AASHTO HDG CHAPTER 4

Abstract

<strong>INTRODUCTION</strong> <br>The function of a culvert is to convey surface water across or from the highway right-of-way. In addition to this hydraulic function, it must also carry construction and highway traffic and earth loads; therefore, culvert design involves both hydraulic and structural design. The hydraulic and structural designs must be such that risks to traffic, of property damage, and of failure from floods are consistent with good engineering practice and economics. This chapter is concerned with the hydraulic aspects of culvert design and makes reference to structural aspects only as they are related to the hydraulic design. <br>Structures measuring more than 20 ft (6.1 m) along the roadway centerline are conventionally classified as bridges. Many longer structures, however, are designed hydraulically and structurally as culverts. Culverts, as distinguished from bridges, are usually covered with embankment and are composed of structural material around the entire perimeter, although some are supported on spread footings with the streambed serving as the bottom of the culvert. Bridges are not designed to take advantage of submergence to increase hydraulic capacity even though some are designed to be inundated under flood conditions. For economy and hydraulic efficiency, culverts should be designed to operate wih the inlet submerged during flood flows, if conditions permit. At many locations, either a bridge or a culvert will fulfill both the structural and hydraulic requirements for the stream crossing. Structure choice at these locations should be based on construction and maintenance costs, risk of failure, risk of property damage, traffic safety, and environmental and aesthetic considerations. Some of the advantages of culverts are better traffic safety and lower maintenance costs than bridges. Culverts do not have bridge railing, which can be a hazard, or a bridge deck, which is subject to deterioration. <br>Culverts are usually considered minor structures, but they are of great importance to adequate drainage and the integrity of the highway facility. Although the cost of individual culverts is usually relatively small, the total cost of culvert construction constitutes a substantial share of the total cost of highway construction. Similarly, the total cost of maintaining highway hydraulic features is substantial, and culvert maintenance may account for a large share of these costs. Improved traffic service and a reduction in the total cost of highway construction and maintenance can be achieved by judicious choice of design criteria and careful attention to the hydraulic design of each culvert.