Where Concrete Ductbank meets Building Foundation

[mshields]

We always specify reinforced 3000psi concrete for electrical details. In recent years, we have frequently been asked by the EC to provide 5000psi concrete and forego the rebar reinforcement. My question is, given that the concrete of the ductbank needs to be pinned to the foundation to avoid disparate settling and therefore sheering force on the ductbank, is the 5000psi without rebar ill advised. OR are there alternatives methods to avoid this potential sheering action.

 

[David Castor]

Differential settlement between a building sitting on piling and a duct bank without piling can be significant issue and needs review and design input from geotechnical and structural engineering and that design should be followed. Generally, a design that allows for some amount of diff. settlement rather than trying to prevent it will be more effective. Your mileage may vary, of course. I have a lot of scar tissue on this particular issue. If it is expected that the building foundation is going to hold up the duct bank, then some type of reinforcing in the duct bank will be necessary (I would think). Seismic concerns may also be an issue.

 

[electrofelon]

We always specify reinforced 3000psi concrete for electrical details. In recent years, we have frequently been asked by the EC to provide 5000psi concrete and forego the rebar reinforcement. My question is, given that the concrete of the ductbank needs to be pinned to the foundation to avoid disparate settling and therefore sheering force on the ductbank, is the 5000psi without rebar ill advised. OR are there alternatives methods to avoid this potential sheering action.

I am no structural engineer, but I have a pretty good handle on forces, physics, and concrete, and increasing concrete PSI to skip rebar generally makes no sense.

 

[drktmplr12]

here's a good explanation as to why removing the reinforcement will substantially weaken the duct bank and increasing compressive strength (3000 to 5000 psi) won't help much

we normally see concrete encasement transition to direct buried once it is beneath the foundation.

 

[Julius Right]

See NSTAR ELECTRIC CONSTRUCTION STANDARD ELECTRIC OPERATIONS