Welding challenge - for me anyway ~ Pipewrench handle

Gerald J.

Posted 12/18/ 14:54 (# - in reply to #)
Subject: Filling the H cross section.

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I didn't get great grades from taking "Strength of Materials" partly because I was taking too many courses that semester and didn't really need the course for electrical engineering but I did retain some fundamentals.

In bending a beam, like that wide flange I beam of the wrench handle, most of the strength is in the flanges. The web serves mostly to keep the flanges spaced. The formula for the moment of inertia, which is proportional to the bend strength, is bhhh/12 for a rectangular bar where b is the width, h is the height. Point is the taller the bar is in the bending direction, the strength goes up by that height cubed, but only goes up by the thickness in width. There is some shear load in the web too, but it only keeps the flanges in place. If you had a beam say 1" high by 3/4" wide with 1/8" thick flanges and web, the web's MOI would be .125 * .75 *.75 *.75/ 12 = .. While the bending strength of the flanges would be .75 * 1 * 1* 1 /12 - .75 * .75 *.75 *.75 /12 = ., or over 8 times contribution to the strength from the flanges. Total about .040.

If you filled the sides of the H with weld metal as strong as the original material, you'd find the weld contribution was .625 * .75 *.75 *.75 /12 = ., 2/3 that of the flanges giving a total strength of .062. The weld metal provides barely over 1/2 the strength of the original beam so if you don't get the original metal welded perfectly, filling will leave you with a very weak spot in the bending beam.

If you added 1/16" to the outside of both flanges you'd gain .75 * 1.125 * 1.125* 1.125 / 12 -
.75 * 1 * 1 * 1 /12 = .. If you added 1/8" to the outside of the flanges you'd gain
.75 * 1.25 * 1.25 * 1.25 / 12 - .75 * 1 *1 *1/12 = . seriously increasing the original bending strength of web plus flange of .040. The total of original metal plus the 1/8" thicker flanges would be .099 practically three times that of the original metal. Providing the weld of the original metal was perfect.

If you added a 1/4" bar top and bottom the strength increase would be even more dramatic because of that h cubed factor. That added flange material would add
.75*1.5*1.5*1.5 /12 - .75* 1*1 *1/12 = .148, 3.7 times the original beam strength giving a total about .188 4.7 times the original.

They do make bigger pipe wrenches, and for a disk a 6' may not be strong enough.

Gerald J.