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RE: Slab Foundation Possible?
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- Subject: RE: Slab Foundation Possible?
- From: "Lutz, James" <JLUTZ(--nospam--at)earthtech.com>
- Date: Tue, 30 Jan 2001 17:33:46 -0800
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 both. 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)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. __________________________________________________ Get personalized email addresses from Yahoo! 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