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This is one of those things that my gut says "how could it be an issue" but am hoping someone has a more engineered answer.

I have a 13k Boat Lift from Boat Lift US. Each side has two 3/8" Aluminum beams, 3.5x7" on a 10' span. Each of these beams are made of two u-Channels on the side, so it is like an iBeam with air in the middle portion (this leaves room for the pipe that rolls the cable up). They have numerous attachments already (about 5 holes) in various places from the factory.

I want to put two 3/8-1/2" bolts through each, one about mid-way and one about 1/4 across the span, which will have a few hundred pound load occasionally on them, pulled at a diagonal. The reason for the largish size for small load is the bolt has to span about 3" in air between the beams where a cable will be attached (the bolt is thus supported on each end but has a bending load in the center).

My concern is not the load on this hole itself or the bolt, the load is small, but am wondering if drilling additional holes through the iBeam significantly weakens it? At present we are not putting anywhere near its capacity on the lift -- but one day we might.

Is there any guidance how large a hole you can drill (well away from other holes) and still have a de minimis impact on strength?

Thanks in advance,

Linwood

Note: Edited to clarify these are not true ibeams

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The reason an “I-beam” works so well is that the “meat” (strength) of the beam is in the flanges, NOT the web.

Drilling a large hole(s) in the web will not decrease its strength, especially if it’s in (or near) the neutral axis. I’d keep the hole away from the flange’s connection to the web. I’d also leave about 10x the thickness of the web between holes.

BTW, don’t let the bolt rest on the flange.

  • Thank you. I actually did this later in the day, though of course will not know until (if) we put a really heavy boat on it. The hole is 20" away from any other hole, and about 1.5" up from the flange. – Linwood Nov 7 '18 at 23:09
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The way to attach to an i-beam without cutting/drilling is called an "i-beam attachment" (google that term), it's a set of bars to grip around the bottom of the beam:

"i-beam attachment"

If that doesn't work for you and you really like drilling into i-beam, read on!

I'm concerned about the way you describe your bolt sticking out 3" from the web, with a cable pulling diagonally on the end of bolt, because the web is not thick and you would be putting a twisting force on it. (If you use a 1/2" bolt, you are using the bolt as a lever and the web will likely bend before the bolt. Not safe.)
Instead, try to find something more like an "bolt on shackle mount", which will have multiple attachment points on the web which spreads out the force:

"bolt on shackle mount or D-ring mount"

  • I explained badly. Each side of the lift has two 3.5" x 7" i-beams, separated by about 3". I also should not really call them ibeams, they really two U-channels on the side, forming one big "ibeam" with air in the middle. The bolt I planned would go through each portion with the middle of the bolt exposed in air, and a cable loop over it. So the bolt is supported at each end, but not in the middle, and is subject to a bending force, so I planned to use a rather substantial bolt, hence the need for a largish hole. I edited the question to clarify. – Linwood Nov 7 '18 at 16:54

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