F=p/A Questions

The screen shot below represent the solution to a practice exam question:

Essentially, I am to find the design load of a pier (not displayed):

Why am I multiplying 32 sqft by 50 psf? Is this referencing the F=P/A formula? If so what value is P in this and what value is F?

@coachjamietischler @coachmarkbailey @coachryanutter

Hi @Shikha ,

I’m afraid I had to delete the photo as we do not allow posting of screenshots of paid content including Black Spectacles’ exams. However, we are allowed to cite or quote paid content.

The screenshot in question came from a Black Spectacles’ question which was asking what the design load on a pier is. The question states:

“What is the design load on pier B? Use 40 psf for the live load and 10 psf for the dead load. Provide your answer in pounds. Round to the nearest pound.”

The answer rational said,
"The answer is 1600 lbs.

First, find the tributary area supported by the pier. The pier will support the deck halfway to the ledger and halfway to the adjacent piers.

Tributary area: 5’-4"x6’=32 feet.
Add the live load and dead load together to get the design load: 40+10=50psf.

Multiply 50psf by 32 sqft to obtain the answer of 1600 pounds"

I’ll let the tagged coaches do a full longer response, but since I’m already here on the page I’ll try to help quickly.

To calculate the full load on a column you need to find it’s tributary area. The tributary area in this instance was found by utilizing the exhibit (not pictured). The tributary area was 32 sqft. Each foot of that area has a live load of 40 psf and a dead load of 10 psf. So, the total load for the area is 50 pounds per square foot for 32 sqft. You multiply 50psf by 32 sqft to get the total load, which is 1600 pounds.

The formula you’ve cited above is for calculating pressure, P(pressure) = F(force)/A(area perpendicular to force).

This information is covered Black Spectacles’ lectures for PPD & PDD if you want to learn more. For those reading this post who might not have a Black Spectacles subscription I also found this old Black Spectacles webinar demonstration Mike Newman did about calculating column and footing loads that directly relates to the question above!

Hope this helps!
-Darion

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So this clears it up a little, but I guess the formula I’m looking at in my notes is Total stress (f) = Total Load (P)/Total Area (A)…when would I use the stress formula?

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@coachdarionziegler any follow up on when to use F=P/A specifically

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f=P/A is the first form for simple direct-stress formula. It is used in stress determinations with stress defined as: force per unit area (unit stress).

It is changed to find out total load (P=f.A) or finding the cross sectional area of a member subject to a force (A=P/f).

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Darion or @Coaches

Does this question fall under the “determine the sizing of structural systems…” objective for PDD?

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Hi @Shikha ,

I think it’s likely this question falls under this category.

-Darion

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