel moment resisting frames, revised seismic provisions were developed to address the vulnerabilities observed in the affected structures. These new design requirements led engineers in the US to favor the use of deep, slender wide flange columns over stockier wide flange sections. However, these deeper and more slender wide flange sections have been shown to be more prone to local and global instabilities due to their larger local slenderness ratios and smaller radius of gyration about the weak axis. Recent investigations into the behavior of deep, slender wide flange column sections have shown that some column sections exhibit significantly less ductility than is expected due to a previously unrecognized interaction between the local and global instabilities, making them more prone to collapse.

While more research is needed to better understand this phenomenon, it is also worth exploring alternatives to improving the seismic performance of moment resisting frames outside of utilizing wide flange sections. A potential alternative is to take advantage of the beneficial properties of hollow structural sections (HSS). These sections offer excellent torsion resistance, good bending strength, and the option to fill their voids with different materials to enhance their performance. Despite these benefits, HSS members are not commonly used in seismic steel moment frames due to stringent local slenderness requirements and a lack of a non-proprietary prequalified seismic moment connection. With these ideas and shortcomings in mind, this research seeks to improve the seismic performance of steel moment resisting frames through the assessment of deep, slender wide flange columns and advanced alternatives utilizing HSS.

To this end, a finite element and experimental investigation of deep, slender wide flange column subassemblies is undertaken to study the behavior of these sections under combined axial and lateral load while in the presence of framing elements such as beams and connections. The results from these tests show how the local and global instabilities can interact with each other to accelerate the capacity degradation in a column section and cause it to not meet the expected ductility requirements. These findings highlight the need to consider the interaction between local and global failure modes in the design process as they are currently checked separately and show similar behavior between previous member level studies and the subassembly specimens.

Following this study, an experimental and complimentary computational investigation on empty and foam-filled HSS beams is undertaken to explore the benefits of utilizing a polyurethane foam as a fill material. The polyurethane foam-fill provides additional energy dissipation capacity while improving the stability of the HSS walls, mitigating the effects of local buckling, and increasing the cyclic performance of HSS beam members. The findings of this study show that incorporating a polyurethane foam as fill material can alleviate the stringent local slenderness requirements and allow more HSS sizes to meet the moderate and high ductility performance criteria.

Finally, a computational investigation of an innovative tube-based collar connection is undertaken to enhance the feasibility of such a connection through economy and efficiency improvements. While the collar connection has previously been experimentally shown to provide adequate ductility, the findings of this study indicate further optimizations can be made to significantly reduce the amount of field welding that is necessary, saving on cost and construction time.