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RE: Slab Foundation Possible?

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A thick mat slab should be feasible, although maybe not practical.  A mat
slab will be stiff enough that you can activate whatever area you need  to
resist overturning forces (with a suitable allowable bearing pressure). For
instance if the footing was 40' long x 10' wide x by 3' thick (30cubic
feet/foot along the length) assuming 150pcf for the concrete you would get
4.5k per foot dead load.  

A 3' or more thick mat slab will also allow adequate development of the
anchors.  I might also thicken up the edges to provide additional resistance
to sliding unless the slab becomes enormously thick.  I am assuming that the
critical overturning direction is across the short direction of the system,
so it may be impractical to just thicken up the footing (and widen it as
necessary to control bearing pressure) to add dead load (overturning
resistance). This being the case, you may be stuck with some form of pier.
Will the pier option really take longer to install if you are only
installing six?


Nicholas Blackburn, PE


-----Original Message-----
From: Robert Valorio [mailto:geotex1(--nospam--at)yahoo.com]
Sent: Tuesday, January 30, 2001 10:42 AM
To: seaint(--nospam--at)seaint.org
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.