Effective Area

[ktulu]

Here's my question:

This is actually question #96 in the "the other board" Civil PE Exam.

What is the effective area of the rectangular footing supporting a concentrated normal force as shwon?

B = 1.5m

L = 3.0m

The normal force is acting at 0.6m in the B direction, and 1.0m in the L direction....

I know that the effective area is the largest area in which the force still acts through the centroid, so I determined that the eccentricity for B equals 0.3m, and 0.5m for the L.

B' = B - 2e( B ) = 1.5m - (2)*(0.3m) = 0.9m

L' = L - 2e(L) = 3.0m - (2)*(0.5m) = 2.0m

Therefore, A' = B'*L' = (0.9m)*(2.0m) = 1.8m^2

I calculated an answer of 1.8m^2 which is a possibility, but wrong according to the solution. No errata for this problem....

Any help? Am I missing anything???

 

[rdbse]

I think your values for 'e' may be wrong.

What is the answer given?

Let me know.

 

[ktulu]

The answer that was given was 2.4m^2

 

[rdbse]

Do they give a diagram?

If not, it apprears they located the normal force from edge of footing in each direction and not from center of footing.

eB = 1.5/2 - 0.6 = 0.15

eL = 3.0/2 - 1.0 = 0.5

Therefore, B' = 1.2 and L' = 2.0

A = 2.4 m^2

 

[ktulu]

The normal force is not in the center of the footing.

That's it....I was calculating the eccentricities wrong.

Thanks for the help, rdbse, I really do appreciate it.....

ktulu

 

[Farmboy491]

I was going to say, watch where the force is located and how you calculate the excentricity.

 

[LakersFreak]

I was going to say, watch where the force is located and how you calculate the excentricity.

yup