Raft foundation (45cm thick) with foundation beams...do i need torsional reinforcement?

[kellez]

I am designing 2-storey RC frame house,according to EC2 and EC8....common practise in my country is to use a monolithic raft foundation (45-50cm thick) with foundation beams connecting the columns.
This is the strongest design possible for a foundation.
Foundation Beams are designed using the capacity design principle so as to be stronger than the columns above


I am a young engineer and all of the projects i have seen up to now (not many) do not use torsional bars for the foundation beams, except in the case of very short beams.

I think this is reasonable due to the huge mass of the raft foundation which makes it hard to get any torsional moments in the members except in cases with very short beams.

The reason I am asking this is because my software gives out some torsional reinforcement therefore it got me thinking.

What do you guys think?

 

[dik]

Depending on the loading conditions, you might. Often not necessary.

Dik

 

[DaveAtkins]

I would use closed stirrups, to resist any torsion which might be present. However, I would normally not worry about torsion in the type of structure you are describing.

DaveAtkins

 

[kellez]

Thank you guys for the fast reply, i appreciate it.

This is all i wanted, a second opinion from fellow engineers.

 

[BAretired]

With a 450mm to 500mm thick raft foundation, why would you need additional foundation beams? Can't the raft foundation act as a foundation beam connecting all of the columns? It seems to be a pretty hefty foundation for a two story house.

BA

 

[dik]

With small torsion, you might look into the combination of top bars with open ties giving you an almost closed stirrup.

Dik

 

[kellez]

With a 450mm to 500mm thick raft foundation, why would you need additional foundation beams? Can't the raft foundation act as a foundation beam connecting all of the columns? It seems to be a pretty hefty foundation for a two story house.

The foundation beams are embedded within the raft foundation,

Think of it this way, the other option you have for the foundations for such a structure in a seismic region is spread footings with tie beams or just use a continuous footing below each column in the form of foundation beams. Then you would have to fill up the space enclosed by the foundations with additional soil, compact it and then build a slab on top of it. Also think of the labour and time required to build the formwork for each footing and beam.

However if you use a raft foundation with foundation beams which is simply the same as the solution mentioned above but with concrete used to fill the gaps, you already have a slab for the house, therefore there is no need to add soil, compact it and then build a slab on top of it, also the formwork is mush simpler. In the end its cheaper, simpler and a lot faster.

 

[Teguci]

Usually, a little cracking to allow the load path to be redirected to bending in the perpendicular beams would be fine. In your case, you need to be sensitive to crack control since the top of your foundation doubles as your finished slab. Not sure what you mean by torsional bars, but a simple "]" shaped stirrup along the edges that is lapped into the top and bottom steel should do the trick.

Looking at how close the columns are to the exterior edge, I'd say you need to be concerned with detailing the steel for an "opening corner joint." Actually even worse since the corner joint will have stress reversals if you have lateral load reversals.