A beam bridge is a rigid horizontal structure on two piers.  The weight of the bridge and any traffic travels downward onto the piers.

The force of compression  happens on the top side of the beam  (or roadway). This causes the upper side to get shorter.

Tension in the lower portion of the deck is caused by the compression on the top side. This tension causes the lower portion of the beam to lengthen.

Many beam bridges use steel beams to handle the load. The height of the beam controls the distance that the beam can span.  High beams have more material to dissipate the tension. 

As the distance increases, the size of the bridge increases until the bridge's weight is so large that it cannot support itself.