November 07, 2011

Devising a frame to support the bond beam pour


Since the start of this building project, I have been thinking about how I would set-up framework to pour a concrete bond beam (description mentioned here) on top of the cob walls at ceiling height.  

In short, the bond beam ties the wall with the roof through the use of vertical rebar from the footing through to the roof structure.  In addition, the bond beam act to distribute the compression load from each viga supporting the roof structure as it rests on the bond beam.

The challenge for this building design is to support the frame around curved walls.  Since the width of the walls for this southeast part of the building are the minimum desirable thickness structurally (12"-17"), this necessitated that the bond beam span the full width of the wall for the entire length of the room.  To achieve this, the support frame had to fastened to the outside of the cob wall.  This fastening method had to be figured-out.

I was also thinking about what material would be flexible enough to bend around the curves of the walls; while at the same time knowing the material had to hold firm during the pouring and curing of the concrete.

I decided to go with 3/16"-thick Masonite (sold in 4'x 8' sheets) for the bond beam frame and 12"-long 1" x 2" furring strips to support the Masonite.

For the frame, I cut 12" wide strips of Masonite, eight feet in length from the sheet.  I pre-drilled three holes in the furring strips in order that the screws securing it to the cob walls, wouldn't split the wood or make the attachment process more difficult.

The screws I chose were 4"-long deck screws (course thread).  I was impressed how the cob (straw/clay) mixture held the deck screws extremely firm when attaching the furring strips in support of the Masonite frame.

The photo at top shows the completed bond-beam form prior to pouring the concrete.  The photo at the left is a close-up of a section of the wall.  The rebar used was 5/8"-diameter and additional furring strips were attached across each side of the wall to provide additional stability to the forms. 

In the top-left quadrant of the photo above, there are two "Simpson Strongties" - a trade name for metal straps that will be secured in the concrete beam and rise vertically to attach the viga to the bond beam.

Upon the completion of the bond beam's support fabrication, I knew I had tackled the challenge of the form's design and implementation.  But I still had to mix and pour the concrete beam, so I was careful not to declare the design a success just yet.

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