Will thin steel bars really help prevent granite from breaking?

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In summary, these metal pieces installed under a granite counter provide tension relief, and do not bend as much as regular wood corbels.
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ddbear
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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
 
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  • #2
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.
 
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  • #3
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?
 
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1. Will thin steel bars really help prevent granite from breaking?

Yes, thin steel bars can help prevent granite from breaking. The steel bars act as reinforcement, providing additional strength and support to the granite slab. This can help distribute the weight and pressure placed on the granite, reducing the risk of cracks and breaks.

2. How do thin steel bars prevent granite from breaking?

Thin steel bars are typically installed within the granite slab during the manufacturing process. These bars are strategically placed to create a grid-like structure within the slab, providing support and preventing the granite from flexing or bending under pressure. This reinforcement helps to distribute the weight and stress placed on the granite, reducing the risk of breakage.

3. Are thin steel bars necessary for all types of granite?

No, not all types of granite require thin steel bars for reinforcement. Some types of granite are naturally stronger and less prone to breaking, and may not need additional support. However, for larger and more fragile slabs of granite, thin steel bars can be beneficial in preventing breakage.

4. How thick should the steel bars be to effectively prevent granite from breaking?

The thickness of the steel bars used for reinforcing granite can vary depending on the size and weight of the granite slab. In general, thinner steel bars (around 1/4 inch) are suitable for smaller slabs, while thicker bars (up to 1/2 inch) may be used for larger slabs. It is best to consult with a professional to determine the appropriate thickness for your specific granite slab.

5. Can thin steel bars be used to repair cracked or broken granite?

No, thin steel bars are not typically used for repairing cracked or broken granite. These bars are primarily used for reinforcing and preventing breakage in new or existing granite slabs. If your granite has already cracked or broken, it is best to consult with a professional for proper repair methods.

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