Understanding how to properly place a footing on the Building Section Vignette is critical when it comes to passing the ARE. This vignette, part of the CDS exam, will require candidates to draw a building section based off of existing floor plans. Since there really is only 1 correct solution for the Building Section Vignette, there is very little margin for error when drawing your solution.
One of the most critical issues on this specific vignette is the placement of the footing (and foundation wall) in relation to the frost depth.
What Is The Frost Depth?
The frost depth is basically the depth of the soil at which water will freeze. This becomes very important in building construction because if a footing and foundation wall are built above the frost line, the chances that heaving will occur drastically go up. Heaving occurs when the soil expands and contracts due to freezing and thawing. When soil expands and contracts like this, it can cause severe damage to a building’s foundation. This is why it is critical that a footing for a building be placed BELOW the frost line.
As I mentioned above, the program that will be provided to you on the CDS vignette will give you the required “Frost Depth”. There is always a big debate on the ARE Forum regarding the placement of the footing in relation to the given frost depth but in my opinion, there is only 1 safe way to draw your footing and foundation wall and that is by placing the TOP OF THE FOOTING at the frost line.
There are some people that feel that the bottom of the footing can be placed at the frost line, but if you think about it, it’s much safer in construction as well as on the vignette to place the top of the footing at the frost line.
Remember, the frost line is measured from the grade line that you place and NOT the top of the slab. Since you will asked to place a slab on grade, many candidates make the mistake of measuring the frost line from the top of the slab. This is incorrect and should always be measured from grade.
To help candidates measure the correct depth, I would recommend using a “sketch line” to draw a line down from grade until you reach the frost depth given to you in the program. Once the sketch line is drawn, then you can use the “Footing and Foundation Wall” tool to draw in your wall until the top of the footing aligns with the sketch line that you drew.
Although placing the footing at the correct frost depth may seem like a minor task on the building section vignette, it’s almost essential because there is such a small margin for error on this vignette. No one quite knows which errors may result in a failed score, but it’s not worth the chance in a case like this. Since the ARE construction documents vignette really one has 1 real solution, it’s important that candidates understand the important factors that may affect their score.
For more information about the CDS vignette and the proper steps it takes to pass it, please see the ARE Advisor ebook series.