Let me use an example since this always seems to be an issue due to people thinking that a 12" sonotube works for all applications.
This is how I operate and how we do plan review for this part of a deck with a roof over it. Your mileage may vary but this is what we do:
Example:
Applicant plans on building an attached 12' x 12' deck to their house with a 12' ledger board properly attached to the house and 2 piers to support the outward end of the deck, one on each corner.
We first take the deck of 12x12 and find the square footage which is 144. Half of that load will be carried by the deck ledger and half split between the two deck piers. Half of the load is 72 square feet and a deck must be designed to a 40# live load and a 10# dead load at a minimum for a grand total of 50psf. If we take 50 x 72 (half the load) we get 3,600 pounds. Since there are 2 piers, each pier will take 1,800# each. Sounds simple, right?
In our area of PA 1,500psf soil bearing capacity is about the norm, sometimes more but without testing, we can safely assume 1,500psf based on the IRC prescriptive requirements.
If the soil bearing capacity is 1,500psf and each pier must be designed to handle 1,800psf then we can easily see that a 12" round pier simply will not work. There are 2 problems with using a 12" round pier. First, it is less than 1 square foot since it has rounded corners and second, due to that fact, it cannot properly transfer the 1,800psf load.
What is the actual bearing capacity of a round pier? Glad you asked, great question. We must first know how to calculate the area of a circle. Pi times Radius Square will give us the area of a circle. (I'm not sure I have those fonts available here so I'll spell it out). The radius is 6" and radius square is 36. Pi=3.14 so 3.14 x 36 = 113 square inches which is not quite the 144 needed for a square foot. It is approximately 78.5% of a square foot. So, with a soil bearing capacity of 1,500# the maximum load that a 12" round pier can handle is about 1177#, way short of our 1,800# design requirement. We would need much better soil bearing capacity in order for a 12" pier to work.
Now that we know we have an 1,800# design load on each pier, lets add a roof like we see all the time.
In our area we have a 40# ground snow load and most construction is a 10# dead load. That happens to work out to a total load of 50psf, the same as the deck so we can safely just double the design load on that pier. That means that each pier must be designed to handle 3,600# of downward pressure.
So this is where my original question comes in. Any architect or engineer will tell you that there is a difference between a ground snow load and its equivalent live load. Luckily the IBC addresses this in Chapter 16 and with the help of the ASCE-7 this can be easily calculated. In our area with different factors applied, our 40#gsl is the equivalent to more like a 34# live load which may make a difference for the piers. Maybe, maybe not. I think on a large job this is extremely important.
I want to look at the difference between the 2. Instead of using 40#, we will use the 34# load. 34+10= 44psf so the same tributary load of 72' x 44 = 3,168 and since the piers will be sharing that load, it is 1,584# each pier.
So for the deck and roof we have the following design load for each pier:
Deck: 1,800
Roof: 1,800 or 1,584
Total load for the piers can be either 3,600# or 3,384#
The question is, prescriptively, what size pier/footing do they need?
