Technical Manual: Conduits through Embankment Dams (FEMA 484)

Chapter 4—Structural Design of Conduits4.2.1 ConcreteConcrete conduits are generally considered to be rigid structures. Plastic and metalpipes used in the construction of conduits are considered to be flexible. A flexiblepipe is generally considered to be able to deflect without structural distress to thepipe or to any coating or lining. A flexible pipe derives its external load capacityfrom its flexibility. Under load, the pipe tends to deflect, developing soil support atthe sides of the pipe. Flexible pipe used in conduit construction within significantand high hazard embankment dams should be encased in cast-in-place reinforcedconcrete to provide shapes that allow for good compaction of embankmentmaterials against the conduit. Flexible pipe used in conduit construction within lowhazard embankment dams is often not encased in cast-in-place reinforced concrete.As discussed in section 2.1, concrete used for conduits is typically either reinforcedcast-in-place or precast concrete. Specific guidance pertaining to these materials isdiscussed in the following sections.4.2.1.1 Reinforced cast-in-place concreteReinforced cast-in-place concrete for conduits is typically designed by either theworking stress design (WSD) or strength design (SD) methods or the alternate designmethod, which is similar to WSD, but includes some SD features. The WSDmethod proportions reinforced cast-in-place concrete members for prescribedservice loads at stresses well below the ultimate and assumes linear distribution offlexural stresses and strains. The SD method requires service loads to be increasedby specified load factors and computed nominal strengths to be reduced by specifiedphi factors. Various editions of the American Concrete Institute building code (ACI318) describe these design methods.Reinforced cast-in-place concrete conduits are normally subject to different loadings,more severe exposure conditions, and more restrictive serviceability requirementsthan buildings. The major embankment design agencies and ACI have morerestrictive versions of WSD and SD methods that are appropriate for reinforcedcast-in-place concrete conduit design. A brief summary of the reinforced cast-inplaceconcrete design philosophies used for conduits by these agencies includes:• Reclamation.—Some existing conduits in Reclamation’s inventory ofembankment dams are as much as 100 years old. Reclamation designed theseconduits with the WSD method. Reclamation has also used the WSD methodfor modifying any existing structures that were originally designed using WSD.The WSD method is preferred for any feature considered to be an integral partof a hydraulic structure, such as a spillway or outlet works, where crack controllimitations are important considerations.89