Wednesday, July 7, 2010

Foundation formwork

The foundation for the two-story rammed earth had to be wide in order to transfer the weight of the walls to the ground beneath. At a density of roughly 130 pounds per cubic foot, the 18" thick, 20' high walls will weigh almost 4000 pounds per linear foot. Soil bearing capacity was estimated at 2500 pounds per square foot. To support the wall and roof weight without settling, our engineer, Nabil Taha of Precision Structural Engineering, designed the footings to be 2'6" wide. The soils engineer wanted them 18" below grade.

The part of the concrete buried in the ground is considered the "footing". On top of the footing, the foundation can narrow down to the width of the wall. This is called the stem wall. The stem wall gets formed with 2" x 12"'s and steel stakes. String lines pulled from the batter boards establish both the line and level of the stem wall form boards. Wooden 1" x 4" "spreaders" are nailed across the tops of the form boards to add extra stiffness. We used additional plywood spreaders to secure the electrical conduits where we wanted them.

We also added an extra strip of wood to the inside of the stem wall forms as a ledger to receive the wall forms. I'll show you this strip clearly in a following post.

The two photos in this post show good details of the formwork, including reinforcing steel and electrical conduit. The vertical steel dowels will tie into the wall reinforcing.

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