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Re: Stack Foundation
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- Subject: Re: Stack Foundation
- From: Neil Moore <nmoore(--nospam--at)innercite.com>
- Date: Sat, 23 Sep 2000 06:42:30 -0700
- Cc: <bryson(--nospam--at)civil.ubc.ca>
Although you are checking the factor of safety for overturning across the flat sections, have you checked the maximum soil pressure across the diagonal? Neil Moore, S.E. At 06:47 PM 09/22/2000 -0700, John MacLean wrote: >Michael Bryson wrote: > ><<You say you use a factor of safety of 1.76 for overturning? But this using >allowable soil pressures or factored soil pressures? If the former, then >you're factor of safety would be higher, yes?>> > >I guess I shouldn't really use the term "factor of safety" with a limit >states code. It's probably not correct, so my apologies. You are right that >you often see a factor of safety of 2 or more for overturning in allowable >stress design. > >Limit States codes are supposed to be clearly superior to allowable stress >codes for overturning. They are supposed to provide a clear and consistent >safety margin in design. I think the inconsistencies in the allowable stress >approach show up in calculating bending moments and shears in the footing. >The downside of the limit states codes is that they seem to generate very >high factored soil pressures. > >Here's an example. Take a 10 ft x 10 ft square footing with a 250 kip-ft >overturning moment on it and a 100 kip concentric dead load. The "factor of >safety against overturning" using allowable stress design would be 100 x 5 / >250 = 2.0. (5 ft being the distance from the centre of the footing to the >outside edge) > >For a limit states load case of 1.5 Wind + 0.85 Dead, the factored >overturning moment would be 1.5 x 250 kip-ft = 375 kip-ft. The factored >vertical load would be 0.85 x 100 kip = 85 kip. > >The factored resisting moment must be >= the factored overturning moment. In >this case 85 x 5 = 425 kip-ft >= 375 so okay. > >To check the soil pressure I revert to specified load levels since the >geotechnical guy will only give me "allowable" soil pressures. The >eccentricity e = M/P = 250/100 = 2.5 ft. Which is outside the kern distance >(10ft/6 = 1.67ft) So my soil pressure is a triangular distribution with a >peak value of (2 x 100) / (3 x (5 - 2.5) x 10) = 2.67 ksf So I'm okay if the >"allowable bearing pressure" is greater than 2.67 ksf. > >To check the concrete I go back to my factored loads. The eccentricity e = >M/P = 1.5 x 250 / (0.85 x 100) = 4.41 ft. Which means under factored loads >the resultant is way out near the edge of the footing. The factored soil >pressure is triangular with a peak value of (2 x 85) / (3 x (5 - 4.41) x 10) >= 9.60 ksf. This would make the geotechnical guy go weak at the knees so I >don't tell him about it. It could be worse. The closer I get to having the >factored overturning moment and factored resisting moment equal the closer >the factored soil pressure approaches to infinity. My gut feeling is that >for this "ultimate" condition with soil pressure concentrated over a small >area at the toe of the footing I'm prepared to go 3 or 4 times allowable >soil pressure and even calculate the pressure as a rectangular stress block >instead of triangular to reduce it. Especially if it all works at allowable >stress levels. > >If I had say a 2ft x 2ft square pier on the footing the factored bending >moment in the footing at the face of the pier would be 85 x (4.41 - 1) = 290 >kip-ft. > >If I was designing using allowable stress I might calculate the moment in >the footing at the face of the pier using the triangular stress distribution >at allowable load levels. This works out to 176 kip-ft by my reckoning. If I >applied a 1.5 load factor to this moment I'd get 264 kip-ft. So the moment >calculated this way is about 10% lower than with the limit states code and >the difference can vary quite a bit depending on the eccentricity e. Shear >differences can be even worse. Bottom line....it's not consistent and >doesn't correspond to a uniform safety margin on overturning. > >I'd like to hear anyone else's opinion on this. > >Cheers, >John MacLean ******* ****** ******* ******** ******* ******* ******* *** * Read list FAQ at: http://www.seaint.org/list_FAQ.asp * This email was sent to you via Structural Engineers * Association of Southern California (SEAOSC) server. To * subscribe (no fee) to the list, send email to * admin(--nospam--at)seaint.org and in the body of the message type * "join seaint" (no quotes). To Unsubscribe, send email * to admin(--nospam--at)seaint.org and in the body of the message * type "leave seaint" (no quotes). For questions, send * email to seaint-ad(--nospam--at)seaint.org. Remember, any email you * send to the list is public domain and may be re-posted * without your permission. Make sure you visit our web * site at: http://www.seaint.org ******* ****** ****** ****** ******* ****** ****** ********
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