STRESS DISTRIBUTION AROUND TOP SLAB OPENINGS
At Delta Precast, we pride ourselves in our ability to answer your technical questions at any time. Each year, we receive several inquiries from reviewing engineers regarding our top slab design process for vaults, and manholes.

The answer is…Delta Precast has been designing top slabs for vaults, and manholes for more than twenty years using a highway bridge code, (AASHTO 17TH Edition, Section 3.24 – the distribution of traffic loads and design of concrete slabs for highway bridges).

IS THE AASHTO BRIDGE CODE SUFFICIENT FOR PRECAST?
Recently, an engineer-of-record questioned the application of the AASHTO highway bridge code for vault top slab design and asked us to prove the theory utilizing structural modeling. The concern was that bridge decks do not possess the same type of openings found in a vault or a manhole top slab. To demonstrate, we used SAP 2000, an integrated 3-D structural design and analysis software.

Below is the vault plan and modeling views revealing the stresses around the square opening.

 

Two-Wheels at Midspan Control Moment-1

Two-Wheels at Midspan Control Moment-1

CONCLUSION
After performing all the appropriate modeling with various wheel load placements, Delta Precast has concluded the application of the AASHTO highway bridge code is an adequate approach in the design of vault and manhole top slabs. If there are openings closer than 4 feet apart, we recommend you consult an engineer to determine the stresses between the openings.

We welcome the opportunity to discuss with you why Delta Precast is the right choice for your next top slab design. Contact Daryl D. Burns, PE, the Director of Specialty Precast Services, at 607-231-6613 today!