CREJ - Journal

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58 / BUILDING DIALOGUE / MARCH 2015
F loating slabs. Don’t just you love that name?
Let’s design floors that “float.” It just doesn’t
sound right, especially for a structural engi-
neer. 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 sys-
tem. 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. Store-
front 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 struc-
tural floor that spans over the soil is typically the alter-
native. 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 ex-
cavation. 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 con-
crete 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-tension-
ing 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 ca-
bles are laid out in both directions. The slab is poured,
typically 5-inches thick. After a three-day cure, the
John
Clarke, PE
President,
RMG –
Rocky
Mountain
Group
TRENDS in Floor Construction