2-story, free-standing porch posts
Indiana_Guy
| Posted in General Questions on
Hello.
We know that we should reach out to a structural engineer for this, but just need some advice to find a starting point.
We are planning a 14′ long (perpendicular to the house) x 35′ wide (parallel to the house) double-story, free-standing porch. The porch will have a stem-wall foundation with footings designed to bear appropriate loads. The porches could be turned to screened-in or 4-season at a later date.
The location is Indiana with roof loads of 30psf.
We plan on using 4×6 beams at the perimeter of the first floor to support the loads from the second floor and the roof.
The question is – would it be sufficient to use 4×4 posts spaced 3′ o.c. to hold up the 4×6 beams ? Or would 6×6 posts be better? If so, at what spacing?
Can someone please help us find a starting point. All tables and charts we’ve come across are for beams and not posts. If we are missing something, please guide us.
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Replies
A ballpark way is to figure out what wall would hold that floor up. For two story, this is typically a 2x6 wall 24OC. You then count how many studs that wall has, your posts need to roughly have the same amount of wood.
Overall, for something you walk on, it is 6x6 post, for roof 4x4 is fine. Working with 2x lumber is better as it won't split as much as a solid post. If you cover it with house wrap and clad it with wood, it will last much longer as well.
If you want solid posts, best to do a cut on a hidden side down to about the center of the post (provided the engineer is OK with this), this will relieve the stress and less chance of splitting as it dries. Consult an engineer no matter which way you go though, this is not a DIY kindof design.
This is the kind of reply we were looking for. Thank you, Akos.
It is pretty easy to build a 2nd story deck that falls down, don't take a chance, get an engineer to spec the details.
Deleted
Columns like this usually fail by buckling (folding in the middle, basically), so bracing becomes important as the columns get tall. Long, slender structures aren't particularly stable. Fatter columns can be longer before they start to fail for the same load, for example. All this means that what might be a safe for a 4x4 that is 2 feet high probably won't be safe for that same 4x4 column that is 14 feet high.
An engineer can work this all out for you. I'd expect some kind of bracing to be required for a 2 story deck. You have similar forces to resist to what causes "racking" in a stud wall, but you probably can't have shear panels to deal with that on a deck.
You don't want to do this as a DIY project since you're getting into a more complex structure than the typical ground-level deck.
Bill
The lumber is really not very expensive in this compared to the engineering and labor - go with as big as you can.
Also: If you do pay an engineer to spec out a tall deck? Go big and over-spec. Decks are magnets for parties, and liabilities involved in a collapse are enormous. 100lb/sf live loading, plenty of posts (4x4 and 6x6 are both way stronger than necessary in vertical loading, but 6x6 is easier to form strong connections out of, and bends less), and a free-standing design that doesn't rely on a sketchy house ledger board. Pay a bunch of extra money to Simpson for the strongest heavily galvanized offset post anchors they make - modern arsenic-free pressure treated lumber does not tolerate being buried in concrete or left on the ground.
One or two 2x12 beams actually ends up being stronger than a 4x6 beam by a large margin. Stiffness increases by the height of the beam cubed, so 12" is 8 times as good as 6" at carrying load without deflecting. The 4x6 is then twice as wide as a single 2x12, so the difference is a matter of 4x, despite the fact that the number of board-feet is the same.
For tall posts bracing and connection details are going to be at least as important as actual amount of lumber in the posts.
For all the reasons stated, hire an engineer. That said, I think intellectual curiosity is a good thing, and one way I've satisfied that with similar things is to model loads with Weyerhaeuser's free, online tool called Forte: https://www.forteweb.com
This is a single member tool, useful for designing beams, floor joist systems, etc. That said, it does do some analysis of posts, and it certainly can give you calculated point loads at posts supporting beams.
There are lots of things to consider like wind, racking resistant, etc., so this is in no way a complete solution for what you're doing, but it might help put you in the ballpark, or at the very least give you data to go cross reference with other specifications.
And in case it wasn't clear, hire a design professional legally authorized to design such a structure in your jurisdiction. :-)