When performing the structural calculations for corrugated steel pipe, one of the central variables is the yield strength of the steel. But pipe is made out of sheet metal, not structural steel like we engineers are used to. What then is the value of the yield strength, Fy? I’ve seen different values used so I will attempt to set the record straight here.
I do not believe the AASHTO LRFD Bridge Design Specifications specify a value to be used in the design of buried structures.
The AASHTO M36 and M167 specifications govern the fabrication of corrugated steel pipe and “structural plate” corrugated steel pipe, respectively. They mandate minimum yield strengths of 33,000 psi (228 MPa) and 27,500 psi (190 MPa), respectively.
AASHTO specifications usually govern on state or federal highway work, but many states have developed specific design standards that take precedence over AASHTO.
This is the ASTM standard that governs the structural design of corrugated steel pipe. Thus it specifies a value to be used for design, rather than a minimum manufactured strength. The values specified by ASTM A796 are:
- For 6″ x 2″ corrugations (150 mm x 50 mm), Fy = 33,000 psi (225 MPa)
- For 15″ x 5.5″ corrugations (380 mm x 140 mm), Fy = 44,000 psi (300 MPa)
- For 16″ x 6″ corrugations (400 mm x 150 mm), Fy = 44,000 psi (300 MPa)
- For all other corrugation profiles, Fy = 33,000 psi (225 MPa)
Most corrugated steel pipe has a corrugation profile of 5″ x 3″ and will thus fall into the “all other” category having a design yield strength of 33,000 psi, or 225 MPa.
The 6″ x 2″ corrugation is generally used for Structural Plate (fabricated in pieces and bolted) CSP, which are used for culverts that are too big to manufacture in one piece with the rolling machine or where the culvert requires a heavier thickness than the machine can roll (see our standard sizes of CSP and SPCSP). The second and third corrugation profiles on the list are only fabricated in big pipes. They’re called “deep corrugated” CSP and are only manufactured for culverts 25 feet and bigger, to give the culvert wall a bit more strength. So if you’re not using deep corrugated CSP, the ASTM standard specifies Fy = 33,000 psi (225 MPa).
At Roseke Engineering, I’ve looked back at some of my designs and we have generally used 300 MPa (44,000 psi). This probably came from the Corrugated Steel Pipe Institute design manual, where the examples all use that value.
If you have any other relevant information about culvert yield strengths, please post it below!