professor ben schafer's thin-walled structures research group - johns hopkins university

 

Direct Strength Method for CFS Design

 

"Users of the main Specification are aware of the complexity involved in current design calculations for cold-formed steel members. A new design method: Direct Strength, has been created that aims to alleviate the current complexity, ease calculation, provide a more robust and flexible design procedure, and integrate with available, established, numerical methods." from the preface to the DSM Design Guide

 

 

Design of Cold-Formed Steel Members Using the Direct Strength Method

As approved July 2003 by the American Iron and Steel Institute Committee on Specifications and finalized January 2004. App. 1 of the North American Spec. for the Design of Cold-Formed Steel Structural Members

Specification (pdf) January 2004 version

Commentary (pdf) January 2004 version

Note, improvements continue to be made and are now continually made available in future editions of the North American Specification. Currently AISI-S100-07 contains the latest Specification approved procedures for the Direct Strength Method. In 2010 a DSM method for inelastic bending reserve was approved.

 

Design Guide for the Direct Strength Method

The Direct Strength Method for the design of cold-formed steel members now has a 170 page design guide, complete with nearly 100 pages of practical examples to help practitioners and students learn more about this new and flexible method for member design of cold-formed steel. The Guide was authored by Professor Schafer through funding by the American Iron and Steel Institute. Purchase a copy of the Guide at AISI or though the Steel Framing Alliance. Learn more about the Design Guide from this paper: Schafer, B.W. “Designing cold-formed steel using the Direct Strength Method.” Eighteenth International Specialty Conference on Cold-Formed Steel Structures, Orlando, FL. October 2006. (PDF)

 

Online learning materials for the Direct Strength Method

In the summer of 2006 Professor Schafer taught a one day workshop for the Metal Building Manufacturers Association on the application of the Direct Strength Method. The complete workshop materials are available here and include an overview of DSM, along with tutorials on the use of CUFSM and hands-on design problems using DSM. In addition a small spreadsheet for performing simple DSM calculations is available. Updates to this information are also available based on additional workshops held for ClarkWestern, ICC-ES, and Dietrich Industries. A webinar was conducted for CFSEI in the summer of 2010.

 

An Overview of the Direct Strength Method

From REVIEW: THE DIRECT STRENGTH METHOD OF COLD-FORMED STEEL MEMBER DESIGN by Ben Schafer: The objective of this paper is to provide a review of the Direct Strength Method for cold-formed steel design. The Direct Strength Method was formally adopted in North American design specifications in 2004 as an alternative to the traditional Effective Width Method. The appendix of this paper provides an excerpt from the North American Specification which provides the Direct Strength Method equations for the design of columns and beams. A brief comparison of the Direct Strength Method with the Effective Width Method is provided. The advantage of methods that integrate computational stability analysis into the design process, such as the Direct Strength Method, is highlighted. The development of the Direct Strength Method for beams and columns, including the reliability of the method is provided. Current and ongoing research to extend the Direct Strength Method is reviewed and references provided. Read the whole paper

 


What is the Direct Strength Method?

 

From the Preface of the Direct Strength Design Method, for implementing rational analysis methods in thin-walled cold-formed steel through the direct use of member elastic buckling solutions. 

Appendix 1: 
Design of Cold-Formed Steel Structural Members by the Direct Strength Method

 Users of the main Specification are aware of the complexity involved in current design calculations for cold-formed steel members. A new design method: Direct Strength, has been created that aims to alleviate the current complexity, ease calculation, provide a more robust and flexible design procedure, and integrate with available, established, numerical methods. Advantages of the Direct Strength Method of design include 
everyday design improvements:
 · no effective properties for strength,
 · no element calculations,
 · no iteration for beams (webs),
 · gross properties of the section used for strength calculations,
 theoretical improvements:
 · interaction of elements in local buckling (e.g., web/flange) is accounted for,
 · distortional buckling is explicitly treated in the design process,
 · reduction in systematic error in portions of the main Specification,
 improvements in applicability and scope:
 · is applicable to wider group of cross-sections than the main Specification,
 · provides rational analysis procedure for sections not previously covered,
 · allows and encourages greater cross-section optimization,
 important philosophical changes: 
 
· numerical methods and rational analysis brought to everyday design, and
 · integrates known behavior into a straightforward design procedure.

Use of the Direct Strength Method requires (1) determination of the elastic buckling behavior of the member and (2) using that information in a series of ultimate strength curves to predict the strength. The commentary to this Appendix provides full details of rational analysis methods, both traditional hand solutions and numerical solutions, that may be used to accurately calculate the elastic buckling behavior necessary for step 1. A freely available program, developed in part with AISI funds: CUFSM complete with tutorials and examples is available to aid in the elastic buckling calculation. This appendix provides the ultimate strength curves along with appropriate safety and resistance factors necessary for the strength prediction of step 2.

The Direct Strength Method is verified with same data and care as the main Specification, and thus no loss of reliability is inherent in its use. The procedure employs the same underlying empirical assumptions as the effective width method used in the main Specification: ultimate strength is a function of elastic buckling stress (or load) and the yield stress (or load) of the material. The method has been extensively explored for beams and columns only. The applicability of the provided provisions is detailed in the General Provisions of this Appendix.


SUPERSEDED Material

The material in this section has been superseded, but is left here for historical purposes.

December 2002

view 2002 Draft Design Manual (PDF - 2993 KB) *includes CUFSM tutorials*

view 2002 Draft Specification, December 6, 2002 draft (PDF - 299 KB)

view 2002 Draft Commentary, December 6, 2002 draft (PDF - 1018 KB)

zip 2002 Design and Elastic Buckling Examples (PDF - 223 KB)

February 2002

(spec

(commentary

(manual

last edited 08/13/10

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