allowable bearing pressure of soil

[sunyaer]

This is from Ontario building code Division B, part 9:
-----------------------------------------------------------------------------------------------------------
Section 9.15. Footings and Foundations
9.15.1. Application
9.15.1.1. General (See Appendix A and A-9.4.4.6.)
(1) Except as provided in Articles 9.15.1.2. and 9.15.1.3., this Section applies to,
(a) concrete or unit masonry foundation walls and concrete footings not subject to surcharge,
(i) on stable soils with an allowable bearing pressure of 75 kPa or greater, and
(ii) for buildings of wood frame or masonry construction,
(b) wood frame foundation walls and wood or concrete footings not subject to surcharge,
(i) on stable soils with an allowable bearing pressure of 75 kPa or greater, and
(ii) for buildings of wood frame construction, and
(c) flat insulating concrete form foundation walls and concrete footings not subject to surcharge,
(i) on stable soils with an allowable bearing pressure of 75 kPa or greater, and
------------------------------------------------------------------------------------------------------------

Question:
Allowable bearing pressure is actually a defined term in Ontario building code, is it equivalent to SLS of soil bearing capacity in part 4? What is the safety factor for checking it against column loading?(ALL geo-technical reports in Ontario would provide SLS and ULS soil capacity)

 

[tmurray]

There would have to be a link from the definition to that section in Part 4 if that is the intent.

There is a fair amount of information in the appendix/notes section of Part 9 detailing practical ways to estimate approximate bearing pressure of soils without the involvement of a geotechnical engineer.

If you are into a situation where it is necessary to involve a geotechnical engineer, it moves the soils from Part 9 to Part 4. Allowable bearing pressures and safety factors would be in accordance with Part 4.

 

[sunyaer]

I found the following information at this link:

Footings

The Ontario Building Code uses an assumed value of 1500 psf (75 KPa) for the bearing capacity of soils. A soils test can be carried out to determine the actual bearing capacity of the soil. The information derived from this test will provide you with the bearing capacity for the soil on your site.

How does the soil's bearing capacity relate to the size of the footings? Lets say that you have a soils report in hand that says the bearing capacity of the soil is 3000 psf. That's the number to start with. From there, you need to work backwards and figure out the loads acting on the footing in pounds per lineal foot (or KiloNewtons per lineal metre KN/m) - plf or KN/m

The footing of a building transmits load to the soil. The lower the bearing capacity of the soil, the wider the footing needs to be. If the soil is firm, the footing can be narrower. Consider some of the following numbers. Sand has a bearing capacity of about 3000 psf (pounds per square foot). This means for instance that a 12 inch wide footing will be able to sustain loading of 3000 plf (pounds per linear foot). Footing calculations are a good example of calculating what is otherwise known as a load path - meaning all of the loads above that are acting on the footing from the roof down. Try a calculation for a typical 2 storey house using common loading conditions to see what this looks like. To do this, you will need to calculate the live loads and the dead loads. After this, you will calculate the length of supported floors that the footings will support. Then you calculate the wall loads and that generally will suffice to contemplate an approximate footing size for your project.


Live loads on roof 20 psf x 1.5 = 30.00 psf
Dead Loads on Roof 15 psf x 1.25 = 18.75 psf
Live Loads on Floor 40 psf x 1.5 = 60.00 psf
Dead loads from floor 12 psf x 1.25 = 15.00 psf

The loading indicated above will vary for the roof loads depending on where you live and how much snow you get. In Toronto, the value is 20 psf so we will use this for the calculations. The live loads from floors are the same across Canada being 40 psf. The dead loads are derived based upon the anticipated flooring material. If you are using tile or hardwood flooring on subfloor with wood joists, the dead load value will be around 12 to 15 psf. If you are using a concrete underlay, the value will be around 20 psf. We will use the typical 12 psf for these calculations.

Then you wonder, why the 'x1.5' and the 'x1.25'? These are the values that are used to factor the live and dead load values. When calculating the loading requirements for floor joists and beams, you need to use the factored loads. When calculating for footing size, you use the specified loads only.