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I need to install some heavy granite contertop (650mm depth) on masonry walls, and I have a lot of angle iron (1 1/2" by 1/8") and U channel (70mmx30mm x 5/64") leftover and thought about welding them myself.

support bracket

I do remember something about levers and pull strenght, but I don't quite remember how to calculate the ratios of horizontal and vertical legs to get a good support. Can anyone enlightmen me?

I thought about using at least 300mm (1 ft) on the vertical leg, but using angle iron I will be able to use only 2 bolts (one at the top and 1 at the bottom, since the middle doesn't really need to be bolt because most of the force is at the top).

Another choice that I have is using 4mm (5/32") plate as the backing (which goes agains the wall) about 100x100 in size welded to a U channel (5/64") cut in a sort of triagular shape (like the image below). This will allow me to have 4 bolts (2 on top and 2 on bottom) on the backplate.

The first design has less bolts but a longer leg, the second has more bolts and large footprint, but a shorter vertical span. What would be stiffer/stronger?

enter image description here

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The first option would be vastly more appropriate for a heavy load. Pullout tension is inversely and exponentially proportional to the length of the mounted leg, so you'll have many times more stress on fasteners with the second option. For that matter, your backplate may not be up to the task of resisting that much torque.

As to how long the legs need to be, that depends on several things. First, what will you use to mount to the block wall? Can it handle the stress? Next, will the blocks in the wall take the stress? If you put all your anchors in the block web you're unlikely to break the blocks. If you were to anchor to the hollow cavity faces, you just might tear them out.

Keep in mind that the length of the horizontal leg isn't as important as is the actual depth of the countertop. I'd see to have a vertical leg that's at least 50% of the counter depth. I'd also put double fasteners near the top, where the most pullout force exists.

It's difficult to offer more specific advice without dimensions, weights, etc., but this should get you in the ballpark.

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  • The countertop will be 650mm long (a little over 2'), in the first case (angle iron) I thought about using at least 300mm (1 ft) on the vertical leg, but using angle iron I will be able to use only 2 bolts (one at the top and 1 at the bottom, since the middle doesn't really need to be bolt because most of the force is at the top). If I went with the second design, the back plate would be a bit bigger than the U channel (so about 4x4 inches), this will allow me to have 4 bolts (2 on top and 2 on bottom). I have more bolts, but also more pulling force on the top... :/
    – Luiz Borges
    Jan 23, 2017 at 18:15

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