Need to know load capabilities of my free standing deck.

by Dave
(Pittsburgh,PA)

My deck is supported by 2 sets of posts (6x6) and beams (2-2x10's). Posts are 7' apart with beam and beams are 9' apart. Can anyone help me calculate the load rating?

Overall deck is 10' out and 12' wide, 18" on center overhang all the way around.

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Jul 03, 2010
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Quick Steps to calculating deck load onto soil
by: Editor - Rich Bergman

Calculating the loads on a structure is not that difficult but there are a couple concepts you have to know first and a diagram really helps explain this. I will be publishing a page on this very topic soon with diagrams to help.

But here goes.

First step is to settle on the total design load for your deck.

Let's start with 50 lbs/ft (square foot). That is 40lbs dead load (weight of structure) and 10 lbs live load (people, furniture etc). So a 10x12 deck will have to be built to hold 6000 lbs (120x50=6000).

Second step is to determine the size and number of Tributary Loading Zones on your deck.

Let's call them TZs. Your TZs are actually flow from where you decide to place your loading points. A loading point is anywhere a load is transfered to from the deck. So in the case of a freestanding deck, it is only the posts. But if you connect to a ledger on the house, the ledger becomes a loading point also.

In your case with a free standing deck its a bit easier because you only have posts and no ledger. You said you have 4 posts, 2 posts per beam. I drew your deck out based on your details and if I have it right, you should have 4 TZs - Tributary loading zones. Each one is 5'x6' because you have everything spaced evenly.

Each TZ has an area of 30 sqft.

Third step is to determine how much of the total load (6000 lbs) is distributed to each TZ.

Take 6000 lbs and divide it by the number of TZs (4). 6000/4=1500 lbs. If the deck is loaded perfectly balanced to its maximum design load of 6000 lbs each TZ will carry 1500 lbs which will in turn be transferred to the deck post which will transfer it to the footing and the soil must then be capable of bearing at least 1500 lbs per sqft.

Fourth step is to know the bearing capacity of your soil.

I used geotechnical engineers every single time I build a house - I take zero chances on screwing up something to incredibly important as the foundation of a home. For a deck you can do the same if you wish but most people don't because it costs a few hundred bucks. There are a few tricks to "estimating" soil bearing capacity but I will share those in another article because I want to keep the flow of thought going here.

Bottom line here is your soil has to be capable of holding 1500 lbs sqft. This is a fairly low number but there are some soils like Leda clay or areas with lots of moisture and bio mass that would be well under this. That is a problem. But you would probably know that because you would see ducks swimming near by reeds growing - well maybe not in all cases but you get the point.

Fifth step is to compare the soil bearing capacity (1500 lbs) with the load (L) distributed to each TZ.

Each L value must be equal to or less than 1500 lbs.

But this is the basic concept.

Anybody else with more good ideas please help out.

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