单选题钢屋架跨中竖杆由双等肢角钢组成十字形截面杆件，计算该竖杆最大长细比λmax时，其计算长度<br/> <br/> <img border="0" alt="" src="https://tuku.weimei168.com/uploads/upimg/20251103/8325372_0.png"><br/> <img border="0" alt="" src="https://tuku.weimei168.com/uploads/upimg/20251103/8325372_0_1.png">