Expert: Stephen Major (Owner--Major Design Group) - 6/12/2007

Question
I want to provide support for a hammoc in a pergola I am building.  To explain what I am working with, I have provided this simple diagram..

 PSSSSSSSSSSSSSSSSSSSSSSSSSSSSSP
 P    K                   K    P
 P  K                       K  P
 PK                           KP
 P                             P
 P                             P
 P                             P
 P                             P
 P                             P
 PHHHH                     HHHHP
 P   HHHH              HHHH    P
 P       HHHHHHHHHHHHHH        P
 P                             P
 P                             P

The posts are specified by letter P.
The 'top support' (will probably be a 4x4) is specified by letters S.
The knee braces are specified by letters K.
The hamoc is specified by letters H.

The two posts are 10' apart.  They are both 8' tall.  The knee braces come down 18" from the top of the posts.

I want to put a hamoc between the posts to hold a 230 lb man (so I guess each post would need to hold 115 lbs).  Will 4x6 posts be sufficient to hold this weight?  To put it another way, would 4x6 posts be sufficient to hold the man in the hamoc and keep the posts from bowing in?

Answer
Rich,

The 4x6 posts are being subjected to a variety of loads in this situation.

(1) The downward push of a static load in the hammock.
(2) The inward pull of a static load in the hammock.
(3) The tendency of the posts to sway laterally (sideways) while the hammock swings.
(4) the added force on (1) and (2) during movement of the hammock.

To deal with (1), make sure that the bottoms of the posts are resting on solid earth, either directly or via a concrete footing.  This is the easiest load to deal with.

For (2), be sure the 6" dimension of the posts is parallel to the long line of the hammock.  This will help to counteract bowing of the posts.

For (3), you need to sink the posts well into the ground, I would guess at least 5', and tamp the backfill well.  Also, a 6x6 (vs. a 4x6) would help in this regard, or the installation of some kind of diagonal strut from the posts to the ground.

For (4), remember that when the hammock is swinging the force exerted, in constantly changing directions, will well exceed 115# per post.  The recommendation for (3) above should be sufficient.

Best of luck and let me know if you have additional questions.

--Steve Major