Concrete Foundation Design - Spread Footing, Spot Footing

INTRODUCTION

• This e-article is about a concrete foundation spread footing
  • Identity
  • Design
  • Construction 

Comment:  Before The Architect (BTA) assumes that these footings as with all other footings are sited on substrates of reasonable bearing capacity and engineered when needed – a condition that, in BTA's experience, is very most often readily identifiable even in the design stage, and, if not, then onsite. 

IDENTITY

• As with most other footings, a spread footing can go by other names
  • Concrete pad
  • Footer
  • Footing pad
  • Individual footing
  • Isolated footing
  • Pad
  • Pad footing
  • Single footing
  • Spot footing 

• There are other footings, don't you know?
  • T-Wall a/k/a Strip Footing reviewed hereunder at "Home Foundation Design - Foundation Plan, Slab-On-Grade and T-Wall" in http://searchwarp.com/swa313149.htm and "Concrete Foundation Design - Strip Footing Foundation, T-Wall Foundation Properties" in http://searchwarp.com/swa320855.htm
  • Grade Beam reviewed hereunder at "Unique Home Foundation Detail – Grade Beam Design and Concrete Pilasters" in http://searchwarp.com/swa210615.htm
  • Modified Grade Beam reviewed hereunder at "Custom Home Foundation Design – Modified Grade Beam Design Detail" in http://searchwarp.com/swa306441.htm
  • Turndown Footing reviewed hereunder at "Concrete Foundation Design - Turndown Footing, Turn Down Slab" in http://searchwarp.com/swa321790.htm 

DESIGN

• This custom home designer's experience is that spread footings are less likely of footings to be engineered, closely tied with turndown footings for being made on the fly on the spot
  • Cheap
  • Easy
  • More often overbuilt at minimal marginal cost increment 

• In BTA's book, a spread footing 
  • Shall be on each side not less than twice the width (or diameter) of that which is supported
  • Shall be in most applications
    • Monolithic
    • Regular in all aspects
  • Shall be reinforced by
    • Not less than 3-Grade 50-#5 rebar in rectilinear pattern
    • With 3/4 linear inch plus maximum aggregate diameter cover and not more than 3 linear inches cover
  • Bottom of face shall be on not higher than the bottoms of face of new, proximate footings
  • If with pier, or pedestal
    • Then shall not be less than half the width dimension of the spread footing
    • Then shall be reinforced on the vertical with not less than 2-#5 rebar from the footing bottom of face plus 3/4 linear inch plus aggregate size cover to the pier top of face less 3/4 linear inch plus aggregate size cover and in either case not greater than 3 linear inches cover 

Comment:  The spread footing in the illustration to follow is notable for at least one indication; namely, it can be envisioned from any orthogonal as the same section.  

Spread Footing With Pier, Section in Elevation 

KEY:
ABV=ABoVe; APX=APproXimately; BEL=BELow; BOF=Bottom Of Face; BOT=BOTtom; CONC=CONCrete*; CONT=CONTinuous; CVR=CoVeR; D=Depth; DIA=DIAmeter; DIR=DIRection; DN=DowN; EQV=EQuiValent; FDN=FounDatioN; FGL=Finish Grade Level; FSN=FaSteN; FTG=FooTinG; GaLVanized; MIN=MINimum; NGT=Not Greater Than; NLT=Not Less Than; PT=Preservative-Treated; RR=RebaR**; SF=Square Feet; SUBS=SUBStrate; T&B=Top & Bottom; TOF=Top Of Face; TYP=TYPical; W=Width; W:C=Water-to-Cement ratio; WL=WaLl
*Please see in this regard hereunder "Home Foundation Design Details – Concrete Design" in http://searchwarp.com/swa218491.htm
**Please see in this regard hereunder "House Concrete Projects, Reinforcement - Rebar Design Standards" at http://searchwarp.com/swa217266.htm 

Comment:  The author notes an historical reference; namely, this drawing executed some time ago specifies #4 rebar, a sizing OK in its time (not so long ago); however, these days, BTA specifies not less than #5 in residential applications of reinforcement. 

CONSTRUCTION

• Where to design-in a spread footing
  • Generally, centered below the load the spread footing bears
  • Specifically, BTA applies spread footings in 3 conditions
    • Below a grade beam
    • Below a modified grade beam
    • Below anywhere else that a point or concentrated load path to earth travels or is otherwise guided to travel 
    • Below a grade beam

• As for a spread footing (with pier) below a grade beam, please see hereunder "Unique Home Foundation Detail – Grade Beam Design and Concrete Pilasters" at http://searchwarp.com/swa210615.htm 
• Here's an example of a spread footing with pedestal, or pier, below a grade beam 

Spread Footing Below Grade Beam at Intersection with Modified Grade Beam, Plan View, Scaled 

KEY: 
ABS=ABSOLUTE; BEL=BELOW; BM=BEAM; BOT=BOTTOM; BOT=BOTTOM; BW=BOTH WAYS; CONT=CONTINUOUS; CVR=COVER; D=DEPTH; FTG=FOOTING; GD=GRADE; HOR=HORIZONTAL; JT=JOINT*; LI=LINEAR INCH; NGT=NOT GREATER THAN; NLT=NOT LESS THAN; PRM=PERIMETER; RR=REBAR; SOG=SLAB-ON-GRADE**; SPD=SPREAD; T&B=TOP & BOTTOM; TOF=TOP OF FACE; TYP=TYPICAL

• Spread footing below a modified grade beam
  • As for a spread footing below a modified grade beam, please see hereunder "Custom Home Foundation Design – Modified Grade Beam Design Detail" at http://searchwarp.com/swa306441.htm
  • Here's an example of a spread footing below a modified grade beam 

Spread Footing Below Modified Grade Beam Intersections, Plan View, Scaled

KEY: 
BOF=BOTTOM OF FACE; CL=CENTERLINE; CONT=CONTINUOUS; CVR=COVER FTG=FOOTING; GBM=GRADE BEAM; INT=INTERIOR; LI=LINER INCH; MIN=MINIMUM; NLT=NOT LESS THAN; OC=ON CENTER; RR=REBAR; SIM=SIMILAR; SOF=SIDE OF FACE; SOG=SLAB-ON-GRADE; SPD=SPREAD; TYP=TYPICAL 

• Spread footing anywhere else that a point or concentrated load path to earth travels or is otherwise guided to travel
  • There can be several conditions in a residential design that are ripe for spot footings, including but not limited to –
    • Generally, load paths interior to perimeters or otherwise not to interior t-walls
    • Specifically among others –
      • Feet of a pool table
      • Interior, bearing columns
      • Exterior, bearing columns
      • Pedestals to certain fireplaces, e.g., Rumfords 
  • Here's an example of an interior bearing column supported by a spread footing
    • This column to-be will be below the terminus of an engineered beam done by others
    • Which beam is double-decked, that is, set atop another such wing wall on L0 and L1 to distinguish entrance to important space on each level 

Spread Footing Below Interior Bearing, Plan View, Scaled

KEY:
AB=ANCHOR BOLT; ABS=ABSOLUTE; BEL=BELOW; BM=BEAM; BOT=BOTTOM; BW=BOTH WAYS; CONT=CONTINUOUS; D=DEPTH; EL=ELEVATION; FTG=FOOTING; GD=GRADE; GRD=GRADE; HOR=HORIZONTAL; JT=JOINT; LI=LINEAR INCH; NLT=NOT LESS THAN; PNL=PANEL; PRM=PERIMETER; PSG=PASSAGE; RR=REBAR; SOG=SLAB-ON-GRADE; SPD=SPREAD; TOF=TOP OF FACE; TYP=TYPICAL  

• Of note –
  • Panel and passage are
    • Referenced to indicate places where anchor bolts shall not be set and for which compensation shall be made (dimensions that site these features are herewith omitted for clarity of expression)
    • Mullioned consonant with respectful observation of Traditional Style befitting this particular exterior (and a common sense approach to framing)
  • Changes in elevation respect the reality of a walk-out, covered deck 6 linear inches below the habitable interior
  • The control, or contraction, joint in the grade beam is off-center, forced so by a need to square-up the joint at its terminus with the wall and still centrally bear critical path loads from the roof, three ceilings and two floor systems

• Please note well that this spread footing is not square in plan view, but rather rectangular – some would call it irregular – because this designer's not entirely sure where in the wing walls will an engineer set the columns either side of a broadly opened portal or even whether more than one column will be applied to either side, since this designer indicated on the L0 and L1 floor plans that more than one column thereabouts would be in-style 

Comment:  While not specified herewith, the custom home designer expects, that in gauging the engineered beam, that engineer will further specify the spread footing as seen fit.

The author of this article has chosen to make this article available with free reprint rights.
Click here to copy this article.

►Reprint Rights◄

Log in to become a member of Ralph Pressel's Fan Club!

No comments yet.