Basically, you're building a swing set for a single swing with a 200lb rider. The difference is that this swing can go a full 360° instead of just back-and-forth.
Treating this as a "shelf" and using the Sagulator, I picked Eastern White Pine as an example of the standard type of "SPF" (spruce, pine, fir) that you're likely going to pick up at the local big box store.
I spec'd a 4x6" top bar* (4" deep "shelf", 6" thick, i.e. placing the 4x6 with the 6" face vertical) with a 200 lb single point load and an 8' span. It says that there would only be 0.01" total sag over the span, which it deems "acceptable" for a shelf and seems quite reasonable.
this is for a static
load. You're gonna be kicking the tar out of this thing, so it's going to be swinging all over the place. That's going to put significant
dynamic load on your "shelf" and will significantly increase the momentary sag and the long term stress on this top bar.
If you adjust your plan to use a 4x6" top bar, I think you'll have a reasonably secure mounting point for your heavy bag.
To support this mounting point (the 4x6"), your triangles are the most stable platform you can design. There is no need for an additional vertical splitting each triangle. You'll probably want to make the diagonals from 4x4 material to resist the dynamic forces you're going to impart. I'd suggest 2 @ 2x4 to connect the bottom of the 4x4s - one on each side to keep them from spreading.
HOWEVER, the one thing still not accounted for is racking forces. Your design will keep this from tipping in the direction of the triangles. There is nothing to prevent it from tipping along the axis of the top bar (one triangle toward the other).
To prevent this, you'll need some diagonal bracing from the top bar to the triangle braces. I'd suggest that on each end, you run one diagonal from the bottom of the 4x6 to the bottom of the triangle on each side. You'd want these diagonals to intersect the top bar at about 1/3 distance (about 25-30" in from each end) and run down to the center of the bottom cross member. Where they intersect the 2x4 at the bottom, I'd put a piece of 4x4 in between as blocking to strengthen and recruit both of the 2x4s to help resist racking.
An afterthought on the diagonal bracing: If you need all the space between the triangles and think that the internal bracing would get in your way, you could build a right-triangle on the outside of each triangle. This would attach at the end of the 8' 4x6 and extend the overall length of the apparatus. I'd use the same 45° angle to extend the triangle outwards, then use some more 2x4 to attach the bottom of this angled piece of 4x4 (one on each side, just like the main triangles) to the center of the 2x4 bracing at the bottom of the other triangles. Connect these using some 2x4 joist hangers.
- Will all this extra and larger lumber blow your budget? Absolutely!
- Will the one-time purchase price be lower than an ongoing gym membership? You bet!
- Will the lumber costs be cheaper than a trip to the ER if this fails on you? Without a doubt.
This will experience tremendous loads as you swing that heavy bag around. Make sure your joints are cut accurately and tightly.
Don't skimp on fasteners.
- There are lumber screws that are designed for construction loads. They're pretty pricey, but they're worth it for this kind of construction. They'll hold better than nails. Nails will pull out under these loads.
- Don't try to use drywall screws (designed for interior drywall) or decking screws (designed to hold thin deck down vertically over joists).
You may want to add some metal mending plates to reinforce joints and some "joist hanger" or "rafter tie" type of plates to hold on the inside corners.
- Some common sense while looking at the types available at your local big-box will help you understand what they have and where to apply them to your construction.
- Pick up a box of "joist hanger nails" while you're there getting your metal reinforcements. They're specifically designed for the holes in the hangers and are short enough to not poke through the other side of the 2x material. You could go whole hog and splash out a few extra bucks for "joist hanger screws", just to be sure. With the fairly small quantity you'll use, it shouldn't be too much more.
* Just out of curiosity, I spec'd a 4x4" and even a 2x4" and it deems them all to be "acceptable" for static
shelf loads. I would think neither of these would take the stresses of the very
live load you're going to have. I wouldn't use either of these.