钢杆的圆截面上作用有轴力<img src="https://static.zxtiku.com/wangke/imgs/topic/287bab05264ec2c8b066ac0a442a97bd.png"/>，弯矩<img src="https://static.zxtiku.com/wangke/imgs/topic/ce7a75b68030d1838bd21c302c85d878.png"/>，扭矩T。若已知许用应力[<img src="https://static.zxtiku.com/wangke/imgs/topic/70ea3e6f8fbeb2f7ed9f15779316d857.png"/> ]，横截面面积A，抗弯截面系数W，则钢杆的强度条件为( )