Need a book? Engineering books recommendations...

Return to index: [Subject] [Thread] [Date] [Author]

RE: Frost footings - whats the big deal?

[Subject Prev][Subject Next][Thread Prev][Thread Next]
For some reason my first response did not go through.  I will send it again. 
 
Frost heave is the concern when it comes to the discussion of "frost depth" and the swell potential that soils have when they are frozen.  Consider that rock (see NFS in the dialogue below)  has no swell potential therefore there should be no requirement for frost depth when a foundation is founded on rock.  Secondly, there is heat that can be added to preclude subgrade from freezing.  Bases for liquefied natural gas (at cryogenic temperatures) can not be adequately protected with insulation alone.  Subsequently the practice in the LNG industry is to provide heating elements in the soil beneath LNG tanks.
 
I have had discussions with the US Army Corps of Engineers and the Cold Regions Research and Engineering Laboratory (CRREL) about this issue.  There was a very good exchange of e-mails that culminated in the following e-mail. I hope that it offers some help. 

Regards,
Harold Sprague

______________________________

Frost penetration depths in Table C-1 and D-1 in UFC 3-310-01, "Structural Load Data" dated 25 May 2005 represent the depth of frost penetration to be expected if the ground is bare of vegetation and snow cover, the soil is non-frost susceptible (NFS), well-drained (i.e., dry) sand or gravel, and no building heat is available. Thus, these values represent the deepest (i.e., worst case) frost penetration expected in each area. Most building foundations can be at a shallower depth without suffering frost action.

 

CRREL did not have direct input to the 25 May 2006 version of UFC 3-310-01. However, our personnel (now retired) provided the original data, analysis, and guidance contained in earlier versions of the UFC.  An earlier version of UFC 3-310-01 dated 30 June 2000 referenced and included the source document, "Load Assumptions for Buildings" as TI 809-01 dated 3 August 1998. That document included information describing how to determine the design frost depth. Unfortunately when the latest version of UFC 3-310-01 was issued the text and figure below explaining how the table values should be used and adjusted was not included.

 

6. FROST PENETRATION. The values shown in tables 1 and 2 will be used to establish minimum design depth of building foundations below finished grade. The depth to which frost penetrates at a site depends on the climate, the type of soil, the moisture in the soil and the surface cover (e.g., pavement kept clear of snow vs. snow covered turf). If the supporting soil is warmed by heat from a building, frost penetration is reduced considerably. The values in tables 1 and 2 represent the depth of frost penetration to be expected if the ground is bare of vegetation and snow cover, the soil is non-frost susceptible (NFS), well-drained (i.e., dry) sand or gravel, and no building heat is available. Thus, these values represent the deepest (i.e., worst case) frost penetration expected in each area. Most building foundations can be at a shallower depth without suffering frost action. (However, other considerations besides frost penetration may affect foundation depth, such as erosion potential or moisture desiccation). For interior footings, which under service conditions are not normally susceptible to frost, the potential effects of frost heave during construction should be considered. Design values for heated and unheated buildings may be obtained by reducing the values in tables 1 and 2 according to figure 1. For buildings heated only infrequently, the

curve in figure 1 for unheated buildings should be used. The curves in figure 1 were established with an appreciation for the variability of soil and the understanding that some portions of the building may abut snow-covered turf while other portions abut paved areas kept clear of snow. Foundations should be placed at or below the depths calculated above. The foundation may be placed at a shallower depth than calculated above if protected from frost action by insulation on the cold side. For more information on foundation insulation, see “Building Foundation Design Handbook” by Oak Ridge National Laboratory.  Additional information on which more refined estimates of frost penetration can be made, based on site-specific climatic information, the type of ground cover and soil conditions is contained in TM 5-852-6.

 

7. CALCULATION OF FROST PENETRATION. In this example the minimum depth needed for footings of a hospital and an unheated vehicle storage building to be built in Bangor, Maine, is calculated to protect them from frost action. The tabulated frost penetration value for Bangor, Maine, is 98 inches (table 1). Using the “heated” curve in figure 1, footings for the hospital should be located 4 feet below the surface. Using the “unheated” curve, footings for the unheated garage should be located 6 feet below the surface.

 

Applying the adjustment for heated and unheated buildings to the 52-inch value for FT Riley, KS, results in 2.8 ft and 3.6 ft respectively for the design depth of the building foundation. The 2.8 ft value is much closer to the 36-inch depth mentioned below. Harold Sprague is correct in saying there are other methods to reduce frost depth by using insulation and non-frost susceptible soils adjacent to and under a foundation.

Hope this helps you to understand the values in Table C-1 and D-1 of UFC 3-310-01 "Structural Load Data" dated 25 May 2005. I have attached a copy of the previous UFC 3-310-01 version containing the source document and will report the missing text and figure problem as a criteria change request via ProjNet.

________________________

----- Original Message -----
From: Jim Wilson
Sent: Monday, November 05, 2007 8:35 AM
Subject: Frost footings - whats the big deal?

Frost footing issues are a real hot-button topic with local inspectors.  But in some cases I think they are going overboard in their interpretation and critical review.  Are there any legitimate references or theories on how or when frost depth requirements can perhaps be relaxed?
 
For example,
-Are frost footings appropriate when adding a small addition to an existing structure not on frost footings?  If a house has worked for 100 years with 18" deep footings, won't adding 48" deep footings next to them cause potential heave differentials?
-If an existing garage slab opening is not on frost footings because the rest of the foundation has 4' of dirt piled around it, and the slab has no cracks after 40 years, isn't it acceptable to leave it alone without underpinning this small area?
-If deck footings go down 24" instead of 48", is that acceptable, even if it is not ideal?
 
These are similar to situations I have seen where inspectors are drawing a line in the sand.  The costs they are subjecting homeowners to to make repairs seems to exceed the risk of some possibly slight foundation movement.
 
Local frost depth in northeast PA is 42" or 48".  Ground is often very sandy and rocky with positive drainage. 
 
Jim Wilson, PE
Stroudsburg, PA
 


Windows Live Hotmail and Microsoft Office Outlook – together at last. Get it now!