Column footing design

[ktulu]

I have an 8 ft square footing with a square 16 inch column. I need to find the allowable shear load for the footing. The answer states that you need to use Eqn 55.17 in the CERM. I see that, but what confuses me is that the answer multiplies Eqn 55.17 by (the critical perimeter of the footing * the length of the column)

Anybody see what I'm trying to say? Why did they add this extra step that was not in CERM Eqn 55.17? :wacko:

Any help would be appreciated....Thanks

ktulu

 

[kevo_55]

ktulu,

What they are trying to calculate there is 2 way shear or "punching" shear. This "extra step" is actually another check.

For a proper column connection to a footing, you must check 3 things. The 2-way shear. The bearing strength of the column itself. And also the bearing strength of the footing itself. Depending on how high your axial load really is, steel can be added to offset check #2. Check #3 almost never governs.

I suspect that you have checked the 1-way shear of the concrete footing itself.

All three of these checks in one way or another relate to the actual load on the column.

 

[GCracker]

Maybe I don't understand the question, but I don't see why you would multiply anything having to do with footing shear by the length of the column.

Do you mean length of the footing? Or, width of the column base plate?

How thick is the footing? Do they give a base plate size?

 

[ktulu]

Maybe I didn't phrase the question well enough. Unfortunately, I do not have the problem here at work. When I can get a few minutes, I can run home and grab the problem and get back to you....

Thanks for the responses, guys. I will rephrase the question a little later. Reading it myself, I was confused too...

 

[GCracker]

This is what I do to check shear in a column footing:

Two-way shear (punching shear): To determine the thickness required to support the load:

1) calculate the bearing pressure (qu): qu= (factored load)/ area of footing

2) calculate critical length(bo): bo= 4 x (width of column + d) where d=depth to reinforcement

3) calculate the shear (Vu): Vu= (area of footing – (bo)^2)x(qu)

4) calculate depth needed for shear(d): d= (Vu)/((.85)x(4xSqrt(f’c))x(bo))

***** Watch your units******* (also note, phi factors are from appendix of ACI, not the new ones from Ch 9)

If you know the thickness of the footing, you can plug “d” into the last equation and solve for the factored punching shear capacity.

One Way Shear (beam shear):

1) calculate the area under shear (A) a distance d from the column:

A=(length of footing/2) – (width of column/2) – (d)

2) Calculate the shear(Vu): Vu= (length of footing)x (A)x (qu) where qu was calculated above.

3) Calculate depth required for shear(d): d=(Vu)/(.85)(2x sqrt(f’c))x (length of footing)

******Watch your units**********

If you know the thickness, you can plug d into the last equation and solve for the factored one way shear capacity.

The lower of the two numbers you calculate for shear capacity will control the design.

 

[ktulu]

Thanks, GC :wacko:

I believe that is what I need to understand the problem. If it doesn't work, I'll let you know...

ktulu

 

[isaac]

This is what I do to check shear in a column footing:
Two-way shear (punching shear): To determine the thickness required to support the load:

1) calculate the bearing pressure (qu): qu= (factored load)/ area of footing

2) calculate critical length(bo): bo= 4 x (width of column + d) where d=depth to reinforcement

3) calculate the shear (Vu): Vu= (area of footing – (bo)^2)x(qu)

4) calculate depth needed for shear(d): d= (Vu)/((.85)x(4xSqrt(f’c))x(bo))

great summary, I noticed when you define 'bo' in step (2), and then use it in step (3) it should be Vu = (area of footing - (width of column+depth)^2)x(qu) , it is probably what you meant,