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Old 11-13-2013, 12:33 PM   #1
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Flitch beam

I'm building a pergola 10' x 10' with 4 posts (6x6 treated pine) with 3.5 ft deep concrete footing. The pergola roof will be a hip roof (35 degree angle) with no sheathing like the one in attachment.

I'm planning to build a porch swing around 5 ft long that will be attached to the pergola, and I'm thinking to build 4 flitch beams as rafters that sit on top of each column.
I will not talk about the beam to which the swing will be attached as I'll leave for alter for now.
The flitch beam (rafters) will be 4" wide x 1/4" thick steel plate sandwiched between (2) 2x6 pine that sits on to of the column and connected through 1/4" steel bracket similar to Simpson strong tie
http://www.homedepot.com/p/Simpson-S...9#.UoPAyPmfgiw I will construct it and weld it myself. It will be much stronger since the base plate (where the U sits) is 6x6 x1/4" steel plate - in order to cover the whole area of the top of the column and attach to the flitch plate with bolt and nuts.
I'm not sure about the size of the flitch beam I'm building. Is it undersized, over sized or just enough.

the steelspecs:
weight: 3.4 lb/ft
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Old 11-13-2013, 02:33 PM   #2
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People who post strength/span questions with prescriptive answers have decent luck on this forum. People who post strength/span questions about custom structures rarely (never?) are lucky. Flitch beam design requires the formula and number crunching skills of an engineer. Even if an engineer within earshot is willing to help you, they don't have enough information to answer your question. What are the load specs for the swinging bench? How exactly will it be hung from the structure? But please take those as hypothetical questions. Call me pessimistic, but I've yet to see an engineer on this site do a free structural analysis of a custom design...and for good reason.

On another note, the centripical forces of a bench swing with, say, four overly-excited and under-monitored kids are going to put that structure to test. The taller the pergola, the bigger the challenge. It might require some serious bracing in addition to the 3.5' deep footers. Just a gut feeling...


Last edited by cortell; 11-13-2013 at 02:36 PM.
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joecaption (11-13-2013)
Old 11-13-2013, 03:13 PM   #3
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I don't see the porch swing weight exceed 700 lbs with 3 adults on it.
Posts are 6"x 6" x 8 ft high. The swing will be carried by 2 steel plates (3/8" thick, 1.5" wide) instead of usual chains. I got a heavy duty hangers http://www.americanswing.com/Swing-H...-Swing-Hanger-. they will be attached to another flitch beam that sits on top on 2 rafters(flitch beams too) around 2 ft from the front 2 posts.
I updated the attachment so it might give clearer picture of what I have in mind

my main concern is how much the flitch plate I have in mind can support as a load. assuming the swing total load is 700 lb, I should probably plan for 1500 lb at least.
All braces will be made from the same steel plate 1/4" thick and either welded or bolted to the flitch plates
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Old 11-13-2013, 03:35 PM   #4
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this is all I can add .....

can't wait to see what your results are. good luck!

"You get what you pay for, and sometimes free costs more!"
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Old 11-13-2013, 05:02 PM   #5
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As per Cortell, you would need an SE to check your proposal.

With a timber/steel plate/timber beam, it would normally be assumed that the steel takes the majority of the bending stress, and the timbers merely stop the thin steel plate from buckling sideways.

No-one here will give you a definitive answer for obvious reasons. Even an SE would be wary, as checking a beam for a dynamic load is more of a problem than for a static load.

And of course there's the structure as a whole to consider as well.

You'll probably end up doing it by gut feeling and it could well be over-engineered, but better that than the other way! Good luck.
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beam , flitch

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