Okay, I’ve scoured the forum looking for insights as to reinforcing and supporting a cantilevered concrete countertop but I’m still not sure about my own application. I’ve also posted this on the decorative concrete network forum but haven’t had any takers. So I’m posting my query here:
I’m just in the design phase of cabinets and countertops for my kitchen remodel, a time when my imagination can defy the laws of physics. Now I want to probe the expertise here on whether what I have in mind is feasible. I’ll try to describe it in words, but I’ll also try attaching a rough digital sketch with dimensions that might clrify the verbal description.
Butting up against the corner of a kitchen island will be a raised breakfast bar. Imagine a letter ‘L’ (in plan view) with legs of 40†and 59†on the outside. The bar top is 16†wide to make ample room for elbows, plates, etc. This ‘L’-shaped top “flows†down vertically on its 16†wide ends and back side (the inside edges of the ‘L’, 24†and 43†long respectively). The front side is open leaving the space under the bar top as a knee well. The concrete is a uniform 2.5†thick.
My concern is about reinforcing and supporting the ‘L’-shaped span that runs 40†, turns the corner and runs 59†more, especially at the corner where the diagonal measures 22 5/8†from the inside corner to the outside (according to Pythagorus).
I’ve read the Cheng bible and was at first considering a rebar/remesh combination but was unsure if 16†(or almost 23â€) supported on one side by 2.5†was beyond the limits. Thinking about airplane wings that are dozens of yards long and cantilevered from the fuselage, I began to think in terms of a “web frame†of steel. Could this frame be welded up from 1/4†or 3/8†rebar, across the 16†width about 4-6†on center? What about adding remesh? Or the carbon grid mesh from TechFab?
Then I began to think about welding up the frame from 1/2†square tube steel stock which would be lighter. But how would it compare in strength to rebar? Across the very vulnerable outside corner I thought maybe I could make the web from 1/4†solid square and ‘skin’ it with 1/4†plate, extending away from the corner in both directions by a foot or so in a chevron pattern.
If this frame (rebar or square stock) were submerged so that it were closer to the bottom of the 2.5†I don’t think I would risk any ghosting.
If you guys have any thoughts regarding the feasibility or the lunacy of my ideas, please let me know. I know there will be the usual suggestions like “Why not put in a support leg at the corner?†“Why not make it 1.5†instead of 2.5� Save a lot of weight†All valid. But for now I want to see if I can engineer a solution to the design as it is. I’ll consider modifying the design when I’ve proven to myself that the initial concept violates the laws of physics and concrete.
Thanks in advance.
Replies
The flat I am in has a spiral cantilevered concrete staircase. The stairs are quick thick so what you are envisioning may be possible. Not sure where'd you get the definitive answer on the span and weight bearing for cantilevered applications based on material. You could build a small model and see if it appears to work properly.
There was also this article:
http://www.dennisallenassociates.com/article_concrete_spiral_staircase.htm
again a cantilevered stair case article but same kind of principle. The steps are 300 pounds each and cantilevered on a 3-1/2" ID pipe. Really cool article. Let us know how your project goes. If you find some kind of engineering span table for cantilevered materials I'd love to hear about it.
MG
I'm envisioning something like these brackets (if I can't get the picture to post, from McDesigns "Cool Portico" # 160) , inverted and using a smaller angle (1 1/2 x 2 or so). Embedd in the concrete 90 degrees to each other at the outside corner. Tie together with a diagonal brace and you've reduced the effective cantilever in half.
And another thought presumes this will not be a pour-in-place top. The tensile strength of the cured material suggests the real concern is holding the inside of the "L" down, countering the forces that would lift it up when a load's placed at the extreme O/B point. How do you intend to anchor it down? Or are the vertical surfaces concrete as well?
I am not an engineer, but have a couple thoughts:
A kitchen I worked on had granite counter tops and one straight piece cantilevered out almost a foot. The fabricator used two steel angles about a quarter inch thick to support it. The angles had their vertical legs bolted to the inside of the support wall (2x4 studs) and the horizontal legs went from the inside of the wall and over the top to support the cantilever.
The other idea is could you mold a sort of "corbel" into that corner (like half of an arch, so it wouldn't interfere with the knees of someone sitting at the counter)? You'd still have to use re-bar above it and probably at the edge of the counter.
Molding ribs into the counter under the "slab" may also help strengthen it--especially if re-rod is in the ribs. (Scallop-looking, or even like a waffle.)
first... you never weld rebar.... second... if you use 1/2" rebar bent just like your counter 3 rows spaced out (or 4 rows of 3/8")... i see no problem if you use a high grade concrete mix either poured in place or cast in a form.. either way it needs to stay in the form for at least 7-10 days... if i was casting it in place i'd make the floating corner maybe 1/8" high when i built my form to put it into compression... I'd also leave a support under it for at least 30 days and try to keep it as damp as the site allowed... if i poured it in a remote form I'm not sure i'd bother building in the slight rise... but again i'd try to keep it damp for 3-4 weeks...
this is all just my seat of the pants approach... would i build it for me like i said and expect it to be fine... yes and would i tell you to do it? NO... just telling you what i'd do and expect it to work...
good luck
p
Just keep in mind that concrete is great in compression and place reinforcements appropriately.
Rebar and mesh can and are welded all the time in high-strength applications. No reason welding wouldn't make the frame easier to keep in the right place.
If it were me I'd go with fiber, remesh, rebar, a water reducer and high strength mix. If so you'll probably be able to have a few people dance on the thing without problem once it's set up for a month.
After placement and consolidation work your fingers or a stick between all sections of rebar or mesh and the form to help break up agregate formations that contribute to ghosting. I know, it's easier said than done.
Beer was created so carpenters wouldn't rule the world.