根据《钢标》7.2.1，柱为焰切边，对y轴b类，$i_y=\sqrt{I_y/A}=\sqrt{7.2\times10^7/1.41\times10^4}=71.46$，$\lambda=l/i_y=8600/71.71=120$，$\lambda/\varepsilon_k=120/0.825=145.45$，查附录D.0.2得$\varphi_y=0.324$。由8.2.1，$\beta_{tx}=0.65-0.35\times\frac{138}{317}=0.498$，$W_{1x}=I_x/y=9.41\times10^8/300=3.14\times10^6$，$\frac{N}{\varphi_y A f}+\eta\frac{\beta_{tx} M_x}{\varphi_b W_{1x} f}=\frac{581\times10^3}{0.324\times1.41\times10^4\times305}+1.0\times\frac{0.498\times317\times10^6}{0.89\times3.14\times10^6\times305}=0.417+0.185=0.602$，选A。