Grade Beam on Piles with Torsional Load 3

[chrisrosebud2001]

I am designing a 4 story building with a basement that will be placed on auger cast piles. The perimeter of the building will be placed on grade beams to support the basement wall as well as a brick facade above. With the exception of the basement wall, the support structure is steel. So, any columns will be placed in the center of the grade beam, but the basement wall and facade will placed to the exterior of the grade beam creating a torsional moment. What is the best way to handle this load? My initial idea was to ensure a moment connection at all pile/grade beam interfaces so that I can assume the beam fixed against torsion at these locations and then design the piles for the additional moment. Does anyone have a better suggestion?

Thanks in advance for any help.

 

[rowingengineer]

Do you have a concrete slab?

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that they like it

 

[Ltwine]

The basement roof/first floor slab should be able to restrict the free rotation of the perimeter grade beam. Pay attention to floor opening adjacent to grade beam, such as stair well.

 

[chrisrosebud2001]

Yes, there will be a floor slab sitting on top of the grade beams. Thanks for the tip.

 

[BAretired]

I don't understand the problem. Could you kindly explain it again?

BA

 

[hokie66]

The basement floor slab, or rectifying grade beams, can be designed to take the bending moments so that the torsion is eliminated.

 

[DaveAtkins]

I agree with what has been said. The couple is easily taken out by a couple at the First Floor slab and the Basement Slab. This is really no different than the more typical case where the Basement wall is supported on a wall footing. You wouldn't design the wall footing for the eccentricity--it is taken out in the wall above.

DaveAtkins

 

[DaveAtkins]

Oops--of course I meant, "The MOMENT is easily taken out..."

And I try so hard not to make mistakes in this forum

DaveAtkins

 

[BAretired]

I think a cross section is needed. I have never seen a grade beam under a basement wall, but if the wall is eccentric to the grade beam, why would there not be torsional moment in the grade beam?

BA

 

[Ltwine]

BA:

The flooring will take out the torsion.

 

[theonlynamenottaken]

If I understand the problem correctly, I don't believe its as simple as the floor slab just assuming all of that torsion... I see the flexural stiffness of the floor slab and the bsmt wall to be paramount. If the grade beam is stiff enough won't it assume torsion anyway?

Kinematically speaking the grade beam rotating will induce diaphragm/membrane forces in the slab. However, if the slab is connected to the grade beam by lying on top with a doweled shear connection, shear friction relies on a tiny bit of initial slip, doesn't it? I see the critical task to be to limit cracking in the grade beam. Cracks in non-soil bearing concrete members scare me due to the likliehood of accelerated corrosion of reinforcing.

 

[Ltwine]

It has to be designed as structural slab - reinforced.
That is not good idea to have a floating slab (SOG) for the basement anyway.

 

[ToadJones]

Ltwine-
Whats wrong with a floating SOG in a basement?
Don't 95% of all basements have this?

 

[theonlynamenottaken]

I believe I was misconstrued. The way its being discussed that the wall and slab effectively negate the torsion altogether ignores distribution of forces based on stiffness of members. The wall above and the slab aren't rigid supports - they have stiffness (and therefore flexibility).

What I'm getting at is that I wouldn't just check/design the wall and slab for the strength required for the additional flexure - I'd put some consideration into the relative stiffness of those members.

 

[Ltwine]

Two reasons for this one:

1. To help resist tendency of grade beam rotation.
2. Prevent water infiltration

 

[BAretired]

The OP did not say the grade slab is structural. Ordinarily, it would be a slab on grade with no connection to the wall or grade beam. In that case, there would be torsion in the grade beam from the eccentric load.

BA

 

[Ltwine]

I am the 5% minority, only had done one floating slab for basement.

 

[ToadJones]

Ltwine-
I am not trying to be smart here.
I have built many a home with basements.
All had plain old slabs-on-grade.

Am I missing something here or saying something wrong?

 

[ToadJones]

..ok....not realyl "grade", since the basement floor is typically 5-6 feet below "grade".

 

[hokie66]

This is a 4 storey building, presumably not a home. The moment/torsion generated by the vertical eccentricity can be quite large. Depending on the ground conditions, it may be feasible to resist the moment by the piles. If not, the moment must be resisted either in the basement floor system or in the wall, or some combination of both. If the wall resists the moment and the basement slab is a light slab on ground, there should not be a soft joint between the slab and wall, as the slab is required to provide the reaction for the push-pull moment in the wall.