Facing problem with biaxial bending footing having uplift 1

[ST04]

Hi
I have problem with biaxial bending footing having uplift, after excessive search I found a reference that has acceptable details regarding the calculation of bearing pressure. (Reinforced concrete design and analysis - by S. S. Ray)

The problem is that after calculating the bearing pressure points, I perform a small static check of Summation of V forces = 0. But I get considrable error. (for example: total applied vertical load = 7500 kN and I get the upward bearing pressure force of around 6000)

Is it something normal since the method given in the reference maybe based on approximation?

For the total bearing pressure force I used Trapaziodal area.

The problem is that I didn't find any other suitable reference for this kind of footing.

Regards

 

[rscassar]

Where does the biaxial bending come from? Have you analyzed the structure with some rotational fixity or is the column load eccentric from the centroid off the footing. I would try to eliminate moment and solve the bearing pressure for P/A.

 

[Lion06]

I Would double check your soil pressure reaction calc. Can you post a sketch of the condition with load, location, and bearing pressures that you came up with?

 

[Lion06]

I think your problem has to with the fact that you have uplift. After taking into consideration ftg weight you can't have a net uplift Arther soul/ftg interface or youre structure is unstable. Also, if you have a
net uplift of 7500 KN you would 7500 KN of soil force pulling down in the ftg, not 7500 KN of force from the ftg bearing on the soil. The statics don't make sense here.

Please provide a sketch.

 

[ST04]

Hi guys
Here is the condition of loss of contact (Zone3) according to the book.

http://files.engineering.com/getfil...e-4bf6-4c1c-a34d-4e50ee1ee55f&file=zone-3.jpg

The way that I calculated the total upward pressure force is as follows (symbols from the attached sketch)

[((P1+P2)*0.5)*0.5]*[(X1+X2)*0.5]*B = pressure force

If I instead of taking the second bracket -(average length)- I took the following (X1*0.5+X2) the error become only 1% but I can't find any mathematical approve to this!

The column is rectangle and located at the centroid of footing, major part of the load comes from the wind but the both axes bending will always happen at the same time. There will be shear force acting along both axes and I have already included them as moments by multiplying by footing depth.

Regards

 

[paddingtongreen]

An additional check that might help. The center of bearing pressure (the centroid of the volume of the bearing pressure diagram) must be directly below the load point.

Without knowing the proportions of ex and ey to A and B respectively, we can't confirm the shape of your bearing pressure diagram.



Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[ST04]

Here are the main data and dimensions:

Dimensions of Column base (in meters): 2.5x1.5
Dimensions of footing (in meters) : 14x9x1.5

Service loads:
Ptotal=5541 kN
Total Bending moments:
about footing major axis: 19820 kN.m
about footing minor axis: 9986 kN.m

Eccentricities:
e_major= 3.58 m
e_minor= 1.80 m

Following the book, the ultimate bearing pressures Pu1 and Pu2 (see the previous sketch) are:
P_u1=313.9 kN/m^2
P_u2=55.8 kN/m^2
X1=13.63 m
X2=2.42 m

Regards

 

[ToadJones]

I have run into this before and found the pressure diagram extremely difficult to determine.
Our friend and excel spreadsheet expert Tomanovich has a footings spreadsheet you can use to check your calc.
Go to Steeltools.org and search for "footings"

 

[ST04]

Hi dear colleagues
I have checked my solution with the excel spreadsheet mentioned by ToadJones. It gives almost the same solution.

So either the static check is not applicable here, or the way that i'm calculating the total upward bearing pressure is wrong!

But if the static check is not applicable, that means if you draw the shear force diagram it will not end at zero and similar for the moment.

any suggestion?
Thanks a lot everybody

 

[ST04]

Okay I think I had a silly mistake causing that huge difference, as I was measuring the ey from the footing border not from the centroid, so the average length was not accurate.

Thanks a lot everybody.
Regards