Reinforced Concrete Pipe Encasement... How to design? 1

[busyanybody]

I have a 24" ductile iron pipe in a 4'x4' reinforced concrete encasement, with an HS-20 live load on the encasement, placed on aggregate base course. I want to design it like a footing with a 2'x2' hollow area in the middle. How do I calculate the maximum live load bending moment in the encasement using the subgrade modulus of reaction?

 

[HTURKAK]

I will suggest you to model, the pipe will be designed for internal pressure and concrete encasement will be designed for soil and traffic loads. Wrapping thick PE membrane around pipe will act as bond breaker and help to this idealization .Regarding the base reaction, you can conservatively model assuming uniform pressure distribution across the bottom slab,or with winkler approach , assigning spring constants to FEM nodes. You may look to The following doc.

https://www.concretepipe.org/secure/webinars/2010/nov4/2010-11.1 LRFD Effect DOTs and RCP.pdf

 

[busyanybody]

Unfortunately I could not get much help from that document. Is it possible to do this calculation in a spreadsheet, or would it need a finite element analysis program like SAP2000?

 

[HTURKAK]

Dear busyanybody ,

I do not know the length of concrete encasement but apparently this protection for a road crossing and the total length of crossing may be in the range of 20 meters..That is, the longitudinal moment will govern and the encasement should be modelled as beam supported on elastic foundation. I will suggest to analyze longitudinally as FEM beam elements supported on winkler springs and transversal (cross section) model the system unit width box culvert supported on elastic found.(winkler springs). Another approach, provide EJ at concrete encasement (in the range of 20 mm) at socket-spigot joints, so the each pipe zone will have some freedom to rotate at joints, and analyze the pipe section 6 m , longitudinally rigid beam on foundation and cross section analysis similarly , take unit width of culvert and assume uniform pressure distribution across the bottom . You can calculate in a spreadsheet in case of second approach. Just by experience, the longitudinal reinforcement will govern....

 

[r13]

You should use "beam on elastic foundation" approach. See Roark's Formulas for Stress and Strain for solutions.