Conventional framing, you have about 1 1/2" of each member to find center on, for a fastener to be installed, and if its a truss, it could be southern yellow pine, which is a bit hard and brittle, miss that center and the fastener will pull out prematurely. I have aligned center by sweeping from each side of the suspected framing member, making a mark on each side and centering between them, yet I do not recommend blindly attaching something a person will sit on, into framing I cannot see first, its asking for trouble, now or later.
A.) check to see the size of the member will support the load between the span, which is the swing and how many people, there has to be a reference for lumber, its an engineer function, but it would seem like overkill, overthinking, but also a safe thing to do, that is entirely up to you.
B.) if after reinforcing, doing some investigating, confirm you should have enough strength and that the fasteners will hold in what you have and remember the fasteners are going to be either in shear and or tension, one had best select the right fastener, do not use deck screws and or even some kinds of cheap lag screws, they can be pot metal, never trust them unless there is a astm designation and made by a reputable mfr.
C.) I got a call from a customer to repair a commercial overhead door at his building and what I determined was mentioned above, I have photos somewhere in this pc, lag bolt fastener off center, split the bottom chord of the truss, the whole thing could have come down, killed or seriously injured someone. Trusses are really not meant for extra loading anyway, especially from the bottom chord, yet people do it all the time, usually not excessive weight, likely no problem but that bottom chord carries most if not all the load and one has no idea what the limits are unless its calculated, checked out and confirmed by same. My fix was to open up the area, you should do the same, and inspect what you have. In this case I decided to make hangers out of steel angle iron, which spanned a couple of trusses each side, I drilled holes and installed threaded rod, used an angle over and under the bottom chord and literally clamped both angle iron sections to the bottom chord, this afforded me the rest of the attachment and firmly stiffened up the entire assembly holding that door, it eliminated further compromising the framing above.
Purely up to you how you do it, blindly nail or attach without inspecting the existing framing opens the possibly much further of what ever it is you are attaching, to fail, which while sitting in the swing could cause serious injury, do a seatdrop from that distance, you could cause a spinal injury, compress a vertebrae, I'm no doctor, or licensed P.E., just common sense to do the right job.
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