Thickened Slab - Footing 1

[RFreund]

See attached sketch.

There are some site restraints and the contractor is proposing to use a thickened slab edge to support a 14' tall fence. With posts at 8' or 6' on center. I have a couple of questions:

1. Should there be an expansion joint between the thickened slab and the 8" s.o.g.? If so is it acceptable to have some uplift on one side of the footing?

2. Instead of an expansion joint can the thickened slab and slab have a dowel between them so that the slab helps resist overturning and any theoretical uplift (as shown in the attached sketch)?

Thanks

EIT

 

[DWHA]

Is the 8" SOG existing? If not, pour them together.

What type of loads are you designing your fence for? I am getting a 1.37k loading at the top of the post to get a net bearing of zero at one corner.

 

[RFreund]

No the 8" is not exiting. As for loads I'm looking at 90mph Exp C so about 20psf.

Moment at base for 8'o.c. spacing M=15.68k
max bearing for 8' long footing x 2 wide approx 2,194psf but the pressure diagram terminates before the other end of the footing (if that makes sense)

So are you saying you would dowel them "together"?

EIT

 

[msquared48]

In that torsioal forces are applied to this grade beam, I would add closed stirrups to the grade beam, with the top leg of the stirrup extending back 4 to 6 feet into the top of the 8" slab. I would also taper the backside of the grade beam at 45 degrees up to the slab using the subgrade as a form. I have used this many times before and it works well.

Mike McCann
MMC Engineering

 

[RFreund]

Mike,

Good call on the stirrups, I should have thought about that...
that sounds like a good though.

Would you say it is ok to have uplift on one end of the footing (meaning the pressure distribution on the footing terminates before the other end of the footing)?

Thanks again.


EIT

 

[msquared48]

As long as the soil is not overstressed, it should e OK.

Mike McCann
MMC Engineering

 

[RFreund]

Mike,

If the moment is below the allowable torsion permitted by the section (ACI - T=phi*sqrt(f'c)*Acp^2/Pcp).

Would you omit the stirrups or keep them regardless?

Thanks


EIT

 

[BAretired]

Wind load can be in any direction, so I think you need to extend both the top and bottom slab reinforcement into the thickened portion.

The slab and thickening should be poured together. Stirrups are a good idea even if not strictly required by code.

I don't think you need to extend the thickened portion so far beyond the fence...just leave enough room to make a proper connection at the base of the posts.

BA

 

[msquared48]

I would keep them regardless to help with the OTM.

Mike McCann
MMC Engineering