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3-3.2 Categories of Structural Materials. Based upon material type, culverts can be divided into two broad structural categories: flexible and rigid. Flexible culverts have little structural bending strength on their own. The material from which they are made, such as corrugated steel or aluminum, can be flexed or bent and can be distorted significantly without cracking. Consequently, flexible culverts depend on the backfill support to resist bending. Rigid culverts, however, are stiff and do not deflect appreciably. The material from which they are made, such as reinforced concrete, provides resistance to bending. 3-3.3 Structural Behavior of Flexible Culverts A flexible culvert is a composite structure made up of the culvert barrel and the surrounding soil. The barrel and the soil are both vital elements to the structural performance of the culvert. Flexible pipe has relatively little bending stiffness or bending strength on its own. As loads are applied to the culvert, it attempts to deflect. In the case of a round pipe, the vertical diameter decreases and the horizontal diameter increases, as shown in exhibit 21. When good embankment material is well compacted around the culvert, the increase in horizontal diameter of the culvert is resisted by the lateral soil pressure. With round pipe the result is a relatively uniform radial pressure around the pipe which creates a compressive thrust in the pipe walls. As illustrated in exhibit 22, the compressive thrust is approximately equal to vertical pressure times one-half the span length (C = P x ~ or C = P ~ R). 2 ~ AS VERTICAL LOADS ARE APPLIED A FLEXIBLE CULVERT ATTEMPTS TO DEFLECT. THE VERTICAL DIAMETER DECREASES WHILE THE HORIZONTAL DIAMETER INCREASES. SOIL PRESSURES RESIST THE INCREASE IN HORIZONTAL DIAMETER. 1 Exhibit 21. Deflection of flexible culverts. 32