SMS:

The reason why these "engineered joists" are shaped like an I-beam is that for an I-beam to bend, then the flange at either the top or the bottom of the I-beam has to stretch in order to conform to the new bent shape of the beam.

By having the flange further from the mid-point of the I-beam, it takes a greater force to stretch the flange because the flange is further away from the middle of the beam where nothing is stretching.

So, taller I-beams are stronger and more rigid than shorter ones, and it's entirely because the thicker and wider the flange at the top or bottom of the I-beam, the more force it takes to stretch that flange. And, the further that flange is away from the middle of the I-beam (height-wise) the more that I beam will resist bending, so the stronger (more rigid) it will be.

The web of the I-beam (which is the piece between the flanges) doesn't have to be nearly as strong. It's job is just to keep the flanges separated by the height of the beam. So, normally the web on an I-beam carries very little stress compared to the flanges at the top and bottom. So, the next time you watch a 1950's movie involving rocket ships, keep an eye out for "cellular I-beams" like these:



Notice that the webs of those I-beams have holes cut in them. Since the web doesn't carry nearly as much stress as the flanges at the top and the bottom of the beam, you can reduce the weight of the I-beam significantly without sacrificing much strength by removing material from the web. However, about the only place where reducing weight is important enough to justify the cost of cutting those holes is in aerospace design where less weight means less fuel needed to take off and/or achieve orbit. That's why every self-respecting 1950's era science fiction movie about rocket ships will have lots of cellular I-beams everywhere inside every rocket ship. They were trying to be scientifically accurate when it came to rocket ship design.

(Since whatever they're building in that picture is going to be too big to fly, my guess is that they're building a ship or a offshore oil rig. Otherwise they'd just be wasting a lot of money by using cellular I-beams instead or regular ones. And, they seem to know what they're doing so I'm guessing the cellular I beams aren't a collossal mistake.)

In fact, you can even reduce the amount of OSB in the web of an engineered wood I-beam without significantly reducing the strength as in this Georgia Pacific XJ85 engineered joist:



But here, the objective is not to reduce weight as it is with rocket ships, but to save on construction time and cost by providing openings through the joists for running plumbing pipes, electrical wiring and maybe even HVAC ducts.

An architect is what you get when you teach an artist engineering.

The fact that the web of I-beams doesn't carry nearly as much stress as the flanges have prompted some companies to product I-beams with GLASS webs. In Sweden, one company is producing decorative wooden I-beams with 1/2 inch thick glass webs:



And the Poles, not to be outdone, are producing I beams with stainless steel flanges bonded to glass webs with adhesives:



Now, neither of these composite beams are built solely with strength and rigidity in mind, but they're not purely decorative either. They provide strength while also achieving other priorities, like allowing better lighting and better "opening up" the occupied space.

Still, it just goes to show that in a traditional 2X lumber joist, much of the lumber near the middle of the joist's height is wasted wood. That's because the lumber near the middle of the joist's height won't stretch hardly at all if the joists bends, and therefore won't help the wood at the flanges to resist bending. So, there is simply no need to have the middle of a floor joist just as thick as the wood at the top and bottom of the joist. We've been making 2X lumber joists the same thickness from top to bottom only because that extra lumber in the middle doesn't do any harm, and it would cost more to remove it. But, suffice it to say that if Mother Nature were in charge, floor joists would be considerably thinner in the middle, but still 99% as strong as with that extra wood in the middle. Mother Nature is not one for wasting anything.