Cantilever basement foundation wall support from perpendicular walls? 1

[TroyD]

I'm assisting a home builder with a permit issue. The attached drawing shows two basement foundation walls that are of concern due to the stairwell - where there are no floor joists to provide support at the top of wall. (I'm using RetainPro software to design the walls). The garage wall can easily be designed with the garage slab reinforcing tied in to the foundation wall via bent dowels to provide lateral restraint at the top. The outer wall needs to be designed as a cantilevered wall, with increased footing width, stem reinforcing, etc. to resist overturning. But I'm curious what support the perpendicular walls (shown bubbled) can provide? Obviously the mid-point between those two perpendicular walls (10'-10" span) will least benefit from those walls. They act as a buttress, right?

I'm familiar with rectangular concrete tank design and the moment coefficient distribution thru the wall height/length. Has anyone applied this design method for situations like this? I'm thinking I might be able to design the mid-point of that wall as a full cantilever wall, and taper the footing width down towards the ends. Any thoughts?

 

[HouseBoy]

I use the rectangular tank coefficients often and I would expect this wall can essentially span 11 feet horizontally without any increase of the footing.
You might also consider that the steps can provide bracing to the wall over a portion of the length, reducing the effective horizontal slab even more.

 

[TroyD]

Thanks for the input. I had also run a scenario considering the stairs providing bracing at mid-height. Good advice.

 

[HouseBoy]

You mentioned about the effectiveness of the perpendicular walls to provide buttressing.
I am convinced that they do.

The "challenge" I sometimes have is in conveying the layout of the corner bars in a manner that will resist horizontal bending. On a recent project when I pointed out the corner bar positioning (that we detailed in the drawings) to the contractor, he sarcastically asked if I thought that all of the other foundations that he has built over the years "...are going to fall down now....." (Since he had never installed corner bars like that before.) Oh well.

 

[KootK]

Agreed, much of what you see in residential construction is only justifiable considering walls to span horizontally in some locations.

What follows is really just some additional thoughts piggy backing up HousBoy's comments.

 

[HouseBoy]

KK
Like your detail but I think that would be even more difficult to "explain" !
Also, would you need it in both directions (2 bars with "U" shape set at 90 degrees to each other in plan)?

Here's what I used. (see details)
SOP around here is to bend both bars 90* at the corners (i.e outside bar bends around the outside and inside bar around the inside, making the interior bars completely worthless as tension bar(I think).
I tried to apply the ideas of strut and tie but honestly I haven't used that approach very much. Still don't have a great feel for it yet.

 

[KootK]

Like your detail but I think that would be even more difficult to "explain" !

No doubt. But this is primarily a technical discussion so I think that the logical starting place is at that which is most technically correct. Although your detail with the diagonal might be better in that respect.

Also, would you need it in both directions (2 bars with "U" shape set at 90 degrees to each other in plan)?

I don't see it. I'm not trying to resist moment at the joint and, going the other way, I'd assumed that there would be diaphragm resistance at the top of the wall.

Here's what I used. (see details)

I could get behind other of those if it meant for significantly happier clients. I wouldn't rate the consequences of failure here quite as severe as a true hanging condition.

For what it's worth, I'd typically be doing this with only a single, interior layer of reinforcement.

 

[TroyD]

For what it's worth, I'd typically be doing this with only a single, interior layer of reinforcement.

My wall model shows that center reinforcing layer works.

 

[bob33]

I assume you will be temporarily bracing the garage wall adjacent the basement and stair to resist the 9' of compacted fill under the garage slab.

I like your corner details but for detail B the earth face horizontals may need to be hooked to develop them just beyond the added diagonals.

 

[TehMightyEngineer]

My wall model shows that center reinforcing layer works.

One thing to watch for with centered reinforcement is the tension surfaces of your section can quickly be stressed well beyond the tensile strength of concrete before the bars become engaged and crack.

I'd consider some cheap insurance of a single #4 L-shaped bar places on the inside of the corner at the very top of the wall (highest stress point I presume).

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL, HI)