Concrete Foundation Design - Turndown Footing, Turn Down Slab

INTRODUCTION

• Among concrete footings, turndown footings have the biggest identity crisis – they're known by many names – 
  • Turn down, turn-down, turndown, turned down, turned-down – each of itself or in various combinations with footing, footer, edge footing, slab, style footing
  • Concrete curb
  • Roll-over or rollover foundation
  • Stiffened or rib-stiffened slab or slab-on-grade or slab-on-ground or floating slab or mat slab or raft slab
  • Raft slab with deep edge beams
  • Mat slab with perimeter or edge beams
  • Orthogonally stiffened slab
  • Footer

Comment:  Raft, mat and the like are names not to be confused with their post-tensioned slab brethren and cisterns, which they appeared not to be when researching this humongo list.

Comment:  The seminal work on turn-down footings in, in this custom home designer's opinion, is "Design of Slab-on-Ground Foundations, A Design, Construction, and Inspection Aid for Consulting Engineers" by Walter L. Snowden, P.E., Wire Reinforcement Institute, 1981.  
     This designer admits to minorly quibbling on a point or two and majorly enjoying almost all of this work.  Engineering aspects of the article seem more appropriate to nonresidential applications.  What residential turned down footing applications witnessed by the author were usually small and most often related only to slab-on-grade perimeters.

• A turned-down footing shall be
  • Not less in width than that would it were a strip footing in the same application hereunder
  • Not less in depth than that would it were a strip footing in the same application hereunder
  • At bottom of face in section the greater of not less than 12 linear inches and 2 times slab-on-grade depth
  • Reinforced not less than would it were a strip footing in the same application hereunder
  • Braced continuously with concrete at joint of horizontal slab-on-grade to vertical footing at
  • 45° or 1:1
  • In length continuous
  • Spaced in the field at not greater than 15 linear feet clearspan, though 12 linear feet is preferred
  • Placed as a monolithic element, that is, the footer is bonded to the slab-on-grade

Comment:  While this custom home designer does little with edge footings, he understands that bottom of face may range generally from 12-16 linear inches and depth to 36 linear inches below slab-on-grade top of face. 

TURNDOWN FOOTER REINFORCEMENT

• Turned-down footer reinforcement shall be
  • Of not less than grade 50-#5 rebar
  • Spaced not less than that for any other slab-on-grade in the field
  • L-hooked from slab-on-grade on the horizontal to turndown on the vertical with cover not less than maximum aggregate plus ¾ linear inch 

TURNED DOWN FOOTER ILLUSTRATION

Turndown Footing, Section in Elevation, Scaled

• This rollover foundation illustration is of an exterior slab-on-grade terminus to earth as in an uncovered driveway
  • The roll-over foundation is not otherwise burdened except by itself with its slab-on-grade
  • No point or concentrated loads on this rib-stiffened slab-on-grade, and then in the latter only if clients insist and this author weakens his resolve, which you, dear reader, can judge is not likely 

Comment:  Would that a point or concentrated load was contemplated atop this rib-stiffened slab-on-ground – usually at a perimeter – that load may be offset by a spread footing. 

• Of particular interest in this drawing of a floating slab is the indication that the slab-on-grade and the mat slab footer shall both be above tamped earth substrates, and that the slab shall be above both sand and gravel substrates
  
• Both the EPDM continuous sheet and the gravel set between this raft slab with deep edge beam bottom of face and earth are to allay scouring

Comment:  This custom home designer recognizes that mat slabs with edge beams are popular in slab-on-grade residential construction, as they can be cheaper to formwork than a t-wall and cheaper to place as a monolith.  

Comment:  BTA refrains from most applications of orthogonally stiffened slabs – notably, interior applications, principally for the element's ineffectiveness of insulation.  In a slab-on-grade, crucially to insulate edges others commonly apply rigid insulation – a safe harbor for crawly critters and a material sure to diminish in efficiency over time.  
     In applying a t-wall with rest for an interior (or, for that matter, an exterior) slab-on-grade, the author lays in Insul-Tarp (http://www.insulationsolutions.com/) as a bond break and thermal break and moisture barrier, edge and slab insulator, and a construction element unfriendly to crawly critters.  At R-10 to the ½ linear inches, reflective, and a PERM of .002, it's such a deal.
     Mat slabs with perimeter beams also can web a slab-on-grade much do BTA's applications of grade beams and modified grade beams.  The difference thereat is that turndowns are bonded monolithically to the slab-on-grade, that is, they must move with the slab or the bond or nearby the bond fails; whereas, BTA's grade beams and modified grade beams are exclusively bond-barriered from a slab-on-grade, which, that is, the slab and it supports can move on their own. 

Comment:  BTA has used the stiffened slab-on-grade's angular buildout as a form of extended pilaster in heavy-equipment bays before design-in initiation of modified grade beam materials and methodology. 

WHERE ARE STIFFENED SLAB-ON-GROUND FOOTINGS APPROPRIATE?

• In what conditions do turndown footers make sense in this author's opinion?
  • Expansive soil – the more expansive, the more appropriate
  • Substrates too obdurate to remove cost-effectively, e.g., immediately subsurface limestone beds
  • Natural, subsurface watertables too high in which to dig
  • Exceptionally stable soils, e.g., sand, marl, etc.
  • Slabs-on-grade largely or entirely unburdened above top of face