Before The Architect - House Foundation Problems - Concrete Strip Foundation Design

Foundat'n Strip

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YOUR CONCRETE STRIP FOUNDATION, T-WALL, OR STEM WALL DESIGN

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What About A Concrete Strip Foundation Design?

This subchapter is about designing a concrete home concrete foundation strip footing with wall, which is a way of saying it's about designing t-wall foundation or stem wall foundation properties

More properly, this is about a continuous, placed, reinforced concrete strip footing and the continuous, placed (or blocked) reinforced concrete wall that sits atop the strip footing, hence 'stem wall foundation' and 't-wall foundation' nomenclature

	Each – concrete strip footing and wall (a/k/a concrete strip foundation_ – operate interdependently, symbiotically – each demands the other to function to support a residential structure
	The strip footing provides the ultimate weight-bearing capacity for the home's weight to earth below finish grade; the wall extends that bearing to level(s) above finish grade

A concrete strip footing (a/k/a continuous concrete stem wall , a/k/a t-wall, a/k/a strip foundation, a/k/a continuous wall strip footing), shall be comprised of

	Strip footing which shall not be less than 12 linear inches wide and 8 linear inches high if not bearing
	Strip footing which shall not be less than 18 linear inches wide and 12 linear inches high if bearing one story or a single wythe or equivalent
	Strip footing which shall not be less than 24 linear inches wide and 12 linear inches height if bearing more than one story or wythe or equivalent
	Strip footing shall be reinforced with continuous and not less than Grade 50-#5 rebar

Comment: Strip footing dimensions vary not only by extent of bearing, but also by environmental conditions, including, among others, soil expansivity, seismicity, etc. Reputable, well-experienced local foundation contractors usually know more about such matters than most anyone else even to subsoil conditions of distinction between areas of a neighborhood. To boot, you might get a two-fer in a foundation contractor who's also an engineer.

In some soils, engineering guidance will be requisite; however, whoever the engineer be, be sure there's no ownership tie to contracting. [The worst substrates this author encountered involved Leda Clay in the Great and Lesser Clay Belts of northeaster Ontario Province.]

Rebar shall be –

	2 continuous in the top if less than 18 linear inches wide or
	3 continuous in the top if not less than 18 linear inches wide and
	2 continuous at the bottom if less than 18 linear inches wide or
	3 continuous in the bottom if not less than 18 linear inches wide and
	Otherwise set at 16 linear inches on-center grids
	With cover not less than 3/4 linear inch plus aggregate's maximum size cover and not greater than 3 linear inches plus aggregate’s maximum size

Strip footing top of face shall not be higher than the greater depth of 18 linear inches below finish grade level or the frost level

Comment: The wall in width is usually ½ the founding strip footing's width, i.e., 24 linear inch strip footing bears a 12 linear inch wall, evenly on the footer centerline. Soil bearing capacity, load, seismicity, etc. can vary strip footing dimensions both absolutely and relatively.

Concrete strip footing shall have

Not less than Grade 50-#5 rebar

On the vertical

From 3/4 linear inch plus maximum aggregate size cover from strip footing bottom of face

To 3/4 linear inch plus aggregate size cover below stem wall top of face

On not greater than 4 linear feet centers in the field

At not greater than 2 linear feet from

	An intersection
	A butt, or terminus
	A change of direction
	A change of height

Without regard as to whether the strip footing and wall placement is monolithic or it is keyed, or cold-jointed

Concrete strip footing shall reveal not less than 4 linear inches and preferably not less than 6 linear inches clear on the horizontal, either side of the wall (or grade beam) it supports

Reinforced concrete wall shall be

Not less than 6 linear inches wide if not bearing

Not less than 8 linear inches wide if bearing

Reinforced with

	Not less than #5 rebar at 24 linear inches on center if less than 12 linear inches wide
	Not less than 2 vertical of #5 rebar at 24 linear inches on center if not less than 12 linear inches wide

With 3/4 linear inch plus maximum aggregate size for cover

Backfill

Shall be clean of building debris, rock and stone, etc.

Shall be tamped damp (not wet) the greater compaction of

	Not less than 50 beats per square foot and
	95% density, modified proctor in conformance with not less than most recent ASTM D-1557 (a/k/a American Society for Testing and Materials, “Substrate Tests for Moisture-Density Relations of Soils, “Substrate Tests for Moisture-Density Relations of Soils and Soil Aggregate Mixtures Using 10-Pound Rammer and 18-inch Drop"
	Alternatively and preferentially, may be applied with high-pressure hydraulic jet

Shall be applied only after

	Not less than 7 days initial, continuously wet and continuously unloaded cure AND
	First floor framing is up to and including ceiling joists

Keyways shall be permitted

	Where a continuous, water impermeable membrane, e.g., EPDM liner extends its entire length before the cold pour
	Where vertical rebar may be applied to further counteract lateral, or shear forces

Vertical faces of the foundation below grade shall be covered by not less than 3 linear inches of compacted material.

Comment: In these regards, please consider “How To Avoid Common Floor Problems: 20 Step You Can Take to Avoid Common Floor Deficiencies” by Metzger/McGuire, Concord, NH, http://www.werc.org/Files/DMX/SETemp/139.pdf

Comment: Please note, dear reader, that in all instances, sub-slab-on-grade foundation elements are continuously separated at their top of face from the bottom of face of a slab-on-grade by a continuous bond break.

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