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?
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.
CONCRETE FOUNDATION GRADE BEAM
DETAIL DESIGN
IT’S A DANGEROUS
WORLD OUT THERE
This home designer
runs into more opposition on this aspect of a home foundation design detail 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."
EXTENDING THE
ALREADY BEEN THERE, DONE THAT
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.
WHO CARES?
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.
BRING ON THE
ENGINEERS
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."
THE SPECIFICS -
DETAIL FOUNDATION STRUCTURE
A concrete 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 bay, as
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 rebar
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
Comment: The AG recalls
when #3 rebar were the rage in residential concrete reinforcement. Now, it’s #5
rebar.
(If this is your first visit to
Before The
Architect, please consider spending a few moments looking over the
Site
Map, in order to get a feel for the website design.
Before The Architect E-mail:
jrp2h2000@yahoo.com.)
