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hammer beam #6320 10/19/99 02:29 AM
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tuffoot Offline OP
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in a hammerbeam truss with a 8/12 pitch will the braces be the same pitch as the
rafter or at a 45 degree angle .
In the workbook it says yes( the same pitch) but I have seen pics of hammerbeams with a lower pitch on the rafters than the braces.
In a 24` wide HB truss with a 8/12 pitch and 9 ` princ.. post puts the hammerbeam at about 6` right ?( 2/3rds the post) well following the pitch of the rafter does not end up the bottom third of the post
(at least not in my drawing) am I doing something wrong? how important
is the bottom placement of the hammer brace into the bottom third of the post
Can I get away with placing it higher sayyyyyy at about the halfway point......help

Mike Evans

Re: hammer beam #6321 10/19/99 04:22 AM
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Reid Costley Offline
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Because I'm an engineer, the answer is always; it depends. The lower knee brace will work most efficiently at a 12/12 slope as any truss member would. The 8/12 solution would tend to bring more of the horizontal force (the hammer!) up towards the rafter/post connection (undesireable).
It all depends on the joinery...the reason for the brace connection at the lower 1/3 of the post is to take out most of the horizontal reaction through the post base rather than the aforementioned rafter/post conn.
In any case you must determine the forces on the joint and design the joinery accordingly or adjust the geometry to force the loads to the desired joint.
I realize this is no answer in specific terms but as I've said I'm an engineer...what did you expect!

P.S. My best guess is to go with a 12/12 brace and make sure the rafter/post conn. has good tension reistance.

Good Luck,

Reid Costley

Re: hammer beam #6322 10/20/99 01:35 AM
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Rudy R Christian Offline
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Mike,

I'm not in a position to answer your specific question, but I would like to offer a little perspective about the truss you are attempting to design.

Traditionally the hammer beam truss was used in structures which either had relatively thick masonry walls, or reinforcing butresses to counteract the spreading of the truss. This failure is due primarily to the lack of a continuos tying beam or chord in the hammer beam truss.

In modern applications the hammer beam is all to often seen perched atop hinged walls constructed of stick framing or timber posts and panel enclosures. Since these walls have the ability to rotate around their bases, the truss is free to spread as much as it needs to. Although many of these roof frames have been effected with some degree of success in modern timber framing, many have failed or are doing so slowly.

My personal recommendation would be to consider a queen post or king post truss. They are quite beautiful and much less prone to failure. If a hammer beam is unavoidable, consider reinforcing the plate connections with wings in the floor plan, or shed construction of some sort. I have seen wonderful examples of shed porches which do a fine job of absorbing the thrust at the plate level.

Truthfully, if this is your first truss, I would recommend at least having the design reviewed by a qualified structural engineer. Trusses are complex and confusing, but quite a joy to behold when effected correctly.

Rudy R Christian


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