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Author Topic: Sanity check on new build  (Read 691 times)

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Offline LRB45

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Sanity check on new build
« on: April 03, 2022, 07:02:07 PM »
Been referencing the boards quite a bit and thought I had my attempt at designing a new outdoor area figured out but figured I should seek advice from those more knowledgeable than me.

My first build is a 22x24 free span pavilion with #1 R/S Douglas Fir in Florida using what I guess is more of a post and beam construction than a true timber frame as there are no M&T joints but instead mechanical connectors.

My primary question is if I can eliminate using typical rafter layout of 16 or 24 oc and rely on my cross beam spacing with purlins for the roofing system.



Current design is:
8x8 posts placed equally along the 24' edges-7'4" OC
Tie, connector and ridge beam are 6x12-Connectors will sit on top of beams directly above posts.
Kingposts are 6x6 to support ridge
4x8 knees opposite each post
2x4 RS for purlins spaced at 36" OC between cross beams per roofing spec
4.5:12 roof pitch.  The roofing material is a polycarbonate plastic that weighs 2.6lbs per sq. ft so very little weight from it and no sheathing required.  And I'm in Florida so no snow load concerns.

I've ran what I think is the correct numbers on the Uniform Load calculator and if I'm right it appears a 4x6 will pass although I am thinking of using a 6x to match size to my other beams . Would welcome opinions on this?

I come up with a total dead load of  1,338 using the below

Total span: 21'-4" divided in half is 10'-8" or 128"
Dead load-roof panel: 3lb per sq. ft (rounded up from actual 2.6 of panel)
Dead Load-2x4 purlin: 14.1 lbs  Based this on 3lb per bd ft with a purlin length of 7'1 spanning connectors.  Not sure of this should be included or not
Rafter sq ft: 78.25' sq.   Connectors are 88" oc so I used 88" as my span load where each beam carries half the span on either side, 44" x 2
Total load: 78.25 x 17.1=1,338 lbs per sq. ft





Hopefully this all makes sense to someone besides me.

Thanks in advance for your input

Offline Don P

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Re: Sanity check on new build
« Reply #1 on: April 04, 2022, 07:19:15 AM »
Welcome to the forum.
I really need a sketch to go along with the word picture, and label your parts, we have a nomenclature thing going on as well that's throwing me.

Some thoughts, when snow ceases to control, wind is usually the controlling load. Basically never use less than 20 psf Live Load +... if you want to use 5-7 psf Dead Load that is probably fine. The poly is 2.6 and the framing probably doubles that, you can figure volumes and weights and keep refining that if desired.

If the rafter pair is truly treated as a truss, the ridge, kingpost and 6x12 tie beam can be revisited or go away. If sunlight is the goal that might be worth looking at. If looking at big beams is the goal, we're probably there.

The math looks like it has an error or two but let's line up the words with a picture.
The future is a foreign country, they will do things differently there - Simon Winchester

Offline LRB45

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Re: Sanity check on new build
« Reply #2 on: April 04, 2022, 02:22:23 PM »
Apologies as when I previewed I thought the images were there and I never checked the actual post.

Calculating for the wind makes sense since it's in Florida so I can rerun adding 30lbs which would account for 20lb live load which is the minimum by Florida building codes and 10lbs for the roofing material and lumber.  I did add the 2x4 purlin in my calculations in the figures below I was thinking since it was uninhabited and simply a cover shelter I could cheat a bit on the load determination.

The overall look is definitely for a large timber appearance which was the reason for going with the R/S DF in these sizes. We looked at having a true timber frame built but that got above our budget real quick. Allowing sunlight/ moonlight through is a must but the family won't be outside if they are sweating so the polycarbonate panels were a good tradeoff.  Plus I'll be adding electricity for some fans to help cool down further.

Definitely appreciate your time Don.







Offline Don P

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Re: Sanity check on new build
« Reply #3 on: April 04, 2022, 07:32:45 PM »
Ahh, I'm coming up to speed. Try a worst case and see how it does. Run 30psf x 78sf and use #1 Dougfir P&T design values unless you know you're getting B&S in that size. Technically B&S starts when the timbers are greater than 5x5 and more than 2" different in width to depth.

Then check the 6x12x 22' tie with the center point load calc, There it would be appropriate to copy down the B&S design values and use them in that calc. The tributary area would be around 88" x 11', with that span and a center point load I think that is the elephant in the room. The ridge and plates carry no more load than the purlins, until the wind blows and the braces go into action.
The future is a foreign country, they will do things differently there - Simon Winchester

Offline LRB45

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Re: Sanity check on new build
« Reply #4 on: April 04, 2022, 08:23:09 PM »
OK. Have patience with the ignorant here.

I ran the uniform load calc using your 30lb x 78sf which came to 2,340 lbs/ sf and that passed using #1 P&T so that would be a good thing I think.
I then ran the single point cal using the same weight but changing to B&S as suggested and I think I am good on fiberstress but over allowances on deflection and shear?? I can't seem to get my head wrapped around these so I may be misinterpreting the results. Both are posted below for referenc

 

 e.

Offline LRB45

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Re: Sanity check on new build
« Reply #5 on: April 06, 2022, 02:07:33 PM »
I spent some more time on the single point calc and if I now understand it looks like I am ok on all tests aside from the floor deflection but I'm assuming that would not apply as this is a roof.

If I'm interpreting this correctly I believe that would indicate I am ok if I change the design to being only (4) beams spaced evenly along the 24' length and avoid all of the additional rafters. 

Offline Don P

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Re: Sanity check on new build
« Reply #6 on: April 06, 2022, 10:47:27 PM »
Sorry to drop out, gremlins in the machine.
On shear in the post above, you are fine, you're running at 24 psi and are allowed 165. deflection is ok for a roof, that is during a hurricane. Bracing that sail is a whole other matter.


Yes, as long as the principle rafters are designed to carry the larger tributary area collected from the purlins, it is a fine roof, just another way to skin the cat.
The future is a foreign country, they will do things differently there - Simon Winchester

Offline LRB45

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Re: Sanity check on new build
« Reply #7 on: April 07, 2022, 04:38:15 PM »
All good Don. I appreciate your time so definitely won't hear me complain about someone else's time between posts.  

So I do have 1 last question (hopefully).  What is your thought if I set my 1 x 3 purlins into the rafter as in the image below. Clearly, it weakens the rafter but enough to be a concern? Would it be safest to simply up the depth from 6" to 8" which would essentially leave me 7" of void free lumber?


Offline Don P

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Re: Sanity check on new build
« Reply #8 on: April 08, 2022, 06:07:28 AM »
What happened to no M&T? ;D
First off, I'll never cut a notch in a beam without good reason, so consider your reasons for notching. If the purlins run undamged over the tops of undamaged rafters they and the rafters will be stronger, and it will form the gable overhangs. Blocking between purlins locks them in place.

Effectively the notching does just that. One way to investigate the rafter with notches is to look at the "worst" one. One of the middle pair with purlins notching in from both sides. The amount of "meat" left between notches on the upper face by the depth of the beam is your "timber". With minimal tenon on the purlin, at best you would have a 3x6 out of the 6x6. Another way is to look at the amount of wood under the tenon by the width and investigate that in flatwise bending. In other words, pretty much the scenario I described above you're just using the beam to create the "blocking".

A tusk or soffit tenon on the purlin would not destroy the upper edge of the rafter and is looked at the same as an unnotched member. The tension and compression "extreme fiber", those straps of most stressed grain, is undamaged and intact.

On the purlin, no notch is best. You may reduce the section in the end thirds by 25% max, a gradual transition avoids stress concentrations.... approaches undamaged strength but does not duplicate it, watch your purlin shear numbers here., if that's pushy, which I doubt here, rethink any notching.

Simpson does make an unobtrusive connector for that situation, I think it'll google under "concealed joist tie". Basically it is a knife plate that is buried in a vertical saw cut in the end of the purlin.

The future is a foreign country, they will do things differently there - Simon Winchester

Offline Jim_Rogers

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Re: Sanity check on new build
« Reply #9 on: April 09, 2022, 06:22:16 AM »
The photo in post #7 is of a drop in floor joist and a sill.
Created by me.

Jim Rogers
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

Offline Don P

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Re: Sanity check on new build
« Reply #10 on: April 09, 2022, 07:05:06 AM »
 
 


Jim and I were looking at these drop in joists earlier this week. There are a few originals left under the house I'm working on. The supports under the sills had failed, the sills tunneled out by termites, heavily bowed between the supports that held, they were done. But none of those 16' long joists had split at the reduction. Further down the floor switched to notched red oak 2x8's. I meant to point it out but our conversation had moved on, there are a couple that split right at the interior of the hard 90 degree inside corner of their notch. There was much less section and the stress concentrated at the hard inside corner. Oh, on the log joists and sills they had a half dovetail drop in at each end, 1st common joist, to help lock the sills.

And the new treated sills. These have continuous support, they are not spanning members.
 

The future is a foreign country, they will do things differently there - Simon Winchester


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