一、简介Introduction
杆轴线为曲线在竖向荷载作用下不产生水平反力。
拱--杆轴线为曲线，在竖向荷载作用下会产生水平推力的结构。
三铰拱的反力只与荷载及三个铰的位置有关，与拱轴线形状无关
The reactions are related to the locations of the hinges and loads, but have no relationship with the form of the axial line of arch.
荷载与跨度一定时，水平推力与 矢高成反比
Given loads and span, thrust is inversely proportional to rise 1/f
三铰拱的内力不但与荷载及三个铰的位置有关，而且与拱轴线的形状有关。The internal forces are not only dependent of loads and the positions of hinges, but also of the form of centroidal axis .
由于推力的存在，拱的弯矩比相应简支梁的弯矩要小。Because of the horizontal thrust, the moment of arch is less than that of the corresponding simple beam.
三铰拱在竖向荷载作用下轴向受压.Under the action of vertical loads, the arch is axially compressed.