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Footing Overturning Factor of Safety 1

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CBrech

CBrech

Structural
Aug 6, 2014
10
Hello-

I've read multiple threads on this subject, but I was looking for a better understanding of the factor of safety use for overturning in a footing. I've read that the .6D+.6W load combination has the 1.5 safety factor already accounted for. Below is an example I would like to use as a point of discussion and any feedback would be greatly appreciated.

Say we have a PEMB moment column and a isolated footing. The wind is producing a moment of 25 k-ft (factored) at the base. We are using the weight of the footing to resist the overturning of the moment.
Let's try a 6'x6'x3' footing at 145 pcf. So 6' * 6' * 3' * 145 pcf = 15660 lbs * .6 for Dead Load reduction = 9396 lbs.
Our resisting moment = 9396 lbs * 3' = 28188 lb-ft
28188 lb-ft / 25000 lb-ft = 1.13

My question is should I take the 25 k-ft of moment and multiply that by the 1.5 safety factor and then size a footing or size the footing the way I've shown?
 
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With 0.6D in your combination there is no longer a 1.5 safety factor external to that.

The way you did it by only assuming 60% of your dead load is in fact using a 1.5 safety factor.
So 1.5W is not the way to do it per current code load combinations.

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Wind loads should be service (ASD) level loads. Dead weight should be actual resisting weight of the footing plus dead load of the structure. Using 0.6DL would be double dipping in my opinion for overturning if you are checking against a 1.5 FOS.

 
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I look at it two ways. I'm assuming that 0.6*WL = 25 k-ft
Method 1: using the 0.9*DL + 0.6WL (i.e. old school)

FOS = 0.9*15660lb*3ft / 25000lb-ft = 1.69

Method 2: Using the 0.6DL + 0.6WL (i.e. new school)
FOS = 0.6*15660*3ft / 25000 lb-ft = 1.13 = 1.69 / 1.5

Therefore, when using the 0.9 factor you need a 1.5 overturning safety factor. But, when using a 0.6 DL factor, you need only a 1.0 safety factor.

For what it's worth, if you're checking your allowable soil bearing for the 0.6 DL case you're still going to end up with a more conservative footing because that partial bearing condition is likely to produce a soil bearing failure for a footing size that just barely worked when using the old 0.9DL combinations.
 
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Remember that regarding the allowable soil pressures, most Geotechs add in a 1.5 safety factor already, so unless you ask for an un-safety factored allowable pressure, you will be using an effective factor of 2.25 and kill yourself...

Just saying...

Mike McCann, PE, SE (WA)


 
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An odd question. Where in the world does one run into a footing sitting on top of the ground and with no weight on it, yet it somehow gets an overturning moment? Footings are usually made to carry some load also. Where is that load here? It also plays into the calculation. Bury the footing and be done with it.
 
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