New hearth slab design question

[DrakeRemoray]

Hi All,

I am planning to pour my structural concrete slab, then a perlite hearth slab per the new design discussed here:
http://www.fornobravo.com/forum/showthread.php?t=499 (Hearth slab construction)

I am planning an enclosure similar to the gable house design. If I do this perlcrete on top, how will I attach my gable house walls to the slab? Will I drill all the way through the perlcrete to anchor in the structural concrete? Should I set some j-bolts in while pouring? I am concerned that the perlcrete is not structural enough to hold my enclosure walls.

Any comments or suggestions?

[jengineer]

I have not built yet but I would tend to put in the anchor bolts for the frame-up as you are pouring your 3.5 inch concrete slab. The Gable enclosure is just a minature house so one could employ the same building techniques.

[DrakeRemoray]

I am leaning toward Jbolts into the 3.5 inch structural slab AND through the perlcrete above it. Then attaching metal studs to the jbolts on top of the perlcrete. Should work right?

[Marcel]

Originally Posted by DrakeRemoray

Hi All,

(M) <snip>

I am planning an enclosure similar to the gable house design. If I do this perlcrete on top, how will I attach my gable house walls to the slab? Will I drill all the way through the perlcrete to anchor in the structural concrete? Should I set some j-bolts in while pouring? I am concerned that the perlcrete is not structural enough to hold my enclosure walls.

Any comments or suggestions?

(M) Unless you build your enclosure walls with rock you might want to keep the strength - rigidity in perspective. My walls are of horizontally placed HardiPlank, a thin cementous material. The purpose of the housing, as James wrote, is to keep the oven dome dry and to contain any poured in dry Perlite.

(M) You *will* need a masonry bit to drill into the dry Perlcrete, and a regular bit for making holes in the sheet metal studs.

(M) If you set machine screws (with washers & nuts on top) in the wet Perlcrete, I suggest keeping them small. Remember that presetting means that lining up the hole positions in your metal "mud sill" that runs horizontally, may be difficult.

(M) The only lifting force on those walls would be from a breeze (unless you live in Katrina land) so heavy bolts seem over engineered. I predriled through the "mud sill", while it was separate from the stand, with a standard bit. I then laid the mud sill on the concrete ( I used the conventional order of Refractory > Perlcrete layers) and positioned a masonry bit over - in the holes I had predrilled in the mud sill. I used the blue colored concrete screws with a hex head to secure the mud sill. Since you will be drilling into a softer Perlcrete, I'd recommend getting as long a concrete screw as practical. They don't need to be wider than 1/4" if you use enough of them. I think I used 6 for each length.

(M) Once you start on your vertical walls be sure to set up your corner studs first so that you have needed "nailing" support at the corners. The studs you buy should be the narrowest available so you have as much space as possible for the insulation you will later add.

(M) I hope that the following images are clear and helpful but I have no closeups of the attachment of the mudsill.



(M) The next may give you some idea of the size of the metal studs which slip into the mud sill:



(M) I hope this is helpful,

Ciao,

Marcel

[jengineer]

Marcel - Ok so being a licensed Mechanical Engineer I may tend to over design. Over kill is lovingly part of my vocabulary. Even so if one wanted to dispense with drilling through the perl/concrete layer and use anchor bolts one could get by with just 2 per side. This would make lining up the sill plates fairly easy. As with most homes it is ok to not have corners at 90 degrees. Depending on where Drake's home is he may get some stiff winds coming off the nearby mountain range. I remember my dad & I flying a kite with close to a mile of line laid out at the Red Rock amphitheatre. It took us 2 hours to pull it back in and my dad's gloves were trashed at the end of the day. That said I would want a couple to a few bolts holding down the house.

My first choice is for overkill, put in the threaded rods as you pour.
My second choice, which gives you a chance to design on the fly, is to drill and use expanding bolts after the dome is finished.

[james]

You can also use wood forms to keep the vermiculite layer under the oven, but not all the way out to the end of the hearth. That way, you can attach your metal studs directly to the concrete layer, and not mess with going through the vermiculite layer -- a solid connection and the concrete layer is more likely to be level and smooth.

James

[DrakeRemoray]

Thanks again for the responses.

I went ahead and put jbolts down into the structural concrete layer, so they stick up through the perlcrete layer as well. And while I am sure 2 per side would have worked, I am definitely in the mode of Over do it or don't do it at all (thanks paulages), as I used many many jbolts. I will remove the forms tonight. I am sooo tired from the pour and the stand is starting to look really really big…but no turning back now!!

Drake