Custom home with 14' foundation walls!! 1

[TroyD]

I have been asked to assist with the structural engineering on a behemoth custom home with some unusual features, including a basement "sport court" with a 12' tall ceiling...wait for it...underneath the garage. The attached PDF shows the sections and the floor plan at this location. (The garage floor is typical precast plank with a 4" concrete topping, 25' clear span). I was legitimately concerned about the extreme depth of this foundation wall...unheard of in typical residential construction, so I hired a local geotechnical engineer that I trust to perform a site visit, and sample and probe the open excavation that was started. The material includes shale and weathered bedrock, which helps immensely with the soil bearing capacity. I am modeling the foundation walls with RetainPro software. I am confident that I can adequately design the walls for the axial loads and lateral earth pressures. I will call for backfilling with clean sand immediately behind the walls to keep the at-rest heel pressure manageable. Perimeter walls will be tiled along the footing. My preliminary wall model requires 14" thick walls with #6 vertical rebar @ 12" o.c. at each face. But the wall I am concerned about is at the stairwell. Unlike the rest of the garage floor area, that wall is unsupported at the top, due to the stair case, and will need to be modeled as a cantilevered wall. For this I am modeling it as a tapered cantilever wall in RetainPro. Looks like it will work but the base of wall might be 24" thick, and the footing ~48" wide. I wonder if there are other stabilization methods, such as soil anchors, that may be more appropriate for this situation.

I'm curious if other professionals have encountered a situation similar to this and if there is any advice or recommendations I may not have considered. I have assisted this custom home builder many times and believe his concrete sub does quality work. This house is 8,700 square feet, so I think we're talking ~$3M construction cost...he probably hires a top-quality mason. But to avoid a potential catastrophic foundation failure I wonder if it is appropriate to suggest the stairs be eliminated there. Any advice is appreciated.

 

[hokie66]

Instead of building the wall beside the stair as a cantilever, you might consider using a horizonally spanning beam at the top. I see no reason to move the stair, but I would query the number of risers without a landing. That would be a code violation where I am.

Another area requiring a lot of attention will be waterproofing of the basement.

 

[EngStuff]

Off topic, but check why RetainPro is calling out Rebar on both faces. perhaps due to shrinkage requirements. I think you're good with it being on the inside face only. Refer to ACI 318-14 section 11.7.2.3.

For your main question. Is the lateral pressure high? I would double check the retainpro software. I have designed and seen walls that are deeper and only require a thickness of between 12" to 16". What if you increased your bar sizes on inside face?

Hokie66 has a good point of designing a beam at top. whether it be a separate beam system or monolithic with the wall. make sure that the horizontal reactions at the ends of the beam can be resisted by the floor.

You can also span the wall horizontally to a certain point and design for the reactions. Or span to the corners if the one corner can handle all that reaction load. Essentially there's a tension load on the inside corner that would require horizontal bars and corner bars. It might just be a little more expensive in this specific case due to the length of the wall. Maybe someone can chime in on this thought.

EDIT: you would also probably need shear reinforcement.

 

[hokie66]

From his plan, it looks like the best way is for the horizontal beam to span the 25' span between the outside walls. On one end, the reaction is compression on the wall, on the other, tension. Cast the beam monolithically with the walls. Whatever the final design of the basement walls, then the wall construction is consistent, without confusing the barsetters with horizontally spanning walls.

 

[TroyD]

@EngStuff:
Yes, I think the heavy-duty reinforcing will be on the inside face only. Might need some smaller bar on the outside face to satisfy temperature & crack control rebar requirements, code minimums.

@hokie66,
Thanks for the suggestion. This has potential...I'll have to research this. I assume this horizontal beam could be located just below finish grade height so it's hidden? I could evaluate it first as a restrained wall, and whatever that restraint force is up at the top of wall is the design lateral load (per linear foot) that the beam needs to resist. Another thought was to possibly construct a foundation pilaster/abutment or "dead man" at mid-span along that wall at the outside face. It could be poured monolithic with the wall (hidden below grade) and designed to resist the overturning forces for the majority of that wall face.

 

[kipfoot]

You might want to consider this as a slab supported on three sides. Hokie makes a good point about not confusing the bar setter, so be clear about the reinforcing in this special case.

 

[DaveAtkins]

I assume this horizontal beam could be located just below finish grade height so it's hidden?

Usually, the beam can just be part of the wall--no thicker than the wall, so there is nothing to "hide."

DaveAtkins

 

[dik]

In these environs... a wall that thick requires reinforcing in both faces... might want to reduce the outer reinforcing by a bar size... keep the same spacing as the inside wall... what horizontal reinf...
Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[r13]

We call this is an "embed beam", yes, it is hidden from view. If necessary, one can provide more than one embed beam, while the load on the upper embed beam gets too large.

 

[EngStuff]

dik,

When would it be ok? Because in ACI it reads out that walls "....greater than 10" except basement walls and cantilever retaining walls, distributed reinforcement for each direction shall be placed in two layers...." I always thought per the statement that any basement/cantilever wall >10" just needs 1 layer of reinforcement, which would be the tension face. I assumed that they didn't care much about the temp/shrinkage cracking at the outer face because the deferential temperatures at the foundation isn't excessive, and if cracking does occur, people wouldn't see it. and also there would be waterproofing.

 

[TroyD]

@DaveAtkins,
Thanks. I have not sized the beam yet, but initially assumed it would be deeper that the wall thickness. Evaluating it first as a restrained wall, depending on the heel at-rest pressure I use, I'll be dealing with a lateral load of ~1,500 to 2,000 lbs/ft up at the top. Conservative values for at-rest heel pressures are ~60 psf/ft. If I'm backfilling behind the wall with clean sand, maybe I use ~30 to 45 psf/ft?? So I will then have several larger horizontal bars, closely spaced, at the inside face of the wall near the top. Those will be the tension rebar for my beam. Is it common to also have equally spaced stirrups/ties, like a typical rc beam?

 

[r13]

EngStuff,

The outer face of basement wall can experience bending, if fixed at the bottom, and support at the top. Which would be the most likely scenario for this tall basement wall.

 

[STrctPono]

TroyD,

I think you stick with your 30 degree friction angle on your backfill material unless your Geotech gives you something different.... So let's say 60pcf at-rest and 40pcf active....

I ran a quick check in my spreadsheet and for a 15ft tall wall (assuming you will need to embed your footing 1ft below interior slab) and active earth pressures I get an 18" thick stem to work out with #6 @ 8" o.c. with less than 1/10" displacement at the top. I don't see any need to go to 24" thick wall. Shear reinforcing in the stem is certainly not needed.

Let me offer an alternative option for the design of these basement walls. Remove the load on the walls by designing the backfill as Geosynthetic Reinforced Soil. This would significantly reduce your wall stem and footing size.

 

[dik]

EngStuff... the wording appears to stipulate one layer of reinforcing... I simply wouldn't do it... sorry.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik