by Deborah Mesher, Architectural Designer and Zoë Hanneman, P.Eng
Deborah: What is foundation benching?
Zoë: Benching an existing footing is a method of lowering a basement without underpinning. The main function of the bench is to preserve and protect the existing footing and it’s bearing condition. The bench itself is a large and wide triangular mass or giant-looking STEP of solid reinforced concrete that extends from the existing foundation wall into the basement space and sits on top of the undisturbed soil that spreads out in a diagonal line from the base of the existing footing.
Deborah: What are advantages to benching foundations?
Zoë: The main advantage of benching is that it is the most common alternative strategy to lower an existing basement slab when underpinning isn’t possible. Some common reasons to bench are:
1. When the existing foundation walls in a basement are not adequate for underpinning and cannot be easily/cheaply reinforced.
2. Unlike underpinning which always requires multiple sequences of individual pins, a typical benched footing in a residential basement can be formed and poured all at once.
3. When adjacent buildings are too close and underpinning is not possible as it will lower the existing foundation walls into soil that is loaded by the adjacent building.
4. When a neighbour does not consent to underpinning a shared partywalls in a semidetached house.
Deborah: Are there challenges with benching?
Zoë: While benching is typically quicker, cheaper, and less risky than underpinning, it takes up a lot of space in narrower homes. The top of the bench must be at least at the height of the original basement slab. The width to height ratio for a benched footing is worse than 1:1 which means an additional foot of basement ceiling height will cause you to “lose” more than 2 feet of basement floor width at the new lower slab elevation. This is a lot of lost space in small urban homes.
Stairs can also be a problem, as a benched footing often forces basement stairs to have a winder which takes up even more space in the floorplan.
Another potential challenge is that, unlike underpinning, a typical benched footing does not add bearing capacity to the existing foundations. As such, a benched footing would not be used for a renovation project that is proposing to lower the basement and include an addition.
Deborah: What do you wish homeowners knew about benching before starting?
Zoë: Describing benching as less risky than underpinning is only true if benching is done properly. The key component of benching that homeowners and contractors must be aware of is that its function is to maintain and protect the existing footing and its bearing condition. Any attempt to reduce the size of the bench or any contractor error when excavating for the bench formwork can quickly create a dangerous situation. Similar to underpinning, benching is serious work. It requires a solid team made up of an architect / architectural designer, an engineer, as well as a contractor who have open and regular communication to ensure the success of the project.
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