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Site Cast Concrete Structural Systems

August 5, 2011
Sitecast concrete is a common structural system. It is typically used for storage facilities, and libraries because it can withstands heavy loads with relatively minimal depths. In the 70’s is was a very popular structural system and can also be found in schools and office buildings built at that time.The most common site cast concrete is for foundations and slab on grades, but this will be covered in another post.
Waffle Slabs:
The most iconic site cast concrete system is the waffle slab. Preformed pans are used to create a form that casts the concrete into a waffle pattern. The typical pan is 4’ or 5’ square and 14” to 20” deep. At the perimeter and around columns the pans are removed so the full thickness of the slab is available for support. When one floor is finished temporary shoring is installed. Shores are temporary adjustable length columns that are used to hold up the waffle pans while the re-bar is places and the concrete sets. After the concrete is set the shoring is removed and used again on another level of the building or on another project. The final step is to remove the form and the iconic waffle slab ceiling is revealed.
Flat Plates:
Concrete can be cast into a thin flat plates as well. With re-bar laid into a grid pattern thicknesses as little as 6’ to 12” can be accomplished. This system is constructed like a waffle slab with shoring, but forms are only used at the edges of the slab and openings in the slab.
Concrete Columns:
Besides the slab, the columns need to be cast as well. Forms are built to contain the concrete and re-bar is installed. Re-bar is typically formed into a spiral or a rectangle. Where the slab and the column meet, additional re-bar in the shape of an “L” is installed to connect to two.
Tilt Up Walls:
One final type of site cast concrete is a tilt up wall system. In a tilt up wall system, walls are cast on the ground near the building and tilted up to place. Tilt up wall systems are not the most aesthetically pleasing walls and are typically used for storage facilities.

Two way systems vs One way systems:
One unique capability of site cast concrete is the two way structural system. Steel and wood can only span in one direction. Meaning when a force is applied to a point on the slab the weight can only go in one direction (north/south) along the joist towards the beams. When re-bar is placed in a in a concrete slab (waffle or flat plate) in both directions, north/south & east west, the load can be transferred in two directions, creating a grid of beams that can carry the loads.

Lastly site cast concrete has made possible some of the most stunning building ever designed. It’s ability to be made into any shape has allows for the ultimate architectural creativity. Below are a few examples of what is possible with site cast concrete.



Boston City Hall by  Kallmann McKinnell & Knowles.
Photos by Brian Foust
TWA Terminal at JFK airport in New York. Designed by Eero Saarinen
(Photo by Wikipedia)
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