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<br />Chapter 1 <br /> <br />GENERAL ISSUES <br /> <br />Traditional definitions of culverts are based on the span length rather than <br />function or structure type. For example, part of the culvert definition <br />included in the 8ridge Inspector's Training Manual 70 states: <br /> <br />"...structures over 20 feet in span parallel to the roadway are usually <br />called bridges; and structures less than 20 feet in span are called <br />culverts even though they support traffic loads directly." <br /> <br />Many structures that measure more than 20 feet along the centerline of the <br />roadway have been designed hydraulically and structurally as culverts. The <br />structural and hydraulic design of culverts is substantially different from <br />bridges, as are construction methods, maintenance requirements, and inspection <br />procedures. A few of the more significant differences between bridges and <br />culverts are: <br /> <br />. Hvdraulic--Culverts are usually designed to operate at peak flows with a <br />submerged inlet to improve hydraulic efficiency. The culvert constricts <br />the flow of the stream to cause ponding at the upstream or inlet end. <br />The resulting rise in elevation of the water surface produces a head at <br />the inlet that increases the hydraulic capacity of the culvert. Bridges <br />may constrict flow to increase hydraulic efficiency or be designed to <br />permit water to flow over the bridge or approach roadways during peak <br />flows. However, bridges are generally not designed to take advantage of <br />inlet submergence to the degree that is commonly used for culverts. The <br />effects of localized flooding on appurtenant structures, embankments, <br />and abutting properties are important considerations in the design and <br />inspection of culverts. <br /> <br />. Structural--Culverts are usually covered by embankment material. <br />Culverts must be designed to support the dead load of the soil over the <br />culvert as well as live loads of traffic. Either live loads or dead <br />loads may be the most significant load element depending on the type of <br />culvert, type and thickness of cover, and amount of live load. However, <br />live loads on culverts are generally not as significant as the dead load <br />unless the cover is shallow. Box culverts with shallow cover are <br />examples of the type of installation where live loads are important. <br /> <br />In most culvert designs the soil or embankment material surrounding the <br />culvert plays an important structural role. lateral soil pressures <br />enhance the culverts ability to support vertical loads. The stability <br />of the surrounding soil is important to the structural performance of <br />most culverts. <br /> <br />1 <br />