Replies

1. 
   Piffin | Sep 11, 2002 10:39pm | #1

   When I did that on a job, the granite man requested corbles for anything over 12" overhang but that was with 1-1/4" granite.

   It seems to me that some large (8") amgle braces would suffice for you and they could be hidden.

   Excellence is its own reward!
2. 
   bobl | Sep 11, 2002 11:03pm | #2

   http://forums.taunton.com/n/mb/message.asp?webtag=tp-breaktime&msg=22700.1

   bobl          Volo Non Voleo      Joe's cheat sheet

1. 
   skylonda | Sep 11, 2002 11:33pm | #3

   Thanks!  Perfect.  Sorry, I usually search the archives before asking my rare questions, but didn't this time...busted!

   /jen

3. 
   Hector45 | Sep 12, 2002 12:01am | #4

   A steel plate, 8'x35"x.5", will weigh about 475 lbs.

   That plate alone attached to the top of your base cabinet, with no granite slab, and a 200-lb person sitting on the very edge of the overhang, would deflect .04".

   If you went with a 3/8" plate, it would weigh about 350 lbs and deflect .09"

   A 1/4" plate would weigh 240 lbs, but would deflect .30"

   For each case, I assumed you attached the plate at the front and back edges of the cabinet.  If you added a third attachement along the middle of the cabinet, you'd cut those deflections by more than half.

   I don't have any material properties for granite, so I can't give you any deflection estimates for it.  From what others have said here, granite varies quite a bit anyway.

   I think I'd feel pretty good with 3/8" plate.  With a little extra care in installation, 1/4" would probably do.

1. 
   UncleDunc | Sep 13, 2002 01:12am | #5

   JJ,

   I found some numbers, but I don't have the equations. :)

   The numbers I found for modulus of elasticity of granite range from 20 to 100 GPa, plus 1 at 7.3e6 psi. The 70 and 100 GPa numbers were from makers of granite surface plates.

   My recollection from previous reading is that granite counter tops break before they show any appreciable deflection.

1. 
   Hector45 | Sep 16, 2002 04:56pm | #6

   Hey Uncle Dunc,

     Sorry I took a while to get back to you.  I went home (Indiana) for a long weekend.

     The equations I used were of the "back of the envelope" variety.  I assumed the countertop was a wide, thin beam.  (I later reconsidered that assumption, but I'll get to that in a minute.)  Assuming beam theory, I split the solution into two parts - 1) the bending of the cantilevered part of the "beam" 2) the moment induced in the countertop where it joins the cabinet.  The total tip deflection is then the bending deflection plus the moment times the overhang distance.  (Does that make any sense?  I didn't explain that very well.)

   The tip deflection of a cantilever beam is P(L**3)/3EI, where P is the applied load (200 lbs), L is the length of the beam (10"), E is Young's modulus (28E6 for steel) and I is the moment of inertia ((1/12)bh**3.  I equals 1.0 for a plate .5" thick and 8' wide, .125 for a 1/4" plate, .422 for a 3/8" plate.

   For the .5" plate, the tip deflection turns out to be .0024"

   The rotation due to the moment induced at the cabinet is M(L**2)/2EI.  For the .5" plate, the rotation equals .0036 radians.  Multiply that by 10" and you get .036" of tip deflection due to the moment.

   Total deflection is then .0024" + .036" = .038"  (I hope that's what I quoted in my first message:))

   The evening after I posted those numbers, I began to question the validity of my assumptions.  It clearly isn't a beam, especially since the load isn't distributed evenly along the edge.  I came to work the next day and made a finite element model to get a more accurate solution.  While I was at it, I studied the defelction due to 200lbs at a free corner, as well as 200lbs over a butt-width area in the middle of the edge.

   To make a long story short (is it too late for that?), a .5" plate would deflect .05" if a 200lb person sat on the corner and .016" if they sat on the edge.  For the 3/8" plate, the deflections were .11" and .038".  Those numbers were close enough to my rough calculations that I didn't post the new ones.

   I also run some numbers with a granite slab on top of the steel plate.  However, I wasn't sure what to do with the numbers once I got them.  Not only is granite quite brittle, it is also not homogeneous - there are veins and such in the material that will crack very easily.

   After considering this over the weekend, I've come up with two conclusions. 

   1) You shouldn't count on the granite for ANY strength.  Size the steel plate to take all the load.  Any contribution from the granite is a bonus.

   2) Even if deflections are large enough to crack the granite, I'm not sure that's such a big deal.  First, a 200lb person sitting on the counter is pretty extreme.  Second, the crack would likely be a very clean break and might not even be noticable once the person moved and the crack closed.

   My recommendations are still that 3/8" plate would be fine, .5" plate is overkill to some degree, 1/4" plate could work but would not support anyone who sat on the corner.

4. 
   nigelUsa | Oct 05, 2002 05:41am | #7

   Sounds like overkill with large steel plate. My way would be 1x1" steel frame made to fit (like a grid) notched into cabinets and the granite built up around the edges to hide the metal.