Calculated Footing Width for Residential and Light Commercial Structures

Introduction

For residential structures that fall outside the limitations of the prescriptive code, and for most of all other structures, our foundation element features will need to be calculated. In this particular lesson we will limit our scope to structures with relatively low loads on soils - or we could say - relatively low-rise structures. This particular focus is convenient, and also appropriate, as you, the Architect, may well want to tackle such designs yourself. Larger structures, on the other hand, will be `over your head' and for which you will likely contract with a Structural Engineer. Footings for larger structures will be covered later. Many of the same design principles apply. And, whether you have your Structural Engineer do all, or just some, of your foundation designs - in any case you still want to be able to `speak the language'.

Note: this article formerly contained introductory material on loads and forces. That material has been moved (here).

Calculated Footing Width Example

Consider the following conditions:

Building: 40 ft wide

Roof: 2 x 12 rafter construction similar to example above with ridge beam down middle

Eaves: 18 in.

2 stories with both floors bearing on ext. foundation and int. slab footing via int. bearing wall down middle.

Frost depth 30 in.

Allowable soil bearing pressure 1500 psf

Roof, walls, and floors: light frame wood construction

Wall heights (floor to ceiling) 9 ft

Loads:

40 psf SL

40 psf LL

Assumed dead loads ...

12 psf for roofs (on plan area)

10 psf for floors

10 psf for ext walls

Let's calculate the load on the exterior foundation wall bearing soil, and determine a minimum footing width.

First, the total load via the exterior wall on the bearing soil ...

RSL ... ½ of 20 ft x 40 psf = 400 plf ...

RDL ... ½ of 20 ft x 12 psf = 120 plf

Eave SL ... 2* x 40 psf x 18/12 ft = 120 plf SL ...

Eave DL ... 12 x 18/12 = 18 plf DL

Upper Wall DL ... 9 ft x 10 psf = 90 plf

Upper FLL ... ½ of 20 ft x 40 psf = 400 plf

Upper FDL ... ½ of 20 x 10 = 100 plf

Main Wall DL ... 90 plf

Main FLL ... 400 plf

Main FDL ... 100 plf

Total Superstructure Load on Foundation ...

DL ... 120 + 18 + 90 + 100 + 90 + 100 = 518 plf

SL ... 400 + 120 = 520 plf SL

LL ... 400 + 400 = 800 plf LL

TOTAL LOAD = TL ... 518 + 520 + 800 = 1838 plf

*Note: SL is generally double over eave for ice damming.

Let's use 8 in. foundation wall thickness ...

Min. foundation wall height (assuming footing thickness of 8 in.) is ...

frost depth + 8 in. clear to top of foundation = wall height + footing depth

gives ... wall height of 30 + 8 - 8 = 30 in.

Now we will calculate the weight of the foundation ... (or estimate it) ...

Stem wall weight ...

ω _{DL, sw} = γ x A = 150 lb/ft³ (8/12 ft)(30/12 ft) = 250 plf

NOTE: we will use γ = 150 pcf for reinforced concrete.

Let's try a footing width of 16 in., and thickness of 8 in.

ω _{DL, sw} = γ x A = 150 lb/ft³ (8/12 ft)(16/12 ft) = 133 plf

So, our line load on the soil is ... 1838 + 250 + 133 = 2221 plf

Now we will use the following equation ...

f_p = ω / width

where f_p = bearing pressure on soil

So,

f_p = 2221 plf / (16/12 ft) = 1666 psf

The design check is ...

IS f_p = applied bearing pressure on soil ≤ the allowable soil bearing pressure? ...

Or, is 1666 psf ≤ 1500 psf? ... No, not quite ...

Let's try 18 in. footing ...

Revising the footing weight ...

ω _{DL, sw} = γ x A = 150 lb/ft³ (8/12 ft)(18/12 ft) = 150 plf

Total line load becomes ...

1838 + 250 + 150 = 2238 plf

f_p = 2238 plf / (18/12 ft) = 1492 psf

Or, is 1492 psf ≤ 1500 psf? ... YES, GOOD!

So, ...

... 8 x 30 in. foundation wall on 8 x 18 footing ... GOOD !!!

Concluding Remarks

We will determine reinforcement and other requirements in the next lesson (here).

NOTE: we have used the FULL Snow load simultaneous with the FULL Live load. The building codes recognize that the full Snow load is not likely to occur simultaneously with the full Live load, and vice versa, and may allow a reduced total when these two loads are taken simultaneously. We could use this provision to design a slightly smaller footing. An example where the reduced combination is used is ... here. This (the linked) example also illustrates use of a truss roof system spanning from ext wall to ext wall and a slab on grade for the main floor.

References

Example Footing Width Calculation, Jeff Filler, Associated Content.