Foundation wall formwork

There is a silver lining in almost everything! My sprained ankle gave me time to think about how to put the formwork for the concrete foundation wall together. I learned about concrete form work during my apprenticeship in Germany, but that was over 25 years ago. It was time for a refresher!

What makes concrete work so interesting is that you get one shot. If something goes wrong with the formwork during the pour, there typically is no fixing it. And starting over is time consuming and very expensive.

We needed an eight inch wide and 54 inch tall foundation wall. The outward pressure from the concrete onto the formwork is considerable – 675 pounds per square foot (psf) at the bottom and 150 psf towards the top. (The pressure was calculated following Lateral Pressure Design Formula in Guide for Formwork for Concrete (ACI 347) by the American Concrete Institute).

I was on a mission to find a reliable formwork construction method that would keep the eight inches wall thickness from top to bottom, despite all the concrete pushing on it. What I settled on is a system with snap ties, brackets and horizontal wales.

formwork-01 formwork-02

 

formwork-03 formwork-04

The snap ties are the spacers between the plywood sheets. The Jahn “A” Brackets slip over the snap tie head and hold a two by four – the horizontal wale. The brackets are tightened up by pushing/hammering the eccentric over the tie head which holds the formwork in place. The snap ties have a working pressure of 3,350 pounds.

But before I get to play with the snap ties and brackets, I have myriad other tasks lined up, competing for my attention.

From the bottom up

The snap ties and brackets can’t be installed at the very bottom of the formwork. Instead, we installed footing plates, which are simple two by fours that get anchored into the concrete footing.

To get to the 54 inch wall height, we added a six inch plywood strip to the 48 inch plywood sheets. I decided to place the plywood strips at the bottom of the wall. This way any associated imperfections (like seams) will be hidden under the future concrete floor.

It also made for relatively simple 12 inch tall framing, with the footing plate at the bottom and the first wale 12 inches up. The 16 inches on center vertical two by fours kept the plywood strip and sheet lined up and prevented it from bowing outwards.

I also had to think about the steel reinforcement and how it fits into the installation sequence.

We started with the outside footing plate, plywood strip and framing. Once completed, I put up all of the #5 rebar, before installing the inside footing plate and framing.

At this point we had most of the tedious tasks behind us and could move on to installing the plywood sheets, snap ties and brackets. More about that in the next post.

The difficulties with architectural documents

This is a subject close to home, as I frequently produce construction documents myself in my capacity as a landscape architect. I personally don’t like to leave details unresolved or un-tested in my drawings. Items not properly resolved or detailed typically lead to pricey bids or change orders during construction because of field changes.

I had installed the footing rebar exactly as detailed in the architectural drawings, but was mindful enough to have Edgar, our porch contractor, stop by to check on everything.

He promptly pointed out that the rebar and the wall alignment on the short east and west footing was about four inches too far out. If I was to keep the alignment as per the drawings, the porch would end up wider than the building.

Edgar’s comment was: “I have to look at the drawing and figure out how to build it. Some of the details and dimensions don’t work as shown.”

Well, I got a taste of that and moved the rebar over by four inches, as you can see at the beginning of the time lapse.

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About Marcus de la fleur

Marcus is a Registered Landscape Architect with a horticultural degree from the School of Horticulture at the Royal Botanic Gardens, Kew, and a Masters in Landscape Architecture from the University of Sheffield, UK. He developed a landscape based sustainable pilot project at 168 Elm Ave. in 2002, and has expanded his skill set to building science. Starting in 2009, Marcus applied the newly acquired expertise to the deep energy retrofit of his 100+ year old home in Chicago.

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