Replies

1. 
   1mancrew | Apr 21, 2003 06:26pm | #1

   I don't know what I am doing wrong, but i cannot get my picture to upload. sorry, i will try it again later, in case the server is not working correctly.

   nick

1. 
   1mancrew | Apr 21, 2003 07:16pm | #2

   One last try, and I think i've got it this time.

   nick

   
1. 
   RodBishop | Apr 22, 2003 01:47am | #5

   Don't know why anybody would want to build something only to have to (maybe) put some supports underneath it to keep it from bouncing. Why not do it right from the get go.

   Frame in the area to support a concrete pour. Build with 2x stock knowing that it'll all have to come out after the concrete cures. 1" plywood is fine as long as you know it has to come out.

   I like to have 4" minimum at the furthes point away from the stack, 6" at the stack itself (wedge shape?)

   You do not have to pour the blocks solid that are there now, in fact, cover the holes with screen so you don't drop concrete down there, unless you LIKE to mix concrete.

   Yes, use rebar. Borrow, rent, or steal a hammer drill with a long bit. Drill into the stack 8", 10", 12", whatever you can get. Position the holes so that they are located in the top third of the pour.

   Here in Connecticut the hearth is not subject to the 2" clearance rule so I'd pour concrete all the way to the sides and front of the hole, up to the height you need to make whatever you are using for a finish material to work. Do you need 2 1/2" for brick, or 1/2" for tile?

   You could substitute the 1" plywood for 1/2" steel plate and leave it in place after the pour.

   Done this many times, works like a charm, no callbacks.

   And no jacks taking up space.

                                                                                                       Rod

1. 
   1mancrew | Apr 22, 2003 02:57am | #7

   Thanks for the reply, but I think I'm still somewhat in need of help. I understand your comment about drilling to insert rebar into the stack, but what will keep the rebar in the holes that I need to drill, and are they used primarily for interlocking the slab to the stack, or to actually support the weight of the slab. What diameter rebar do you use for this type of job? It seems like the outer edge of the concrete would act like a lever arm, if not given a little resistance to droop from the framing. I know that the framing should not be the only means of holding the slab up, but is there harm in letting it "help out" with the stability of the slab. Even with large diameter rebar, with all that weight cantilevered out away from the hearth, I'd be nervous about tearing out my framing. I guess that's the reason for pouring it thicker at the hearth, so the extra weight bears directly to the footing....right????

2. 
   jayzog | Apr 22, 2003 02:00am | #6

   The extension hearth is supposed to be self supporting ,off the masonry mass of the chimney.

   Normally when building a new fireplace the hearth is poured all at once with rebar in both directions underneath the firebox , and cantelevering for the extension hearth.

   To meet code , the hearth cannot be supported by framing, nor can framing be supported by the fireplace.  In your situation ,you need to bring new blockwork up from the footing to support the hearth

1. 
   1mancrew | Apr 22, 2003 03:05am | #8

   Thanks for your comments,

   What you say would be bad news for me, since I cannot bring up new block work in this area. In a way I am sorry that I broke out the old hearth extension, since it would have been grandfathered, but the whole area was a mess. That pour was heavily dependant on the framing to hold it in place, since it was actually isolated from the mass of the chimney. The many years of flexing with the floor, also caused the facia brick to fall off the front of the fireplace opening. That's how I got started on this in the first place. It was obvious something had gone wrong, and the more you look..........

1. 
   jayzog | Apr 22, 2003 04:34am | #10

   Dont get me wrong- if this was my house ,I would hang the hearth on the framing as you describe, but I would never do it that way for a cliet - cause it aint up to code.

2. 
   1mancrew | Apr 22, 2003 01:36pm | #12

   Jayzog, your reply intimidated me a bit at first, but I think that the real issue here is to pass code or i wouldn't have raised this question in the first place, so I will work things out from that angle.

2. 
   CloudHidden | Apr 22, 2003 03:44am | #9

   Jayzog, what you describe is what I've always seen.

   I'd want to see this from different angles, but if what I'm picturing is accurate, it should be possible to drill a half-dozen 1/2" holes straight down into the block in the front of the chimney base. Go at least 1' deep. Make an upside down L of #4 bar, with the leg being 18" or so. Epoxy this into the drilled holes so the vertical part of the L is about an inch from the finished floor surface and comes within an inch of the front of the hearth. Then build a temp form 3" or 4" or so below the finished floor surface. Tie a coupla 4's perpendicular beneath the L's. Pour the hearth. Looks like there's enough room to form the wedge needed to support the hearth with the concrete in compression.

   That's the armchair version. I WOULD NOT DO IT JUST LIKE THIS. I would use this description in my first discussion with my favorite structural engineer, and go from there.

1. 
   1mancrew | Apr 22, 2003 01:32pm | #11

   Perhaps the best thing would be to combine both yours and Jayzog's advice. If I am understanding you right, you would want to set the vertical rebar in the core holes closest to the hearth. I would not have to drill for this as most of the cores align enough to pass a decent length down into the wall. I could then fill those cells containing the rebar to embed them. If I tied off some more rebar coming out of the stack to these "L's", and added the perpendicular pieces, I can't see that it would be able to go anywhere. I'd rather not incur the expense of an engineer, since around here it is difficult to get one to come out for a small project, without a great deal of advance notice and expense.

2. 
   CloudHidden | Apr 22, 2003 02:09pm | #13

   From the picture the core holes looked filled. There's always things that ya can't see in a picture, which is why the caveat for an engineer, etc, etc. Just make sure the rebar is encased in concrete for a good depth down those cores and filled solid, that the concrete is in compression and the rebar in tension, that the block is in good enough condition to support the added stresses. You could probably taper the slab part of this from 4 to 2 or 2.5" up front. That would help keep the center of gravity back as far as possible, which will help your cause. I'd likely form the vertical part of the pour out as far as the front face of the front block, so that my "wedge" is as far front as possible.

3. 
   1mancrew | Apr 22, 2003 02:32pm | #14

   OK, I was doing fine until you said make sure that the concrete is in compression and the rebar is in tension. Could you explain what that means further. Also, is it OK to bend the rebar using a torch, or should I rent a bender.

4. 
   CloudHidden | Apr 22, 2003 04:09pm | #15

   Don't bend with a torch. With #4's you can likely bend it enough with your hand and foot or rig up something to bend against. Doesn't need to be precise or pretty. Just needs to be placed right.

   Concrete has great compressive strength....it's pretty hard to crush it. Takes >2500 psi to crush most mixes. But it's not good in tension. Think of a 6" wide by 2 inch deep non-reinforced slab that extends out 3'. Put some weight on the free end--such as, stand on it--and you'd have just the weight alone crack that cantilevered slab off where it starts to cantilever. You wouldn't crush it, but you'd essentially be pulling it apart from itself.

   Rebar, on the other hand, is pretty hard to pull apart. You grab one end and have a buddy grab the other, and play tug-of-war. You'd have to pull pretty darn hard before it would stretch and fail--thousands and thousands of pounds worth of pulling depending on the size of the bar. On the other hand, it bends relatively easy.

   Two materials that have big flaws but significant strengths. Put them together in the right way, and reinforced concrete becomes especially useful. A classic case of the whole is greater than the sum of the parts. But the devil is in the details, or in this case, the engineering. That is, how thick is the concrete, and where is the rebar, and which forces are being applied from which direction.

   Imagine building like the picture on the right. What would happen when downward force (weight) is applied to the furthest end of the slab? Well, probably, the slab would want to fracture somewhere near where it changes direction. Since the rebar is toward the bottom, with very thin concrete there to stop it, the rebar would bend there and act as a hinge allowing that cantilevered lip of concrete to just flop down and hang there.

   Now build like the picture on the left. Apply downward force. What happens. Concrete still wants to crack at much the same place, and the rebar will still start to bend in much the same place, only it won't get to bend much at all. With the rebar where it is, the wedge of concrete beneath it will press against itself. But concrete is good in compression, so it can take a good amount of pressing against itself without crushing itself to dust. So what's holding it together now is the rebar. The leg of the concrete is holding the rebar from pulling out of the block, while the cantilevered chunk of concrete is trying to pull it out. A tug of war. But rebar is good in tension, and if it was sized right, and properly embedded in the concrete, it is stronger in resisting the tension than the forces being applied to it.

   View Image

   Pretty sure I have the basics right. Might have a word or two wrong, and hope someone will correct me if I do. Don't know enough about site conditions to know if your chimney base is suitable for this or not, so that's my caveat on the use of this method.

5. 
   1mancrew | Apr 22, 2003 05:41pm | #16

   I truly appreciate the effort that went into your response. This particular type of project is one that I have not attempted before, so the instruction/education you are giving me is very helpful (very cool visual aids). So your analysis begs another question: other than experience, how does one size the rebar for a job like this? My first inclination would have been to get the heaviest rebar I could bend, relative to the thickness of the slab, but your comments seem to indicate that the rebar's job is not about its tendency to bend or its thickness, as long as it is placed properly, and can hold the concrete together adequately when it wants to pull apart. So does tensile strength needs alone dictate the size, or is the total surface are just as important? And as was mentioned earlier in this thread, is there an advantage or disadvantage in leaving my forms in place once I have done this work? The floor in this area is now quite stiff, so I don't expect it to "rock" the slab like it probably did in the past.

6. 
   CloudHidden | Apr 22, 2003 06:44pm | #17

   You're welcome. Tensile strength is the biggest consideration. Where to learn other than experience? That's why they have engineering schools. There are ways to measure all of this, but I don't do calculus anymore! #3's at maybe 8" would probably work and would be easier to bend, but 4's (maybe 8" to 12"--just a guess) are readily available and would certainly be sufficient. There's also a book on engineering basics for builders--try http://www.amazon.com/exec/obidos/search-handle-form/002-7327997-1378466 for some titles. Taunton may have their own....can't remember.

   Hopefully some others will chime in with their own thoughts or even all new designs. Mine was at best armchair engineering. You'd want some sort of consensus on an approach before committing to it, and a single data point (even though I've done similar to this and it's worked) is not consensus. And remember, I haven't seen the base of the chimney, so I'm not commenting on its suitability for this method. (Like all the caveats?--CYA at its finest! :) )

   I see no advantage to leaving the formwork in place (code generally requires that formwork be removed anyway). Gotta verify that the concrete properly covered the rebar.

7. 
   r_ignacki | Apr 23, 2003 12:06am | #18

   I had an inspector reject a hearth with the rebar tied the way you have it. He wanted the cross pieces on top.

8. 
   CloudHidden | Apr 23, 2003 02:12am | #19

   Hmmmm....have an explanation behind that? I'm trying to think of why.

9. 
   CAGIV | Apr 23, 2003 03:13am | #20

   I've bent rebar with a conduit  bender, worked pretty wellView ImageGo Jayhawks..............Next Year and daaa. Blues View Image

10. 
    CloudHidden | Apr 23, 2003 03:49am | #21

    Wimp.

    <G>

11. 
    CAGIV | Apr 23, 2003 03:58am | #22

    no lazyView ImageGo Jayhawks..............Next Year and daaa. Blues View Image

12. 
    r_ignacki | Apr 24, 2003 12:09am | #23

    He insisted it would be stronger that way. That's what he wanted, that's what he got. I don't know if the concrete knew the differance, I wouldn't know.

2. 
   stonefever | Apr 22, 2003 12:43am | #3

   As man y here know, I'm not a professional...

   But you make me beg.  What concrete?  What pour?

   How I've seen these built:

   The masonry supply stores have a T iron (about 1" wide) that spans that 18" (or whatever).  It is placed level on one side of the ledger (to which you refer) and the other to the fireplace core ledge.   The mason then simply lays concrete block on their sides between these irons.  No core is visible.  Additional blocks (4", 6" or whatever is needed) are simply laid on top until you get close to the level desired.  Then the stone or brick being used to face the fireplace is wet mortared to these stationary blocks.

   This is successful when no movement occurs within the hearth.  If that edge of the opening farthest from the fireplace is bouncy or weak, prop a couple of floor jacks (posts) underneath to stabilize. 

   Any other ideas?

1. 
   1mancrew | Apr 22, 2003 01:02am | #4

   Wow, that is really wild. I certainly believe you, but it is hard to imagine that it could be done that simply. I will look into the device you mentioned, and thanks for the input.