A bridge structure consisting of beam, girder or truss spans supported upon bents. The bents may be of the piled or of the frame type. When of framed timbers, metal or reinforced concrete they may involve two or more tiers in their construction. Trestle structures are designated as "wooden", "frame", or "framed", "metal", "concrete", "wooden pile", "concrete pile", etc., depending upon or corresponding to the material and characteristics of their principal members.