Question 13.85: Determine the largest length of W8 × 31 structural A992 stee...

Determine the largest length of W8 × 31 structural A992 steel section if it is pin supported and is subjected to an axial load of 18 kip. Use the AISC equations.

 

 

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Section properties: For W 8 \times 31 \quad r_{y}=2.02  in. \quad A=9.13 \mathrm{in}^{2}
Assume it is a long column:

\sigma_{\text {allow }}=\frac{12 \pi^{2} E}{23\left(\frac{K L}{r}\right)^{2}} ;\left(\frac{K L}{r}\right)^{2}=\frac{12 \pi^{2} E}{23 \sigma_{\text {allow }}} \\\frac{K L}{r}=\sqrt{\frac{12 \pi^{2} E}{23 \sigma_{\text {allow }}}}

Here \sigma_{\text {allow }}=\frac{P}{A}=\frac{18}{9.13}=1.9715 \mathrm{ksi}

\frac{K L}{r}=\sqrt{\frac{12 \pi^{2}(29)\left(10^{3}\right)}{23(1.9715)}}=275.2>200

Thus use \frac{K L}{r}=200\\\frac{1.0(L)}{2.02}=200\\L=404 \mathrm{in} .=33.7 \mathrm{ft}

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