In 1952, University of Tennessee professor R.E. Whitmore developed an effective width for design of gusset plates. AISC LRFD 3rd Ed., p. 9-3, says "the width of the Whitmore section is determined at the end of the joint by spreading the force from the start of the joint 30 degrees to each side in the connection element along the line of force. The Whitmore section may spread across the joint between connection elements, but cannot spread beyond an unconnected edge." Additional information can be found in the Steel Tips paper "Seismic Behavior and Design of Gusset Plates" by Astaneh, from the AISC web site.
