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Tracking the Progression of the AISC Spec
Posted by MSC on October 13, 2008 at 3:49 PM.

By James Fall and Jie Zuo

 

Every few years, AISC releases a new specification for steel buildings. New research often breeds significant improvements to the specification, and it is important to understand what changes were made and how they affect the design of future steel buildings.

 

Several major changes have been made between the 1989 Specification for Structural Steel Buildings—Allowable Stress Design and Plastic Design and the ASD side of the newer 2005 Specification for Structural Steel Buildings, which also contains LRFD. (An extensive look at the detailed changes between the two specifications can be viewed online at www.aisc.org/crossrefASD.)

 

First, it is important to understand what sparked the industry to switch from allowable stress design to allowable strength design, a change reflected in the 2005 Specification. Allowable strength provisions are based on forces and moments that are absolute capacities; units are usually in kips or kip-ft. On the other hand, allowable stress works with proportional stress capacities, which usually indicates units in kips per unit area. By eliminating allowable stresses and introducing the safety factor, Ω, the provisions in the 2005 Specification can be applied to both ASD and LRFD, unifying the two design philosophies with each producing similar results.

 

Another important update is the introduction of the term limit state to the 2005 Specification. Since previous LRFD specifications have always used the term limit state to define the boundaries of which a building is adequate for its intended use, it was only fitting that it should be incorporated into the 2005 Specification. Interestingly, traditional allowable stress design was also formulated based on limit state principals, but the term was never used.

 

Globalizing Measurement

 

One problem with the 1989 Specification was that it dealt exclusively with U.S. customary units. Since most countries use metric units, this hindered the use of the specification outside of the U.S. Now that many design firms are working on international projects, the inclusion of metric units into the 2005 Specification facilitates design outside the U.S. by avoiding pesky conversions. For example, in Table J2.4, Minimum Size of Fillet Welds, millimeters are provided in parentheses next to inches. This is typical throughout the 2005 Specification.

 

Taking it one step further, all ratios in the 2005 Specification were non-dimensionalized by factoring out the modulus of elasticity of steel, E. Once again, this change allows flexibility for designers to use either U.S. customary or metric units in their calculations. High-temperature design, which allows for a smaller E value, has also benefitted from this change.

 

On that Note

 

Although not considered part of the 2005 Specification, User Notes were inserted throughout, with useful information to assist users. The content of these User Notes consists of helpful design tips, general rules-of-thumb, approximations, recommendations, and references to relevant documents. To avoid confusion, they appear in shaded boxes to help segregate them from the actual specification.

 

In addition, several important topics were added to the 2005 Specification, most notably Chapter K, Design of HSS and Box Member Connections. Originally, the provisions that governed HSS design were included in a separate specification titled AISC Specification for the Design of Steel Hollow Structural Sections, which was last published in 2000. The provisions were abridged and combined with the 2005 Specification. The appendices were completely revamped, and the most obvious change is the addition of six entirely new appendices, some of which reflect new research and newly developed methods, such as the appendices on structural design for fire conditions and the direct analysis method.

 

While new content accounts for the majority of the differences between the 2005 and 1989 Specifications, some parts of the 1989 version were entirely removed, most notably Chapter N, Plastic Design and Appendix F, Beams and Other Flexural Members, which covered the design of web-tapered members.

 

Besides the removal and addition of content, the actual organization of the specification was overhauled. Some sections moved from one chapter to another, while others were combined into a single section. For example, former Sections B4 and C1 were combined into C1.1, General Requirements.

 

James Falls is an undergraduate student at the University of Florida in Gainesville, and Jie Zuo is an undergraduate student at the University of Illinois at Urbana-Champaign. Both were 2008 summer interns with AISC.


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