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Need Help Sizing Main Beams For Pergola

 
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Old 12-16-2019, 01:26 AM   #1
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Need Help Sizing Main Beams For Pergola


Does anyone know of a beam sizing calculator that would tell me what's required to span 21' for a pergola? Redwood or doug fir beams and matching 8 x 8 posts secured to concrete patio with Simpson or equivalent knife plates. 3x8 x 12' cross members on top, client wants to grow wisteria over top which should weigh ~5 pounds/sf when mature.

Any chance a 6x12 would be enough? Really hoping not to need flitch plates for this! Thanks for direction and/or advice,

Rob
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Old 12-16-2019, 11:29 AM   #2
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Re: Need Help Sizing Main Beams For Pergola


Professional would be Woodworks Sizer program for PC.

You can also look up NDS values for the wood you're using and use a calculator like this one:

http://www.timbertoolbox.com/Calcs/beamcalc.htm

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Old 12-16-2019, 07:10 PM   #3
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Re: Need Help Sizing Main Beams For Pergola


Where can you even get redwood?
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Old 12-16-2019, 11:14 PM   #4
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Re: Need Help Sizing Main Beams For Pergola


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Where can you even get redwood?
He's in North Cali., redwood is readily available in these parts.
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Old 12-17-2019, 12:59 PM   #5
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Re: Need Help Sizing Main Beams For Pergola


We can get redwood same price or cheaper than cedar many times around here.

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Old 12-18-2019, 01:37 AM   #6
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Re: Need Help Sizing Main Beams For Pergola


This version of the calculator seems to say that 6 x 12 redwood beams will work at 21 feet assuming I input the data correctly:

http://www.timbertoolbox.com/Calcs/ddsimplebeam.html

Very cool tool!
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Old 12-18-2019, 11:59 AM   #7
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Re: Need Help Sizing Main Beams For Pergola


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Originally Posted by RLH_CT View Post
This version of the calculator seems to say that 6 x 12 redwood beams will work at 21 feet assuming I input the data correctly:

http://www.timbertoolbox.com/Calcs/ddsimplebeam.html

Very cool tool!
That one is for a set deflection. I'm guessing it's L/360, but I didn't see it documented on the page. Something like a pergola may only need L/240 or less.
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Old 12-18-2019, 01:17 PM   #8
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Re: Need Help Sizing Main Beams For Pergola


The calculated deflection with the projected weight was negligible - less than .3" over 252 inches. I could get up to more than 2,200 lbs per beam before getting to the .7" of deflection that would represent L/360 - so I will play with the beam sizes and load estimates to get ideas for the client.
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Old 12-18-2019, 08:47 PM   #9
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Re: Need Help Sizing Main Beams For Pergola


Remember, You want it to work 30 years from NOW...

so one or two sizes greater then the minimum...

And draw it to scale to see if it "looks" safe.

Imagine a heavy growth of Ivy or grape vines with a freak snow load...
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Old 12-18-2019, 11:22 PM   #10
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Re: Need Help Sizing Main Beams For Pergola


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Remember, You want it to work 30 years from NOW...

so one or two sizes greater then the minimum...

And draw it to scale to see if it "looks" safe.

Imagine a heavy growth of Ivy or grape vines with a freak snow load...
Roof systems in houses are designed at L/240, so he's already above minimum. AG buildings may be L/180.
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Old 12-20-2019, 08:53 PM   #11
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Re: Need Help Sizing Main Beams For Pergola


Deflection due to self-weight is your main factor and that's not likely to show in any chart. I'd be concerned about a 6x12 over that span. We would probably go 14-16". Another option is to reduce the thickness to 4-5" to reduce the weight but that's an issue with most of our timber joinery. If you're using brackets it could work well.
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Old 12-21-2019, 01:53 AM   #12
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Re: Need Help Sizing Main Beams For Pergola


I just bought some 18' 6 x 8' con heart redwood beams and after a couple weeks of air drying (in fall weather, so mostly dry but not that hot) they weighed less than 150 pounds - I was surprised at how light they were. Assuming that the weight is proportional, the 21' span would weigh 225-250'; the 12' long 3 x 8's horizontal rafters will each weigh ~75 lbs green and 55-60 or so when aged. Each beam will be carrying the equivalent of 6 "rafters", so another 300-400 lbs of stucture. Doesn't sound like a lot for a 6 x 12 to carry, but I appreciate the 'voice of experience', it's why I enjoy these forums.

Last edited by RLH_CT; 12-21-2019 at 01:55 AM.
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Old 12-21-2019, 01:56 AM   #13
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Re: Need Help Sizing Main Beams For Pergola


Ethan, what brackets would you suggest for these post->beam connections?
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Old 12-25-2019, 09:42 PM   #14
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Re: Need Help Sizing Main Beams For Pergola


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Ethan, what brackets would you suggest for these post->beam connections?
Everything we do is either traditional pegged timber joinery or custom brackets. Simpson probably makes something that would work and be cheaper for your situation. We do all the engineering so custom is actually less for us.

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