This one looks pretty sturdy...It looks like the top bar rests on a few wood braces that are bolted in... I suppose the top bar must be secured somehow, but the wood braces look like adequate support...
Or as Pat suggested, look for specialized swing set brackets, such at this:
hehe - as usual, Pat is more thorough than I. I spent all of 30 seconds Googling 'swingset plans' and 'swingset brackets'.
But as she says, some sort of diagonal braces should be added to the top configuration to add lateral support - an undertaking that will indeed require some higher mathmatics in determining the angles...
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