Finally,some bridges are movable eithervertically or horizontally to allow traffic flow underneath if the available clearance is not sufficient.
首先，可以根据交通荷载对公路、铁路或人行天桥进行分类，但要知道这三者可以进行任何组合。
According tothe bridge axis there are straight and curved bridges.
According to the position of thefloor carrying the traffic there are deck bridges,with the floor on top of the mainmembers,or through bridges,whe bottom chord of the mainsupporting members.Bracing is provided on the plane of the top chord above the traffic,but if the height is notsufficient,the space above is left open;such structures are calledsemi-through or pony truss bridges. Sometimes a combination in the form of double-deckbridges is used.
根据桥轴线，可以分为直桥和弯桥。
根据地板承载交通的位置，有甲板桥，地板在主要构件的顶部，或通过桥梁，甲板连接到主要支撑构件的下弦杆。在交通上方的上弦杆平面上提供支撑，但如果高度不够，则上方的空间应留空；这种结构被称为半贯通或小马桁架桥。有时采用双层桥的组合形式。
根据主受力构件的腹板组成，可以识别全腹梁或梁、箱梁桥和空腹桁架桥。
简单结构设计
2.桥梁
桥梁可以分为几种类型，通常一个桥梁结构代表两种或两种以上类型的组合。
First a classification according to the traffic load can be made under which there are highway, railway, or pedestrian bridges, with the understanding that very often any combination of these three is possible.
如果桥梁结构的系统线是直线和水平的，则桥梁为梁式；如果是曲线，则为拱桥。
可以对上述每一组进行进一步细分。较旧的钢桥采用铆接，较旧的钢桥采用焊接和/或螺栓连接。
最后，如果可用的间隙不足，一些桥梁可以垂直或水平移动，以允许下方的车流通过。
Which type of bridge is to be used is a major design decision,and localconditions---traffic,site,geological,geographical,and aesthetic---are to be considered.Available clearances,erectionpossibilities,and foundation construction are also major factors affecting decisions concerning the type and span length of a bridge.
要使用哪种类型的桥梁是一个重要的设计决策，要考虑交通、场地、地质、地理和美学等局部条件。可用的间隙、安装可能性和基础结构也是影响桥梁类型和跨度的决定的主要因素。
According to the makeup of the web of the main load-carrying memberswe can recognize full web-girder or beam or box-girder bridges and open-webtruss bridges.
If the system line of the bridge structure is straight and horizontal,thebridge is of the girder type; with a curved line it is an arch bridge.
Further subdivisionswithin each of the above groups can be made.Older steel bridges were riveted whilenewer ones are welded and/or bolted.
第2次作业（E-C）
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Design of Simple Structure
2.Bridges
Bridges can be classified in several ways.Often a bridge structure represents somecombination of two or more types.