Tella Firma: a Cost-Effective Alternative to ‘Floating Slabs’
John Clarke, P.E. President at RMG Engineers
Floating Slabs: don’t just you love that name? Let’s design floors that “float”. It just doesn’t sound right, especially for a structural engineer. But, we all know that when you place a floor directly on our sensitive Colorado soil, it is going to move. So we create what we call a floating system. Since it is cheaper to use a slab-on-grade for a commercial floor, we design for the anticipated movement. The lower level walls are hung from the framing above, leaving a gap along the bottom to allow the slab to move without distressing the wall (or set the wall on the slab with the gap at the top). This void system can get complicated in addition to the plumbing and mechanical systems which must be sleeved or made flexible for the slab to move. Storefront doors also need to be installed in such a way to be isolated from the slab to avoid the constant scape at the bottom. And, by the way, the floor is still going to move so be aware of tile or other rigid type floor toppings.
Okay, so how expensive can the options be? A structural floor that spans over the soil is typically the alternative. But, just the name sounds expensive! Typically a wood framed floor is used consisting of floor joists and beams. This requires a crawlspace for the 18” clearance above the soil, per code. The framing and crawlspace requires a raised floor level or a deeper excavation. This space must also be ventilated to control mold issues. You can use steel joists to cut down on the clearance, but now you are escalating the cost of the structural material.
A structural concrete floor can significantly reduce this depth, since the only clearance needed is a gap to allow for soil heave. But, conventionally reinforced concrete slabs are limited in their span lengths requiring additional supporting beams and pier supports. These supports are in addition to the foundation pier system. The slabs must also be supported by formwork during the pour and curing process, which can be expensive.
This brings us to a proven alternative system that was introduced to Colorado last year: Tella Firma. This is a structural slab system that is poured directly on the ground and then lifted into place, avoiding the costly formwork. The slab also utilizes proven post-tensioning systems that span directly between isolated piers without the use of costly deep beams. Piers are laid out in a grid fashion under the entire footprint, typically eliminating 20% of the piers of a conventional pier and grade beam system. Structurally rated lifting mechanisms are placed at each pier. Post tension cables are laid out in both directions. The slab is poured, typically 5” thick. After a 3 day cure, the cables are pulled and the slab is raised by the embedded lifting mechanisms 6” to 10”, depending on the soil swell potential. The building is them constructed directly on the structural slab.
The advantages of this system include:
- Elimination of the crawlspace
- A shallower floor thickness
- A reduction in the number of piers
- Future adjustability of the floor
- And, the end of the “floating slab”!
This system provides a main level commercial floor that will not move due to supporting soil heave. Storefront doors scraping on floating slab floors are no longer an issue. The cost of this system is cheaper than conventional pier and grade beam systems and most over-excavation schemes. Colorado Foundation Solutions (CFS) is the sole provider of the lifting mechanisms for this patented floor/foundation system in Colorado. The slab can be installed by any adequately trained crew. The design can be provided by any structural engineer familiar two-way flat slab post-tensioning systems. RMG Engineers is the only Colorado engineer designing these systems to date. Contact either of these firms for more information on this Tella Firma system that eliminates floor slab movement.