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106 shapes, range of sizes, and common uses for each type of cul- existence of adequate soil support. This may be the weakness vert or pipe are summarized by Ballinger and Drake (1995). of flexible culverts, in case of earthquakes, in that the soil support can be reduced or lost during liquefaction or other permanent ground failure mechanisms associated with 9.2.1 Flexible Culverts and Pipes seismic events. Significant distortion or collapse of the cul- In general, culverts and pipes are divided into two major vert cross section is likely if soil support is reduced or lost. classes from the static design standpoints: flexible and rigid. Flexible culverts and pipes typically are composed of either 9.2.2 Rigid Culverts and Pipes metal (for example, corrugated metal pipe (CMP) made of steel or aluminum) or thermoplastic materials (for example, Rigid highway culverts and pipes consist primarily of rein- HDPE or PVC). Flexible culverts and pipes respond to loads forced concreted shapes that are either precast or cast-in-place. differently than rigid culverts and pipes. Because their oval- Unreinforced concrete culverts and pipe structures are not rec- ing stiffness is small, relative to the adjacent soil, flexible cul- ommended for use in seismic regions. The sizes of reinforced verts and pipes rely on firm soil support and depend upon a concrete pipe (RCP) range (in diameter) from about 1 foot to large strain capacity to interact with the surrounding soil to 12 feet. Larger RCP can be precast on the site or constructed hold their shape, while supporting the external pressures im- cast-in-place. Rectangular four-sided box culverts can be fur- posed upon them. nished precast in spans ranging from 3 feet to 12 feet. Larger For static design, current AASHTO LRFD Bridge Design spans can be constructed cast-in-place. Three-sided precast Specifications require as a minimum the following main design box culverts can be furnished in spans up to 40 feet. considerations (in addition to the seam failure) for flexible cul- Unlike the flexible culverts and pipes, the strain capacity of verts and pipes: (1) buckling (general cross sectional collapse rigid culverts and pipes is much lower. Rigid culverts must as well as local buckling of thin-walled section), and (2) flexi- develop significant ring stiffness and strength to support ex- bility limit for construction. Except for large box structures or ternal pressures. Hence, they are not as dependent upon soil other large spans with shapes other than circular [per McGrath, support as flexible culverts. et al., (2002) NCHRP Report 473], the flexural strength con- For static design, the primary design methods used for pre- sideration (that is, bending moment demand) is generally not cast concrete pipe, either reinforced or unreinforced, include: required for flexible culverts and pipes. (1) the Indirect Design Method, based on the laboratory three- Neither current AASHTO LRFD Bridge Design Specifications edge bearing test, known as the test; (2) a more direct design nor the McGrath, et al. (2002) study has addressed seismic de- procedure that accounts for bending moment, shear, thrust/ sign concerns for culvert structures. From the seismic design tension, and crack width (bucking is generally not an issue standpoint, there are two main factors that must be considered: with rigid converts and pipes) around the periphery of the cul- vert wall; and (3) methods employing computerized numer- 1. Bending moment and thrust evaluations: Seismic loading ical models accounting for soil-structure interaction effects. is in general nonsymmetric in nature and therefore may re- For box culverts the static design uses the same criteria as sult in sizable bending in the culvert structures (even for other reinforced concrete structures (for example, beams and circular shape culverts). Furthermore, the behavior of thin- columns). In general, the effect of surrounding soils is ac- walled conduits (such as for the flexible culverts and pipes) counted for by applying the soil pressures (active or at-rest) is vulnerable to buckling. This behavior differs somewhat directly against the wall in the model, instead of fully taking from that of a rigid concrete culvert structure, for which advantage of the soil-structure interaction effect. Most cur- bending moments are often the key factor in judging struc- rent commercially available computer software can perform tural performance. For buckling, thrust (that is, hoop the structural analysis required for this design. For other force) is the key factor and seismically induced thrust can structural shapes, consideration of soil-structure interaction be significant, particularly if the interface between the cul- becomes important and therefore is generally accounted for vert or pipe structure and the surrounding soil is consid- by using computerized numerical models. ered a nonslip condition (Wang, 1993). Therefore, it is im- portant that both seismically induced bending and thrust 9.3 General Effects of Earthquakes be evaluated using published solutions for circular tube and Potential Failure Modes (Moore, 1989; Janson, 2003) as failure criteria for evaluating the seismic performance of CMP and polymeric conduits The general effects of earthquakes on culverts and pipe (for example, corrugated HDPE pipes). structures can be grouped into two broad categories: ground 2. Soil-support considerations: Implicit in the current shaking and ground failure. The following sections discuss AASHTO design assumptions for flexible culverts is the each category. As it will be demonstrated, soil-structure inter-