Question: I am designing a moment frame using HSS tubes for a seismic region (California). The plan checker is refusing to accept it as an “Intermediate Moment Frame” (IMF) because it does not comply with AISC 341. Do you have any information regarding the use of HSS moment frames and how they can be classified for seismic design as “ordinary”, “intermediate” or “special” (OMF, IMF SMF)?

Answer: A seismic force-resisting system, using moment-resisting frames with HSS columns, and either wide flange beams or HSS beams, can be designed to be an Ordinary Moment Frame (OMF), as defined by AISC 341 Chapter E, with some attention to the connection details and reinforcement. However, in order to be classified as an Intermediate Moment Frame (IMF) or a Special Moment Frame (SMF), as defined by AISC 341 Chapter E parts E2 and E3, the connections to be used are subject to a “conformance demonstration” which can involve specific laboratory testing or the use of “pre-qualified connections”, as approved by AISC 358. There are a number of public domain connection solutions for moment-resisting frames under high seismic loading available as prequalified designs in AISC 358, however none of these pertain directly to hollow structural section members. There are, however, two proprietary seismic moment connection types at present that do have prequalification status, for use with HSS columns.  They are Sideplate (www.sideplate.com) and ConXtech (www.conxtech.com). Please refer to their websites and AISC 358 for additional information.