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12/7/2011 10:02:55 AM
I'm doing some research to update our deck building handout. When it comes to what size post are required, there appears to be no sound answer. The IRC states the minimum size post shall be 4x4. AFPA requires a minimum 6x6. Other standards appear to base the post size on a tributary load using a dead load of 10 lbs/sf and live load of 40, which makes sense...at least for those outside of the snow belt. For the rest of us, do we need to factor in the additional 25, 30, 40lb snow load or do we just assume that once the snow hits everyone hibernates and thus we do not need to be concerned of the snow factor unless it exceeds the 40 lb live load?
12/7/2011 5:04:26 PM
Our decks are required to meet the snow loads (46 psf) and include snow drifting. A lot of summer homes so the snow just builds up with no removal. Ever see the damage from 12 inches of wet snow sliding off a 60 ft X 28 ft metal roof and dropping 16 ft to the deck below? It wasn't pretty
ICC does not stand for Intentionally Clear and Concise
12/8/2011 5:49:15 PM
I was always taught to design the post based on the worst case tributary loading. For those with snow loading to consider, use the loading combination of dead load, live load and snow load. The design snow load should be available through the local building department. And the loading combinations are available in the latest IBC.
12/9/2011 9:43:00 AM
I provide the following link to anyone building a deck in my jurisdiction and then do my inspections with it in hand. It makes it pretty easy.
Like mtlogcabin our decks are required to meet the snow loads (40 psf for us) and include snow drifting.
12/20/2011 12:30:08 PM
You should also check the NDS for post sizing, lateral restraint, etc. There have been many studies done on deck framing and the conclusions have flown in the face of the methods that have been employed for years. One such situation is the fastening of the beam(s) to the posts - it is commonplace to see the post "sandwiched" between 2-2"x10" or whatever size is required. The research shows that even bolting through the beams and posts with carriage bolts does not provide the necessary load resisting values.
1/7/2012 2:32:10 PM
By what authority? This is not a referenced standard in the IRC; And some of the suggested solutions exceed code.
1/18/2012 5:51:58 PM
In reference to the question of by what authority; I apply (although not often) more restrictive requirements where I feel it is needed. I simply have written right into city ordinance that the adopted ICC codes along with the "Guidelines For Construction Book" (which references this publication) shall hereby be considered the Code. If any provision of the adopted ICC codes is in conflict with said Guidelines, the more restrictive shall prevail.
2/21/2012 11:53:53 AM
Post (column) sizing can be a complicated topic. Or, it can be very simple for general purposes.
Two issues need addressed for column sizing. One, the allowable compression parallel to grain and two, the allowable un-braced length. Simply stated, a short column (e.g. 2 or 3 feet) is VERY strong. For instance, a DF 4x4 up to 6 feet or so in length can support around 11,000 lbs. But, when the un-braced length gets to 10 feet, the allowable axial load drops to about 5,000 lbs. The difference is the potential for buckling.
For more on buckling see Design of Wood Structures by Breyer, Cobeen, Fridley & Pollock (McGraw/Hill), the NDS, Simplified Engineering for Architects and Builders, by James Ambrose (John Wiley and Sons), etc. There are some pretty intense and lengthy calculations to factor down the allowable axial load based on a derived coeffecient for the un-braced length.
The good news is, there are charts and graphs that show this info in pre-calculated form that should work for most decks, carports, etc. The attached file is an example and is from the Simplified Engineering book mentioned above. As you can see, with a 6x6 x 6 feet long being good for 27,600 lbs, short columns are very strong. One item to keep in mind is that to be considered "braced" from buckling, the column needs to be braced in both directions.
Hope that helps.
2/27/2012 3:28:47 PM
Nice post. Between this post on post size considerations and the other post on how to build a deck, seems deck construction and post sizing is about covered....
3/22/2012 3:17:24 PM
thats a good quistion, but I allways factor in the snow wait because it would be much stronger. go the extra and in the long it run will pay off better.