Tapered or non-prismiatic wall

NRO · Jul 4, 2018

Hello all,

For an RC wall of 5.5m(18ft), there is the option of having the wall taper from top to bottom, or having the wall of two different thicknesses. From experience, which would be the more preferable in terms of: cost; formwork construction, erection and removal.

Thanks in advance!

JAE · Jul 4, 2018

NRO - I had a similar question a while back and called a good friend of mine that was a contractor superintendent with >35 years experience.

I asked him about the options of a straight, 19 ft. tall retaining wall vs. a tapered wall. He indicated that a tapered wall would be no problem.
That you'd essentially be setting your one side form at an angle with adjustable wall ties and you'd save some concrete material (not a big money deal).

He said a straight wall thickness would be "better" as the ties would all be set the same but the tapered wouldn't break the budget.

A stepped option? I would think that would almost force the wall into two placements (assuming one step) and require awkward rebar configurations.
It would also then result in a construction joint on the exposed wall face - or a special joint reveal or something to hide the potential concrete irregularity.

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SlideRuleEra · Jul 5, 2018

NRO - Contractors will compute their bid price by evaluating several factors. The value given of each factor will depend on that company's resources and will not be the same for each bidder:

1) Cross section of the wall. Do the company's workers have the skills to form a wall that is not a constant thickness?

2) Height of a concrete placement. Eighteen feet is a lot for a single pour (2700 psf, hydrostatic concrete pressure... worst case). The Contractor will have to place the concrete slowly (vertical feet/hour) to keep form pressure reasonable. Another way is to use forms built like battleships. The length of each wall pour and wall thickness will be needed to determine if the concrete volume of the pour is reasonable for "slow" placement. "Slow" placement creates other problems, but will skip that for now.

3) Two placements (say, 9' high each), allows the Contractor to use less total square feet of forms(saving $$$) assuming the forms will be reused for the second lift. However, two pours will take longer (more days cost $$$ for labor and equipment on the job).

4) Temporary scaffolding may (probably) be needed whether one lift or two. A Contractor's resources could may one way or the other the "best" for them.

5) Quality control. Internal vibration of concrete inside an 18' deep wall form can be done (lot of rebar in the way)... but the deeper the forms, the more difficult.

As a former bridge contractor we built tall railroad crash walls as part of certain overpasses. Similar forming to a retaining wall. My recommendation is keep it simple, put aside "saving concrete"... IMHO, that is false economy.
My first choice, consider a wall with one constant thickness; each bidder has the option for one horizontal construction joint at an elevation proposed for your approval.

If a single thickness is not acceptable... I believe you will get more bidders and lower prices for a two-lift wall with two thicknesses.

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