Development of free flange moment connection.

Abstract

Premature failures of steel beam-to-column moment connections, with welded flange-bolted web detail, during the 1994 Northridge earthquake showed that conventional beam theory may not be applicable for their design. Extensive parametric study has been conducted since then in order to determine the reasons for brittle failure of their connection types. Analytical study showed that the main reasons of inferior behavior of the pre-Northridge connection are: actual stress distribution and force flow in the connection region are quite different from what has been assumed in design work as well as use of low toughness weld metal and poor welding practice. In this study, a new design of fully restrained steel moment connection, called the free flange connection, is developed based on the results of analytical studies using finite element analysis and experimental results. The concept of free flange connection is introduced and a unique free flange connection configuration and its role are presented. The results of five full size free flange connection tests show that the free flange connection possesses outstanding ductile behavior with adequate plastic deformation capacity without premature fracture in the connection region. Through a comprehensive parametric analytical study and full size specimen tests, important design factors, such as optimal free flange length, optimal geometry of the free flange connection, and optimal strength of the column panel zone for the free flange connection are obtained. Based on these optimal design factors, final design procedure for the free flange connection is developed. This design procedure includes the effects of actual stress distribution and force flow in the connection region and employs sufficient notch-tough weld material and improved welding practice.