Louis F. Geschwindner, PhD, PE
Professor of Architectural Engineering
The Pennsylvania State University
University Park, PA 16801

Robert O. Disque, PE
Consultant
Gibble Norden Champion Brown Consulting Engineers Inc.
Old Saybrook, CN 06475

Introduction:

It seems that there has been confusion among structural engineers about the type of construction referred in the AISC LRFD Specification (1), since 1986, as Partially Restrained or PR. The general concept has been of interest to the authors for many years and has been the topic of several of their papers. The purpose of this paper is to reacquaint the profession with a longstanding and successfully applied approach to structural steel frame design here called "Flexible Moment Connections (FMC)," and to compare this approach to the Specification defined PR approach. In addition we hope to show that although much has changed in the profession, including specifications and our tools for their application, FMC design is still an acceptable and economical approach for steel structures.

The "Flexible Moment Connections" approach has been permitted in this country and around the world since at least the 1910's (2). The basic principles of the FMC approach are to treat the beams as simply connected under gravity loads but as moment connected under lateral loads. The approach used for these historic designs has been referred to as "Type 2 with wind," "Semi-rigid," "Smart Connections," "Flexible Wind Connections," or with the British term "Wind-Moment Connections"(3).

These historical approaches appear to have first been given formal approval in the US through the AISC Specification in 1946 as Type 2 with wind. What may be the first paper to discuss the actual response of frames with semi-rigid connections designed with this approach was that of Sourochnikoff (4) in 1949, although the method had been in common use for more than 40 years at the time. Another early paper which addressed the seeming paradox of connections knowing when to resist moment and when not to resist moment was presented by Disque in 1964 (5). An uncounted number of buildings have been successfully designed with this approach, including such well-known structures as the Empire State Building, which also included a braced core and stiff walls, and the UN Secretariat as well as a large number of unnamed buildings of a common nature. It appears that there have been no strength or serviceability problems associated with this approach to design. Thus, it seems appropriate that the approach be reassessed and, if it proves viable in today's world of structural engineering, as we believe it will, that it be brought back as a tool for today's designers.