basementframingsoffit
I'm finishing my basement and need to build a soffit around the HVAC trunks, pipes, and a wood beam. I'm planning to build a ladder on one side of the ducts and another between the ducts and the beam.
I'm hoping that I can just screw the drywall into the beam to make one large soffit.
Go ahead and put that wall right on under that beam. The beam is a bearing surface for what is up above the beam. The wall you are adding is a non bearing wall that will not hinder the bearing capacity of the beam. Nail it in place using standard nailing practice. In other words, don't load it up with no more nails than you need, one nail staggered every 16", 2 on the ends.
The thing about the non-bearing wall you are building, it can never be made a bearing wall unless you do something with the floor it sets on. The reason why I bring this up, it is possible that in the future, and it is highly unlikely that somebody at some time sees this wall under the beam and thinks "well this wall is under this portion of this beam and I need to run such and such through this area, I will just cut it out since there is this wall under it." WRONG. The beam MUST remain intact as if there is no wall under it.
I usually use wood for everything. This is how I would put it together with 2x4s. Personally, I think 2x4s are overkill for this, but that's just me. I suggest using screws as your fasteners, and with screws your "bottom plate" is probably sound. Screws will be fine in tension. If you are using nails, I'd stick to the sketch so that your nails are loaded in shear.
The main code issue you are going to be dealing with here is fireblocking. You have this open plenum that connects all the joist cavities which allows fire to spread quickly across the joists. You'll want to stuff insulation in areas that will prevent airflow from one joist (and wall) cavity to another.
Best Answer
In general, there is no problem in screwing drywall (or most other materials or light weight fixtures) into any framing members. This includes 2X studs, beams, steel studs or other variants on these.
There are restrictions on notching and drilling large holes. Dimensional lumber is most forgiving of these modifications, but manufactured beams have especially strict rules about what size and shape holes can be punched through them, and also restrict where they can be put.
Screws, especially thin drywall screws, present no such compromise. If there is some reason why you don't want to screw into the beam, you could use construction glue, but this is much harder to remove if there is some need to change or repair the area.