BEFORE THE ARCHITECT – CUSTOM HOME DESIGNING BACKGROUND – HOUSE DESIGN ARTICLES

ALL ABOUT HOUSE FOUNDATION PROBLEMS and

DESIGN OF

CONCRETE FLAT SLAB, OR GRADE SLAB,

FOUNDATION CONSTRUCTION

By Before The Architect  Copyright 2009

YOU MAY FREELY QUOTE THE AG WITH PROPER ATTRIBUTION

Symmetry?   The eye looks for it, the mind's eye expects it, the mind is comforted by it.  It's a designer's habituation.  Reined in this side of obsession, it suits principles if not purposes.  Be not smitten.  It may be likened to the frame around a painting and may be artful; it's not art.  At its best, it counterpoints inspiration, its complement.  At its worst, it’s inspiration’s antonym, a kiss from your sister. Before The Architect

INTRODUCTION

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.

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 ILLUSTRATION

Turndown Footing, Section in Elevation, Scaled

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. 

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?

Comment: That is to say that in other instances where a slab-on-grade may be contemplated with a turned-down footing, this designer would much prefer a crawlspace format either on posts or t-wall.