lumber
I want to use a framed doorway to do chinups but I'd hate to come crashing down to the floor on my tail bone. This doorway is framed but not drywalled.
I realize there are a lot of factors but not sure how to calculate this. The stamp on the 2×4 says "A. F. P. A. 134 S-P-F xxxx KD-HT 2" where xxxx is illegible. I don't know what any of that means but maybe it's useful.
I weigh about 210 lbs.
From the similarity of the lines, I'd say you pulled something out of the old 2x4 (nails) and dragged them across the board, perhaps as you twisted and pulled to separate/remote the board from wherever it was before. I don't believe there are any insects that travel in perfect unison like that.
No. It may support two aquariums without collapsing, but it will sag substantially. Adding legs in the middle would resolve that problem, but I'd also consider doubling the beams, front and rear. I'd have confidence in that setup as well (though you'll still probably see some sag).
Yes, the front and rear beams each carry half the load. This is a rare example of a very evenly-distributed mass.
Other notes:
It appears as though your upper bolts are very near the top edge of the beams. SPF (spruce/pine/fir) lumber of the last few decades is very soft and prone to splitting. It won't take much to rip those bolts out of the upper edge of the lumber. I'd leave at least 1" between the edge of the lumber and the bolt holes, or add two more in between.
SPF lumber can vary quite a lot in terms of strength and stiffness. By using a single beam front and rear you incur some risk that either the front or rear beam will sag more than the other, resulting in a twist in the aquarium. That's what breaks glass. With something like this, rigidity is key, not just strength.
Here's my 55 setup. notice that it really only has 4 primary points of contact, but they're in key locations and provide a flat plane of support.
Best Answer
I expected to see a double 2x4 header while you have a single; I'd make it a double. FYI, you can look up weight it can support on beam span table/calculator. You can rely upon the jack studs to support the weight but I'd add metal brackets (e.g. Simpson Strong Tie) to add additional support (i.e. for the not strictly vertical forces) since you're jerking around and brackets are less than $1 each. Bracket the king studs to the header and two more brackets for the cross-beam. P.S. Your stamp means it's kiln-dried southern pine/fir