TECH CORNER:
Frost-Protected Shallow Foundations (FPSF)
We recently had the opportunity to implement a not-so-commonly-used provision of the Building Code that allows foundations to be placed above the frost line by installing rigid insulation and crushed stone beneath the building footprint.
The project is a small, unheated ticket booth intended for seasonal-use only at a performing arts venue in Westchester County. The ticket booth is located in a part of the site with difficult access and limited room to work so the cost of a conventional frost-depth foundation would be a disproportional percent of the total cost of construction and more than the limited budget would allow.
Fortunately, IBC Section 1809.5 Frost Protection includes a provision that allows foundations to be placed above the frost line if the guidelines outlined in ASCE 32-01 “Design and Construction of Frost-Protected Shallow Foundations” are followed. (Interestingly, the same Section allows free-standing, light frame buildings that are in Risk Category 1 and 600 sq. ft. or less with an eave height of 10 feet or less to be constructed without frost protection.)
Using the ASCE 32-01 guidelines, we designed and detailed a simple haunched slab foundation that was installed over a layer of crushed stone and rigid insulation. The benefits of this approach are a reduction in site disturbance (less excavation and backfill) and simplified formwork and placement of concrete.
Prior to implementing the design, we cleared the concept of a shallow foundation with the local Building Official who quickly recognized the suitability of the application and accepted it.
We were pleased to be able find a way to make design decisions that improve the project in a cost-effective way. We look forward to the opportunity to do the same on future projects.
