Foundation Design for Electricity Pylon 1

[010185]

A 5m x 5m x 5m concrete foundation has to be designed to bear the load of a typical electricity pylon.

What forms of and how much steel reinforcement bars have to be used?

Do bars have to cross through the whole cube or do they just have to form a cage???

 

[csd72]

Are you sure you got the depth right? The weight of the concrete alone would be 125kN a square metre on the soil and this does not include the effects of overturning!

There are other material issues too with casting such a big lump of concrete!

 

[010185]

Yes, I'm certain it's correct.
I had doubts aswell with the dimensions, but they are apparently required for the purpose of supporting the pylon.

 

[csd72]

It sounds like the footing has been designed by an electrical engineer!

I would be concerned about 3 things in particular:

1. The amount of concrete used.
2. The bearing pressure on the soil(some soils would not be strong enough to support the self weight of the concrete).
3. The heat of hydration in the concrete will cause large differential temperatures throughout the concrete(google mass concrete ) also

http://www.dot.ca.gov/hq/esc/tollbridge/SFOBB/EastSpan/012024/MaterialsHandout/cd2/concret1.pdf

http://www.usace.army.mil/publications/eng-manuals/em1110-2-2200/c-6.pdf

As per your original question, I believe this is one of the few situations where you would provide internal layers of reinforcement. Cant help you any more, sorry.

 

[csd72]

Also

http://www.cement.org/buildings/pdh_se_mass.pdf

 

[naughtyhorse]

if i am reading this right, you seem to be proposing a cube design of foundation (5*5*5m * concrete mass * G/25m^2) ~125kNM^2.

tower foundations (usually) use pyramid & chimney foundations, which reduces the load imposed by the foundation considerably.

Broadly this design uses the area of the base to carry the compressive load

and the depth of the foundation to cope with any uplift (outer legs on section towers, with significant deviations experience uplift - in the case of broken wires and wind - a lot of uplift)

when considering uplift the mass of a fustrum of soil extending from the edges of the base of the foundation out at an angle of 30 degrees up to fgl is added to the mass of the concrete. thus

base area - manages compression
depth - manages uplift.

a recent HDD crash resulted in the loss of all of my foundation spreadsheets etc. but i have copies at work....somewhere

to your original point about rebars - as far as i'm aware (and that aint far in the case of reinforcement) i believe that reinforcment (usually) takes the form of a cage.

alternatively augered foundations can be used, or even multiple augered foundations with a pile cap - these definately have cage reinforcement.

 

[civilperson]

I disagree with the statement,"required for the purpose of supporting the pylon". A moment as well as an axial load can be designed for with a variety of configurations. Electric transmission towers can use drilled shafts, spread footings, cylindrical rings with a center filled with soil, etc. The massive block of concrete described needs superior bearing capacity beneath as well as provisions for thermal gradient change due to hydration heat. If the concrete is designed as plain concrete, then no reinforcement is required.