Mod'd Grade Beam

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BEFORE THE ARCHITECT

HOUSE DESIGN ARTICLES

ALL ABOUT HOUSE FOUNDATION PROBLEMS and YOUR HOME DESIGN FOUNDATION PLANS MODIFIED, CONCRETE GRADE BEAM FOUNDATION DETAIL STRUCTURE

By Before The Architect  Copyright 2009

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QUESTION: WHASSUP WITH HOUSE  FOUNDATION PROBLEMS?

ANSWER:  PRETTY NEAR NOTHING WITH A MODIFIED, CONCRETE GRADE BEAM FOUNDATION DESIGN DETAIL.

MODIFIED, CONCRETE GRADE BEAM DETAIL STRUCTURE IN A HOME FOUNDATION PLAN

Introduction

·         BTA has added to its custom home foundation plans portfolio with a modified, concrete grade beam

o       Same deal:  it's part of a BTA home foundation plan

o       More suitable than proliferating ‘regular’ grade beams 

·         A modified, concrete grade beam foundation involves (subject to modification by qualified engineering) –

o       Continuous 12 linear inch square concrete strip of

o       3500psi compressive strength at 28 days

o       Reinforced with 3-Grade 50-#5 deformed rebar evenly distributed

o       Top and bottom and

o       Similarly reinforced crosswise at not greater than 16 linear inch centers (as short lengths of support on the horizontal, top and bottom) continuously run

o       At cover to all bars not greater than 3 linear inches and not less than maximum diameter of the mix aggregate plus ¾ linear inch

o       Set centered below contraction, or control, joints with

o       A continuous bond break between slab-on-grade bottom of face and modified grade beam top of face 

Comment:  Applications open to atmosphere – covered and not covered – shall involve only epoxy-coated rebar.  

·         Further, a modified, concrete grade beam foundation shall, itself be –

o       Supported at termini by masonry pilasters

§         The width of the grade beam and

§         The depth from stem wall strip footing or grade beam spread footing top of face to modified grade beam bottom of face and

o       Along a modified grade beam's length at not greater than 12 linear foot clear spans, a spread footing 20 linear inches square by 10 linear inches deep with 4-grade 50-#5 deformed rebar with cover as above

§         Evenly distributed

§         In a square pattern

§         Top and bottom

§         With not less than 2 similar rebar bent to lap the spread footing rebar and the modified grade beam rebar, the latter on centerline    

Comment:  Again, applications open to atmosphere – covered and not covered – shall involve only epoxy-coated rebar.  

Siting Modified Concrete Grade Beam Foundation

·         Therefore, it's the rules of arranging the sites of contraction, or control, joints that direct siting the modified grade beams, whereby control joints  – 

o       Shall involve linear places in concrete placements

o       Shall include in surface area not greater than 275 square feet

o       Shall be in width of slab-on-grade section in elevation not greater than 1.5-times length

o       Shall be formed

§         As nearly square as possible (though not at all necessarily same-sized or asymmetrical in layout)

§         At 90-degrees intersection ideally

§         But not less than 60-degrees if at all possible

§         To intersect at floor drainage and other penetrations of a slab-on-grade

o       Shall be in a perpendicular, horizontal dimension of linear feet not greater than 3-times slab-on-grade depth in linear inches

o       Shall be in configuration straight and true and continuous

o       Shall be in depth of joint

o       Not less than 1/3 of the slab-on-grade's depth but

o       In any case not less than 1 linear inch in depth

o       Shall be in width not greater than 1 linear inch

o       Shall be saw cut 

Comment:  In regard to saw-cut specifically and slab-on-grade care-giving generally, please consider "CRACKS AND DETERIORATED JOINTS IN YOUR CONCRETE FLOOR" by Metzger/McGuire, Concord, NH, http://www.metzgermcguire.com/cracksdet.htm and "How To Avoid Common Floor Problems: 20 Step You Can Take to Avoid Common Floor Deficiencies," by Metzger/McGuire, Concord, NH, http://www.werc.org/Files/DMX/SETemp/139.pdf 

·         Modified grade beams do not take critical path loads.  ‘Regular' grade beams, spread and strip footings and t-walls take critical path loads 

Modified, Concrete Grade Beam Foundation Express Structure and Function

·         Structure arises in support of a slab-on-grade on both sides of its weakest lines – the contraction or control joints, by allowing slab movement linearly (permitted by sleeved or smooth rebar properly applied across control joints and, hopefully, not impaired by welded-wire mesh or fabric – this custom home designer is not fond of such mesh and fabric amendments)

·         Function arises in that the foundation residential contractor is alerted to the planned importance of the control joints and their siting . . . like a wake-up call, and delivers substantial redundancy to offset all sorts of mistakes, misunderstandings, and misdeeds in forming a slab-on-grade substrate and the slab itself

Modified, Concrete Grade Beam in a Home Foundation Plan

·         Here are all the concrete elements indicated and then some –  

Modified Grade Beam et al. in Home Foundation Plans Segment, Plan View

Key: EL=ELEVATION; TYP=TYPICAL; Rn=RADON PIPE

·         This pic is of a segment of a home foundation plans covering the upper Left Of House Garage at L1 (main level) [yes, there's another garage area at L0]

·         Whassup?  Please note that –

o       Not only does the modified grade beam support the meeting of slabs-on-grade at a control joint, but also the ‘regular' grade beam does, too

o       Dimension statements for grade beams run from footing sides of face to grade beam centerlines, while dimension statements for control, or control, joints run from t-wall sides of face to joint centerlines

§         That's because in setting up formwork for grade beams, strip footings go in first

§         Whereas, control joints are applied once the footings are covered up with slab-on-grade placement and all that's then observable of foundation elements are the t-walls and their sides (and tops) of face

Modified Grade Beam (left) and Strip Footing and T-Wall, Section in Elevation, Scaled

  

. . . where, B=T-wall, C=Strip Footing, D=Rebar, I=Pilaster,  K=Moisture and Gas Retardant Barrier, M=Modified Grade Beam 

Note well:  In all instances and rebar cover shall be 3/4 linear inch plus maximum aggregate diameter and not more than 3 linear inches cover

Comment:  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 continuous bond break.

. . . . . . .

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