C-D1D1 Slenderness limitations

The advisory upper limit on slenderness, $L / r$, of 300 in the User Note is based on professional judgment and practical considerations of economics, serviceability, ease of handling, and care for the integrity of the member. Slender members are more susceptible to inadvertent damage during fabrication, transport, and erection. This damage could have a detrimental impact on member capacity, although less so for tension members than for compression members because out-of-straightness within reasonable tolerances does not affect the strength of tension members; applied tension tends to reduce, whereas compression tends to amplify, out-of-straightness. Note that rods may be less susceptible to these problems and so are excluded from the recommendation. Slender members may exhibit noisy vibrations when subjected to wind forces as in an open truss or when supporting vibrating equipment such as fans or compressors, and under even minor wind or seismic loading may result in "slapping" with cyclic buckling. Very slender members may sag excessively under self-weight. This sag would not affect the tensile strength of the member. However, the sagging appearance of such members may be aesthetically unacceptable if they are part of an open structure. The sagging of very slender brace members used in a tension-only braced frame may prevent the member from immediately engaging in tension, resulting in larger than predicted lateral drift levels and potential frame stability concerns.

For these reasons, it may be advantageous to specify members with slenderness ratios not exceeding this limit. The User Note references fabricated length. If tension members are spliced in the field, it is possible that the installed length may be greater than the fabricated length. In all cases, engineering judgment should be used to determine whether to apply the limit; if not, steps must be taken to eliminate the serviceability problems, and special care should be exercised by the fabricator and erector to protect the structural integrity of the member. Conformance to the maximum slenderness limit of 300 does not guarantee that these detrimental conditions

will not occur, but experience has shown that they can be significantly mitigated by conforming to this limit. For single angles, the radius of gyration about the $z$-axis produces the maximum $L / r$ and, except for very unusual support conditions, the maximum effective slenderness ratio.