Shallow Frost Footing

[Keystone]

Existing dwelling constructed 2001 with finished basement and 7' backfill.

Placing a walkout door, 4' landing, poured concrete stairs & side walls to grade.

Excavation reveals existing house footing and proposed 4' landing on solid rock, concern of frost protection (36" depth required) for the excavated house footing and basement slab.

Any code compliant suggestions?

 

[Sifu]

Not sure of the question but if I understand what you are asking....IRC 403.1.4.1 allows the solid rock to be the frost protection. (2009).

 

[Msradell]

If it's in hard contact with the rock I've seen places where they actually doweled into the rock and tied into the rebar in the footing. One location I know this was done in upstate New York haven't had a problem in over 20 years.

 

[Keystone]

IMO, soil has a moisture content which creates expansion, solid rock would not expand.

A thermal barrier may be placed between the existing footing and new stair landing.

 

[Keystone]

Also found this archive, not specific to my question but follows inline

http://www.thebuildingcodeforum.com/forum/archive/index.php/t-7560.html

 

[north star]

= | = | =

Keystone,

Are you asking if placing a foundation on solid rock is allowed [ by the adopted code ],

or if it can be done at all, both or other ?

= | = | =

 

[Keystone]

north star - I have encountered a situation where a footing is on solid rock, the footing on solid rock will not be at frost depth. Is this code compliant?

 

[north star]

& ~ ~ &



From the `09 IRC, Section R403.1.4.1 - Frost protection: "Except where otherwise protected

from frost, foundation walls, piers and other permanent supports of buildings and structures

shall be protected from frost by one or more of the following methods:

1. Extended below the frost line specified in Table R301.2.(1);

2. Constructing in accordance with Section R403.3;

3. Constructing in accordance with ASCE 32;

4. Erected on solid rock [ <----- ].

Exceptions:

1. Protection of freestanding accessory structures with an area of 600 square feet (56 m2)

or less, of light-frame construction, with an eave height of 10 feet (3048 mm) or less shall

not be required.

2. Protection of freestanding accessory structures with an area of 400 square feet (37 m2)

or less, of other than light-frame construction, with an eave height of 10 feet (3048 mm)

or less shall not be required.

3. Decks not supported by a dwelling need not be provided with footings that extend below

the frost line........Footings shall not bear on frozen soil unless the frozen condition is

permanent."

I do not know your adopted code, but the `09 IRC Edition appears to allow a foundation to be

on solid rock.



& ~ ~ &

 

[mtlogcabin]

Excavate as much dirt as you can. Level the area under the landing, wall and steps with 3/4" washed gravel 12" thick if you can get that much and then place the concrete you should have no problems with frost heave.

The washed gravel will allow the moisture under the concrete to freeze and thaw without heaving the concrete above.

 

[Sifu]

I have seen this done many times. Some engineers have specified padding the bedrock with sand, some have specified cleaning it off entirely, some have specified dowels. Kind of all over the place from designers but done on a regular basis. Either way code does allow it.

 

[SCBO1]

I have seen 2-inch blue Dow used over solid rock for expansion not sure it was used for a thermal barrier, but it was designed by an engineer and stamped.

As far as gravel, 3/4-inch clean would have voids that would allow moisture to wick down.

Pc1

 

[Keystone]

Got it, thanks all...