构造筋下料长度=梁净跨长+2*15D抗扭筋下料长度梁同纵筋，即当直锚时=梁净跨+2lae 弯锚时=梁净跨+2*（0.4lae+15D）-2*2D 一端弯锚一端直锚时=梁净跨+lae+0.4lae+15D-2D
