I am trying to rough out a project plan, and my “spook hole,” the little 10 x 16 mechanical room basement, 9 foot wall at the highest, might look like the pic attached. It is formed and poured reinforced concrete, and wall thickness is 8″, except for the thickened wall section at front, which is 12″. Note the curves. Inside radius is 32″. I set my CAD program to render the curves in segments that are about the width of 4″ lumber, so one could envision the formwork staved like barrel segments, or done with curved plywood. How long might some good formwork carpenters take to build this, all ready for the pour? Don’t ask me why the shape; an architect dreamed it up. The outside of the wall above this foundation is stone veneer on wood frame and the inside houses the staircase. Both are major features of the design.
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Architech dreams= Builders nightmares.
Gene
Can't help much with a time estimate to form up your wall. However, fitting the bracing to a vertical curve will be a real pain.
You need to think very seriously about what you want the finished curved surfaces to look like. If you use staves, you'll have a stepped curve and cement motor will rib in every joint - is this what the customer wants? Or does it have to be a "perfect" curve. Round here ply used for formwork is 3/4in. Bending for a 32in radius ... forget it. If customer wants a perfect curve, then plastering/grinding the off-form wall would appear your only option - but you may be able to use staves wider than 4in thus simplifying the form-up. I can't see how any ply thin enough to bend arround your curve can be supported well enough so that when the concrete is in the form it does not become distorted. Now to be a real pessimist, for a "perfect" off-form vertical curve you may have to engage a furniture maker to build you 4 coopered panels for your formworker to install.
Just had another thought.
Which curve is the critical one, the inside or outside?
Have you though about getting buying a concrete pipe of the right diameter.
32in inside diameter for curve, 8in wall - 7 foot diameter pipe might just do the trick.
Hire a diamond saw, cut the pipe in half lengthways, then slice one half in half again. Voila, you now have two curves of the right external diameter that can sit at junction of two simple to form straight walls. Should work a treat unless the curve section MUST be load bearing - a structural engineer should be able to advise if a quarter pipe section can be load bearing. In this context MUST be load bearing means that you can't hide the true load bearing member behind a ceiling, bulkhead, etc.
good luck
ian
Thanks for the input, Ian. Here is some more info. The exterior of the wall is hidden and entirely below grade. The interior is just unfinished basement, and if formed with, say, 1x4 staves, the board edges will want to be sawn with the right bevel so the joints are reasonably tight against the 'crete. We will just hammer off any crack seepage dross on the inside surface, after taking down the forms. It is all load bearing, and being underground, we want to pour it monolithically so as to be waterproof.
Gene
It sounds as though the architect has just continued the above ground curved down to the foundations.
I'm with Piffin. Square or triangular corners are much easier (read faster and cheaper) to form, bend and tie the reo for, than curved walls. I would only build a curved wall where I had no other choice. From your description, the interier curve could be dispensed with or done later in rendered brick or dry wall. As a rough rule of thumb, where I live, the cost of 1yd of concrete is aproximately equal to 4 hours work by a one form worker. As one on my teachers once said "the cost setting up a one-off is usually very much more than the value of the materials you're trying to save.
For me it's a no brainer ... radically simplify the shape, so you can use rented forms, use a bit more concrete, save probably 2, maybe 3 days work by the forming crew plus materials that will end in the fire. Not sure how to add a picture, so will try a description: on the outside, form the wall with one 45 deg angle that is tangential with the architect's curve - for a 9 ft high wall you'll use maybe an extra 1/4yd of concrete on each "curve". for the inside wall, form it from 12in wide panels, which if you don't already own, you should be able to rent.
The whole set up should only take a couple hours more than it would to form up two 90 deg corners.
Ian
Thanks for everyone's input. I think I have it now. We will segment it, using standard-width wallform sections on the inside, and one site-built flat section on the outside. Thus two sheets of ply and some 2x4 stock will go on the burn pile. We can probably save $ and use a couple 18s or 20s in standard form sections on the outside, but I did not want to work out the geometry here. All this concrete is underground, with the outside seeing backfilled earth, and the inside is cellar stair walls. My sketch, attached, shows things in plan view. I show the stair arrangement, so you can see we have sufficient space at the turn landing, with this degree of segmentation. I needed to get this thought out well and on paper before I give it to my poured foundation guy for a quote. I believe he has standard wall form sections in 2" width increments, and certainly must have 18s.
Personally, I still think you are making it too complicated by segmenting the inside to three pieces, not all parallel to any other form section.
But I understand the walls relationship with the stairs now. It is landing and not steps that abutt the curve..
Excellence is its own reward!
Here's a line view of the interior of what I was talking about.
Excellence is its own reward!
Gene,
I would use bending plywood and support it with semi-circular ribs made of 3/4" CDX sandwiching 2x stock scraps. I'd run vertical 2x4s as whalers to stiffen the back side of the ribs. Two guys should probably be able to cut, assemble and place the formwork for the curves in one day while another sets forms for the straight runs.
Eric
And thanks to you, Eric. I will try to expedite the effort by making two templates out of some 1/2" scrap baltic birch ply I have. We will use them for tracing the curved backer shapes on 2x and CDX materials. I'll make sure we have the bandsaw in the trailer when we need it. By bending ply, I presume you mean what we call wacky wood here, and we will do the form faces in that. The attached pic shows your concept, without the vertical braces, which we will use wallties to affix. The continuous 2x4s running vertical on each end of the forms can be used to bolt the form edge to the adjoining straight section.
Gene,
I envisioned leaving a little extra meat on those ribs (ie: the inside radius would be solid half-circle ribs and the outside would look like a triangle with one semi-circular side) for added stiffness. Are you planning to use snap-ties to hold this all together? Careful placement will allow you to "borrow" some of the stiffness of the inside ribs to help with the outside form, which in my opinion, will be the hardest one to control.
Eric
If I understand what you are doing right, there is no need to form and pour the subterranean wall curved. I drew this with 8" walls except for the angled spur which is a 12". The walls could be shaped right at the top with form inserts or if the concrete slab overlays first, it could be shaped with a curve easily.
Compare for yuorself the difference in curved over curved vs curve built over angled.
BTW a 32"radius inner surface seems tight to me for a stair to follow. A larger radius would of course need a thicker base wall to do this way, but it still seeems easier to form than an actual curve.
Excellence is its own reward!