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Altogether，three forces can act on a structure:vertical-those that act up or down;horizontal-those that act sideways;and those that act upon it with a rotating or turning motion.Forces that act at an angle are a combination of horizontal and vertical forces.Since the structures designed by civil engineers are intended to be stationary or stable，these forces must be kept in balance.The vertical forces，for example，must be equal to each other .If a beam supports a load above，the beam itself must have sufficient strength to counterbalance that weight.The horizontal forces must also equal each other so that there is not too much thrust either to the right or to the left.And forces that might pull the structure around must be countered with forces that pull in the opposite direction.
老师布置的作业，有可能考试考到，请英语高手帮翻译下，狂谢~~~