Beam Size For Long Span On Pergola

 
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Old 03-08-2010, 01:48 PM   #1
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Beam Size For Long Span On Pergola


I have been asked to build a pergola that is 32 feet wide between posts and the owners do not want a middle post blocking their view of the Sea of Cortez.
So I need to use a 36 foot beam but I am not sure how thick it needs to be. It will be carrying 4 x 8 x 16 rafters and 1 x 2 pine. Also the posts are already in place as we are tearing out the existing pergola with center post. They are 9" - 10" diameter pine set in concrete and with a concrete skirt 36" high. Will these posts support the beam.

I appreciate any advice you can give me. We have built many pergolas here but nothing with a span so long.

Last edited by Bajawood; 03-08-2010 at 01:51 PM. Reason: bad spelling
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Old 03-08-2010, 02:05 PM   #2
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Re: Beam Size For Long Span On Pergola


Call your lumber salesman, he should be able to help you.

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Old 03-08-2010, 02:14 PM   #3
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Re: Beam Size For Long Span On Pergola


That's going to be one big arse beam. Get an engineer to spec something for that kind of span. Maybe a big glulam or steel wrapped w/1x.

The biggest span I ever did on a pergola was 20', it was a 6x14 RSC.

4x8 rafters & 1x2 pine? I can't picture that looking good or lasting very long, the 1x2 pine part.
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Old 03-08-2010, 02:21 PM   #4
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Re: Beam Size For Long Span On Pergola


If you were really good you could make up a flech beam that would make that span.

Now if the posts/foundation are up to the load is another thing. J.
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Old 03-08-2010, 04:02 PM   #5
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Re: Beam Size For Long Span On Pergola


We are in La Paz, Baja Califonia Sur , Mexico and not only does the lumber seller have no idea but the architect told me I had more experience in this than him as I am the carpenter. We are building to basically match the existing structures in a development here and we can not make big changes. The 1 x 2 rough Mexican pine spaced 1" apart over beams is typical for shade down here. The pergola is usually finished a dark color to match the old style of finishing with tar and diesel fuel.

My greatest resource here is the internet as getting any information locally is problematic.

I have gone to some span calculators but don't feel like I have a good answer yet. Anybody have experience spanning 32 feet with a load similar to 4 x 8 beams.

I am including a photo of a pergola in the same development so you can see the style.
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Old 03-08-2010, 04:14 PM   #6
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Re: Beam Size For Long Span On Pergola


Well snow load is probably not an issue.
What are the beam calcs sayng on the span with say a 20 pound dead load? that should suffice for carrying that weight? Also what does the local building code people have to say. You could always get an engineer to spec the beam.
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Old 03-08-2010, 04:23 PM   #7
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Re: Beam Size For Long Span On Pergola


Will this be a round beam or rectangular. Give me a bit and I'll figure out the loads/size required.

I also need to know the species of pine - southern yellow, or western white?
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Old 03-08-2010, 04:44 PM   #8
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Re: Beam Size For Long Span On Pergola


Baja - how many 4 x 8 beams going on top?
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Old 03-08-2010, 05:23 PM   #9
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Re: Beam Size For Long Span On Pergola


I cant see any wood spanning 32' even with-out snow the deflection under its own weight is heavy. I would bet steel has to be involved and perhaps have a small arch in the design.

Bajawood nice work welcome to Contractortalk. How did you tie your Pergola down, for uplift?
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Old 03-08-2010, 05:26 PM   #10
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Re: Beam Size For Long Span On Pergola


I take it back, the 1x2's don't look bad. I'm digging the tree truck post.

I don't think even a ipe 4x8 would span 36', no wood will do that. Those 4x8 beams in the pic look overspanned, but no noticable sag in them.

I'd tell them 36' is not possible with 4x8 & they need a post in the middle, pull the two end post in catilevering the beam 2', then you'll be about the same span as those in the pic
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Old 03-08-2010, 05:26 PM   #11
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Re: Beam Size For Long Span On Pergola


Tom - I can calculate the required beam size if I can get some more info
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Old 03-08-2010, 07:34 PM   #12
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Re: Beam Size For Long Span On Pergola


Baja - here's your calculation. I based this on Southern Yellow Pine. Dead load only, as there will be no snow load.

Southern Yellow Pine weighs 40.6 lbs/cu.ft. I went with your picture, half the weight of 4x8 beams on top, every two feet.

Formula -[3 x l x w], divided by [4 x f x h squared] = breath ( width )

l = 384"

f = 1100 lbs/sq.in (compression)

b = breath

h = height

w = load

The load in this case is half the carried load plus the beam weight.

Using 24" then as a height, the breath becomes 24"

3 x 384 x 4328 = 44985856 divided by 4 x 1100 x 576 = 2534400 = 1.97,

or 2' - 24"

I doubt very much you will find a beam of this size, and it would look horrible.

My recommendation would be a steel beam, clad in the pine.
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Old 03-08-2010, 07:41 PM   #13
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Re: Beam Size For Long Span On Pergola


I thought you could just use the 1" for every 1' ergo he would need a 4"x32" or 2-2"x32"'s.....
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Old 03-08-2010, 07:49 PM   #14
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Re: Beam Size For Long Span On Pergola


That might be a little light. As the size increased, so did the dead load. I tried a 16 x 24 first, but wouldn't handle the load.

I did this calc. just for fun.

If anyone ever needs to know load specs on any species of wood, I have it in an engineering book I have.

For example - pressure treated lumber ( jack pine ) will support 750 lbs. per square inch. So a 6 x 6 will handle 22,687 lbs. Who would have thought?

That's on edge - as a post.
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Old 03-08-2010, 07:50 PM   #15
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Re: Beam Size For Long Span On Pergola


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Old 03-08-2010, 07:54 PM   #16
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Re: Beam Size For Long Span On Pergola


Wow!!! I like that. Is that stainless or what?
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Old 03-08-2010, 08:11 PM   #17
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Re: Beam Size For Long Span On Pergola


Quote:
Originally Posted by katoman View Post
Wow!!! I like that. Is that stainless or what?
Light weight, great stuff
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Old 03-08-2010, 08:36 PM   #18
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Re: Beam Size For Long Span On Pergola


Very cool project Mexico dude. I can just see me sitting under that having a Lone Star and a green chilie burger.


Now just for you and none other ....for that free span you build a.......Drum Roll berrrrrrrrrrrrrrrrrrrrrrrrrrr Bing!!!

Flech Beam.

What is a Flech Beam JonMon???? Well little buckaru its wood with steel inside. You get yourself some beams like in the picture cut them in half,screw 1/2'' x10'' steel flat stock to one of the rips. Then drill/ thru bolt the whole thing together.

Know what you got now Paco??? a 32 ' free span beam with all the strength you need to hold up that pond er rosa pine bubba thats what you got. Now its up to you to figure out if the posts/foundation will work and get that little girl conected up there. J.
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Old 03-08-2010, 08:55 PM   #19
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Re: Beam Size For Long Span On Pergola


I don't think 1/2" X 10 flitch plate will do the job on a 36' span ...it will bend.
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Old 03-08-2010, 09:01 PM   #20
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Re: Beam Size For Long Span On Pergola


NO way dude.

Fastened corectley it will provide way more strength needed for that over lay. Might bounse a little durning install thats bout it. J.

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