Frost Protected Shallow Foundation Design Specifications

These foundation design specifications were developed under the auspices of a project funded by the Minnesota Housing Finance Agency (MHFA) for the three award winning homes designed for the "Tomorrow's Homes Today Design Competition".  While this financial support is gratefully acknowledged, the Principal Investigator assumes complete responsibility for the contents herein.

Prepared for:
    Minnesota Housing Finance Agency
    400 Sibley Street, Suite 300
    St. Paul, MN 55101
    Program Manager: Han Lee

By:
    College of Architecture and Landscape Architecture
    Building Research Group
    Principal Investigator: L. F. Goldberg

Revision Date:
    September 16, 1999

 


A.   DESIGN ZONES

For frost-protected shallow foundation design purposes, Minnesota is divided into 4 zones as shown in the map of figure 1 which is extracted from section 403 of the 1995 "Cabo One and Two Family Dwelling Code".  This map depicts the 100-year return air freezing index (AFI) contours for Minnesota ranging from 4250 FFDD (Fahrenheit freezing degree days) shown by the dashed line in the far northwestern corner of the state through 2500 FFDD which passes through the Twin Cities, isolates a panhandle along the Mississippi River in the south-eastern corner of the State before following the Iowa boundary westwards.  The 3000 FFDD contour begins at Duluth and traces a roughly south-west-westerly route through Brainerd and Alexandria while the 3500 FFDD contour runs mostly southwest from International Falls to Moorhead.

Figure 1  100-year return air freezing index contours for Minnesota
(extracted from CABO 1 & 2 family dwelling code, section 403)


The design parameters are given for each of the above four AFI contour values.  Since insulation thicknesses are given in increments of one inch (following standard manufacturer's sizing), the recommended interpolation procedure is as follows:

Location AFI Contour Design AFI Value
location <= 2500 2500
2500 < location <= 3000 3000
3000 < location <= 3500 3500
3500 < location 4250

If in doubt as to which side of a particular contour a given location falls, the higher AFI design value should be selected.

 


B.   HEATED BUILDING DESIGN

The design for a heated building is given in figures 2 and 3 and in table 1.  In this design, a heated building is defined as any structure in which the air spaces above the slab are maintained at a temperature of 55oF or greater throughout the year.

Table 1 Heateda frost protected shallow foundation design parameters

Air Freezing Index- AFI
(FFDD)

Common Vertical Insulationd Thickness
(in.)

Wing Insulation

Thermal Latencyb

Along Walls

Corner

Start Timec

Days

Thickness
(in.)

Width
(in.)

Length from Corner
(in.)

Thickness
(in.)

Width
(in.)

ae

b

c

f

e

d

2500

2

2

24

60

2

36

12/23 - 12h00

34.0

3000

2

3

24

60

3

42

12/23 - 10h00

43.0

3500

3

3

30

60

3

42

12/22 -15h00

36.4

4250

4

4

36

60

5

48

12/22 - 9h00

35.0

Notes

  1. Heated implies a design interior temperature of 55oF throughout the year.  In practice, this means that the interior temperature should not fall below 55oF at any time.

  2. Thermal latency is defined as the time between the start of a heating system failure and the penetration of the frost front (32oF isotherm) to the base of the structural foundation.  The thermal latency profile is defined as an exponential decrease in interior temperature from 55oF to 25oF over 24 hours followed by a steady-state of 25oF.

  3. The thermal latency begins 15 days (360 hours) before the time of maximum frost depth within the structural foundation perimeter.

  4. All insulation is extruded polystyrene (ASTM C578 Type X) with a derated design R-value of 4.5/in. (nominal value is R-5/in.).  This is a minimum insulation specification - higher density (with higher load bearing capacity) extruded polystyrene (such as ASTM Types IV through VII) must be substituted as necessary.

  5. Dimensions a, b and c refer to figure 2, d, e and f to figure 3.

 

Figure 2  Heated building mid-wall design cross section

Figure 2 shows the cross-sectional layout of the design at a mid-wall location.  As depicted, the design gives the minimum acceptable dimensions which can be exceeded at the designer's discretion.  In particular, the base of the structural foundation (including slab, sand and gravel drainage layers) should be no less that 22 in. below grade.  While medium sand and 3/8 in. gravel layers are required below the slab thickened edge and wing insulation, the balance of the fill is at the designer's discretion.  On a thermal basis, the design permits the use of sieved native soil and requires only minimal excavation beneath the interior portions of the slab, so minimizing costs.  This fill may be replaced with a structural fill (sand and/or gravel) at the designer's discretion.  The following non-discretionary design details should be noted:

  1. The drainage system including sand and gravel layers separated by a filter fabric which prevents the gravel drainage layer from becoming clogged.  In addition to maintaining the required design moisture content profile beneath the footings, the drainage components also function as the sub-slab depressurization radon mitigation system.

  2. The 2 in. by 4 in. buffer block insulation (insulation schedule no. I3) glued to the vertical insulation above the wing insulation mitigates thermal short-circuiting caused by settling of the soil and wing insulation over time.  Such settling can be exacerbated by improper foundation soil compaction.

  3. The 2 in. high density extruded polystyrene block beneath the sill plate (insulation schedule no. I2) necessary to prevent a thermal short circuit at the outside slab edge that can have a notable impact on the frost-protection system performance.  This block has subsidiary advantages of providing a gasket between the framing and foundation so minimizing infiltration and significantly reducing the likelihood of condensation on the slab around the building perimeter.  Note that, in figure 2, this block has a minimum specification of ASTM C578 Type VI which has a compressive strength of 40 psi.  If this is insufficient, type VII (60 psi) or type V (100 psi) may be substituted as necessary.

    As an alternative, the sill plate detail shown in figure 2.1 may be substituted.  While not quite as effective as the detail in figure 2, the negative impact on the frost protection performance is minimal and can be ignored.  It is important to note that the exterior insulation should extend at least 3 in. above the top of the sill plate assembly (that is, if two or more plates are used).

  4. The rigid wing insulation protection layer necessary to prevent structural damage to the wing insulation during gardening activities, for example.  In the design, this layer is specified as 1 in. of concrete, but any substitute with similar rigidity and resistance to degradation by water is satisfactory.

Figure 2.1  Alternate sill plate insulation detail of heated building, mid-wall 
design cross section

The wing insulation is sloped away from the thickened edge at a nominal slope of 1 in 12, however, this is not critical, and smaller (but not larger) slopes are satisfactory. The intention is to prevent pooling of surface water above the wing insulation.

The design parameters a, b and c are given in table 1 for the 4 AFI design values.  Also shown in table 1 is the thermal latency performance of the design. This value is an indication of the "safety margin" of the design and is given in terms of the amount of time between the failure of the heating plant and the penetration of the frost front beneath the structural foundation (which always occurs first at an external corner).  The thermal latency is computed assuming the failure occurs fifteen days prior to the maximum frost penetration beneath the structural foundation (although such penetration is always above the foundation base during normal operation), so it represents the theoretical worst case scenario.

The 4 in. diameter drain tile should empty into a centrally located and sealed sub-slab sump.  It is recommended that the sump be vented to the outside via a sealed stack with provision for insertion of a low power fan at a later date.  This system will allow sub-slab depressurization permitting any radon accumulation beneath the slab to be safely vented if necessary.

With the exception of the 4250 FFDD design parameters, as shown in figure 3, the only difference between the corner and mid-wall heated foundation parameters is that the corner wing width is larger than the mid-wall wing width.  The sole exception is for 4250 FFDD where the corner wing insulation thickness is 1 in. greater than the mid-wall thickness.  The design parameters in figure 3 also are given in table 1.

Figure 3  Heated building corner insulation layout


C.   UNHEATED BUILDING DESIGN

An unheated building is defined as a structure in which the air spaces above the slab are kept at ambient temperature but are sheltered from the wind.  The design is given in figure 4 and table 2 for the 4 AFI values.

Table 2  Unheateda frost protected shallow foundation design parameters

Air Freezing Index- AFI
(FFDD)

Gravel Thickness
(in.)

Sub-Slab Insulationc

Maximum Frost Penetrationb Relative to Foundation Bottom
(in.)

Thickness
(in.)

Wing Width
(in.)

hd

g

i

2500

6

4

48

+.31

3000

6

5

48

+.68

3500

6

5

60

.00

4250

6

7

72

+.32

Notes

  1. Unheated implies an interior temperature equal to the ambient temperature with still air convective slab heat transfer conditions throughout the year.

  2. Positive values indicate the maximum frost penetration is located above the bottom of the structural foundation.

  3. All insulation is extruded polystyrene (ASTM C578 Type X) with a derated design R-value of 4.5/in. (nominal value is R-5/in.).  This is a minimum insulation specification - higher density (with higher load bearing capacity) extruded polystyrene (such as ASTM Types IV through VII) must be substituted as necessary to meet slab loading conditions (as for garage slabs).

  4. Dimensions g, h and i refer to figure 4.

Figure 4  Unheated building design cross section

The design is quite simple and self-explanatory.  The mid-wall and corner insulation requirements are identical.  The only detail to note is the extruded polystyrene buffer block (insulation schedule I1) glued (or otherwise attached) to the spread footing.  In this case, the buffer block prevents thermal short-circuiting of the wing insulation in the event of a shear in the horizontal insulation at the thickened slab edge owing to ground settlement.  Also note the rigid protection layer above the wing insulation which, as above, is specified as being of 1 in. thick concrete, but, substitutes with similar structural and water degradation characteristics may be used.

Table 2 also reports the maximum frost penetration relative to the foundation base.  A completely optimized design yields a maximum penetration of zero, that is, the protection system is sufficient to just contain the frost front within the structural foundation (as in the 3500 FFDD case).  Any margin of safety can be added at the designer's discretion by increasing any or all of the design parameter values depending on what is most economical.

 

D.    HEATED/UNHEATED BUILDING INTERFACE DETAILS

The preferred standard interface is shown in figures 5.1 and 5.2.  In this case, the vertical insulation is kept intact all the way to the heated slab edge with the exposed insulation protected with a suitable covering.  Note that this design includes the case where the unheated and heated slabs are co-planar so that only the narrow horizontal strip of insulation in the gap between the slabs requires protection (figure 5.2).  In the event that the interface experiences significant structural loading (such as at a door opening) the protection plate should be of a structural material, preferably steel.

Figure 5.1  Standard heated/unheated building interface at stepped slab location

 



Figure 5.2  Standard heated/unheated building interface at coplanar slab doorway location

 


E.   FROST CONTOURS

The maximum frost penetrations for the heated building frost-protected shallow foundations are shown in figures 6 through 9 for the four design AFI values simulated.  Each figure gives a cut-away, 3-dimensional perspective view of the outside corner of the structural slab showing the frost line on each face of the slab and the intersection of these lines beneath the external corner with the deepest frost penetration occurring at the intersection.  It should be noted, that these contours define the maximum frost penetration under continuously heated conditions, so the frost depths are about 8 in. (or greater) above the base of the structural foundation in all cases.  This arises because of the design necessity for a 28 day or greater thermal latency.  During the latency period, the frost contours deepen until they reach the base of the gravel layer, at which point structural failure can occur.

Figure 6  Heated shallow foundation at AFI = 2500 FFDD showing cut-away corner view of maximum frost penetration beneath structural foundation
(7.52 in. above base on 1/7 -12h00)

 



Figure 7  Heated shallow foundation at AFI = 3000 FFDD showing cut-away corner view of maximum frost penetration beneath structural foundation
(8.37 in. above base on 1/7 -10h00)

 



Figure 8  Heated shallow foundation at AFI = 3000 FFDD showing cut-away corner view of maximum frost penetration beneath structural foundation
(8.97 in. above base on 1/6 -15h00)

 



Figure 9  Heated shallow foundation at AFI = 4250 FFDD showing cut-away corner view of maximum frost penetration beneath structural foundation
(11.54 in. above base on 1/6 - 9h00)


The maximum frost penetrations for the unheated building shallow foundations are shown in figures 10 through 13 for the four design AFI values.  The layout of the figures is the same as that for figures 6 through 9.  In this case, since there is no necessity for any thermal latency, the maximum frost penetration is seen to be within an inch of the base of the structural foundation in all cases.

Figure 10  Unheated shallow foundation at AFI = 2500 FFDD showing cut-away corner view of maximum frost penetration beneath structural foundation
(0.31 in. above base on 2/10 - 4h00)

 



Figure 11  Unheated shallow foundation at AFI = 3000 FFDD showing cut-away corner view of maximum frost penetration beneath structural foundation
(0.68 in. above base on 3/4 - 9h00)

 

Figure 12  Unheated shallow foundation at AFI = 3500 FFDD showing cut-away corner view of maximum frost penetration beneath structural foundation
(0.00 in. above base on 2/16 - 2h00)

 



Figure 13  Unheated shallow foundation at AFI = 4250 FFDD showing cut-away corner view of maximum frost penetration beneath structural foundation
(0.32 in. above base on 2/14 - 8h00)

 

E.   IMPLEMENTATION

In terms of the Minnesota amendments to the Uniform Building Code, section 1305.5400 (A.), frost-protected shallow foundations of the type described above may be built in Minnesota under the sentence reading "In the absence of a determination by an engineer competent in soil mechanics, the minimum allowable footing depth in feet due to freezing is five feet in Zone I and 3-1/2 feet in Zone II." .  Thus in order to build a frost-protected shallow foundation in Minnesota using these guidelines, the following procedure should be followed:

  1. Prepare a set of foundation drawings using these guidelines.

  2. Have the drawings reviewed and approved (stamped) by a structural engineer registered in the State of Minnesota.

  3. Submit the approved drawings to the local building code official.

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