1 September 1999
3-5. FOOTINGS AND FOUNDATIONS. Footings and foundations will be built on undisturbed
soil or compacted fill material.
a. Frost Protection. The minimum design depth of building foundations to protect
against frost penetration will be in accordance with Tables 1 and 2 of TI 809-01 "Load
Assumptions for Buildings". The minimum depth when not governed by frost protection
requirements will be 300 mm (12 inches). The DOD Weather Manual (a Tri-Service document)
provides additional information that can be used to determine frost depth penetration.
b. Design. Footings will be so designed that the allowable bearing capacity of the soil is
not exceeded and that differential settlement is minimized. The minimum width of footings will
be 300mm (12 inches). Footing design will be in accordance with TM 5-818-1 / AFM 88-3,
Chapter 7, or Navy Design Manual (DM) 7.02, "Foundations and Earth Structures." Footing
design will also meet the requirements of ACI 318, except loads and load combinations will be
per the requirements of Chapter 1.
c. Seismic Footing Ties. Where a building is assigned to Seismic Design Category D,
E, or F, as defined in FEMA 302, "NEHRP Recommended Provisions for the Seismic Design of
New Buildings and Other Structures," individual spread footings founded on soil will be
interconnected by ties in accordance with the provisions of FEMA 302 and TI 809-04, "Seismic
Design for Buildings."
d. Gable Bent Footing Ties. The gable bent type of moment frame requires a horizontal
reaction force at the foundation to resist horizontal spreading. Because often it is unreliable to
count on the soil surrounding the footings to provide this reaction, (i.e., excavation next to the
building may reduce lateral bearing resistance), footing ties are advisable. These ties, may
consist of reinforced concrete tension tie beams that are located below the slab-on-grade, or
for short span frames may be reinforcing steel which anchors the gable bent footing directly to
the slab-on-grade. When gable bent frames are anchored to the slab-on-grade it is imperative
that the location of the ties be coordinated with the slab-on-grade jointing to assure tie capacity
is not reduced or impaired by the joints. Reinforced concrete tension tie beams are required for
gable bent frames with spans of 15 meters (50 feet) or more.
e. Footings on Expansive Soils. Expansive soils change volume from changes in water
content leading to total and differential foundation movements. Seasonal wetting and drying
cycles have caused soil movements that lead to long-term deterioration of structures. Soils can
have large strengths and bearing capacity when relatively dry. Expansive soils consist of
plastic clays and clay shales that often contain colloidal clay materials. They include marls,
clayey siltstone, sandstone, and saprolites. Some of these soils, especially dry residual clayey
soil, may heave under low applied pressure but collapse under higher pressure. In certain
cases, clay soils may not exhibit swelling characteristics if undisturbed. This same soil, when
dried by manipulation and re-compacted at less than the initial moisture content, may exhibit
some swell. Other soils may collapse initially but heave later on. Estimates of the potential
heave of these soils are necessary for consideration of the foundation design. Information
regarding the design of foundations on expansive soils can be found in TM 5-818-1.
Additional information on expansive soils can be found in EM 1110-1-1905 and Navy Design
Manual (DM) 7.02.