Residential Monolithic Footer - Eccentricity 1

[rcast044]

So my concern is the Florida Building Codes monolithic footer. I am applying a 5 kip load from the truss above down to the monolithic footer. Well, technically it is applied at a 4.25" eccentricity (12" width - 4.25" to center of sill plate). However, if you take this eccentricity into account, the footer definitively fails even on low loads. I was wondering, what was your guys way of analyzing this specific foundation? See attachment for picture.

 

[rcast044]

An assumption is that the 3-1/2" slab is tied to the other side so it acts like a tension tie so it can't rotate, so just analyze it as if the load was centered about the footing for bearing pressure.

 

[CANPRO]

If I understand correctly, you're saying the center of the wall is 4.25" from the edge of concrete and the footing is 12" wide? If that is the case your eccentricity is only 0.5*12 - 4.25" = 1.75"

Something else to consider - you're determining the eccentricity on the footing due to the applied to see how that effects bearing pressure distribution, but it doesn't appear that you're taking into account the self weight of the footing, which will pull your net reaction closer to the center line of the footing.

If your slab is sufficiently tied to the footing, I agree with your assumption in your follow up post.

 

[ARPASEVAN]

Hello, dowels can be used to address overturning moments due to eccentricity of the load. Below is a sample calculations I use for L shaped footings at masonry walls.

 

[hoshang]

Mot= 20469 lb-ft/ft

It's 20469 lb-in/ft.

 

[ARPASEVAN]

Thank You hoshang.

 

[rcast044]

The footer is 12" wide so the center is 6" from edge. The sill plate is 4.25" from that or 1.75" from edge like you stated. The FBC does not have any rebar tying into the slab-on-grade. I am not taking into account the net weight from the footing, that is true. Was just wondering if I even need to assume an eccentricity for the house

 

[Ron]

First, let me get my pet peeve out of the way!.....It is a FOOTING not a FOOTER! You're an engineer, not a contractor working from the back of a pickup truck.



If you need to engage more of the slab for uplift resistance (code only gives you 6" prescriptively with WWF), then use hook bars into the footing.

You will only have 5ksf bearing every 48 inches, where the truss corresponds to a stud. Everywhere else, you have have less, with a low of 3.3ksf when split between two 16-inch spaced studs, assuming you have shear failure in the concrete at the edge of the thickened section/slab interface. If you reinforce across that interface (or increase the slab thickness to prevent shear failure), your bearing area increases and you can reach a point where bearing is no longer an issue and uplift is no longer an issue.

Now for your slab.....I would not specify a 3-1/2 inch thick slab. With the sloppiness of residential construction and the fact that you will not likely have a flat and compacted subgrade, your slab thickness will vary quite a bit, increasing "random cracking" from drying shrinkage. I would suggest the following:

Minimum slab thickness: 5"
Tolerance for slab thickness: -1/4", +3/8"
Subgrade flatness tolerance: No more than 5/8" in 10 feet with no abrupt changes in subgrade flatness that exceeds 1/2" in 5 feet.

One of the fallacies of residential construction is that it is always assumed the quality doesn't have to match that of commercial construction! That makes no sense. Quality is quality....without regard to the class of construction. It costs no more to do it right! If you specify properly what should be done, then you can rest that you have complied with a reasonable standard of care. If you don't, you increase your liability.

 

[Ron247]

Ron,
Great comment on Footer. I wrote my own foundation program for metal buildings. I sarcastically named it "Columes and Footers" because I hear those words more than "Columns and Footings". That is a hard "u" in Columes. But it is not "the back of a pickup truck", its the back of a pickup truck that has no tailgate, the cab is white but the truck bed is blue.

The slab tie strength and allowed passive soil bearing are 2 of the more needed values. They will help resist overturning. Without those, it is hard to put a load on the edge like that. B/6 = 12/6 =2 but you have 6-1.75 = 4.25" eccentricity. Without some overturning resistance, that is hard to overcome.