Steel Trestles are commonly found spanning fairly shallow, but wide, valleys. They consist of a series of plate girder spans that are supported by trestle bents and towers. Towers have four legs, which makes them free-standing and they resist forces that come from all directions. Trestle bents have only two legs and cannot resist any forces that act along the length of the bridge. Long trestles and tall trestles commonly use only towers to support the spans. Short bridges often will use only bents. Trestles using combinations of bents and towers are common.