CREJ - Building Dialogue - March 2015
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 as to be isolated from the slab to avoid the constant scrape 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. OK, 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-inch clearance above the soil, per code. The framing and crawlspace requires a raised-floor level or a deeper excavation. This space also must be ventilated to control mold issues. Steel joists can be used to cut down on the clearance, but now the cost of the structural material is escalating. 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 also must 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: SlabTek. 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 percent 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-inches thick. After a three-day cure, the cables are pulled and the slab is raised by the embedded lifting mechanisms 6 inches to 10 inches, depending on the soil swell potential. The building is then 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 overexcavation schemes. While Colorado Foundation Solutions is the sole provider of the lifting mechanisms for this patented floor/foundation system in Colorado, any adequately trained crew can install the slab. And any structural engineer familiar with two-way flat slab post-tensioning systems can provide the design.