Commentary — Appendix 7 Alternative methods of design for stability

Appendix 7 includes the following major addition in this edition of the Specification.

(1) A limitation on the required axial compressive strength of nominally horizontal members in moment frames has been added to the conditions for using the first-order analysis method.

The effective length method and first-order analysis method are addressed in this appendix as alternatives to the direct analysis method, which is presented in Chapter C. These alternative methods of design for stability can be used when the limits on their use as defined in Sections 7.2.1 and 7.3.1, respectively, are satisfied.

Both methods in this appendix utilize the nominal geometry and the nominal elastic stiffnesses, $E I$ and $E A$ , in the analysis. Accordingly, it is important to note that the sidesway amplification, $\Delta_{2 n d \text {-order }} / \Delta_{1 s t \text {-order }}$ or $B_{2}$ , limits specified in Chapter C and this appendix are different. For the direct analysis method in Chapter C, the limit of 1.7 for certain requirements is based upon the use of reduced stiffnesses, $E I^{*}$ and $E A^{*}$ . For the effective length method and first-order analysis method, the equivalent limit of 1.5 is based upon the use of unreduced stiffnesses, $E I$ and $E A$ .

C-7Commentary — Appendix 7 Alternative methods of design for stability