Monolithic footing with fiber mesh (In Florida)

[Eng.V-C]

In Residential Construction in Florida,
A lot of houses (1 and 2 story)are designed with Monolithic Footing with fiber mesh on top.
I have read multiple forums and re-searched online and asked around in my office and
I have not found calculations for monolithic footings with fiber mesh on top.
So far I have come across 2 approaches (See attached picture for reference):

1st approach: Assumes that the wall load will create a rotation at the heel of the footing and therefore will create a moment that needs to be counter-balanced with the slab. This approach ends up requiring top steel around the perimeter of the footing in order for the slab to work with the thickened edge. In most cases the area of steel required is more than wire mesh, and in theory this approach requires top rebar around e perimeter of the slab.

Pros of this approach: It Seems to be more in alignment with the footing design principles found in text books.

Cons of this approach: Having that perimeter steel(rebar) is not a common practice for 1 and 2 story houses construction in Florida.

2nd approach: Assumes a projection of the load from the base of the wall to the base of the footing at 45 deg. This approach seems to disregard the eccentricity from the load since the load ends up being distributed at the base of the footing as if the load was at the center of the footing.

Pros of this approach: It fits the construction practice of building (1 story houses and 2 story houses) with no rebar around the perimeter of the thickened edge slab.

Cons of this approach: There is no textbook or code (that I am aware of) that validates this approach for monolithic footings.

Please let me know which approach you go by and what is your basis. Please share any resources or calculation examples if possible.

 

[DaveAtkins]

You cannot defy statics. Eccentricity should not be ignored. I have used the first approach for buildings without traditional frost wall and footing.

Alternatively, you can try to design the thickened edge/slab connection as plain concrete.

DaveAtkins

 

[DaveAtkins]

Another possibility is assuming a trapezoidal or triangular bearing pressure below the thickened edge.

DaveAtkins

 

[phamENG]

I agree with Dave - you can't ignore the eccentricity. But, both approaches work. You either have to design the turndown/footing for eccentricity or you have to resolve it with part of the slab. Whether you do that with rebar or through plain concrete principles is up to you.

 

[Eng.V-C]

Thank you for your responses DaveAtkins and phamENG.

How can you account for load eccentricity using the approach #2. When you spread the load at 45 deg?

 

[phamENG]

You don't really 'spread the load'...your applied load is at the center line of the wall (unless it's eccentrically applied at the top of the wall, in which case you have to chase that down)...you'll end up with a triangular or trapezoidal reaction under the footing with the centroid of that reaction aligned with the applied load. You will not have a uniform load distribution under the footing.

 

[Eng.V-C]

Thank you PhamENG.

 

[XR250]

I don't think the IRC requires any rebar. It also treats all the footing widths the same regardless if they are concentrically or eccentrically loaded. Typically, I just call out "turned down slab per code". I have a spreadsheet that calculates the amount of slab I need to engage to balance the eccentricity. I use it sometimes when the loads are high.

 

[Eng.V-C]

XR250,
Would you mind sharing the spreadsheet that you used to balance the footing eccentricity?
Which section of the IRC are you referring to?

 

[XR250]

Section R403.
If you email me, I can send it to you. You can decipher it in my info page.
It basically just does a moment balance around the center of the footing. You would then have to calculate the rebar required to support the slab "lever"

 

[SE2607]

I have seen some engineers use passive pressure on the inside face of the footing to resist eccentricity, but that will result in tension in the slab therefore requiring at least mesh reinforcement.

AFAIK, fiber reinforcement is only used to reduce spider cracking as a result of curing too quickly. It's not (again, AFAIK) structural reinforcement.