Help wanted on Beam Span and Strength

[dlincoln]

Ive decided i to add a screened in porch to my house. I have access to a mill to rough cut some pine timbers.
My plan is to use for the most part 8"x 8" timbers for the project but im unsure of the span and safety. My biggest span will be 16'. Ive attached some autocad drawings to show my plan so far. The structure is 24'x16'. The top truss is 8' tall. The 2 spans im concerned about is the 16 foot span that is on the first picture on top of the columns and the second 1 is the top beam running for the roof joist.
Any one have any thought on the design? The timber will be rough cut pine.
Thanks for the advice in advance.


 

 

 

[Mike W]

First, welcome aboard dlincoln, plenty of really smart guys on this forum, one will be along shortly to help out soon I am sure.  

what little I might able to add in your two areas of concern are.  you really don't have a free clear span of 16' that i can see.  it appears you have support posts about mid span or 8' so your span is relative to that.  The ridge beam is also spanning 8' from rafter to rafter.  if your concerned with the king post being unsupported where your larger opening is shown offset for access it appears, you could also look at a queen post design to spread the load closer to those posts somewhat.  would also suggest lowering your collar tie (center rafters) to within 1' foot above the beams to be an actual tie, any higher as you have it shown now is not a tie but rather in compression keeping the rafters from sagging into each other and doesn't do much to prevent the center wall sections trying to spread out, moving it to within a foot does just that, it will tie the two together and prevents mid wall spreading.

anyway, as mentioned, someone with some smarts will be along shortly to help your design and build become reality.

enjoy the stay, lots and lots of great info compiled on this site, as well as great help from the fellow forum'ers'

Mike

[Don P]

If the joinery is designed to make those functional trusses, the bottom chord and the king post could be cables. They really aren't subjected to bending loads.

The bottom chord is a tie to keep the rafter feet from spreading, now visualize it as a rope fastening those legs together. The kingpost is pinched between the top, rafter, chords. Visualize it as a rope tied to the peak and tied around the bottom chord to prevent it from sagging.  The webbing Y braces are compression elements, posts, that spring from the kingpost and bump up under the rafter chords to support them in midspan.

The 8x8 should be plenty. This is a pic of what the heeljoint might look like. See how the bottom chord is really just keeping the top chord from spreading.



The kingposts, notice how the top joint is designed to pinch the king between the top chords and the lower tenon to drop into the bottom chord and when pegged will keep it from sagging, kinda counterintuitive compared to the way most people visualize the load working.


 


The assembled truss, these didn't need webs, their real purpose was to support the ridge there.


 



Your middle truss should probably be the same as the gables, or as Mike said, lower the tie to the bottom.I'm not sure what you are doing for roof sheathing, are there purlins across the length of the roof then boards up and down the slope?

Which pine? , What is the location or snow load in psf?

[Mike W]

dlincoln,

As mentioned, you'll get all the expert information you could hope for here on the forum, aside practical information able to be provided, they'll direct you to some really good resources to tap those elements outside there wheel house.

Don,

great post as all of your's are, been reading hundreds of your comments on multiple topics as I am at the he infant stages of designing my first heavy timber frame, not there yet to engage your time, but coming shortly. your previous responses to others have driven the design in both visual as well as loading limitations, my experience to date is a lifetime of conventional framing with some ole barn renovations.

[dlincoln]

I was hoping to instead of cutting the wood trusses to fit notched together like that just use the quarter inch thick plates with good screws to tie all the trusses together. 
The center truss was really just for looks but if possible i wouldnt mind just not having it.
The snow load isnt a concern. Im in north Mississippi so what snow we do get is gone in a couple days.
My plan so far for the roof was just using like 2x6s running from ridge beam to the side beam with a plywood on top to roofing material. The inside i plan to do a decorative thin wood finish.

[Don P]

Thanks Mike, hundreds? You've probably read the three things I know a hundred times each :D

Let's play with it a little, the kingpost trusses at each end support a structural ridgebeam that has common rafters supported by it and the lower plate beams. Species is SYP, Live load is controlled by wind at 20 psf, dead load is 10 psf for a total of 30 pounds per square foot load.

The 16' ridge beam span is carrying half the roof load, or 12' wide, each plate beam is carrying 6' of tributary width plus probably a foot of overhang. So the ridge is supporting 16'x12'x30psf=5760lbs
http://forestryforum.com/members/donp/beamclc06b.htm
You are looking at an 8x12 Ridge Beam

The plates are carrying 7'x8'x30psf=1680, 8x8 is fine

Rafters, the code calc is here;
https://awc.org/codes-standards/calculators-software/spancalc
2x8's at 2' on center would be correct.

I would probably set the ridge on top of the kingpost like in my pic and frame the rafters above the trusses.

What is the pitch?

[dlincoln]

Would i be better off on the ridge beam buying a laminated like 4x12?
It the king post is 8' and the beams on top add about a foot im looking around at 36 degree roof on the 24' span

[Don P]

A glulam is stronger, dry, engineered and tested. Let's see what it would take.
They are specced by lbs per lineal foot so taking the total ridge load from before 5760 lbs/16'=360 plf

Then go to the glulam beam tables;
http://www.aitc-glulam.org/pdf/Capacity/SP_48a.PDF
In a SYP glulam you're looking at 3-1/8" x 11" or larger. The supplier can do the final engineering on that free as part of the service of the sale on an engineered product.

Need to go fire up the mill but its gusting to 50 out there, miserable :D

[dlincoln]

Man you dont have to tell me. Im in MS, going from a week of 70 degree weather and 9 inches of rain to 30 degrees this morning and sunny... it doesnt make sense.
Is that pre fab board high in cost?

[Don P]

I just looked online real quick, it looks like about $15 per foot for an 11-7/8x 3-1/2" one at Lowes.

I swung by the job on the way to class tonight, the porta john had blown about halfway across the parking lot, still upright thankfully! One security light busted, and a call on the machine asking if I wanted some blowdown pines and an oak, it was rocking last night.

[dlincoln]

Alright so thats they way il go for the ridge beam. On connection to the king post with the ridge beam. Would you cut a notch out of the king post for the ridge beam to slide down in and bolt it all together? The ridge beam would be pretty much sandwiched between the 2 pieces of the truss coming into the king post.
Also on the bigger open side should i still run the 2 extra beams about 4' apart under the truss or is the 2 beams at the end of the 24' span sufficient  to hold it all?

[Don P]

I was thinking something along these lines for the roof structure;


 

[dlincoln]

Well that answers all those questions.
I owe you a beer. Hopefully i can get this done this summer when the weather clears up.

[dlincoln]

maybe a couple more questions.
i plan to make a u shapped bracket to bolt to the slab and then bolt beams. I dont want to pour into the concrete because the beams wont be treated.
Yall think im ok?

[Mike W]

If the concrete is not in place yet, you could just buy wet set base anchors with stand off plates to bolt or lag the post to.  There are a variety of embed knife blade anchors with standoff plates as well.  Key is to have the bottom of the post off of the concrete or ground contact, both will wick water if its in contact with either.  a 1" stand off is adequate and most common with the box store anchors and such.  The standoff plate will also allow room to anchor the base after the concrete is in place with a wedge anchor or epoxied application.  Would still treat the bottom of the post with some sort of preservative even if the balance of the wood remains untreated.

Mike 

[dlincoln]

I thought about using embeds but just thought it would be easier to anchor bolt them down after the fact. 
I plan to stain with thompsons waterseal on all the wood

[Don P]

In a Simpson that would be a CPT88Z post base (strongtie.com)

Thinking about easy ways to make the truss heel thrust connection, this is one idea. Notch the truss bottom chord 8x8 1½" over the posts and drop the 8x8 common rafter plate into the notches to lock it from sliding outward. The truss top chord 8x8's jam into the 8x8 plates to resist their thrust. Run a common rafter alongside the extended truss bottom chord and peg or nail rafter to bottom chord well as a lock on the rafter plate. It can't slide or roll then.