Enercalc Footing Design

[MommaRe]

Does anyone know when Enercalc analyzes the reinforcing if it takes the total reinforcement for a moment footing? Example, 6' square footing should be ok with 7 #5's. If you enter in 14, is it assuming half on top and half on bottom? Of course, I'm talking about 14 in one direction.

 

[ash060]

If it is a single footing why would you need top reinforcing?

 

[MommaRe]

It's one of a group of footings for a metal building. It's going to see shear and moment

 

[ash060]

Well if the single footing has a triangular load distribution, like a typical moment footing, it should only have tension on the bottom.

 

[j19]

Many metal building footings have very large uplift forces. At least compared to the gravity forces. For these cases wouldn't it be proper to also include reinforcing at the top of the footing?

 

[swivel63]

if it's on piles, i would think. unless soil has some tension capacity that i don't know of.

 

[structuralhawaii]

Try to read the enercalc manual, i think it should be there but normally we used it as a single layer. You can get the As from the actual moment to check if the rebars are sufficient.

 

[ash060]

Remember you can only pick up the weight of the footing and most codes require a S.F. of 1.67 against uplift. So the footing never would see these uplift (Tension on the top face) forces. Draw FBD and you will not see much tension on the top face.

 

[Lutfi]

j19, if the uplift is less than the gravity load, this means no negative bending in footing. Therefore, it is my opinion that top bars are not be required. They may be a good practice.


ash060, I use SF of 1.5 against uplift. Where do you get the SF = 1.67?


Regards,
Lutfi

 

[ash060]

0.6DL + 1.0WL out of the Florida Building Code

 

[j19]

Lutfi - Thanks for the info. I guess I'm thinking that if the applied load from a column is the center of a footing is an uplift load, and this load is resisted by the dead weight of the footing and maybe some soil on top of the footing, wouldn't the top of the footing be the tension side of the resisting "beam"?

 

[ash060]

The footing has to bear or there would be no factor of safety

 

[JoshPlumSE]

ash060 -

Take a footing with eccentricity (M/P) greater than 1/6th the width of the footing. At that point, your footing goes into partial compression where there is partial uplift on one side of the footing. Increase your moment some more and the portion of the footing in uplift can become siginifcant.

When a good portion of that P load came from the self weight of the footing slab and the soil overburden above it, there could be significant negative moment on the uplift side of the footing. Therefore, you will often NEED to add top reinforcing to your footing to resist this moment.

 

[ash060]

Sand can not hold down footing. Weight has to be greater than uplift force or the system is not in equilibrium. In the end the net force is down, therefore you have a footing with a large moment and small axial force. Cut FBD at face of column with soil distribution on it get tension on bottom. If take FBD at other face you will have self-weight and soil weight which will be very small compared to thickness of footing (if uplift is the reason footing was sized) will work as plain concrete in 99 % of cases. In uplift case footing will never see that real uplift because of the safety factor.

Also in the case of steel columns anchor bolts are typically designed to resist the uplift on the column not to pick up the weight of the footing. Anchor rods will pullout before footing "lifts up" so footing won't see that load.

 

[haynewp]

j19

If there is uplift on the concrete I always use bars in the top. For a footing with a large plan area under pure uplift, the middle of the footing experiences uplift first and engages the remainder of the footing through bending to resist the total uplift.

 

[ash060]

I really do consider it an engineer's preference. Unless the numbers really tell me I need top bars, I don't put them in.

But I have done it before for "feel good" reasons.

 

[j19]

haynewp - thanks. You said it like I thought it. I don't design very many footings and I have found that fairly wide shallow footings work best in many instances (no frost depth problems here in the south). I almost always use bars in the top.