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