Shear keys for basement walls:
pros and cons
The familiar key and keyway at the wall-footing joint
present some construction problems
ngineers and architects frequently show a shear key at the
joint between a footing and a basement wall. Usually the detail is like
that in Figure 1, with the keyway or
g ro ove formed in the footing and
the shear key cast as part of the wall
extending into the slot in the footing. Many standard references include these keys—undimensioned—in their drawings of typical
structures.
Reinforcing bars usually resist
bending. Howe ve r, reinforcement in
excess of that required for bending
can also resist shear, and thus bars
can be an alternate to the shear key.
The yield strength of the reinforcing
bar governs the load capacity of this
connection. Since the yield strength
of the bar is clearly established, the
capacity of the connection may be
easily calculated. It is necessary
however to anchor the bar into both
the footing and the wall to use its
full strength. Vertical re i n f o rc i n g
bars, whether used for shear or
bending should be accurately
placed and firmly supported before
concrete is placed in the wall forms.
Purpose of the shear key
How to form the keyway
Shear keys are used to resist lateral forces such as earth and water
p re s s u re s, earthquakes, and wind.
In engineered construction the architect or engineer will design a
connection of sufficient strength to
resist this horizontal shear and assure wall stability. In residential
work where lateral forces are often
less critical, a standard detail may
be followed.
Contractors have always considered keyways to be a nuisance. Although plastic keyway formers are
now available, a wood 2x4 or 2x6 is
generally used to form a shear key.
The sides of the wooden member
are beveled to make form removal
easier. Traditionally this keyway former has been supported from the
side footing forms as shown in Figure 2. Frequently the beveled 2x4 is
splintered during form removal.
An easier method consists of the
following steps:
• Place and level the footing concrete to the desired elevation.
BY BRUCE A. SUPRENANT
ASSOCIATE PROFESSOR DEPARTMENT OF
CIVIL ENGINEERING AND MECHANICS
UNIVERSITY OF SOUTH FLORIDA
TAMPA, FLORIDA
E
Alternate ways to resist shear
Although a shear key cast as part
of the wall is the most frequently
specified connection, there are other ways to resist shear. Vertical reinforcing bars or a combination of
shear key and bars are also satisfactory. An upturned shear key, cast as
part of the footing, is sometimes
used but usually not in residential
work.
Figure 1. Shear connections between a
basement wall and its footing may be
made by: (A) a concrete shear key at
the base of the wall cast in a formed
keyway in the footing; (B) a shear key
plus reinforcing bar; or (C) the
reinforcing bar without a shear key.
• Where the keyway is to be formed,
place an 8-foot-long beveled 2x4
on the concrete surface.
• Use a 2-pound hammer to drive
the 2x4 into the fresh concrete,
pushing up concrete from under
the wood.
• When the 2x4 is flush with the top
of the footing forms, screed off
excess concrete from the top surface.
• As the concrete begins to stiffen
remove the wood to reveal a
formed keyway.
This simplified method won’t
work where reinforcing bars are
used with the shear key. If there are
vertical bars in the middle of the
shear keyway, as is often the case,
holes must be drilled in the beveled
2x4 at the exact spacing of the bars.
The 2x4 is then supported from the
footing side forms and re i n f o rc ement is positioned in the drilled
holes before concrete is placed.
To avoid the expense of drilling
holes, you may be able to shift the
position of the shear key if the engineer or architect approves. You
might also request approval to split
the wood piece and form a half key
on each side of the vertical bars. But
if coarse aggregate particles are too
large to fit into each half-key opening when the wall concrete is
placed, this solution would not be
acceptable.
How effective is the shear key?
Some engineers question the actual resistance of the shear key. Theoretically, its strength is calculated
by multiplying the cross-sectional
Figure 2. The keyway may be formed by a beveled wood strip supported from side
forms for the footing. A simpler way of forming the footing is described in the text.
failure area by the shear strength of
the concrete. Forming a shear key
with a 2x4 should provide a 31⁄2-inchwide failure surface, but estimating
the strength of concrete in the key is
a problem.
Have you ever watched as ready
mixed concrete leaves the truck?
What is the quality of the first concrete from the truck, and where is
that concrete placed? The answers
to these questions indicate why
some engineers avoid shear keys.
Because of inadequate mixing or
improper batching and delivery,
concrete from the first portion of a
truck load may be too wet or too
rocky. When deposited directly into
the shear key, this lower quality concrete will reduce the capacity of the
connection. Also if consolidation is
poor, the key may sometimes lack
strength because it is not entirely
filled.
COST COMPARISON: SHEAR RESISTANCE PROVIDED BY
SHEAR KEYS VERSUS #4 REINFORCING BARS
Wall height
Factored shear
force to be
resisted, pounds
per foot
Shear resistance
provided, pounds
per foot
3900
4 feet
8 feet
Method
Concrete key
Cost,
dollars
per
foot
0.50
544
1
544
#4 @ 7 ⁄2 feet
0.10
3900
Concrete key
0.50
2176
#4 @ 221⁄2 inches
0.39
2176
NOTES: Shear key assumed to be built by simplified method described in article.
Cost data provided by Heritage Development and Construction Company,
Kankakee, Illinois.
Comparing the costs
Finally, cost is an important aspect of shear resistance. The table
shows the shear force to be resisted,
the shear resistance provided, and
the cost to provide it in some typical residential walls. The needed
shear resistance is a shear force calculated assuming an equivalent fluid pressure of 40 pounds per cubic
foot and applying a load factor of 1.7
as required by current concrete design pro c e d u re s. For a 4-foot wall,
the resistance needed is 544 pounds
per lineal foot. For an 8-foot wall,
2176 pounds per lineal foot is needed.
A concrete key formed by a 2x4
provides 3900 pounds of resistance
per lineal foot. A single vertical #4
bar acting in shear can provide 4080
pounds of resistance. The concrete
shear key offers more than enough
resistance, but being continuous, it
cannot be adjusted to match more
closely the needed strength. The reinforcing bars, on the other hand,
can be spaced to match closely the
needed resistances. This spacing,
shown in the table, gives the ve rt ical bars a cost advantage in residential construction.
The final choice
Engineers and architects are
questioning the strength capacity of
a typical shear key. And contractors
consider the key a construction nuisance. While a shear key is frequently shown on standard basement wall
details, the cost for this practice
should be investigated. Vertical bars
Figure 3. Where vertical bars are required in the center of the keyway, the beveled
wood strip may be drilled with holes at the exact spacing of the bars (A). Less
expensive methods may involve (B) shifting the keyway location if the engineer or
architect permits, or (C) splitting the keyway former.
used to resist both bending and
shear forces are easy to place and
can be spaced exactly to match the
loading re q u i re m e n t s. They also
avoid the problem of estimating the
strength of poor quality concrete
which may be placed into a shear
key.
PUBLICATION #C870620
Copyright © 1987, The Aberdeen Group
All rights reserved