Unique Home Foundation Detail – Grade Beam Design and Concrete Pilasters

• One home foundation detail extends a foundation design in limited application today
• The other home foundation detail seems brand new. 

• What if you did not perfectly prepare the home foundation substrates to a slab-on-grade and then perfectly reinforce and place and joint the slab-on-grade?
• The best slip sheet on the planet won’t protect your stone floor from displacing its smooth surface plane or garage floor from cracking down at the corners or strip oak from rolling.
• You likely needn’t be concerned much about a distorted concrete slab-on-grade if you’re building on marl in the Florida Keys or over ledge limestone north of Austin, TX. 
  • Most don’t get so lucky. 

GRADE BEAM DESIGN

• This home designer runs into more opposition on this aspect of a home foundation design than any other. 
  • “Well, I’ve been doing it this way for 10 years and never got a call-back." 
    • That’s typical of builders who like what they know. 
    • As well, that’s not a proof statement.
      • When’s the last time anybody heard of a call-back of the general contractor in regard to foundation problems, notably, cracked-up slabs-on-grade? 
      • In 40 years or so, this home designer recalls 2 – 1 leaker, 1 horrid substrate prep - neither remediated.
  • "We don't do it that way around here."

• What’s more, Before The Architect’s approach to grade beaming a garage slab-on-grade never, never gets second-guessed. 
• And we extend that approach to other slab-on-grade elements. 
  • 30, 40, 50, 60 linear feet and more of uninterrupted, reinforced concrete 4-5 linear inches thick supported by earth can crack up and deflect in an imperfect world. 
  • How much?  Ask an engineer.   

• In a well-finished space over a slab-on-grade of sizeable extent, you’ll care when the paneling distorts, doors catch. 
• You'll care after looking across a sea of pricey, terra cotta floor tile or Spanish-red pavers over a slab-on-grade when the isolation membrane has said “No mas," and cracks with high-low sides begin their differential journeys. 

• In fact, grade beams should, ultimately, be specified by an engineer.
• Before The Architect’s approach to grade beam application is one among others, including but not limited to the 2-way flat slab in Basic Concrete Engineering For Builders, Max Schwartz, orig. 1922; Craftsman Book Company, 2000, pp.147-150. 
• Whatever the engineering latitude, these grade beam design variables need address -
  • Beam spacing
  • Beam width
  • Beam depth
  • Beam sizing
  • Reinforcement
  • Pedestal or pier sizing
  • Spread footing sizing
  • Footing depth
  • Substrate preparation
  • And never bond a grade beam to the concrete slab-on-grade that it's supporting

Comment:  Beware all ye who pass this way – this is not about post-tensioned slabs slab-on-, with names, such as, mat slab, stiffened mat, stiffened slab, raft foundation, and, doubtlessly, others.  

• When obliged to withdraw reference to grade beams supporting slabs-on-grade [note: this is different from grade beams supporting bearing walls, which support is commonly and widely accepted], this designer writes on the foundation plan, “Grade beam design done by others with engineering latitude." 

• A grade beam 
  • Shall not be to a dead end except that it terminates in a spread footing and pier
    • Which footing shall not be less than 24 linear inches x 24 linear inches x 12 linear inches
    • Which footing shall be reinforced by not less than 3-#5 rebar continuous on long axis and evenly spaced with 1-3/8 linear inch cover from top of face (3/4 linear inch + an assumed 5/8 linear inch aggregate) and similarly with not less than 1-3/8 linear inch cover from beam bottom of face (3/4 linear inch + an assumed 5/8 linear inch aggregate)
    • Which footing bottom of face shall be on not higher than the bottoms of face of proximate footings
    • Which pier shall not be less in its four sides than the grade beam width
    • Which pier shall be reinforced with not less than 2-#5 bent rebar on the vertical from the footing, or pier, bottom of face with 1-3/8 linear inch cover up to the grade beam top of face (3/4 linear inch + an assumed 5/8 linear inch aggregate) and similarly with not less than 1-3/8 linear inch cover to footing bottom of face (3/4 linear inch + an assumed 5/8 linear inch aggregate)

Grade Beam with Footing Below Slab-On-Grade, Section in Elevation   

Comment:  In the illustration Grade Beam with Footing Below Slab-On-Grade, Section in Elevation, notice that the grade beam is drawn to support either a 4 linear inch or 6 linear inch slab on grade, that a bond break is required between beam and slab, and that the “FGL", or finish grade level, is implied because the owners were, at plan set submission time, still uncertain as to certain site grade levels, particularly at passages between interior and exterior.  Note also that tamping of earth includes earth below the footing. 

• Shall tamp all disturbed and amended substrates 
  • The greater compaction of
    • Not less than 50 beats per square foot and
    • 95% density, modified proctor in conformance with not less than ASTM D-1557 (a/k/a American Society for Testing and Materials, “Substrate Tests for Moisture-Density Relations of Soils and Soil Aggregate Mixtures Using 10-Pound Rammer and 18-inch Drop")
    • In layers, or lifts, not greater than 6 linear inches
  • Which pier with footing shall not be greater than 12 linear feet on center
  • Which pier with foot shall be directly below each point, or concentrated, load, or concentrated load bearing on a grade beam
  • Which pier shall not be connected to the supported slab-on- grade in any manner or reinforcement or any other material
  • Which pier top of face shall have a continuous bond break applied between the top of face and the supported slab-on-grade bottom of face
• May terminate at a continuous concrete stem wall (a/k/a t-wall, a/k/a strip footing, a/k/a strip foundation)
  • Which termination shall be by not less than the continuation of lengthwise reinforcement into the stem wall as dowels or
    • Which termination shall be by bending and lapping the grade beam and stem wall rebar as specified elsewhere in this document
• Shall be applied 
  • As support to a load-bearing partition interior to a perimeter foundation (though not necessarily in lieu of a t-wall)
  • As support to a load bearing partition exterior to a perimeter foundation (though not necessarily in lieu of a t-wall)
  • As support to a 6 linear inch thick slab-on-grade floor
    • At not greater than 24 linear feet span except
    • At not greater than 20 linear feet side-to-side in a garage bayAs support to a 4 linear inch thick slab-on-grade floor with not less than 16 linear feet span
• Centered
• In interior and exterior applications
  • Shall not be less than 16 linear inches on a side wide and 12 linear inches high
  • Shall be reinforced with continuous and not less than #5 rebars
    • If not less than 18 wide
      • 2 lengthwise in the top if less than 16 linear inches wide or
      • 3 lengthwise in the top if not less than 16 linear inches wide and
      • 2 lengthwise in the bottom if less than 18 linear inches wide or
      • 3 lengthwise in the bottom, and otherwise
    • Set at 12 linear inches on center both ways, i.e., crossed with not less Than #5 rebar at 12 linear inches on center
    • The 2 layers shall have 2 linear inches cover each
• Shall be supported at joints to stem walls 
  • With a masonry pilaster of concrete, block, or brick
  • In width equal to the grade beam width
  • In depth continuously from grade beam bottom or face to footing top of face
  • In thickness equal to the footing reveal 

PILASTERS

• A pilaster
  • Shall function to support an exterior slab on grade at its joint to a foundation 
    • 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
  • Shall function to support each grade beam butt to foundation
  • Shall be not less
    • In height than from a footing top of face to the bottom of face of the supported element
    • In width not less than 8 linear inches, not less than the width of a supported grade beam 
    • In depth not less than the depth of the footing reveal on which the pilaster rests
  • May be of 
    • Brick
    • Block or
    • Placement
  • If of brick or to brick, then
    • Secured to a foundation wall
    • With wall ties at not greater than 6 linear inches on center on the vertical and
    • With wall ties not greater than 12 linear inches on center on the horizontal

1. Support the slab-on-grade either side of contraction, or control, joints (long, dashed lines), unbonded.
2. Support the slab-on-grade at either side of a cornerpoint, unbonded.

 

• Center on the vertical and not greater than 12 linear inches on center on the horizontal
• If of block or to block
  • Then secured to a foundation wall with deformed rebar of diameter as applied in proximity, as dowels,
    • At not greater than 6 linear inches on center on the vertical and
    • Not greater than 12 linear inches on center on the horizontal
    • Or equivalent
    • Or with wire mesh as in block wall material on not greater than 8 linear inches centers on the vertical
• If by placement to placement either cold joint or monolithic
  • Then secured to a foundation wall with deformed rebar of diameter as applied in proximity, as dowels,
  • At not greater than 6 linear inches on center on the vertical and
  • Not greater than 12 linear inches on center on the horizontal
  • Shall not be secured by any means (except as by compression) to a grade beam and
• Shall be separated by a continuous bond break between the pilaster top of face and the grade beam bottom of face

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