how deep to sink arbor posts

i want to put up a 3 post triangular arbor and sling a hammock between the outer posts. i'm looking for a 2 person hammock, so the posts should be able to support up to about 350 pounds. how deep should i sink the posts? i'll be using 2X8 pressure treated wood to form the triangle at the top of the arbor, so there will be additional support where the two outer posts would want to sag in to the middle.
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It depends where you live (e.g. climate) and the type of soil (clay, coral, heavy organic, sand, etc). 42 inches is generally the deepest needed to stay straight barring a hurricane, earthquake, tornado or high straight-line winds. You can rent a power or hand auger. Make sure you know where the underground utility and sewer lines are. Most areas have a service that mark these for free and your city should be able to tell you where the sewer line is in your yard. Also, ask locally about any codes regarding decks. This is a good reference for depth of a weight bearing arbor.

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