My unusual umbrella design uses some “Euclidean egg” shaped ribs erected around a central tower. vaults will span between the ribs forming an umbrella like structure that will distribute the earth loads down to the ribs. These interesting architectural features allow me to have an open plan layout without needing to span the space in one large vault.
The idea is to precast these flat on the ground and erect them into place. If I can find a precast concrete company to do the work for me at a reasonable price, I will gladly pay. But I suspect it will be expensive and I will need to do this work myself.
If I can order from an architectural precast company, I can relax knowing that they have lots of experience working with precast concrete. They know about the mixes and the finishes and they have all the equipment to cast the pieces and move them around, as well as the space to store them until I am ready to position them. We we are ready, they will deliver them at what ever rate we can handle assembling them (all on one day I would guess).
If I precast the ribs myself, the first concern is having the space to setup the molds. The ribs are heavy and cranes are expensive, so it would have been nice if I had the space to setup the mold, pour the rib and then just leave the >5000 lb rib right where it is sitting. I would move the mold to another spot on the floor, pour again, etc. Then I would only need to call the crane out for one day to erect and position all the ribs at once. I have a lot of ribs to make, so if I only used one mold to make them one at a time, it may take me 6 months while the rebar is placed and the concrete cures in sequence. I am thinking that I should probably make 3 or 4 molds to speed up (parallelize) the process. I could fill several molds at once from a single cement mixer delivery or I could be setting up the rebar in one while the previous one is curing. Laying on their sides, The ribs are only one ft deep, but cover an area about 13′ x 16′. I could fit only 4 at a time on my garage slab. I don’t plan to pour the floor for the rest of my house until much later, so I will probably need to move one set out of the way before I can pour the next. This may mean several additional crane rental days at probably close to $1000 per day.
I also must consider that if I do end up using my garage slab to pour the ribs, I won’t be able to erect the Quonset hut and close it in for storage purposes until all the ribs are done.
Unlike the bridge example below, my ribs will be architectural elements of my home. This keeps them in a a more gentle environment in terms of temperature swings and corrosive elements like salt or acid rain. However, the arches will be under much closer visual scrutiny by my guests (some of whom may be concerned about the tons of concrete and earth over their heads). I am sure any flaws will be even more visible to my critical eye. So the final finish and other small details will be important.
Visually, I am most concerned with the difference in the finish between the steel troweled side and the the other three sides that will be poured against the mold. Most precast architectural components are designed so that the steel troweled side is the “back” of the piece and never seen. Speaking to various experts, I am told that the difference will be minimized if I use a lighter and finer cement. I actually got some samples (shipped in a 7 lb box) from one of the architectural precast companies and Sherri and I compromised on “sand stone”. I also plan to sand blast the all sides of the final piece to remove that poured concrete look and simulate something that looks more like the texture of sand stone.
Structurally, I have the engineered drawings for the rebar spine that should be more than strong enough once the rib is in position and loaded as expected, but I am a bit concerned about the process of putting it into place. I will need some sort of rebar hooks in the spine for the crane to lift it by. I asked the engineer to position these, but he is under a fixed price contract and declined. he said that was something the precast shop should work out. I plan to cast #5 rebar loops right into the spine of the ribs which will complicate the mold a little so it can be demolded after the hooks are set in place. I have some ideas. ;^) The hooks will end up helping to tie the ribs into the vaults and will not be visible in the final home.
Placing the precast rebar will take a crane. I have already shopped around for crane rentals. I can rent one to use myself for about $600/day or $4000 for 4 weeks. I will also look for one that comes with a skilled operator. The cranes will lift 25 tones 45 feet into the air, so it shouldn’t have much problem positioning the ribs. I had hoped to use the Articulating Manlift that I will already be renting to help setup the rest of the arches and put the shotcrete crew in the right positions, but it will only lift 1/10th of what the ribs weigh.
I will illustrate how it was done on the Hang Tua Bridge in China. Sorry for the quality of the images, but the process was the best documented of any example I could find.