Earthquake Resistance of Open WEB Steel Framing Systems

Ahmad M. ITANI

This report summarizes studies conducted on existing open web steel framing systems and improvements to their seismic performance. A two-part program, experimental and analytical, was carried out on open web frames designed in accordance with the requirements of the 1988 Uniform Building Code. Based on this investigation a complete design procedure was recommended for the open web framing system, which will ensure their adequate performance during severe ground motions.

An open web frame for a four-story building was designed as an ordinary Moment Resisting Frame (OMRF) with R_ω equal to six. Three subassemblages from this building were tested under cyclic deformation to determine their hysteretic behavior. Based on the experimental results of the existing style frame subassemblages it was found that the behavior was greatly influenced by the web members because when a web diagonal member buckles it produces a large drop in the load carrying capacity and stiffness of the subassemblage. The analytical study representing this type of framing system concluded that it becomes a very flexible structure after buckling of the joist web members causing large story drifts.

Guided by the results from this existing framing a complete design procedure was developed for open web frames, which produce improved seismic performance. The basic concept of this new design procedure is to limit inelastic activity to predetermined members that are able to withstand large deformations. This concept was implemented by using X‑pattern joist web members in the four middle panels. These panels are designed for the specified forces and all other members are designed for the ultimate capacity of the middle panels. Therefore, the middle panels provide the yield link during severe ground motion. Once they yield they will dissipate most of the inelastic energy.

Experimental and analytical investigations were carried out on open web frame systems designed using this modified procedure. Two full scales steel joists were tested under cyclic loading to verify the ductility and stability of the hysteresis loops. The response of this system was compared analytically to a solid web framing system designed as Special Moment Resisting Space Frame (SMRSF). The results showed that the recommended design for open web frames is more economical and produce a stiffer framing system than solid web frames when designed for the same lateral forces.