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Re: Foundation A.B. Capacity

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jake-
imho you're going to have more trouble with the posts than the foundation. check the
combined tension and bending at the reduced section thru the bolts. i'd pair up
phd-8s or use mbr rods.
chuck utzman

Jake Watson wrote:

> Seaintonln(--nospam--at)aol.com wrote:
> >
> > Jake, here is some comments:
> >
> > 1. Typically, the concrete will fail before the steel does. I am not sure
> > what type of foundation you have, but the deeper the embedment, the larger
> > the cone (concrete cone failure). If you are considering a stem wall for a
> > raised foundation, your capacity should not (in my opinion) consider the stem
> > as helping to resist the tension. I would develop the capacity in the footing.
>
> I agree with you, but here is the exact problem:
>
> 8" FNDN Wall
> 4" Edge Distance (2 sides) ed = 4"
>
> 12" Embedment (le = 12")
>         Projected area = 106.4 in^2 (Ap), this has taken into account that the
> wall is only 8" thick (I truncated the shear cone).
>
>         Therefore, phiPc = (0.65)*(1)*(4)*(106.4)*(2500^(1/2)) = 13.83 kips
>
>         Edge distance reduction, (13.83)*(4/12)*(4/12) = 1.54 kips
>
> 30" Embedment (le = 30")
>         Projected area = 251.32 in^2 (Ap), this has taken into account that the
> wall is only 8" thick (I truncated the shear cone).
>
>         Therefore, phiPc = (0.65)*(1)*(4)*(251.32)*(2500^(1/2)) = 32.67 kips
>
>         Edge distance reduction, (32.67)*(4/30)*(4/30) = 0.58 kips
>
> See the dilema?  Note the edge distance reduction seems to overpower the
> increase in depth.  Because it is on two sides, the reduction must be
> taken twice.  As a side note. This also happens with only 1 truncated
> edge, but not to the same degree.
>
>
> > 2. This generally means designing the footing of the all to resist bending so
> > as to distribute enough load into a rectangular foundations to allow for
> > sufficient concrete to act as a "deadman" before failing as a beam.  Although
> > this is starting to get a bit beyond my talents (since I would have created a
> > deeper deadman type resistance for the anchor) I would assume that you can
> > use the stem of the foundation wall and model the foundation as a "T" section
> > in bending.
>
> I have designed the footing / foundation wall as a grade beam to
> compensate for the uplift.  The real problem is getting the forces into
> the foundation in the first place.
>
> Can anyone see a flaw in my logic (or bad math?)
>
> Thanks again,
>         Jake Watson, E.I.T.
>         Salt Lake City, UT
>