Basement retaining wall and Grade beam - Torsion only or still design for Flexure

[Jrswett]

I'm designing a grade beam under a basement retaining wall which I'm modeling as fixed (grade beam / bottom) and pinned (top / floor joist). I checked the foundation retaining wall and it can span between supports with no shear reinforcement required, carrying the full bending moment, and has a deflection of 0.00 inches. About what I would expect for a 7ft tall x 8 in thick conc wall.

So I designed my grade beam for the torsion applied by the fixed bending moment at the bottom and the shear. Ended up with a 18" wide x 24" deep grade beam, (2) #5 bars top, mid and bot, and #4 stirrups @ 6" o.c.

My question is whether I should also require the grade beam to carry the bending moment? This would increase the top and bottom reinforcement to (3) #6 bars top & bot to hit the minimum flexural reinforcement ratio.

And to be fair I haven't got to the drilled shaft design so all this may be revised but the concept for modeling still applies.

I guess the retaining wall doesn't meet the specifications for beam flexural reinforcement ratio with #5 long bars @ 18" o.c. (temp / shrinkage) So I would have to increase the reinforcement in the foundation retaining wall to meet the flexural reinforcement ratio for a beam... which would require more reinforcement than the (3) #6 bars in the grade beam. And also be slightly confusing to specify on the plans. And probably not in line with engineering standards of practice.

I may have just answered my own question.

Image of the detail: [URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1681160454/tips/Foundation_Rtg_wall_and_Torsion_Grade_Beam_gxsdr3.pdf[/url]

Thanks

 

[Greenalleycat]

I'm pretty confused by the setup here. Typically you'd have a footing returning in front of the wall or behind it, so you develop the vertical wall flexural reinforcement into the footing and then design the footing to take that bending moment
There is no torsion present in this model

If you're saying torsion, I'm envisioning something like a wall over a strip footing that is supported on piles at discrete intervals?
In which case you would design the footing for the torsion only and there wouldn't be any bending moment (apart from a very small self weight major axis moment as it spans between piles)
If this is your model then I would be dubious of the validity of the fixed base assumption though
I suspect designing for compatibility torsion and assuming a pin-pin wall is a more realistic scenario

Can you post a sketch

 

[KootK]

My question is whether I should also require the grade beam to carry the bending moment?

I vote no. For any loads applied after the wall is installed, the wall will be the stiffer thing by a huge margin. So it's the "beam" in my mind. The wall can utilize the rebar in the grade beam as is flexural reinforcing, noting the following:

1) The flexural demand will likely be pretty small given the proportions involved (depth of the "beam") and;

2) Strut and tie may be appropriate for the flexural rebar give the proportions involved.

 

[Jrswett]

Yup the grade beam is supported by cast in place reinforced concrete drilled shafts at fixed intervals.

It's definitely supposed to be modeled as fixed - pinned. And the geotech has specified that retaining walls shall be supported on grade beam and cast-in-place drilled shafts. The wall should have no problem applying the torsion to the beam.

Added a pdf of the detail (draft) to the original post.

Oh strut and tie model!! of course. Sounds right. And I was just thinking that the grade beam long bars would also count for flexural reinforcement of the wall modeled as one large beam. Thanks Kootk. I haven't designed this particular setup before.

 

[KootK]

It's definitely supposed to be modeled as fixed - pinned.

What leads you to that conclusion? I would say that it is more common to assume that the wall is pinned pinned. And the proportions of your grade beam certainly don't scream "torsion resistance" to me.

The wall should have no problem applying the torsion to the beam.

The challenge with these setups is the connection that takes the torsion in the grade beam and delivers it as flexure in the piles.

 

[Greenalleycat]

OK, now that I understand it

I have designed something similar once
I conceptualised it as vertical pier elements aligned with each pile then the wall spanning horizontally between
So run continuous/lapped vertical steel out of the piles and up to the top of the wall
The grade beam is therefore basically just a spanning element to carry gravity loads in the construction load case for the wall
This seems more realistic to me than the grade beam being torsionally stiff enough to provide proper fixity, especially when considering the full systems effect of the beam-pier junction

 

[Jrswett]

The challenge with these setups is the connection that takes the torsion in the grade beam and delivers it as flexure in the piles.

especially when considering the full systems effect of the beam-pier junction

Hmm so you essentially run the piers to the top of the wall? I thought the cast in place drilled shafts with the long bars fully developed into the beam and the column tie at the beam bottom bars provided a fixed support?

This is light frame 2-story residential construction and my backfill height is about 6ft so we're not talking huge loads here. I just like to be thorough.

And the reason I am modeling this as fixed pinned is because

https://www.concrete.org/portals/0/files/pdf/e702.4_buried_concrete_basement_wall_design_2013-04.pdf

And also I have typically modeled cantilever walls on grade beams as fixed at the base, as this is required. Can you recommend a design guide for the beam / column torsional connection?

 

[Greenalleycat]

I don't know what your pile size and steel content is, but I probably wouldn't literally extend it full height - I imagine it would be quite bulky
Consider it to be more of a design philosophy
My philosophy is that there are stronger vertical elements aligned with each pile that are designed to take the tributary load of the wall either side spanning horizontally to it
The grade beam then only supports its self weight + construction weight of the wall above, and nominal out-of-plane spanning demands as it spans between the tops of the piles

 

[Jrswett]

Strut and tie model worked like a charm. Haven't done one of these since college. Many thanks KootK