36" in Seattle

[redmen4]

Re: 36" in Seattle

kebwi,

I was like you in the beginning analyzing and analyzing everything. I read the pompeii plans about 10 times. I finally said to myself that I am not constructing the Empire State Building, in your case the the Space Needle, and just plowed ahead.

My thread is under the Pompeii folder "New Oven in Connecticut"

JQ

[kebwi]

Re: 36" in Seattle

My ever-evolving hearth-rebar design. In the diagrams the red cores are empty, and thus cannot contain any vertical rebar.

First, if you compare to my earlier designs, you will notice that I beefed up the "weak" corner by converting from a 3-half-block 'L' to a 2x2 half-block column. That space was useless for storage anyway.

The Pompeii directions suggest 1/2" rebar on a 12" spacing. It was subsequently suggested in this thread that 3/8" rebar on a 6" spacing will be functionally equivalent (thanks).

Problem is, it was also suggested that the horizontal rebar be hooked down into the vertical filled cores, and the centers of those cores are on an 8" spacing, not 12" or 6". Instead of stretching it to 16", thus going weaker than any design or recommendation, I went to an 8" spacing (perhaps an alternating mix of 1/2" and 3/8"?). Consequently, only every *other* horizontal rebar can hook into the vertical cores because the remaining horizontal rebars are suspended over empty cores.

Before people suggest that I connect up some of the open corners into long winding rebars, bear in mind that I need to get this stuff home. Thus, I really can't buy anything much longer than 9'-10' without doing something fairly elaborate on my roof rack to prevent the rebar from flopping around. Lashing/taping it to a 12' 2x12 seems like a possibility I suppose.

If you aren't completely drained by the tedium of this discussion, I would appreciate any additional input.

Thanks.

[kebwi]

Re: 36" in Seattle

First gravel dump. I bought this batch, carefully creaming the large stones off the underlying clay so this is extremely "clear", as they say in the gravel industry (apparently). The rest of the gravel will come from a scrap gravel pile in my yard that was left over from an earlier job (see album photos). It has a high quantity of "fines" in it (gravel colored clay/dust). I think once it's all mixed together it'll make a nice base.

[kebwi]

Re: 36" in Seattle

Well, I finished digging out and leveling the foundation. This photo doesn't look much different from the earlier photo of the foundation, but it is actually considerably deeper and perfectly bubble level now. I already dumped some gravel in it too. Photos forthcoming.

[Neil2]

Re: 36" in Seattle

End area is all that counts. Structural design usually just stipulates the percentage of the cross section that is to be steel, the size of the individual rebar pieces doesn't matter (with a few exceptions).

Rebar works solely in tension or compression when part of a reinforced concrete structure. It does not resist load through any kind of "flextion" per se, although I understand what you mean.

[kebwi]

Re: 36" in Seattle

I suppose one could ask -- although this is venturing deep in academic obscurity -- is it easier, harder, or equally difficult to flex a single rebar of X cross section or multiple smaller rebars whose cross sections sum to X? If flexing the bundle is easier, then the bundle is "weaker".

It seems to me that total cross section is not necessarily directly equivalent to flex resistant strength. For example, instead of rebar, consider flat metal slats (like one side of an angle iron. In fact I see this stuff sold next to angle iron everywhere). Lay the slat flat and it's easy to bend vertically (but hard to bend horizontally). Stand it on edge is it hard to bend vertically (but easy to bend horizontally). This is how I-beams work of course. In their vertical orientation they provide excellent resistance against vertical flex.

The point is, total cross-section wasn't the only determining factor since the slat had the same cross section in both cases. Vertical height of the slat played a crucial role.

Back to the rebar example: A single 1/2" rebar is thicker, thus "taller" than two 3/8" rebars next to each other. Thus, as per my example, one might imagine that the single 1/2" rebar is more flex resistant than two 3/8" rebars.

None of this really matters too much of course. I'm just wondering "out loud". That's what I do. Sorry.

[kebwi]

Re: 36" in Seattle

Everett Steel doesn't bend rebar, although they will cut it for me (which is ironic since I'm not too worried about cutting it with a grinder). They really are one of the only options I've found around Seattle actually.

Home Depot and Lowes (and Ace) have rebar for a much higher price, and obviously don't bend rebar either.

Thanks for the info. It's coming together.

Cheers!

[Neil2]

Re: 36" in Seattle

3/8 inch rebar has a little over half the end area of 1/2 inch rebar. If forno bravo is recommending 1/2 inch at 12 inch spacing with 1/2 inch rebar, you will get the same steel ratio (or a bit better) with 3/8 at 6 inch spacing.

I suggest 3/8 inch because it is a whole lot easier to cut, handle and bend than the 1/2 inch rebar, especially with home improvised tools (grinder and a length of pipe). I even like to use the 1/4 inch rebar but this is hard to get.

As for embedding or overlap I would go with at least 12 inches with 3/8 rebar. Or you can "button hook" the end. The dead end of a straight piece of rebar doesn't "grab" the rebar fully for about the first 12 inches or so. The rebar should always bend around the corner and overlap the next piece - avoid "dead ending" it. Tie the overlap with two pieces of rebar wire.

And again, your local rebar supplier will almost certainly have a bending tool on site for customer use and will show you how to use it. You can also rent rebar benders - they are easy to use. These tools will cut, bend and button hook with precision. Or you can wrestle your rebar with a piece of pipe.

The 6 inch WWF (welded wire fabric) is primary for use in crack control. It doesn't' hurt to throw it in but don't rely on it for structural strength.

I also see on your plans that your are casting an integral approach slab. This will crack in the vicinity of the oven wall. Consider casting this in two steps - this will provide a construction joint so the crack is controlled.

[kebwi]

Re: 36" in Seattle

Here's my current plan for the foundation rebar arrangement w.r.t. the first course of concrete blocks. The foundation is a weird shape because it fits into a corner against the retaining wall (see yard rendering and progress photos in my album). It will also have the standard concrete mesh (what is that, about 6" spacing and 1/8" diameter?).

[dmun]

Re: 36" in Seattle

You can also use a couple of lengths of pipe (like half inch black iron gas pipe) as a lever pair to bend lengths of rebar.

[KINGRIUS]

Re: 36" in Seattle

You can run the rebar vertically at the corners all the way to the foundation slab and leave about 18" extra rebar sticking up. Pour your four corners and wait for the cement to get good and hard (@least 36-48 hrs). It will be much easier to bend with it anchored in the cement.
I managed to bend my 1/2" rebar with a 4 lb. sledge by leaning one end up on a brick and repeatedly smacking it till the bend was started. But I realized that it would've been much easier to just set it in the cement and let my foundation hold the rebar whilst I smack it over. I you use 3/8" it'll be that much easier.
You can use the wire ties and tie these vertical bent rebar to your horizontal grid. I hope that makes some sense.

Darius

[kebwi]

Re: 36" in Seattle

So on this rebar business, when you suggest bending all the cross bars down into the cores (obviously only the cores that are filled, which is every other core), how far down should it go? I ask because, if it extends all the way to the floor, it seems redundant with the vertical rebar already in the core (as per the Pompeii directions). Should they be separate pieces: full height vertical straight and separate cross-hearth hooked a few inches? Or should it just be one super long piece: up one core across the hearth, and down the opposite core?

Grrr, sorry, I'm not trying to be pedantic. I suspect people will tire of offering advice before I get the best design settled in my mind and at that point I'll just go with the consensus where ever everyone's patience with me peters out. My apologies.

:-)

[Les]

Re: 36" in Seattle

I used 1/2 inch spaced 11 - 12 inches. You can go down to the box stores and buy a couple of feet of pipe - it's not that expensive. You do want to bend it and drop it into the cores. I didn't do that, but had my numbers ran through a PE. If I had the bend, it would have added a huge amount to what the hearth could support. Well worth the effort.

Les...

[kebwi]

Re: 36" in Seattle

Sorry, I misunderstood you. When you said hooks, I first thought you meant J-hooks at the bottom of the "lintel" bars. Upon rereading your response I now think you meant a ninety-degree turn on every cross-hearth bar, just like the lintels...which means my previous response didn't quite make sense.

Same problem stands though. I need an efficient way to make numerous clean ninety-degree bends.

Sorry for the confusion.

[kebwi]

Re: 36" in Seattle

Well, I had originally planned on using 1/2" since it's the Pompeii prescription and seems naturally stronger. I can go with 3/8" if folks think it will be strong enough for the job. You suggested 6" spacing. I thought Pompeii suggested 12" but maybe you're accounting for the 3/8" being weaker?

Should I use 1/2" for the foundation and 3/8" for the hearth? Or just 3/8" all around to make things easier to work with?

As for the hooks you suggest at the ends, I'll be lucky if I can figure out how to make a clean ninety degree bend for the top. I don't have any pipe lying around. I was kind of hoping to get away with no rebar bending at all on this project. I'm a little worried now that I'm getting all this advice to bend the rebar. I mean, anyone can wrap it around a tree and make a mess of it, but to get a nice tight corner, I just don't know. I need pipe, right? Is there any other way to do it (without a $3000 bending machine)?

Most of my 3D modeling is done with Meshwork. Bryce makes things look prettier but it is virtually unusable for actually building models. Sometimes I make the models in Meshwork and then render them in Bryce if I want a near-photo-realistic appearance, but for this project it hardly matters, maybe for the picture of my yard (in my album) I should redo it in Bryce. I dunno.

Thanks.