Steel in the News
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Posted by Geoff Weisenberger on May 22, 2008 at 10:17 AM.
Important changes to the AISC specification can be easy to lose track of, and often the challenge is to understand when the changes were made. Design Guide 15: AISC Rehabilitation and Retrofit Guide (DG 15), Appendix A1 provides a comprehensive source of historical information that references changes made to the specification.
Currently, DG 15, Appendix A1 provides a list of changes to the specification through the 1999 LRFD Specification for Structural Steel Buildings. An update to this list, including the changes from the 1999 LRFD specification to the 2005 specification, is now available at www.aisc.org/crossref99. An overview of some of the more prominent revisions is outlined below.
The 2005 specification contains numerous unifying changes, which can be seen in Chapter A, General Provisions. While this list of updates only pertains to the LRFD portion of the specification, the most noticeable change is the combination of the ASD and LRFD provisions. Additionally, the scope of the specification has been expanded to include “other structures”, which are defined as “those structures designed, fabricated, and erected in a manner similar to buildings, with building-like vertical and lateral load-resisting elements.” Less noticeable but equally useful to the 2005 specification is the inclusion of the specifications for single angles and HSS sections. Incorporating these specifications has mitigated the need for other provisions.
Revisions to Chapter C, Stability Analysis and Design, reveal major organizational and substantive changes; most notable is the requirement to address second-order effects in the analysis and design. A new procedure, the Direct Analysis Method, which is described in Appendix 7, will satisfy the requirements of Chapter C. Additionally, stability based on plastic design must follow Appendix 1, which also includes other provisions for inelastic analysis and design.
Chapter F, Design of Members for Flexure, has also been renamed and reorganized. The chapter is now divided into sections based on member type and the axis of bending. Table User Note F1.1, Selection Table for the Application of Chapter F Sections provides a summary of the chapter by illustrating each cross-section addressed and stating the applicable limit states for that member.
The updates to Chapter I, Design of Composite Members, reflect research results and allow the use of higher strength materials, as well as provide better consistency with ACI 318-05, Building Code Requirements for Structural Concrete. Shear stud strength is now dependent on the location of the stud in the flute of the metal deck, the number of studs welded within one flute, and the orientation of the metal deck with respect to the beam. Composite column design is based on new interaction formulas that better reflect behavior and strength.
Some changes have been made to Chapter J, Design of Connections, and one of the most notable organizational revisions is the inclusion of the effects of concentrated forces previously appearing in Chapter K of the 1999 LRFD specification. The 1999 LRFD specification combined concentrated forces, fatigue, and ponding into one chapter. The 2005 specification separates these sections where Design for Ponding is located in Appendix 2 and Design for Fatigue is located in Appendix 3.
The all-new Appendix 4, Structural Design for Fire Conditions, provides much-needed criteria for the design and evaluation of structural components for fire conditions. This appendix discusses the effects of elevated temperature on materials and accounts for these changes in the design.
A review of the complete list of changes will help engineers using LRFD to become more acquainted with the 2005 specification. To get up to speed with all the changes to the specification, be sure to visit www.aisc.org/crossref99.
- By Matthew Fadden and Jill Rajek