I'm currently working on designing a temporary stream crossing for a construction project in Southern California. The crossing is currently planned to consist of multiple corrugated metal pipes (CMPs) installed in one layer, at the same elevation across the width of the stream channel, with a TBD spacing between each pipe. Based on the hydraulic analysis I performed, a total of eight (8) 36" diameter CMPs will adequately accommodate the design flow and rip rap will be placed at the entrance and outfall (wall thickness of the pipe is TBD). The portion of design I'm struggling with pertains to verifying the structural adequacy of the CMPs because most reference publications seem to focus on highway design loads (e.g. AASHTO HL-93, HS-20, etc.) but the design load for this project is a piece of heavy equipment on tracks with an operating weight of nearly 500,000 lbs. The tracks are each 3 feet wide with 20 feet of soil contact length on the ground. AASHTO's LRFD Bridge Design Manual and the National Corrugated Steel Pipe Association (NCSPA) Design Manual both provide guidance on design for a single pipe but contain little, if any, guidance on load analysis for multiple pipes for non-highway loads. These design manuals do provide information on general spacing of a multiple pipe configuration but this seems to apply to highway load conditions only. Does anyone know where I can find structural design criteria for multiple CMPs with a special loading criteria as previously outlined?
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Mark Berns P.E., M.ASCE
San Francisco CA
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