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,
Note: Edited to clarify these are not true ibeams