Questions from the Field is a selection of questions NPCA Technical Services engineers received from calls, emails and plant evaluations.
WOODY ASKS: Can the requirements of AASHTO M306 “Standard Specification for Drainage,
Sewer, Utility and Related Castings” for traffic rated casting be applied to the underground precast structure?
NPCA TECHNICAL EXPERTS ANSWER: While the design load used in AASHTO M306 and in the design of precast manholes and structures is AASHTO H-20/HS-20 live load, to apply AASHTO M306 directly to the underground precast structure would be a misapplication of the standard.
AASHTO M306 is a standard written expressly for the manufacture of iron castings. The 40,000-pound proof load described in Section 5.1 is for “first article inspection” or “quality conformance inspection” of the cast iron components. All castings subjected to the proof load are to be subsequently destroyed.
The same AASHTO load factors used for the design of roadways and bridges also are used to design underground precast structures, which are higher than the AASHTO M306 proof load when it comes to the design of top slabs. In other words, an underground precast structure designed to withstand AASHTO truck loading is sufficiently capable of supporting one or more cast iron rings and covers fabricated to withstand AASHTO M306 test loading.