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Will thin steel bars really help prevent granite from breaking?

  1. Oct 1, 2013 #1
    This construction practice made me curious. As soon as I lifted the 3/16 inch thick by 2 inch wide steel bars (now routed into the plywood subtop under our granite counter overhang) I noticed that these 12 foot long metal pieces bend considerably under their own weight.

    It is common practice to place these under the granite, spaced about 1 foot apart perpendicular to the length of the overhang, to reduce or eliminate the need for corbels (those wood support pieces you hit your knees on at a bar).

    Even a 4 foot section of steel bar will bend a little bit. I don't understand how this makes a significant difference in preventing the granite from cracking when someone decides to jump up and down on the countertop.. since the steel bends.

    Is this because the steel bends more uniformly across the entire span, whereas plywood can bend at sharp points wherever the plywood has a weak spot? Is it because these steel bars routed into the plywood subtop and glued with epoxy help more with tension and compression, rather than just resisting bending?

    Here is a similar discussion but I don't think anyone had a good engineering answer yet:

    http://diy.stackexchange.com/questi...-countertop-overhang-with-embedded-steel-bars
     
    Last edited: Oct 1, 2013
  2. jcsd
  3. Oct 1, 2013 #2

    UltrafastPED

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    Granite (and most stone) is strong under compression, but weak under tension.

    The steel rods provide "tension relief" ... the expansion of the steel is small enough that the granite does not need to feel the tension. A similar idea is used with reinforced concrete:
    http://www.constructionknowledge.net/concrete/concrete_basics.php
    - reinforcing concrete counter tops.
     
    Last edited by a moderator: Sep 25, 2014
  4. Oct 1, 2013 #3

    nvn

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    ddbear: When you lifted your uninstalled steel rectangular bar, alone, you were applying high local bending stress to the steel bar, about the steel rectangular bar cross section minor axis. Therefore, bending deflection, about the steel bar minor axis, occurred. If you instead apply high, uniform, axial tensile stress to your steel bar, then you will find the steel bar does not deflect (elongate) hardly at all.

    If you install a very thin steel bar only at the top of the plywood cross section, where the overhang begins, and if the plywood is very thick, then the steel bar is subjected only to uniform axial tensile stress, not local bending stress.

    The steel bars can be quite thin (say 0.5 to 2 mm thick), if your plywood is quite thick. The steel bars will not really be effective if the plywood is too thin; i.e., if the steel bars are too close to the plywood midplane. How many millimeters thick is your plywood? How thick is your granite? And what is the distance from the beginning of your overhang to the outer edge of the overhang? If you already had the exact flexural strength (MPa) for your particular species and type of plywood, the exact shear strength of your epoxy, and the exact tensile ultimate strength of your granite, then one could perhaps compute, in advance, whether or not a particular design would be adequate.

    By the way, post 1 does not state whether or not the granite is bonded to the sub-top, which makes a big difference in the answer. Above, I assumed the granite is not bonded. If the granite is instead bonded to the sub-top, then your steel bars will be almost at the midplane, in which case they will be completely ineffective. Is your granite continuously bonded to the sub-top? Or not bonded?
     
    Last edited: Oct 1, 2013
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