Pier diameter? 10"? 12"?

[McClain]

I am using the Little House Plans kit for my "main" cabin area.  However I need to build a 14' by 24' structure to cover my travel trailer from winter snow.

I plan on using concrete piers to 12' posts, to beams.  This will give me a carport like structure that I can later enclose into a guest cabin/shop.

My question is how to decide how many piers to use? and how large their diameter should be?  I have written out plans which use 8x 10" by 48" concrete piers to the 12' posts.  These would then be spread 8' on center, 4 on either side.  How do I know if this is suffecient?

[jraabe]

You might want to consider doing a pole frame building where you bury perhaps 4' of a PT pole and then come up to your beam and roof supports.

One of the main concerns for a carport structure, especially one 12' high, will be wind and other racking forces.

You should review your sketch with someone locally. These projects are designed individually based on your layout and the local wind, snow and earthquake loads.

A small house is actually easier to design and build than an open carport without walls. It has much more built-in strength.

Now, having said that, a 10" or 12" diameter concrete pier resting on a footing at frost line is quite strong. You could even bring it up 3' out of the ground before it bolts onto the post (I would use at least a 6x6). That would mean the posts would only need to be 9' tall and if you bolted in 45º knee braces between the post and the beams, then you have a good start on building in some rigidity.

[McClain]

Thanks John,

I dont know why I said the post were going to be 12', they are going to be 10'....  

Also, as a follow-up question:  Since I am going to pull a trailer in this structure for the first few years, I cannot build a floor, I can however build walls on most sides (like a gargae with only one opening).  The question is whether the fact that I am going from pier to post to roof beams, makes my structure less strong (i.e. ability to withstand wind) then a structure which has a subfloor in between the piers and roof beams?

I know this is a general question for a complex engineering problem, however, I just want to get some ideas of what I want/need before I get someone locally involved.

Thanks for any more advice you can give...

[glenn-k]

If the footings are well anchored the diagonal bracing or plywood shear walls provide the wind bracing, transferring the strength of the anchor point at the ground up into the walls and upper part of the structure.  The big triangle is what provides the bracing whether through diaphragm action of plywood and nails or a diagonal brace.  Squares or rectangles can squash from corner to corner.  Triangles can't.

If your posts or stud walls are not braced through shear panels or diagonal braces they can just hinge at the ground and fall over.  A sub floor does not make a whole lot of difference, nor does a second floor.  You have to stop the above ground hinging effect by some sort of diagonal bracing.

The posts John mentioned do the same thing but the bracing is the dirt acting on the bottom of the post and there is no hinge point as the post is continuous.  Hope this makes sense.

[peg_688]

  Use a 12 inch sono tube  say three feet deep into the gnd.  put a gavl post saddle 4x4  or 6x6 depends on post size.  tie the top of the post to the beam with a Simpson saddle , see any Simpson book look for stout brackets .  tie the beam to the Rafters / roof  beams   you called them if they are 4x they are still rafters again simpson makes a hanger / hold down to do the job , More than likely .  As Glenn said add knee braces from the post to the beam , use the fall off from the post , keep them as long as you can w/out making them head knockers , sort of depends the height of your building , lag bolt or better yet thru bolt from one knee thru the post and out the other knee 5/8 threaded rods w/ washers . that will give you the rack strenght you need . And also up into the beam , long lag bolt 1/2 X6 or 8 inchs .  If you want to get fancy do all mortise and tendon joints instead of the bolts  Post and beam PEGED together , barns in New England have stood for 150 plus years with out the concrete piers .   HTBH  ;)PEG    See the side note      Simple  

[Daddymem]

Goooo Simpson!
Sorry, my uncle is a distributor for them  

[McClain]

It sounds like I will get more lateral strength from putting my posts in the ground three feet, instead of on metal concrete anchors (eventhough they are from Simpson  ).  I would think that the wood posts would last longer on top of the concrete then in the concrete, is this right?  How long would one expect a 6"x6" PT post to last with the bottom three feet in conrete?


PEG688 - Could you explain this a little more "And also up into the beam , long lag bolt 1/2 X6 or 8 inchs".  I think I know what you mean but I just wanted u to clarify.  Thanks

[jraabe]

If you want open sides then yes going into the groud will add stiffness. If you put the PT posts in packed crushed rock rather than concrete it will last considerably longer. Concrete holds moisture and the gravel can drain.

Moisture is what does in wood. Lifetimes for PT in concrete might be 20 to 100 years depending on climate.

Also get foundation grade posts not landscape timbers.

[PEG688]

  The knee braces , ah I guess I assumed you knew what they where.  They go from the post to the beam that holds the rafters , Generally a 45 deg cut on both ends , like I said there length should keep them above head height , say the beam is ten feet off the slab the knees would be about 36 to 46 inchs long running from post up to the" roof beam"(the one the rafters sit on ) the lag would bolt it ( the knee)upware into the bottom of the beam making the triangle effect that will provide shear .  Go to your local library and get a   timber framing book and look at what they will call knees .. Hope that helped , HTBH  ;)PEG

[McClain]

Thanks guys, I really appreciate it