bridge unit

-force- : any action that tends to maintain or alter the position of a structure (loads).

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-compression- : a force which acts to compress or shorten the thing it is acting on.

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-tension- : a force which acts to expand or lengthen the thing it is acting on.

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-shear- : this is compression gone wrong. This is when things begin to break.

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-gravity- : this is the natural force that pulls things to the ground.

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-span- : the distance between two bridge supports, whether they are columns, towers or the wall of a canyon.

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-load- : is a force a bridge must overcome to stay standing.

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-displacement- : this is the ability of a bridge to spread the forces out evenly over the whole bridge, so that no one spot has to bear the brunt of the concentrated force.

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-stable- : the bridge has the ability to resist a collapse. And be able to carry a realistic load without collapsing or deforming significantly.

Based on the distance on where they build a bridge depends on what kind of bridge they build.

(This is known as span.)