短語詞組短語extrados springing 拱背起拱線點extrados radius 拱外弧半逕extrados springing line 拱外弧起拱線radius of extrados 外拱圈半逕Arch building on extrados 拱式拱上建築extrados of arch 拱外圈extrados cable -stayed bridge 矮塔斜拉橋girder arch building on extrados 梁拱式拱上建築 返回 extrados