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overhead concrete design stuff

[486]

[486]

ugh, that guy again?
Joined
May 19, 2020
Member Number
94
Messages
5,320
Loc
pine city MN
Been thinking further about my concrete plans for next year once this shit thaws
Was looking at buying a few containers and tossing a steel roof between them, then ran the numbers and for only a little more I can do poured concrete.
Figuring on making a building that is 30x40.
I really like the idea of earth sheltered stuff, so it'll be all poured with an arched roof.
Imagine an 8-10' tall silage bunker with a roof on it, then a few feet of dirt on top of that.

I'm having a ton of trouble finding information on the roof arch design. Almost like there's schools out there making tons of money on this shit.
Basic shit I've found says that the arch height should be at least 1/10 the span, and more height atop that reduces thrust pushing the walls out
so at a 30' span I'd want at least 3' of height in the middle
I'm fine with running a few tension rods at the base of the arch through the building, so I figure I'll keep it near that minimum figure

the question comes in here
I plan on having a wall run down the middle of the arch, dividing it into two 15 foot wide bays
conventional wisdom would have me just make two 15 foot wide arches, would certainly be easier to form up, but it makes for trapped water in the center
Curving the two arches in the other direction as well would solve drainage but jeez louise I don't know if I can figure out how to make all that not horribly wavy and shitty looking.
If I support the center of the arch does it make it into some sort of form with no strength that'll collapse and kill me?
I'm thinking it might put some of the roof in tension rather than all of it in compression.
Figuring on 8-10" thick walls, then the center divider being something like 6" because I figure it'll be taking less of the load than the side walls due to how the arched roof works?

inb4 hire a structural engineer
 
Buy a section of spancrete or use that steel deathmachinefab used on his building addition. This is the easy way no engineer.

The arch I really wouldn’t worry to much about as the abutments the arch sits on. Those guys are what is gonna want to spread.
 
Just use metal deck for your roof and poor it as well. Any standard 3" cell deck will handle 4" of concrete over 15' easy
 
2big bronco said:
Just use metal deck for your roof and poor it as well. Any standard 3" cell deck will handle 4" of concrete over 15' easy
Click to expand...

I'm no Engineer, but......
A 15' foot span is pushing it, even with 20 ga. deck
Plus, having decking rolled to the arch is gonna be $
Plus, add the weight of a couple feet of soil

As was said, Hire an Engineer
Or die under it
Your choice
 
[486 said:
;n244979]

Big G. Aiden str. eng. esq. :flipoff2:
Click to expand...

His name is Big Gay Dan get it right.


latest?cb=20120314220555.png
 
Forget the arch. Pour the three walls with beam pockets dividing it into 6 bays per side of your center wall. Beams run across the 30' way. Metal deck, weld studs and two mats of rebar. All this can be pretty light stuff because the spans are small.

Metal deck is going to be cheapest when you figure in all the lumber you would use for forms, and the added labor.
 
ThePanzerFuhrer said:
Buy a section of spancrete or use that steel deathmachinefab used on his building addition. This is the easy way no engineer.

The arch I really wouldn’t worry to much about as the abutments the arch sits on. Those guys are what is gonna want to spread.
Click to expand...

I've been looking at the steel decking for other places, figure it ain't the best here as I'm looking for at least a 15' span and to do that I'd need support steel underneath it to do it flat. Dunno about getting the decking rolled for an arch in order to do a further span, probably not an approved design application lol
Spancrete is way cool, but I get stuck on its cost. It worked out to way more than ten thousand dollars, plus trucking, plus a crane rental.

On the walls wanting to spread, I figure I'll have 3/4" rods with turnbuckles tying the walls together in the direction they'd want to spread.

Got off on a tangent about having just stubs of the rods (tied into the rebar) sticking out of the walls, then using the AC arc welder through a big mean step-down transformer to heat the rods up like a soldering gun before welding them in place in order to tension them with the thermal shrinkage. That probably won't make the final cut haha.

On the formwork side of things I'm thinking about a couple options
A relatively normal dismountable set of plywood forms to cast it in panels one full width arch "panel" at a time, almost like how a quonset hut is assembled.

The other option is from looking at videos on "bubble deck" where they make these precast decking panels sorta like spancrete that they set in place and then pour concrete on top to lock it all together. It is sorta like the bottom half of a spancrete panel, but with a bunch of large hollow plastic balls arranged within the wire mesh to provide the voids.
I figure I could make a form to cast a half arch on the ground and then drag two into place and pour the top half of a set of two at a time. This would be pretty neat as it'd reduce the weight of the roof by quite a bit (napkin sketch calculations say that it'd be something like 27 yards instead of 39). Trouble is finding 1400 rigid plastic basketballs for under a buck apiece (much more than that and there ain't a cost savings)
 
Dethmachinefab said:
Forget the arch. Pour the three walls with beam pockets dividing it into 6 bays per side of your center wall. Beams run across the 30' way. Metal deck, weld studs and two mats of rebar. All this can be pretty light stuff because the spans are small.

Metal deck is going to be cheapest when you figure in all the lumber you would use for forms, and the added labor.
Click to expand...

Yep, hes over complicating it
 
How do you plan on waterproofing it? I live in an area where basements and such dont exist but I like the idea of underground bunker type things
 
Dethmachinefab said:
Forget the arch. Pour the three walls with beam pockets dividing it into 6 bays per side of your center wall. Beams run across the 30' way. Metal deck, weld studs and two mats of rebar. All this can be pretty light stuff because the spans are small.

Metal deck is going to be cheapest when you figure in all the lumber you would use for forms, and the added labor.
Click to expand...
This is a really compelling option except that the arch lets me be much more of a cheap fuck on paper. The trouble with engineered solutions is that you can never break them away from the time is money mindset.

I'm really leaning heavily toward making a curved form out of cement-stabilized dirt to cast 4'x15' panels on to set on the three walls like decking before pouring atop those
Support and formwork would be one end form the size of the roof thickness, and a few tree trunk timbers underneath to support mid-span

Figuring on setting up the roof panel forms at the same time as forming up the slab, then as I get concrete for the walls and floor delivered in smaller loads I can make two panels a week with each delivery. Gonna take like 120yd all together, so there'll be plenty of separate deliveries for me to space out long enough between to do that kinda shit.
 
2big bronco said:
How do you plan on waterproofing it? I live in an area where basements and such dont exist but I like the idea of underground bunker type things
Click to expand...

my brother's a flat roofer
I figure dumpster picked roof membranes, couple three layers oughta do it

ETA: you've got the advantage of being in sand, with a slope to boot. Drainage shouldn't be any problem for anything you build. I'm on clay and pretty flat land to boot
 
Also thinking on rolled up dumpster carpet to use as void forms in the roof
 
[486 said:
;n245089]

This is a really compelling option except that the arch lets me be much more of a cheap fuck on paper. The trouble with engineered solutions is that you can never break them away from the time is money mindset.

I'm really leaning heavily toward making a curved form out of cement-stabilized dirt to cast 4'x15' panels on to set on the three walls like decking before pouring atop those
Support and formwork would be one end form the size of the roof thickness, and a few tree trunk timbers underneath to support mid-span

Figuring on setting up the roof panel forms at the same time as forming up the slab, then as I get concrete for the walls and floor delivered in smaller loads I can make two panels a week with each delivery. Gonna take like 120yd all together, so there'll be plenty of separate deliveries for me to space out long enough between to do that kinda shit.
Click to expand...

Whats your rebar plan for the arch? Your going to need a crane to set them too. Deck is really quite cheap. Your going to have a much better product and much easier time putting it together.
 
DOOOOOD, you could really use a shotcrete machine. Bend bedsprings into whatever shape you want, then hit it with a light coat of shotcrete. Let cure, rinse and repeat until your desired thickness is reached.

Look it up!

Oh, and fawk engineers! :flipoff2:
 
Dethmachinefab said:
Whats your rebar plan for the arch? Your going to need a crane to set them too. Deck is really quite cheap. Your going to have a much better product and much easier time putting it together.
Click to expand...
Figure I'll make mats out of 3/8" that fit the curve of the form

set one into the first 3-4" thickness that'll be cast on the ground, dunno if I should go 6 or 12" spacing on the long runs, probably just go 6" because "reasons", then the short direction I'll just have 4 runs, just enough to keep everything straight
then once the panels are up I'll probably wet-set a set of bars at the peak where the two panels join when pouring the top half of the roof atop the reinforced panels
undecided on if I want to tie the panels together in the short direction, as there shouldn't be much force going in the non-arched direction.

I don't think I'll run rebar in the top side of the roof, as the top should all be in compression
 
on the crane bit, I'm thinking on trying to skidloader them into place after half-filling in the walls with dirt
 
[486 said:
;n245126]on the crane bit, I'm thinking on trying to skidloader them into place after half-filling in the walls with dirt
Click to expand...

If your set on this design, I think 2 mats of rebar spaced 6" oc. and cast the whole thing at once. Don't do it in two parts. Then set it on a grout bed on top the walls. Even better if you key the top of the wall and the panels to lock them in. You need the upper mat when you crane this thing up if you do a pick towards the center.
 
Dethmachinefab said:
If your set on this design, I think 2 mats of rebar spaced 6" oc. and cast the whole thing at once. Don't do it in two parts. Then set it on a grout bed on top the walls. Even better if you key the top of the wall and the panels to lock them in. You need the upper mat when you crane this thing up if you do a pick towards the center.
Click to expand...

3" thick is even pushing it with what my little babcat will lift
was figuring something like 8" thickness when it is done and in place, with 5-6" diameter voids running along the arch on 10" or so centers,

if I cast them full-thickness I'd certainly need a crane of some sort, probably set loops into them near the ends for lifting and use a spreader bar to try and keep them loaded in the correct direction in transit
I really hate the idea of wasting rebar, as the stuff in the top of the panels isn't really doing anything once it is in place
 
Muckin_Slusher said:
DOOOOOD, you could really use a shotcrete machine. Bend bedsprings into whatever shape you want, then hit it with a light coat of shotcrete. Let cure, rinse and repeat until your desired thickness is reached.

Look it up!

Oh, and fawk engineers! :flipoff2:
Click to expand...
missed this one somehow
my grandpa was on a few shotcrete jobs, back in the '70s when the university of mn was exploring underground building to deal with the population crisis or some such shit
man, he's been dead like 4 years now. :\
Don't really know anyone else IRL that's into that sort of stuff, it's all you fake internet friends.

I'd be all over shotcrete over top of a quonset hut, as it is what most people do for these earth sheltered things
it just ain't near so cheap as conventional forming
 
[486 said:
;n245173]

if I'm going with any sort of new-agey rebar it'll be old galvanized guardrail cable that a local scrapyard has a bunch of spools of
Click to expand...

Fiberglass rebar has been around for a long time. It was commonish when I went through my ironworker apprenticeship 18 years ago. Cable is an interesting idea... what if you were to overspan deck at 15' and then post tension it?
 
2big bronco said:
Fiberglass rebar has been around for a long time. It was commonish when I went through my ironworker apprenticeship 18 years ago. Cable is an interesting idea... what if you were to overspan deck at 15' and then post tension it?
Click to expand...
I've seen a few youtube people using it, but other than that all I ever see driving by jobsites around here is that green epoxy coated steel. Can't imagine it being cheaper than boring old bare steel.

I was thinking about post-tensioning stuff a bit back in relation to the house's footings, I can't remember why I ultimately decided I didn't like it.
it does net some really cool results for sure though
be easy enough to do with pex and cable (or maybe even just hot rolled round bar coated in wax to keep the concrete off it) and a porta-power

I'd bet my dislike of it is that there is the possibility of the cables snapping, or drilling into the cables when putting up lights or whatever
where if you just dump a large amount of dumb cheap steel into it there isn't as much to go wrong
 
Fill the void with dirt mounded in an arch. Lay your rebar in a square pattern on chairs. Pour concrete over your mounded dirt. Let it dry real good. Excavate the dirt out. Walla you have your arched roof. No forms to buy. No cranes to pay for. Just concrete trucks and a pump truck. :grinpimp:
 
you know it's funny I was thinking about dirt forms and half filling the thing with dirt in order to reach with the skidsteer
that seems way easier. Thanks for the genius idea, man.
need a lot of borrow dirt though, might have to get the tubing for a ground source heat pump trench and put one of those in at the same time

also, 2bb, now I'm thinking that the foundation pour needs a couple pieces of 3/4" hot roll stuck in it with threads cut on the ends for some post-tensioning
 
RusM said:
:eek: #3 bar? For a load supporting structure? :lmao:
Click to expand...

I dunno man, it shouldn't really be doing much of anything with the arch there
and it feels like I'm planning on using a lot of it for a relatively short span
your thoughts? easy to say to use 1/2 when you aren't paying for it :flipoff2:
 
[486 said:
;n245248]you know it's funny I was thinking about dirt forms and half filling the thing with dirt in order to reach with the skidsteer
that seems way easier. Thanks for the genius idea, man.
need a lot of borrow dirt though, might have to get the tubing for a ground source heat pump trench and put one of those in at the same time

also, 2bb, now I'm thinking that the foundation pour needs a couple pieces of 3/4" hot roll stuck in it with threads cut on the ends for some post-tensioning
Click to expand...

tensionsing should be done with cables, it needs to be able to stretch pretty good so that it can actually generate some tension.

that was my initial thought on reading through this read, for a 30' span, do a flat roof with a support at 15' and PT through the roof, but it also depends on how much dirt you are planning to run on the top. if you are really fighting a 7' frost depth, do you need 8' of dirt or will 3' still provide any sort of benefit?

Forming up an arch like an old military style ammo bunker/magazine takes a whole lotta concrete and metal and doesn't net you as much usable space as a box does, but would be plenty strong and is nice and hollow. dirt forming it would be an absurd amount of dirt, either way this is not going to be a 1 man pour
 
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