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RE: Load combination 7. .6D+W+H

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Because PEMB's have little DL, uplift resistance can be a problem.  Uplift resistance in the PEMB world is resisted by the footings with:
1.  The dead load of the footing
2.  The dead load of the soil and slab above the footing
3.  The shear resistance offered by the slab and footing
 
In this manner you can have additional uplift resistance by spreading the uplift load out over a greater area predicated on the shear strength of the soil and slab.  Additional reinforcing can provide more shear resistance in the slab.  If more resistance is required, the foundation can be lowered to engage more soil. 

Regards, Harold Sprague



> Subject: RE: Load combination 7. .6D+W+H
> Date: Tue, 4 Nov 2008 09:44:39 -0800
> From: YI(--nospam--at)summit-sr.com
> To: seaint(--nospam--at)seaint.org
>
> I had a project recently where we designed the foundation for a metal
> building. I used 0.6D load combination while keeping a 1.5 uplift
> ratio. It was a big footing, and I wasn't quite convinced if this was
> necessary. But I couldn't find any language about this one way or
> another, in IBC / ASCE7.
>
>
> YI YANG, S.E.
>
>
> Do you really need to print this email?
> -----Original Message-----
> From: William Haynes [mailto:gtg740p(--nospam--at)gmail.com]
> Sent: Tuesday, November 04, 2008 9:02 AM
> To: seaint(--nospam--at)seaint.org
> Subject: Re: Load combination 7. .6D+W+H
>
> The combination is for stability too. You use the combination 0.6D+W+H
> and that is it. You don't have to subsequently use a 1.5 safety factor
> or any additional dead load factors for stability after you have applied
> the 0.6D+W+H load combination (it's not cumulative).
>
> Footing weights and retaining wall dead loads must also be multiplied by
> 0.6 when using this combination, it's not just for concrete columns.
> This combination applies to uplift and overturning. Dividing
> 1/0.6=1.67 so the traditional 1.5 safety factor is more than accounted
> for already.
>
> Will
>
>
> On Tue, Nov 4, 2008 at 11:35 AM, Joseph Eribarne
> <jeribarnese(--nospam--at)bak.rr.com> wrote:
> > Sirs:
> >
> >
> >
> > We have some confusion on the application of ASCE7 ASD load
> > combination 7, .6D+W+H.
> >
> >
> >
> > ASCE 7 Section 2.1 indicates that the load combinations shall be used
> > for "a particular construction material throughout the structure." I
> > understand that the .6D allows for the sometimes overestimating of the
>
> > dead load and that concrete columns, for example, should be
> > proportioned for this load case as the reduced dead load sometimes
> leads to the governing condition.
> > Are these load combinations for the design of the materials only or
> > are they also to be used for the overall stability of the structure
> itself.
> >
> >
> >
> > Two quick examples:
> >
> > 1) We do many light framed steel structures and also foundation
> design
> > for prefabricated metal buildings. If, for example, the resulting
> > .6D+W load combination results in a 6000# uplift, how much concrete
> > footing weight (along with slab and soil overburden) must be placed.
> > Is it 6000# or must it be 10000#. The latter will result in
> > inordinately large footing on a very small building using a factor of
> > .6D on a material weight that is known quite accurately with very
> little chance of over estimating its weight.
> > Anchor bolts and any attachments of course, should be designed for the
>
> > full .6D+W load.
> >
> > 2) Retaining walls: Must the retaining wall stability also be
> designed
> > for this case. Should there be a case where the unit weight of the
> > wall, footing and soil is reduced 40% by using a .60 factor to resist
> > overturning. This would even be more magnified where the retaining
> > wall extends above grade and receives wind load (or is all of this
> > ignored and the traditional FS of 1.5 is used for sliding and
> > overturning? Or is it
> > cumulative?) Or even just a free standing 8' high block wall fence
> > receiving wind load. Should overturning be checked by reducing the
> > unit weights of the wall? When using a program such as Retain Pro,
> > one should redefine the unit weight of the materials to .6 of their
> actual values?
> >
> > Again, I am asking about the overall stability of the structure, not
> > the design of a particular element.
> >
> > Thanks for reading this and for your thoughts in advance.
> >
> >
> >
> > Joe Eribarne
>
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