Post & Beam Idea to be troubleshot

[mainewoodsguy]

Hey guys, Chris in Maine here. Trying to come up with a quick and easy way to build a structure with the material I have on hand. Hoping you guys can help trouble shoot it for me.

I have enough 8X8's on hand to for the uprights on a 18X32' building. I have enough 2X18" on hand to make trusses. My question is: Would the structure be strong enough if I "half lapped" the top of the 8X8's to have a double bottom cord and a double top cord, in a king post style. It would kind of make a bent that could be stood up after? So the top of the 8X8 would be 4" plus the 2ea 2X's on each side.

[Ljohnsaw]

Welcome Chris,

Not an expert but you have a lot of questions to answer...

So the truss will be 18' long?  Roof pitch?  Snow load?   Will the "joists" be used for storage (live load vs. dead load)?  You say post and beam so, to me, that means bolts and plates, correct?  How big will the bays be?  That's all I can think of now besides what will the building be used for (garage/barn or house)?

[mainewoodsguy]

Snow load in my area by code is 90lbs/sqin. No live weight. 4/12 with metal roof should work. Could go six twelve if needed. I wasnt planning on using plates just double "half lapping" the 8X8 on the top and placing both the double bottom cord and double top cord sandwiching the 8X8. The king post would be double "half lapped" and sandwiched between the double top cord and double bottom cord as well. I have been searching for a picture of something similar but have yet to come up with one. I would probably run 2X6 purlins 16OC between them. I will keep looking for a pic or fab up a mini version for you guys to look at.

[mainewoodsguy]

 

  

 

[mainewoodsguy]

So that is a picture of some homesteaders in Alaska. I was wondering if I did something close but notched the top of the 8X8. What kind of issues I may run into.

[Ljohnsaw]

Snow load in my area by code is 90lbs/sqin. No live weight. 4/12 with metal roof should work. Could go six twelve if needed. I wasnt planning on using plates just double "half lapping" the 8X8 on the top and placing both the double bottom cord and double top cord sandwiching the 8X8. The king post would be double "half lapped" and sandwiched between the double top cord and double bottom cord as well. I have been searching for a picture of something similar but have yet to come up with one. I would probably run 2X6 purlins 16OC between them. I will keep looking for a pic or fab up a mini version for you guys to look at.

Probably more like 90lb/sq-ft ;)  If you are worried that you are taking too much out of the 8x8 to house the 2x18s, you could probably get away with only letting the 2x18 in 1" or so - but full in, a 4x8 post (what the cross section you are left with) can still hold a substantial amount of load.

[mainewoodsguy]

I figured it would work, esp. since these yahoos were doing this in Alaska. Just wanted to see if anyone was worried, or if 2X18" was too much or too little for a 18' span, or if anyone knew the minimum 2X(XX) I could get away with, in case I run out.

[Ljohnsaw]

The 2x18 is plenty for the joist.  The issue is the rafters.  You didn't specify the spacing of the posts/rafters.  First blush I'd say the 4/12 is a bit flat for that snow load.  Even a 6/12 with the steel might not be steep enough for it to shed the snow.  You need to do the snow load calc (depends on roof type (metal), hot or cold roof and pitch) to see what the lb/sq-ft final load is on the roof (roof snow load - RSL).  Then you figure the load that an individual rafter will be taking (depends on spacing).  Then you need to look at what your purlins need to hold.  That may be your issue - a 2x6 flat won't hold much across any substantial distance.

For instance, my ground snow load (GSL) is 341 psf.  My roof is a 3/4/5 (53°) slope of cold metal.  My RSL came down to a 143 IIRC.  Still pretty substantial but doable.  I have 5x9 common rafters every 24" with 2x6 planking.  They are sitting on large purlins (beams) and only span (horizontally) 6 feet.  The calc said I could use 4x8 rafters but I wanted the more cabin look of the 5x9s.