Calculator Question: Actual load imposed on the soil versus the maximum soil bearing capacity?

by Mark
(New Jersey Coast)

Great site, learned a ton here for footings/loads/psf etc. When i use the load calculator I get numbers for the tributary area and their actual loads vs the max load. The middle area is cutting it close (2725<3000psf). I am wondering if this is too close a number of psf or is this adequate?

My solution would be to use a bigger footing in the middle area to lessen the actual load (2725 psf) to 1575. This of course is about 10 more bags of concrete i have to lift, which i would like to avoid if i have to.

Editor's Comments

You have got the right idea. Glad the deck load calculator is helpful.

You are still almost 300 psf less than the maximum so you are pretty safe. However, the middle footings of any deck almost always carry a heavier load given the tributary area is larger.

Just curious though because you indicate the soil in your area is capable of supporting 3000 psf. That is pretty high. In all my years I have normally seen 2000 psf as pretty near the maximum. There are ? icons you can click for each component of the calculation to explain what numbers to input.

There is nothing wrong about adding another footing in the middle or enlarging the surface area of the existing footing to further spread the load. As you can see it makes a big difference. I would do it.

I built a deck many years ago that was about 25' by 20' and it was in an area that was mostly silty soil. So the compression for it was lower than I thought. A year and a half later I had to come back to the customer and dig another footing in the middle of the deck to prop it up as it had sunk about an inch and a half in the middle of the deck.

Had to remove deck boards and use a clam shell digger to surgically dig out another 4' deep hole and pour cement and re-level the central beam. It worked out just fine.

But this was a first hand lesson that I never forgot.

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