I'm framing half of my basement. Through the middle of the house runs a 24' main beam made of 4 2x8's, held up by 3 metal posts. Can I use the main beam as a top sill for the 2x4 studs? Or do I need to attach a 2x4 to the bottom of the main beam to use as a top sill? The ceiling will be up against the joists, which is obviously higher than the main beam, so the side of the main beam will be the top of the wall. Hope I explained this clear enough.
Thanks
Tom

It can be done either way. Because it's a non-load-bearing partition, it doesn't matter. It IS going to be a non-load-bearing partition, isn't it? Meaning, the steel posts will still remain in place?

yes. the steels posts will be holding up the beam, and they will be hidden inside the new wall I build.

Thanks

If you run the studs up to the underside of your girder, it doesn't matter if you use a 2x or not, but it IS going to act as a load bearing wall whether you want it to or not. The fact of the matter is that as yoru girder gets loaded and deflects that it is going to load your wall.
It will likely only be for live load since the house is built and the it is already seeing the dead load deflection, but I would try to come up with a detail that will allow the girder to deflect WITHOUT loading the wall. You could have serious consequences if the wall gets loaded, such as cracking of your basement slab. You could also have some not-as-serious ones, like your drywall cracking from the deflection of the girder.

Assume the spans of the girder are about equal. Therefore, in 24 feet, with three posts, there are four 6-foot spans. Let's assume the worst, that it's a two-story house, with attic floor, second floor and first floor loads carried by the girder. That's a total of 30+40+50 pounds per square foot total load. What's the dimension of your house the other way? Let's assume 28 feet, so that the girder carries that load for 14 feet. That's a total of 1680 pounds per foot on the girder. What kind of wood was used for framing? Let's assume No. 2 Douglas Fir - Larch. You would need a LOT more than 4-2x8's as a girder, so let's say it's only a one-story house. Now we're at 1120 pounds per foot, and now your 4-2x8's just make it as a girder...well, it isn't that close. If my calculations are right, you can expect a total deflection under absolute full TOTAL load of 0.004 inches...and that would only be at mid-span. It would decrease in each direction as it nears the supports. The shrinkage of your framing lumber would be more than that. See what I mean about overkill?

If it will help you sleep better at night, use a galvanized steel stud track fastened to the underside of your girder, and frame the walls in sections, slipping them up into the stud track, but not fastening them, so that there is about 1/8 inch space between the top of the top plate and the inside of the stud track. That will allow the four thousandth of an inch deflection to occur without pushing your bottom plate to China.

I don't know where you came up with that number, but I just ran the calc two ways myself. Once assuming the bema is continuous over all supports and finding the right skip loading to produce max deflection and once assuming all spans are simply supported. I used DF-L No.2 with E=1,600,000 psi and 4-2x8's with Ix= 190.52in^4. I get a different answer than you both times. The deflection is small, but can't be neglected.

The object of this forum is to help people, and that is what some of us do, to the best of our ability, using every bit of training and experience we have. Others arrive here to blow their own horns. So be it. Those of us who help will go right on helping people. Our messages of thanks are all the payment we desire, and there have been lots of them. Somehow, I don't think the horn-blowers will collect quite so many.