Concrete Foundation Design - Strip Footing Foundation, T-Wall Foundation Properties

INTRODUCTION

• This e-article is about designing a concrete foundation footing, which is a way of saying it's about designing t-wall foundation properties
  
• More properly, this article is about a continuous, placed, reinforced concrete strip footing and the continuous, placed (or blocked) reinforced concrete wall that sits atop the footing, each – footing and wall – operate interdependently, symbiotically – each demands the other to function to support a residential structure
  
• The 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 

DOING THE LINGO THING

• There are, to this custom home designer's thinking, two home design and building subjects with murky lingo – electrical grounding and concrete foundations
  
• Let's talk concrete foundations

WHAT'S IN A WORD

• Most sources in the literature incorrectly define no more than three basic concrete foundations (in the field, they know better), sometimes defined by their footing, that is, by that foundation element in ultimate bearing to earth, and sometimes not
  • Each has names, yes, names, including strip footings
  • Strip footing a/k/a continuous spread footing, stem wall, and t-wall
    
  • In short, the other three – yes, three – are spread, grade beam, and turn down, again, each with synonyms
    
  • Ready?  They're all commonly referred to as ‘footers.' 
    • Just like the vaguely descriptive ‘rim board,' footer can, indeed, mean all sorts of foundation elements
      
  • Here, strip footing gets addressed

Comment:  This home designer addresses spread and turndown elsewhere subsequently.

• Grade beams are addressed hereunder in "Unique Home Foundation Detail – Grade Beam Design and Concrete Pilasters" at "http://searchwarp.com/swa210615.htm, and in
• "Custom Home Foundation Design – Modified Grade Beam Design Detail" at http://searchwarp.com/swa306441.htm
  
• And lastly, there's the monolithic term ‘monolithic,' oft associated with concrete foundations
  • That is, a one-time deal, placed altogether
  • Strip footings are rarely monolithic, that is, the footing is placed at an event distinctly separated temporally by, usually, days before the wall application, whether blocked or placed
  • It is another misfortune that the separately placed wall atop the previously placed footing is most often referred to as 'keyed' or a ‘cold joint' or . . . ‘cold pour' and not a cold placement.  Yep.  (Blocked walls are, of course ‘cold,' too; however, this custom home designer has neither heard nor read such a reference to block over footing.)  The other footers are usually monolithic placements

Comment:  A few might argue arguably that a slab-on-grade is itself a footing.  Disputation collapses with the notion that it, as with the foundation elements above, slabs-on-grade definitionally bear directly on earth (or on layers of substrates – sand, gravel, and such – that bear ultimately, directly on earth. 

     This custom home designer regards such disputation as diddly – a slab-on-grade in residential construction is, to this designer's knowledge, never intended to support house weight greater than, say, a non-bearing wall or a bog or a footfall.  To wit, BTA puts its foundation design in the breach with: t-walls and grade beams interior to perimeter walls where interior walls bear; pilasters at slab-to-wall termini of grade beams, modified grade beams, and control, or contraction, joints; modified grade beams along most all contraction joints; and spread footings alone and with piles and piers, and thickened slabs-on-grade below point or concentrated loads. (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 bond break.)
     Please see hereunder in these regards "House Foundation Design Detail - Slab-On-Grade Design Basics, Scored Concrete Supplement" at http://searchwarp.com/swa210956.htm and "Home Foundation Design - Foundation Plan, Slab-On-Grade and T-Wall" at http://searchwarp.com/swa313149.htm.  

WHAT ABOUT A STRIP FOOTING

• A strip footing (a/k/a continuous concrete stem wall, a/k/a t-wall, a/k/a strip foundation), shall be comprised of
  • Footing which shall not be less than 12 linear inches wide and 8 linear inches high if not bearing
  • 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
  • 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
  • Footing shall be reinforced with continuous and not less than Grade 50-#5 rebars 

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 sure there's no ownership tie to contracting.

• 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
    
• 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 footing's width, i.e., 24 linear inch footing bears a 12 linear inch wall, evenly on the footer's centerline.

• Footing shall have
  • Not less than Grade 50-#5 rebar
    • On the vertical
      • From 3/4 linear inch plus maximum aggregate size cover from 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 footing and wall placement is monolithic or it is keyed, or cold-jointed
          
• 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
  
• 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 aggregate size for cover 

AND THE WALL ATOP THE FOOTING?

• Wall width is determined
  • In part by footing width
  • In part by bearing to code
    
• Wall (and footing) shall be
  • 3500psi compressive strength at 28 days
  • Continuously wet and unloaded not less than 7 days
  • Backfilled only after the lowest floor ceiling structure is firmly in place
    
• Wall top of face not less than 6 linear inches (8 linear inches in some jurisdictions)
  
• Wall shall have
  • Grade 50 #5 deformed rebar
    • Not greater than 32 linear inches on the vertical and
    • Not greater than 24 linear inches on the horizontal
  • Tied in place
  • J-hooked on the vertical to the horizontal centerline in the footing to tying off at first bar intersection 

Comment:  For both the wall's vertical rebar and for the anchor bolts, the author has migrated from "L" to "J" forms to assure fastening to horizontal reinforcement run through them and to avoid low levels of cover in reversing the "L" forms (especially in the anchor bolt applications) 

T-Wall Bearing Two Slabs-On-Grade, Section in Elevation, Scaled

Key to abbreviations: ABV=above; BEL=below; BOF=bottom of face; BOT=bottom; BW=both ways; CONT=continuous; D=depth; DP=drainpipe; DR=door; DVY=driveway; EL=elevation; EPDM= ethylene propylene diene monomer; FDN=foundation; FIN=finish; FTG= (reinforced, concrete) footing; G=garage; GL=(finish) grade level; GVL=gravel; HDPE=high-density polyethylene; ISO=isolation (or expansion joint); JST=joist; LF=liner foot; LI=linear inch; NGT=not greater than; NLT=not less than; OC=on center; OA=overall; RR=rebar; SF=square foot; SOG=(reinforced, concrete) slab-on-grade; TOF=top of face; TYP=typical; WL=wall 

Comment: This T-Wall figure is an example of a continuous concrete stem wall section, supporting a garage slab-on-grade on right and driveway slab-on-grade on left. 

Comment:  This T-Wall figure would include a continuous, masonry pilaster from footing top of face to slab-on-grade bottom of face – not bonded – below control, or contraction joint, intersections with wall and below grade beams and modified grade beams intersection with wall, and otherwise at not less than 6 linear inches both sides of a change in direction of the slab, at the entirety of concentrated loads (as a step) plus 1 linear foot along wall line each side of load, otherwise at not greater a clear span in feet than 3 times slab thickness in inches, except where masonry finish clad flooring is to be applied, then at the more numerous of not greater than half a clear span in feet than 3 times slab thickness in inches where masonry finish clad flooring is to be applied below contraction joints, sharing slab section burdens equally. 

• Several noteworthy points
  • The 1 linear inch rain dam from of garage floor top of face at overhead door seat (cut to ½ linear inch in adaptable home applications)
  • Pitch downward to left of garage floor at 1/8 linear inch:1 linear foot and driveway at 1/4 linear inch:1 linear foot
  • High-density polyethylene sheeting as moisture and vapor barrier below garage floor (indicating no interest in heating garage, else the HDPE would have be changed-out for InsulTarp)
  • The turndown edge of the 4 linear inch driveway slab, for lateral stability
  • #5 deformed rebar both ways on 12 linear inch centers in the garage slab-on-grade and 24 linear inch centers in the driveway's slab (noting separately that the driveway's rebar reinforcement shall be epoxy-coated
  • No mesh or fabric reinforcement – more trouble that it's worth in this home designer's opinion, that is, almost always pushed down too low in a placement profile to serve well and when serving at all it's there to hold together already cracked slabs – based on BTA's foundation designing, that's one to many redundancies, bordering on ill omen
  • Compacted layering of garage floor substrates, including earth below both slabs-on-grade and below footing, the latter compaction being standards ops for Before The Architect, but not for any others to which its principals have ever borne witness
  • Foundation footing perimeter drainpipe and stone surround within particular form of geotextile – both stone and pipe
  • Continuous EPDM sheet at wall-footing keyed joint in order to prevent wicking 

Comment: Note the myriad details of cross-references to key specifications and notes in re highlights of application.  Cross-sections of each unique foundation element shall be so presented.  Even in modest applications, these sections can number upwards of half-dozen.   

• Another prototypical application of stem walls involves the following –
  • Support on the interior to a wood frame floor – sawn or trussed
  • Support on the exterior to a masonry wythe

T-Wall Bearing Frame Floor and Masonry Wythe, Section in Elevation, Scaled

KEY:
ABV=above; BEL=below; BOF=bottom of face; CONT=continuous; CVR=cover; EPDM= ethylene propylene diene monomer; EXT=exterior; FDN=foundation; FIN=finish; FL=floor; FTG=footing; GL=grade level; GVL=gravel; HWD=hardwood; INS=insulation; INT=interior; LF=linear foot; MIN=minimum; NLT=not less than; PT=preservative-treated; RR=rebar; SF=square foot; SOG=slab-on-grade; SUBS=substrate; T&B=top & bottom; TOF=top of face; TYP=typical; WL=wall

• Of note –
  • Floor system at right, atop the t-wall
  • Provision for masonry wythe on left, atop the t-wall
  • Please see the downset of masonry shelf to aid against moisture intrusion
  • Designed by Before The Architect as common practice whether to slab or t-wall – a commercial format not otherwise witnessed by the author in application to home design and construction and makes much good sense
  • Drainpipe on exterior at footing and interior above footing, the latter to let excess moisture from an unvented crawlspace
  • The interior drainpipe lets to the exterior drainpipe
  • Both pipes and stone surrounds are wrapped continuously in a specifically prescribed geotextile fabric
  • Other features and elements are similar to the earlier stem wall section in elevation 

WHERE DOES THE STRIP FOOTING GO?

• Strip footings go below major lines of bearing
  • At exterior perimeters overall
  • At selective interior lines of bearing
    
• Here's a plan view of a Foundation Plan colored green for strip footings and red for the reinforced concrete wall on top of the strip footings 

Foundation Plan Highlighting T-Wall Elements (Green=Strip Footing, Red=Wall), Plan View, Scaled

• Note, please –
  • T-wall around the entire exterior perimeter
  • Banding porch over wine cellar at center Front Of House, side porch to left and two concrete decks at left Back of House, garage on L1 at right Front Of House and garage on L0 at right Back Of House
  • Other points of interest –
  • Diagonal markings indicate grade beams
  • Dark marking indicate modified grade beams
  • Cross-check markings indicate thickened slab-on-grade
  • Dark interior outside and inside corners of green footing are pilasters
  • Squares all over the place are spread footings by any other name
  • Diagonal pairs of lines in interior outside and inside corners are rebar reinforcements to slab-on-grade  

CLOSER LOOK AT T-WALLS

• Let's more closely inspect a small chunk of this Foundation Plan, being the area about the side porch at right 

Foundation Plan, Side Porch Environs, Plan View, Scaled

• Please note –
  • The side porch foundation is bounded by strip footings (green) on right
  • Elevations of the top of the t-walls varies
  • EL± = L1 floor level and thereat comprises a part of the L1 garage gas curb
  • EL-20 = level of t-wall top of face relative to L1 floor level, to let for resting 20 linear inch wood, open-web, metal plate-connected floor trusses
  • The little squares with the littler hex figures within are 3x3 bearing plates in lieu of washers below hex nuts associated with anchor bolts.  Please see hereunder "Home Foundation Design - Anchor Bolt Installation" at http://searchwarp.com/swa313180.htm  
  • D is for clothes dryer, E is a cross reference to how the author wants the side porch reinforced, pitched, and stepped
  • The double-crossed squares denote posts set upon stainless steel pins to ally shear 

Comment:  Note well that compensation for sill plates and mudsills is assumed overall and without exception at same-depth, very most likely 1 ½ linear inches true.  Porch finish topface shall be adjusted onsite to develop 6 linear inch finish flooring lift from porch to interior L1.

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Comments on this article:

» left by Anonymous (218 days 19 hours ago.)

This is a great article.

Respond to this comment

» left by Ralph Pressel from Georgia (218 days 18 hours ago.)

Dear Anon.,

Thanks.

As with other many matters of home design and home building, I write in self-defense.  I already knew or needed to learn what I could not find anywhere else altogether, so I chose to share the knowledge.

Your interest is deeply appreciated,

Ralph

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» left by JANE from PHILIPPINES (133 days 16 hours ago.)

WHICH FOUNDATION SETTLED MORE ON THE SOIL, THE SQUARE OR CIRCULAR ONE? (HAVING THE SAME AREA, LOADS AND SOIL BEARING PRESSURE)

Respond to this comment

» left by Ralph Pressel ^(48,060) (132 days 16 hours ago.)

Dear Jane,

Now, this old boy's not an engineer, so it'd be a swell idea to get the straight-up answer from somebody who is an engineer.  Just looking at the geometry of it, you'd think there is no difference if load, footing depth and surface area were the same, wouldn't you?  I am sure an engineer can set us both on the right track.

Thanks,

Ralph

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