We stopped at London Bridge, a double-arched formation whose outer span collapsed in 1990, stranding a couple on the other side so that they had to be rescued by helicopter.
我们在伦敦桥停下车来,这是一座双拱桥梁,外延部分在1990年垮塌了。 当时有一对男女被困在桥那一边,只能由直升飞机前来救援。
It is to be square-a span long and a span wide-and folded double.
这胸牌要四方的,叠为两层,长一虎口,宽一虎口。
FIG. 12 illustrates a timber double post roof truss. This type of truss is constructed where a wider span or a less restricted roof space is required.
图12所示为一个双柱木屋顶桁架。这种桁架主要建于跨度要求较大或屋顶空间限制较少的地方。
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