16'X32' Cabin Project - Post & Beam Foundation Input Please

[MichMidwest]

Hello Everyone,

I am new to posting but have read many post on here in the post. We are starting our project this weekend and will be posting photos beginning this next week. I am building a 16'X32' single story Cabin and would like some input. The cabin will be on a post and beam foundation. I plan to use PT 6x6 post spaced 8' apart. in three rows running the 32' length of the cabin. I will be using double PT 2x10's with PT 1/2 plywood spacer and the top of beams will be 4' above grade. The floor joist will be 2'x8'x16' @ 16" centers. The decking will be 3/4" tongue and groove, The walls will be 2x4x8 with a bottom and two top plates. The roof will be a 6/12 pitch and the rafters will be 2"x6"x12' roughly. I am thinking a 12 overhang and metal roof. On the interior I plan to use some wood finish, maybe tongue and groove. The ceiling will be open and finished the same as the walls with some type of wood.

Will the 4x10 beams be sufficient for the weight, would the 6x6 post at 8' centers be enough or should I space them closer and go with triple 2x10 beams?

Thanks for any input.

[Don_P]

These are the pertinent code sections

R404.1.9.3 Masonry piers supporting braced wall panels.
Masonry piers supporting braced wall panels shall be designed in accordance with accepted engineering practice.

Girder spans are here;
http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_5_par019.htm?bu2=undefined

R802.3.1 Ceiling joist and rafter connections.
Ceiling joists and rafters shall be nailed to each other in accordance with Table R802.5.1(9), and the rafter shall be nailed to the top wall plate in accordance with Table R602.3(1). Ceiling joists shall be continuous or securely joined in accordance with Table R802.5.1(9) where they meet over interior partitions and are nailed to adjacent rafters to provide a continuous tie across the building when such joists are parallel to the rafters.

Where ceiling joists are not connected to the rafters at the top wall plate, joists connected higher in the attic shall be installed as rafter ties, or rafter ties shall be installed to provide a continuous tie. Where ceiling joists are not parallel to rafters, rafter ties shall be installed. Rafter ties shall be a minimum of 2 inches by 4 inches (51 mm by 102 mm) (nominal), installed in accordance with the connection requirements in Table R802.5.1(9), or connections of equivalent capacities shall be provided. Where ceiling joists or rafter ties are not provided, the ridge formed by these rafters shall be supported by a wall or girder designed in accordance with accepted engineering practice.

Collar ties or ridge straps to resist wind uplift shall be connected in the upper third of the attic space in accordance with Table R602.3(1).

Collar ties shall be a minimum of 1 inch by 4 inches (25 mm by 102 mm) (nominal), spaced not more than 4 feet (1219 mm) on center.

Rafter span tables are here, pay attention to which live load table you are in;
http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_8_sec002.htm?bu2=undefined

[MichMidwest]

Thank You Don !

I will go over the information you provided. I have decided to go with 6x10 beams (three 2x10s with 1/2 plywood spacers). We have been planning this for some time but at last minute im questioning our design. Id rather be safe than sorry. The major concern of mine is our cabin will be located in oklahoma. Our land is flat with not many trees nor on the many acres that surrounds ours. The post will be pt 6x6 sitting on concrete footings and being four feet in height. Im concerned about wind shear as we have alot of wind here. I have thought about going with 8" sonotube concrete piers instead of the wood post. Im trying to keep the cost in budget but I would rather spend more than to pay additional later. I thank you again for the information you provided.

[Don_P]

You certainly should be concerned about the foundation, neither of those is a good idea without an engineer on board. I doubt you will find one to approve this, one that I spoke with about these methods calls it a collapse mechanism.  A continuous footing with full perimeter walls or a pressure treated wood foundation, again with full perimeter walls can resist the wind, a slab is also a good way. The intent there is to collect the horizontal loads through the braced walls and transmit them safely to the ground through the foundation. With piers the closest way to do this prescriptively is with a pier and curtain wall foundation, braced walls. Work your way through chapter 4 in the links above to read about the different prescriptive (non engineered) methods. In high wind, low and wide with a hip roof is the best way to survive a storm. I'm not at all against using an engineer but very often I look at the cost of bringing one on board versus doing it prescriptively and it makes more sense to put that money into materials rather than paper and just build prescriptively. The American Plywood Association has high wind construction guides on their website, apawood.org

[nailit69]

I'm curious as to why so many of you go with or prefer to go with a post and pier foundation rather than going with a poured concrete or cement block foundation?  Is it that conditions onsite require that method?... a lack of skills/abilities or tools to do a traditional foundation?... freezing/flooding?... personal preference?  I hear almost everyone say they regret the fact that they didn't go w/a crawlspace or basement foundation as they're attempting to do it after the fact. 

My current house is on post and pier and it scares me to think of what would happen if something were to fail during an earthquake or some other situation... i'd probably be dead in the lake.  It did alright during the last major quake but I wasn't here for it and it was very small here... the house I was building at the time (closer to the epicenter) shook like a mo-fo and scared the life out of me and it was on a massive concrete foundation.

A concrete/basement foundation seemed to make the most sense to me on my project... 300+ sq. ft. of useable and valuable extra space... this is where i'll put all my plumbing and mechanical systems, 500gals. of house water, 250gals. of greywater, a composting toilet tank, extra food/drinking water, snowmobile/quad storage, tools, pull the front of my truck in and work on it if needed, and I can even get my boat in there.