Grade Beam

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BEFORE THE ARCHITECTCUSTOM HOME DESIGNING BACKGROUND – HOUSE DESIGN ARTICLES

ALL ABOUT HOUSE FOUNDATION PROBLEMS and YOUR HOME DESIGN FOUNDATION PLAN, CONCRETE GRADE BEAM FOUNDATION

By Before The Architect  Copyright 2009

YOU MAY FREELY QUOTE THE AG WITH PROPER ATTRIBUTION

Unique Home Foundation Detail Structure – Concrete Grade Beam Foundation Design

Redundancy is Before The Architect’s friend.  Before The Architect 

INTRODUCTION

This article is about concrete grade beam foundation design detail structure.  

FEELING LUCKY, EH?

bulletWhat 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?
bulletThe 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.
bulletYou 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. 
bulletMost don’t get so lucky. 

CONCRETE FOUNDATION GRADE BEAM DETAIL DESIGN

IT’S A DANGEROUS WORLD OUT THERE

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

EXTENDING THE ALREADY BEEN THERE, DONE THAT

bulletWhat’s more, Before The Architect’s approach to grade beaming a garage slab-on-grade never, never gets second-guessed. 
bulletAnd we extend that approach to other slab-on-grade elements. 
bullet30, 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. 
bulletHow much?  Ask an engineer.   

WHO CARES?

bulletIn a well-finished space over a slab-on-grade of sizeable extent, you’ll care when the paneling distorts, doors catch. 
bulletYou'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. 

BRING ON THE ENGINEERS

bulletIn fact, grade beams should, ultimately, be specified by an engineer.
bulletBefore 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. 
bulletWhatever the engineering latitude, these grade beam design variables need address -
bullet Beam spacing
bullet Beam width
bullet Beam depth
bullet Beam sizing
bullet Reinforcement
bullet Pedestal or pier sizing
bullet Spread footing sizing
bullet Footing depth
bullet Substrate preparation
bulletAnd 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.  

bulletWhen 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." 

THE SPECIFICS - DETAIL FOUNDATION STRUCTURE

bulletA concrete grade beam 
bulletShall not be to a dead end except that it terminates in a spread footing and pier
bulletWhich footing shall not be less than 24 linear inches x 24 linear inches x 12 linear inches
bulletWhich 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)
bulletWhich footing bottom of face shall be on not higher than the bottoms of face of proximate footings
bulletWhich pier shall not be less in its four sides than the grade beam width
bulletWhich 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 plans set submission time, still uncertain as to certain site grade levels, particularly at passages between interiors and exteriors.  Note also that tamping of earth includes earth below the footing. 

bulletShall tamp all disturbed and amended substrates 
bulletThe greater compaction of
bulletNot less than 50 beats per square foot and
bullet95% 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")
bulletIn layers, or lifts, not greater than 6 linear inches
bulletWhich pier with footing shall not be greater than 12 linear feet on center
bulletWhich pier with foot shall be directly below each point, or concentrated, load, or concentrated load bearing on a grade beam
bulletWhich pier shall not be connected to the supported slab-on- grade in any manner or reinforcement or any other material
bulletWhich 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
bulletMay terminate at a continuous concrete stem wall (a/k/a t-wall, a/k/a strip footing, a/k/a strip foundation)
bulletWhich termination shall be by not less than the continuation of lengthwise reinforcement into the stem wall as dowels or
bulletWhich termination shall be by bending and lapping the grade beam and stem wall rebar as specified elsewhere in this document
bulletShall be applied 
bulletAs support to a load-bearing partition at interiors to a perimeter foundation (though not necessarily in lieu of a t-wall)
bulletAs support to a load bearing partition exterior to a perimeter foundation (though not necessarily in lieu of a t-wall)
bulletAs support to a 6 linear inch thick slab-on-grade floor
bulletAt not greater than 24 linear feet span except
bulletAt not greater than 20 linear feet side-to-side in a garage bay, as support to a 4 linear inch thick slab-on-grade floor with not less than 16 linear feet span
bulletCentered
bulletIn applications - interiors and exteriors
bulletShall not be less than 16 linear inches on a side wide and 12 linear inches high
bulletShall be reinforced with continuous and not less than #5 rebar
bulletIf not less than 18 wide
bullet2 lengthwise in the top if less than 16 linear inches wide or
bullet3 lengthwise in the top if not less than 16 linear inches wide and
bullet2 lengthwise in the bottom if less than 18 linear inches wide or
bullet3 lengthwise in the bottom, and otherwise
bulletSet at 12 linear inches on center both ways, i.e., crossed with not less Than #5 rebar at 12 linear inches on center
bulletThe 2 layers shall have 2 linear inches cover each
bulletShall be supported at joints to stem walls 
bulletWith a masonry pilaster of concrete, block, or brick
bulletIn width equal to the grade beam width
bulletIn depth continuously from grade beam bottom or face to footing top of face
bulletIn thickness equal to the footing reveal 

Comment:  The AG recalls when #3 rebar were the rage in residential concrete reinforcement.  Now, it’s #5 rebar. 

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