AISC


4.1. AISC Specification Requirements

4.1.1. Can the 2005 AISC Specification be used to design structural shapes not listed in Part 1 of the 13th Steel Construction Manual?

Yes. AISC's 13th Edition of the Steel Manual is written based upon the 2005 AISC Specification for Structural Steel Buildings and provides design aids and charts for commonly used structural shapes: those listed in ASTM A6/A6M as well as hollow structural Sections [HSS] (square, rectangular, and round). This coverage is not intended to exclude the use of other shapes that may be available. Other shapes and combinations thereof may be designed and used within the requirements of the 2005 AISC Specification.

4.1.3. It is indicated in the 2005 AISC Specification Section B7 that the slenderness ratio of a tension member should preferably not exceed 300. Why? Is this requirement strength related?

From the 2005 AISC Specification Commentary Section D1, Users Note, this recommended limit is "based on professional judgment and practical considerations of economics, ease of handling, and care required to minimize inadvertent damage during fabrication, transport and erection." It is further indicated that this requirement is not strength related.

Essentially, this rule also ensures that a non-pretensioned structural brace will be stiff enough that significant lateral deflection of the structure will not be required to activate the strength of the brace. As such, this requirement does not pertain to rod bracing, cable bracing and other light bracing systems that are fabricated and installed with draw to activate the brace.

4.1.4. What is the allowable bending stress for weak-axis bending of a channel with compact flanges?

ASD Section F2 addresses weak-axis bending of I-Shaped members, solid bars and rectangular plates. However, it does not address channels.

The LRFD Specification has provisions for weak-axis calculations, provisions that are not found in the ASD Specification. For bending about the weak axis, the lateral-torsional limit state is not applicable. With thick, stocky flanges and webs thick enough, flange- and web-local buckling are not likely to control, in which case the channel can be designed for flexural yielding. In LRFD, 0.9FyZ is appropriate.

If desired, the LRFD provisions can be converted by comparison to an equivalent ASD format. In ASD, this corresponds to 0.66FyS. If flange- or web-local buckling does control, the equations in Appendices B and F may apply.