aluminum I-beam weight reduction

[letterman7]

Mods, if this isn't the correct forum, please move.

Hi all,
Just getting into this forum with my project camper. For those not following my only other question yet posted, I own a vintage Ultravan, an all-aluminum, stressed skin motorhome. Built like an airplane, it has no chassis. The strength comes in from the formed aluminum ribbing and, surprisingly, the gas and water tanks that go side to side under the coach. In my coach the gas tank is aluminum, while the potable water and black water tanks are fiberglass. Over the years the water tank had been high-centered at some point and punctured, so that has to be replaced and the gas tank was punctured by the transporter to my home in such a way that it, too, needs replaced. So.. since I have to replace two tanks, I'm going to have a custom sized gas tank welded up a bit smaller than the original (in width, length will be the same), and order a plastic potable water tank.
In order to maintain strength in the floor (did I mention the flooring plywood rests directly on the tanks? ) and to add rigidity to both the suspension mounts and the coach itself, I'm planning on adding three pieces of I-beam spanning the width of the coach. The ends of the beam will have plates welded on to which I can then solid-rivet to the coach's structure, much like the tanks were installed. Actually, the tanks were installed with sheet metal screws, but I digress. The beams are not large as I wanted them the same depth as the tanks - 5", non-filleted webs. But they still will add some weight. In this coach, weight savings is everything with the powerplant... the less weight, the better, hence the newer, smaller tank capacities (old capacities were about 42 gallons each).
So.. being built like an airplane and seeing how most aircraft wing spars have holes in them for both weight reduction and systems wiring, I'd like to do the same for the I-beams. I found a great article https://www.google.com/url?sa=t&rct=...59oKNA&cad=rja that outlines just such a project, so that does give me some confidence, but I'd like some real-world input from anyone who has done anything similar or has some engineering knowledge that could provide some insight.

Rick

[Joe Hartson]

Rick, Google honeycomb aluminum panel, you might find this interesting. High strength and light weight.

[letterman7]

Hi Joe,
Yep, familiar with honeycomb panels. I was considering them for the flooring but most are difficult to source in singles or very small quantities. How they are designed are a bit different than an I-beam, relying the whole to provide strength. Strip a piece of one side off a honeycomb and that section collapses.

[sblack]

Hneycomb is the same as an ibeam really. The face sheets are the flanges that take tension and compression when the part is bending and the honeycomb holds the face sheets in place and takes the shear, just like the web on the ibeam. The whole idea of both is that you concentrate your material as far from the middle as possible so as to have the maximimum stiffness and strength per weight.

If you remove the flange off one side of an ibeam it will collapse too.

[letterman7]

Ah, I see the correlation now. Yeah, makes sense!