Sloped vs Stepped Footings
First published in Concrete International Magazine, March 2009
Generally, it's most economical to place wall footings
at a constant elevation. If the site or finished grade
slopes along the length of the wall, however, the footing
may end up a considerable distance below finished
grade. This is clearly not economical, as it requires extra
excavation and material. Two other options are therefore
preferred (Fig. 1):
- Slope the footing with the site so its depth below the finished grade is nearly constant along its length; or
- Step the footing so its depth below finished grade is not excessive at any point along its length.
SLOPED FOOTING ISSUES
The sloped footing option may seem appealing
because of the simple geometry and apparent ease in
formwork construction. It does, however, create the
following construction issues (Fig. 2):
- Vertical wall bars above the footing will have different
lengths, creating major challenges in the fabrication
plant and on the job site. Two of these—managing the
inventory and placing the bars in their correct locations—
can be eased by detailing the bars with variable lap
splice lengths. This will, however, increase the quantity
of vertical reinforcement;
- Horizontal reinforcing bars in the lower portion of the
wall will also have different lengths because they are
interrupted by the sloped footing. If constant length
horizontal bars are used at the wall base, they can be
fanned out, but this will create a variable vertical
spacing of the reinforcing bars;
- Sloped footings will require trapezoidal formwork.
This will require modifications to standard rectangular
formwork;
- A sloped footing could be unstable, particularly on a
very steep slope; and
- Concrete placement and finishing could be difficult,
and a stiff concrete mixture might be required to
prevent the concrete from flowing downhill, which may
lead to segregation. Alternatively, the top of the form
may have to be closed.
Because of these challenges, most engineers and
contractors prefer to use stepped footings instead of
sloped footings.
CONSIDERATIONS FOR STEPPED FOOTINGS
As with any aspect of a design, cost should be
considered before a system is selected. If the slope of the
finished grade is less than 2 ft (0.6 m) for a 20 to 30 ft
(6 to 9 m) long wall, a lower but constant bottom bearing
elevation may be more economical than a stepped
footing. For a very long wall, however, even a 1 ft (0.3 m)
variation in the site elevation may make a stepped footing
more economical. Communication with the contractor
during the design phase regarding the number and length
of steps can be very helpful.
It's generally more cost effective to minimize the
number of steps. For example, it may be more economical
to design for a 6 ft (1.8 m) change in elevation using three
2 ft (0.6 m) steps or two 3 ft (0.9 m) steps rather than six
1 ft (0.3 m) steps. This minimizes the number of wall
sections to be detailed and formed. Before deciding on the
footing step locations, however, consider the horizontal
distance between them. Distances should preferably be
multiples of available or standard form lengths.
Before completing a design, it's a good idea to
communicate with area formwork contractors. The
horizontal runs should be dimensioned in 2 or 4 ft (0.6 or
1.2 m) increments to conform to standard plywood or
form system dimensions. Unless the site slopes drastically,
try to keep a minimum horizontal run of 10 ft (3 m) for
each step, if possible.
Keep the detailing simple. Avoid using Z-shaped bars
(Fig. 3). Their geometry may make it necessary to slant
the riser out of plane to meet cover requirements for
the treads.
It's also prudent to evaluate other footing options. For
example, the individual spread footings or piers supporting
grade beams shown in Fig. 4 may be more economical
than a continuous spread footing option. Because the wall
can span between footings or piers, similar configurations
can be constructed without the grade beam.
Situations can vary along the wall length, so it's
prudent to show specific details rather than generic
details. This will expedite placing drawing preparation
and perhaps minimize requests for information (RFIs).
CLOSURE
The use of sloped or stepped
footings depends on site conditions,
finished grade elevations, finished
wall slope, and various reinforcing
bar placement and construction
issues. Regardless of the footing
system selected, the engineer is
required to follow the design
requirements of Section 15.9 in ACI
318-08.[1] Section 15.9.1 requires that
the angle of slope or depth and
location of steps be such that the
design requirements are satisfied at
every section. Additionally, Section
15.9.2 requires footings designed as
a unit to be constructed to ensure
they act as a unit.
Acknowledgments
Thanks to Joint ACI-CRSI Committee 315
member Javed Malik, Jacobs Carter Burgess
Engineering, Houston, TX, for providing the
information in this article.
References
1. ACI Committee 318, "Building Code
Requirements for Structural Concrete
(ACI 318-08) and Commentary," American
Concrete Institute, Farmington Hills, MI,
2008, 465 pp.
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