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RE: Slab Foundation Possible?

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Here's a simple procedure that may be of help.

Assume a slab maybe a foot beyond the anchor bolts as a trial. Calculate the
area of the footing (B x L)and the section modulus about the two
centerlines, BL^2/6 and LB^2/6. Ask the manufacturer to furnish you with the
gross weight and center of gravity location, and with the moments about each
axis which were assumed for the anchor bolt reactions. Pick a trial footing
thickness, say 12 inches. Compute the total gravity load "P"(including the
footing), and the total moments in each direction, including any effects due
to eccentricity of the equipment weight. You can then calculate the soil
pressures at each corner of the footing as q=P/BL +/- Mx/Sx +/- My/Sy. If
the pressure at any corner is more than the allowable, increase the
dimensions. If the pressure at any corner is negative or less than the unit
weight of the footing, you have liftoff or tension in the top of the footing
which will require either increasing the dimensions or the slab thickness or

Once you have a satisfactory pressure distribution, all positive and all
greater than the footing weight at the corners, take the largest value and
pretend it is uniform over the slab and design the slab as if it were a
one-way slab in the short direction spanning between the two bolt lines
along the long edge of the skid, or between the webs of the skid frame,
whichever is greater. 

You will need top steel for the interior moments and bottom steel for the
slab zone outside the skid. You may need to thicken the slab or be able to
reduce it depending on the forces involved. This is a simplified procedure
but your equipment doesn't seem all that heavy. It's easy to set this up on
a spreadsheet and just fiddle with the dimensions until it works.

It gets more complicated if you need to restrict the dimensions and allow
part of the slab to lift-up. AASHTO has some charts that allow you to
calculate the soil pressure for those cases.

-----Original Message-----
From: Robert Valorio [mailto:geotex1(--nospam--at)]
Sent: Tuesday, January 30, 2001 10:42 AM
To: seaint(--nospam--at)
Subject: Slab Foundation Possible?


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

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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