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Re: Slab Foundation Possible?

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Robert, can you just add some bolt-on structural outriggers after arrival on
location, to provide a suitable "footprint" to resist transverse overturning
without uplift?

Mark D. Anderson PE
----- Original Message -----
From: "Robert Valorio" <geotex1(--nospam--at)>
To: <seaint(--nospam--at)>
Sent: Tuesday, January 30, 2001 9:42 AM
Subject: Slab Foundation Possible?

> Hello:
> I am looking for some help on a foundation for an
> enclosed, skid-mounted landfill flare unit (i.e.,
> burns off the methane from garbage degradation).
> Anyway, the unit is self-contained because it is
> mounted on a 40ft long x 8.5ft wide trailer skid and
> will be delivered to the site and set upon a prepared
> foundation. After which, a 40ft tall (ground level to
> tip of stack) flare stack will be bolted on. So, what
> we have is a long narrow footprint with a tall chimney
> that can produce a large overturning moment due to
> wind loads.
> >From the flare manufacturer, wind design was for 100
> mph winds, exposure C, and I = 1.05, which is
> reasonable for the site and unit. The manufacturer
> also provided loads at the bearing for the foundation
> design. There are six bearing points in two rows of
> three (either side along the length of the skid). So,
> without a picture to show, here's how they're
> positioned: picturing a rectangle 40ft x 8.5ft, one
> bearing is positioned at each corner of the rectangle
> and the two intermediate bearings are placed 8ft from
> the end of the skid along the same line as the other
> bearings. (Note, all descriptions imply centerline
> dimensions.) Loading from the manufacturer is as
> follows:
> At one set of end bearings and the intermediate
> bearings 8 ft from them:
> Wind Shear = 4kips
> Download = 12.6kips
> Uplift = 5.58kips
> The bearings at the opposite end are:
> Wind Shear = 2.1kips
> Download = 6.6kips
> Uplift = 0kips
> Now the dilema, foundations for these units are
> normally cookbook for me. For this loading, and
> typical soil conditions encountered, I've always used
> drilled or poured piers beneath each bearing point.
> However, this particular installation, the owner is
> strongly pushing a foundation slab of some sort. I
> have no experience with designing a slab to handle
> this sort of equipment and it's loads, which the
> overturning potential 'feels' too great to me for a
> slab given the narrowness of the footprint (Note, no
> guywires will be installed either to further support
> the stack). Can anyone offer me some advice for design
> or at least persuading the owner not to go with a
> slab? I've searched through ACI, PCA, and Corps of
> Engineers literature, but nothing has given me any
> level of comfort for this kind of design. I was told
> to buy Ringo and Anderson's book, "Designing Floor
> Slabs on Grade: Step-By-Step Procedures, Sample
> Solutions, and Commentary," which will arrive
> tomorrow. However, the owner is pushing for a fast
> turnaround, so I'm limited on where I can seek out
> further advice. The manufacturer cannot help as they
> want no liability for the foundation design. I've been
> reading the mail list for sometime, but this is my
> first post and I really could use the guidance of
> those of you who are more experienced than I.
> Thanks in advance!
> Rob Valorio, PE
> Cherry Hill, NJ
> P.S. - Also, if it helps in your response, the
> foundation soils are ideal being a virgin deposit of a
> very dense gravelly sand with a high modulus of
> subgrade reaction.
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