I’m doing some planning to build a three season porch that is attached to my house. I’ve been reading various deck books including the ones published by FHB. I understand the concept that you calculate static loads to see how much weight is at one post to calculate the footing size. That is, the footing size depends on the soil bearing capacity and the load.
So I can use one of those Bigfoot forms that is the right size depending on my calculations. My questions is how to size the pier that sits on top of the footing? How do you know if you should use 12″ or something bigger or smaller?
Replies
My questions is how to size the pier that sits on top of the footing? How do you know if you should use 12" or something bigger or smaller?
The short answer is to hire a licensed professional engineer to design the structure.
IF you live in a municipality that does not require a PE to stamp your plans, you could consider doing the design yourself.
The general method is to determine the maximum live and dead load on the structure, the support the soil can provide, add a safety margin, and then distribute the load across the piers.
You calculate the total dead load, and determine what bit of it would be assigned to each footing. (I'm sure there are formulae for this, for a uniform deck, but you'd have to do some more complex figuring if the deck is non-uniform.) Once you have the load on each footing, you divide the PSI of the soil into that load to calculate the square inches of footing needed. Since footings only come in certain sizes, you're most times going to be 50-100% oversized on a footing, but it's best to have them reasonably "balanced" so that any subsidence will be uniform.
You calculate the total dead load
Code and safety requires including both the dead and live load in the calculations.
His question concerned how to size the piers rather than the footers. Assuming he is using 24" diameter footers and that they meet the load requirements, 12" diameter piers will meet the footer offset requirements and SHOULD also meet the load requirements. However, without more information such as the size of the structure, load requirements, etc. it's impossible for us to determine the required size for the piers.
still investigating this requirement
Hi, thanks for all the replies thus far. I'm still researching this topic and have done a lot of reading. I've also asked questions of the local building department who have been very helpful.
I understand the concept of calculating the dead load, etc. It seems that I need to figure out the layout of the porch itself (e.g, what size joists, spacing, and the size of the support beam).
After looking at a lot of decks, I see that they were built with a 4x4 post sitting on top of a pier. So for the sake of this example, let's say that we determine that a 4x4 post is sufficient for the load requirement. It seems that theoretically, you could get by with a 4" diameter concrete pier becasue wouldn't a 4" column of concrete be stronger than a 4x4 post? Or is that not true? I could understand the want to use 8" or 12" pier instead so that you have some margin of error in centering the post on top of the pier.
No it will not be a 4-inch concrete column
A concrete coulmn wil require reinforcing steel, at least one longitdinal bar, and probably one est of hoops, or a helix.
They will not fit into a 4-inch diameter column.