Technical Manual: Conduits through Embankment Dams (FEMA 484)

Conduits through Embankment DamsFor high-flow-velocity, high-flow-volume structures, cracking can causesignificant hydraulically induced structural problems, such as cavitation, uplift,and binding of gates or valves. This is considered especially important where aconduit passes through or under an embankment dam and seepage could bedetrimental to the safety of the dam or where shutting down a structure formaintenance or repair can be difficult or very costly. Embankment dams andappurtenant structures are usually designed for larger load factors than thoseused for buildings. Allowable stresses of 1,800 lb/in 2 (compressive strength forconcrete) and 24,000 lb/in 2 (minimum specified tensile stress for Grade 60reinforcement) are used in WSD.With the issuance of ACI 318-02 (2002), the alternate design method wasremoved from the code. However, ACI 318-02 Commentary Section R1.1states, “the Alternate Design Method of the 1999 code may be used in place ofapplicable sections of the 2002 code.” In order to fully address therequirements for hydraulic structures for its dams, Reclamation is currentlydeveloping guidance for the design of reinforced cast-in-place concretestructures with unique design requirements. This guidance will includerecommended codes, design aids, and references for use in design. Thisguidance will allow use of the strength design method for structures wherecrack control and/or deflection limitations have been adequately addressed.Designers may want to consider this guidance when it becomes available, foruse in future design work.• USACE.—The USACE uses the SD method in accordance with ACI 318, asmodified in USACE’s Strength Design for Reinforced Concrete Hydraulic Structures(1992, pp. 1-2 and 1-3). Load factors that bear a close resemblance to ACI 318are modified by a hydraulic factor to account for the serviceability needs (crackcontrol) of hydraulic structures. This modification factor is intended to ensurethat the resulting design was as conservative as if the working stress design wereused.• NRCS.—The NRCS uses either the WSD or SD methods for site cast “servicehydraulic structures,” which includes conduits through embankment dams.The current NRCS WSD criteria follow ACI 318-77 (1977) with severalexceptions, including (1) allowable concrete compressive stress limited to0.40 f'c and (2) allowable steel tensile stress limited to 20,000 lb/in 2 .The current NRCS SD criteria also follow ACI 318-77 with several exceptions,including (1) single load factor of 1.8 applied to all loads and (2) steel designyield strength limited to 40,000 lb/in 2 for all grades. Current NRCS WSD andSD criteria are styled to produce basically the same concrete design results.Both are intended to provide lower stress levels than the ACI Code for90