Expert: Ted Barnhill (Principal, Design45 LLC) - 9/20/2007

Question
Hi Ted,

I live in Baltimore Maryland.  I want to add an attached garage to my home.  The first floor of my house is ground level in the front of the house.  The yard slopes down along the sides so that my basement is ground level in the rear of the house.  I want to extend my driveway along the side of the house and put the garage there.  I've had contractors propose various ways of supporting the garage floor.  Since it will be above ground level, all of the contractors are suggesting that they build a perimeter wall and then add fill of either crushed stone or crushed stone and dirt.  The wall will be ground level in the front of the house and about 6 feet above the ground in the rear.  The garage floor will be cement.  The garage dimensions will be pie shaped.  14 feet wide in the front, 21 feet deep and 23 feet wide in the rear.
I need help figuring out if the proposed construction techniques are sound.
1.  The first proposal is to build a perimeter cinder block wall and anchor it with steel rods attached to the cinder block and tethered to cement footers located in the center of the area.  The rods are to keep the wall from bulging outward over time.  Then dirt and crushed stone will be layered and tamped.  The fill will support the concrete floor.
2.  A different proposal was to have poured concrete walls and four pillars established to support the floor.  There will also be fill of stone and dirt but as it settles it will be the four pillars that actually hold the floor up.
3.  A third proposal is to build a cinderblock wall, use stone/dirt fill, and establish 2 pillars to support the floor.  The 2 pillars will hold the floor up.

Can you help me decide if all of these proposals are OK or if there should be any concerns about the proposals.  Please advise.

Answer
You have some creative contractors to choose from!  I would be very wary of structural schemes by non-engineers, especially untested ones.  The bottom line is that your proposed raised garage requires a non-standard structural engineering solution, and the best person to provide this solution is a structural engineer.  Perhaps the contractors you have spoken with are planning to provide this engineering in order to acquire building permits?

On the specific schemes, #1 sounds feasible, but the configuration of the anchor footers is a big question, as the lateral forces on the foundation wall may be quite large as you are parking near the back wall.  #2 and #3 call for a cast-in-place slab to support vehicles, and to span 10' or so.  This is doable, but requires careful design of the reinforcing, concrete strength, etc.  The fill is used as formwork only, not to bear any load.  A more common method, in my experience, is to use precast concrete planks bearing on the block (or poured, it is equivalent) wall.  These are commonly available around the country and come in a variety of thicknesses.  The spans of such planks are known for any given load condition, so there would be no guesswork on the contractor's part.  Your irregular geometry would require cutting planks or ordering custom shapes.  This method eliminates the need for interior piers under the slab, and wouldn't require any unusual lateral bracing of the perimeter wall, nor would you have to provide fill dirt.  (Also, you could design in a lower level workshop or something should you choose to.)  Good luck with your project.