Question on Large Carport Roof Attached to Barn

[MushCreek]

 Is there a way to determine from the codes (prescriptive) how big and how many piers one needs for a given load? I know soil type comes into play here. As an example- I'm planning a huge porch/carport off the side of my barn. The roof is 12' deep by 48' long (plus overhangs), and there will be 7 piers/posts on 8' centers. How do I determine what size they need to be? And how do I convince the inspector that they are adequate? At a generous 20 psf snow load, plus a 10 psf dead load, it works out to only 1475# per post. I was planning on using 20" Bigfoot forms, which should be way over-kill for the application. I'm actually more worried about uplift, hence the Bigfoot's (Bigfeet?) This is in SC, with little snow, and no frost line.

[MountainDon]

My guess is this is getting into the area of needing to refer to a geotech and/or structural engineer.  
This will be placed on our list of things to do with piers, posts, bracing and the like.


I believe you are correct to worry about upload resistance; there may also be anti-racking concerns. A roof, attached to a building, may also have some back and forth forces out there along the line of the posts, 12 ft. out from the building. Where the posts attach to the poured concrete footings and piers becomes a bunch of hinge points. There may have to be some brace walls along that outside line to stiffen that line of posts. Or maybe that can be designed into the roof by an engineer.

[Don_P]

I'll step in with the little I know and let those who know more add or correct me when they have time.

"R403.1.1: The size of footings supporting piers or columns shall be based on tributary load and allowable soil pressure"

Early in chapter 4 is an allowable bearing capacity chart. Most building depts will allow 1500 psf prescriptively for good soils. So generally that can be done prescriptively.

There are wind pressure and uplift tables in chapter 3. A porch is technically outside of prescription, the roof is acting as a diaphragm plus any other bracing you add to resist racking ... All of which is outside of the simple methods in the codebook, but I've built plenty of non engineered porches here. Connections for uplift are my greatest concern. 20 psf live load whether from snow or wind + 10 psf dead load is the minimum prescription.

[MushCreek]

I plan on having diagonal braces at every post. No sheathing for the toof, though- just tin over purlins. I could let in  a couple braces on top, too. The smallest Bigfoot is just over 2 square feet, so I think the area is plenty for my posts. I'll use a post bracket, so the pier will be connected well to the post, and the posts will be strapped to the roof structure. I guess it's a matter of how deep into the ground I should go with the Bigfoot to resist uplift. While not a high wind area in general, I'd hate to lose my nice porch due to an over-zealous thunderstorm. Eventually, this area will have a slab, but I had to cut it from the budget for now. Perhaps a continuous footer would be better?

[rwanders]

I believe 12" or even 8" concrete columns and BigFoot footers will be sufficient for the foundation. I would place them at least 3' feet deep. It seems to me that the connections between the piers and columns, between the columns and the roof framing, the connection at the roof barn intersection and, finally the between
the purlins and the perimeter beams and their connections to the metal roof will represent the most likely failure nodes----I would go the extra mile at those points. Some stout X bracing between the columns to counteract racking forces would also be advisable. A 48+ foot long roof will be able to generate a lot of twisting stress if the wind is from the right (wrong) direction. Some don't see Simpson connectors as a cure all, but I believe they add a great deal of strength when properly chosen and installed.

[PEG688]

I'll chime in with the others your issue is uplift resistance.

The Bigfoot's  set deep  or a continuous footing with piers coming out of it being better.

  Nice big posts 6x6 min. with CB66's at the bottom poured in.

A built up  / laminated beam where the joints are staggered over different posts with EC66  and CC66   post to beam connectors , thru bolted , or some  CS16 strap's up the post over the top and down the other side of the post to resist uplift should make any inspector happy.

That or hire a engineer to spec it out for your max area wind loads. 

With just metal roofing on it the roofing will tear off before you'd reach uplift , so that may be a good idea. Sort of a fail safe weak link option, let the roof blow off BEFORE it tears the thing out by it's roots.

Good luck. 

[MountainDon]

Is the metal roofing you are considering rated: structural?  OSB or plywood used for roofing is.  My metal roofing was not even though it is 26 gauge.

[MushCreek]

I'm not sure if my roofing will be structural or not, but I'm assuming not. I'll frame it so it doesn't need any help from the tin. I could sheath it, like the rest of the barn, but I want to be able to sit out and listen to the rain. I'm not even sure any of this needs to meet code- in SC, a building designated 'for agricultural use' requires no inspection. I'm not sure if having a full bathroom moves me out of the 'agricultural' category, though. Either way- I want it sturdy for my own purposes, whether the inspector requires it or not. I'll definitely be using Simpson hardware to tie everything together. Since it's a rustic building anyway- maybe I could put a couple big diagonal braces on the underside of the rafters? Between that and diagonal bracing on the posts, it shouldn't be able to rack.

I'll get a price from my concrete guy for a continuous footer. That makes more sense if I'm eventually going to pour a slab there.