(area_moment_of_inertia_I_beam_example.jpg) Attachments - area_moment_of_inertia_I_beam_example.jpg (70KB - 840 downloads) A bigger moment, means stronger in that direction. The formula at the bottom, I subscript X, is the one you would use, since the webs (verticals )are constant, regardless of width, you don't even need to calculate that, if all you want to do is compare. Image is of an I beam, but the principle is the same ( in the up and down direction ), just have two webs instead of one. Which is exactly what Exit is referring to regarding mass moment of inertia. It will take 1.5 times more effort to stretch the bottom flange, and 1.5 times more effort to compress the top flange, due to the increased area over the 8". The distance between the top and bottom plates will gain you the most ( by memory, I think the distance relationship is squared,whereas increases in area have no multiplier, 1 to 1 ), but since you are keeping that constant, and only increasing the width, you will still get more strength from the 12" wide. West Central Alberta Coldest, wettest edge On an I beam.ĭonno if tubes and I beams act the same, or different? Twocompeting theories here. Since the skinny tube weighs less, the bridge could carry more.īut you are saying the strength comes from the horizontal sufaces stretch/compress, the vertical is just the spacer. He's talking tube, not I beam, so he is figuring the vertical walls hold the same, while the horizontal walls are just spacers.
Or at least that's what they taught me back in engineering school. I-beams strength is in the flanges, and the 12x12 will have more flange than the 8x12. Section modulus or Area moment of inertia is what you need to look up.
Am I right or wrong? What do I need to type into google to find more answers? Using the I beam designs, I feel the rectangle would carry more net. Which would carry more net weight in the center of the bridge?
Same wall thickness.īoth tubes have the same vertical measure, but the rectangle would have the vertical sides closer together than the square. Using just arbitrary numbers for easy figuring, use a 12"x12" square, and a 8"x12" rectangle on edge. You can choose between a square tube or a rectangle tube for the frame.
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