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<br /> <br />Exhibit 23. Concrete thrust beams may be used as longitudinal stiffeners. <br /> <br />3-3.4 Structural Behavior of Rigid Culverts. <br /> <br />The load carrying capability of rigid culverts is essentially provided by the <br />structural strength of the pipe itself and little benefit from the surrounding <br />soil is required. When vertical loads are applied to a rigid pipe, zones of <br />tension and compression are created as illustrated in exhibit 24. With the <br />exception of non-reinforced circular pipe, reinforcing steel is added to the <br />tension zones to increase the tensile strength of the pipe. Shear stress in <br />the haunch area can be critical for heavily loaded rigid pipe on hard <br />foundations, especially if the haunch support is inadequate. Because rigid <br />pipe is stiffer than the surrounding soil, it carries a substantial portion of <br />the load. <br /> <br />The weight of earth that must be carried varies with soil characteristics and <br />installation conditions. The installation conditions can have a significant <br />influence on the loads that must be carried by a rigid culvert. There are two <br />major classes of installation conditions: 1) trench, where culverts are placed <br />in natural ground or compacted fill with a controlled trench width and 2) <br />embankment, where culverts are placed in or covered by an embankment. <br /> <br />34 <br />