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Additional research on the topic of steel frames with partially restrained (PR) connections has been conducted over the past ten to fifteen years, in recognition of the fact that connections in real structures do not possess the idealized characteristics typically used in design codes around the world of being fully restrained (rigid) or simple (pin-ended). Several studies have demonstrated the potential for improved stability, strength and serviceability descriptions for structures. Despite significant research and development efforts, usage of PR principles has nevertheless been very slow in coming to the profession. This is clearly and primarily caused by a lack of understanding and appreciation of the applications of PR principles on the part of designers, but also due to a lack of suitable software. Practical implementation of the use of PR connections in structural systems by designers is a real possibility, but to get to that stage requires a well-planned
technology transfer effort.
A one-day workshop was organized and conducted by the Structural Stability Research Council (SSRC) in Atlanta, Georgia on September 23, 1998. Support for the workshop was provided by the National Science Foundation. The Program Committee was chaired by Reidar Bjorhovde (The Bjorhovde Group), and included as members Wai-Fah Chen (University of Hawaii), Andre Colson (ENSAIS, Strasbourg, France), Nestor R. Iwankiw, (American Institute of Steel Construction), Roberto T. Leon (Georgia Institute of Technology), David A. Nethercot (University of Nottingham, England), James M. Ricles (Lehigh University), and Riccardo A. Zandonini (University of Trento, Italy). The following five topics related to PR connections were addressed in the workshop:
Session I - Frame Stability
Session II - Connection Performance
Session III - Influence on Seismic Effects
Session IV - Analysis and Design Technologies
Session V - Practical Implementation and Technology Transfer
The invited prominent speakers were leading experts involved in research and design of structural steel connections. A book containing 14 contributed papers presented at the workshop has been published and is available through SSRC. The papers in this book present the state-of-the-art and recommendations for implementation and technology transfer into practice the results of research on PR connections. Researchers and designers will find these papers to be a valuable resource.
Following the presentation of the papers of each session, organized discussions among the participants took place. A general discussion session was also held at the end of the workshop. The findings and observations during the workshop discussions are summarized below for each session. Recommended research and development needs suggested by the participants are given following the summary.
SUMMARY
Session I
Frame Instability
Significant research on partial restrained connections has been undertaken in the last 15 years in the U.S. and Europe. Areas of research include: database, connection modeling, and classification.
The 1993 AISC LRFD Specification has two basic connection classifications. These are fully rigid or partially rigid. Although this classification appears in the Specifications, there is a lack of design specifications, design aids, and software for design engineers to use for partially restrained connections.
Current design methods require the classification of columns as either leaner (gravity) or restraining (part of lateral framing). It has been found that the use of the column alignment charts for effective length factors produces inaccurate results in partially restrained frames.
The use of a linear connection moment-rotation relationship as an approximation for modeling the non-linear behavior of partially restrained connections can be unconservative, particularly when combined lateral and gravity loading are applied to the frame.
Session II
Connection Performance
Extensive studies in Europe on partially restrained connections have recently been completed or are currently in progress. The topics of investigation include: isolated partially restrained connection tests; testing of sub-frames and full-scale frames with partially restrained connections; testing of composite floor systems with partially restrained connections; development and verification of design methods.
In order for practicing engineers to design frames with partially restrained connections, the connection design models must be simple and straightforward.
The additional restraint of the column by a partially restrained connection with a composite floor slab results in additional demand on the connection. The additional strength gained by a partially restrained connection through composite action must be carefully compared to the additional demand imposed on it.
Consideration should be given to investigating the use of frames with partially restrained connections as special moment resisting frames. Partially restrained frames may have more ductility, but also a reduced strength when compared to fully restrained frames.
Session III
Influence of Seismic Effects
The SAC Steel Project is sponsoring a major research project that addresses several aspects of connections: material (base metal; weld metal); inspection; frame analysis; connection testing; and development of fracture mechanics-based design guidelines.
The over strength of composite construction with partially restrained connections has not been fully established. Unbraced frames controlled by drift design may have significant over strength under lateral loading.
New innovations in seismic resistant design to avoid weld failure are currently in development. One of these uses a post-tensioned partially restrained connection. The issue of fire protection however must be carefully considered, since a loss of post-tensioning can occur in the event of a fire.
Future U.S. seismic design criteria will become more sophisticated and be based on performance. The equivalent lateral load analysis will be replaced; in addition, the design criteria for the connections will be fracture mechanics-based.
Session IV
Analysis and Design Technologies
Most engineers can probably design a fully rigid connection system in half the time required to design partially restrained systems. This is mainly attributed to the lack of design tools and prescriptive specifications for PR frames.
There is currently a lack of standardized partially restrained connection design data.
There is currently a lack of knowledge of the cyclic behavior of partially retrained connections and partially restrained systems. The research being conducted under the SAC Steel Project will provide more information in this area.
The fabricator is an important participant in the construction of partially restrained frames. More cooperation between fabricators and design engineers of partially restrained framing systems is necessary if this form of construction is to become common.
Session V
Practical Implementation and Technology Transfer
There are several problems facing designers of partially restrained frames. These include: the design is inelastic, not elastic; no cost data base of actual buildings with partially restrained frames is available; there is a lack of good computer software to design partially restrained frames; there is no single reference on connection curves; there are several misconceptions about partially restrained frame behavior; no seismic design criteria exists.
There is the need for an effort to develop and promote the use of partially restrained frames in design. Such as effort would require five steps: (1) establish standardized partially restrained connection curves; (2) develop software for use by the industry; (3) publish a theory of design and design guide to use the standardized partially restrained connection curves and the developed software; (4) educate the profession with design and analysis seminars and/or short courses; (5) promote the use of partially restrained frames in design.
RECOMMENDATIONS
Several studies have demonstrated the potential for improved stability, strength and serviceability descriptions for PR structures. Despite significant research and development efforts, usage of partially restrained principles has nevertheless been very slow in coming to the marketplace. Several needs and recommendations for research and development were suggested in the workshop to enable the practical use of partially restrained connections in the design of structural systems.
Develop a design methodology that better reflects the true behavior of a structure with partially restrained frames.
Develop simple, straightforward design guidelines, models, and software for practicing engineers to design frames with partially restrained connections.
Perform additional analytical studies to examine the connection demand and over strength of partially restrained frames by nonlinear static and time history analysis.
Develop design guidelines for seismic resistant design.
Develop commercial computer programs for the design and analysis of partially restrained frames by practicing engineers. These programs should use robust solution schemes and element formulations, and have member design and comparative cost analysis modules.
Develop documentation of design guidelines and education programs for practicing engineers to enable them to use partially restrained connections in frame design. This task must be undertaken by an agency such as the American Institute of Steel Construction (AISC) in order that the design guidelines be accepted and adopted as an industry standard.
James Ricles
Lehigh University
Reidar Bjorhovde
The Bjorhovde Group
SSRC Chairman
Nestor Iwankiw
American Institute of Steel Construction
SSRC Vice Chairman
CONTENTS
Session I - Frame Stability
Frames with Partially Restrained Connections (Semi-Rigid Frames) by W. F. Chen (Keynote Speaker)
In-Plane Buckling Behavior of Pitched Roof Steel Frames with Semi-Rigid Connections by N. Silvestre, A. Mesquita, D. Camotim, and L. Silva
Overall, Local and Distortional Buckling in Pallet Racks by N. Baldassino, C. Bernuzzi, R. Zandonini, and G. Hancock
Consideration of Leaner Columns in PR Frame Design by C. O. Rex and A. Goverdhan
Session II - Connection Performance
Connection Performance by D. A. Nethercot (Keynote Speaker) and R. Y. Xiao
Slab Design in Connection Zone of Composite Frames by A. Plumier, C. Doneux, and J. G. Bouwkamp
T-Stub Connection Tests by R. T. Leon and J. A. Swanson
Experimental Studies of Seismic Behavior of Shear Connections with Floor Slabs by J. Liu and A. Astaneh-Asl
Session III - Influence of Seismic Effects
Evaluation of Over-Strength for Partially Restrained Unbraced Steel Frames by C. M. Foley and K. D. Gillis
Behavior of Beam-to-Column Joints in Moment-Resisting Steel Frames by C. Bernuzzi, C.A. Castiglioni, S. Vajna de Pava, and L. Calado
Post Tensioned Seismically Resistant Connections for Steel Frames by M. M. Garlock, J. M. Ricles, R. Sause, S. W. Peng, C. Zhao, and L.-W. Lu
Session IV - Analysis and Design Technologies
Design Methods for Frames with Partially Restrained Connections by G. H. Couchman (Keynote Speaker)
Session V - Practical Implementation and Technology Transfer
Computer-Automated Design of Steel Frames with Partially Restrained Connections by L. Xu, A. N. Sherbourne, and D. E. Grierson
A Suggested Methodology to Enhance the Use or PR Connected Frames by S.D. Lindsey (Keynote Speaker)
Summary and Recommendations of Partially Restrained (PR) Framing by J. M. Ricles, R. Bjorhovde, N. Iwankiw
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