Hey Guys,

Remodeling my infinished basement on new home. As usual, when the builder built the home, all duct work, vents, gas lines and electrical was installed BELOW the main floor floor joists, and these items hang below the top plate at edge of concrete wall. The concrete wall is almost 9' high all the way around basement.
I have started the project with 1 small room (10x12) with 2 exterior walls that butt up to the concrete wall. The concrete wall does have 1/2 insulation board on it from builder. I have constructed 2"x4" stud walls for this room and am planning on nailing them to the concrete floor. I have also nailed them to each other. I will be installing 2"x6" framing for ceiling joists to hang drywall with approved hangers to the top plate (which I have already found is acceptable by code due to the small span of the room, and up to 24" OC).
My question is... How can I fasten the stud walls to the outer walls? Can I just fasten them to the concrete like I will do the floor (with a nail gun). And if so, will this be acceptable to code for fastening. If not, how can I fasten them that will be acceptable.
Look forward to your answers and appreciate your help
Ohh, I also have a small interior wall and doorway that runs directly under a steel I-beam. What is acceptable way of fastening wood top plate to steel beam?

I'd recommend that you use pressure treated lumber for your floor plate before you start using that nail gun. Normally your wall should be built laying flat on the floor, then raised into position nailing to the floor and the ceiling(floor joists.) If the ducts are blocking your way you can box them in with a 2x4 frame. This will also give you a nailing surface for your drywall. The steel beam should get a nailing surfaced attached to it. Some nail guns can attach to steel beams but you chances of injury starts to climb. I have chosen the chicken way out a few times by drilling the nailing surface and the steel beam. Bolts will hold forever, especially if you add a bit of construction adhesive for insurance.
Hope this helped.

I did use treated wood for the floor plate and have built the walls on the floor and raised them into position. Can they (the 2x4 wall frames), be anchored into the concrete wall (as top plate does not go all the way up to the ceiling/floor joists of main floor)? And is this acceptable as a fastening method, for code?

You can anchor to the walls but you'll need to insure a straight, flat wall. Your concrete will not be flat enough for what you are doing. I would anchor a short piece of 2X4 to the wall near the top. Use your nail gun and a bit of adhesive. The 2x4 should be immediately on one side or the other of several of your wall studs. You can now use these 2x4 pieces as a nailing surface. Use a string to insure you are plumb and a long straight edge or string to insure the wall is flat.

Two other issues when doing basements. Have you created a vapor barrier between your insulation and the wall. Also, most remodeling site recommend that you not insulate the bottom foot or two of the wall cavity. This allows some heat to reach the foundation wall and floor joint. There have been a few cases where the soil froze outside that joint and buckled the foundation. A lot depends on your foundation tile and how deep the frost line reaches in your area.

Good advice was given in the previous posts. I would like to add an important point . Whenever you build a wall that only has paneling/sheetrock/sheathing/furring strips / whatever , only attached to one side then there is strong tendency for the studs over time to twist as they dry out.
To combat this , install " fireblocking". Which is a horizontal run of 2x blocks , about half way up the wall , fitted between each stud.
The sheetrock nails/screws pop and mar the surface when a single sided wall is not blocked.

Another way to attach the top plate to the existing concrete is to attach a metal clip ( like a 90 degree framing anchor) into the concrete wall by means of a masonry screw.
One flange of the framing anchor resting snugly against the underside of the top plate . So that you can see it easily . Manipulate the plate until you have a plum, true & straight wall. Then screw through the flange up into the plate