Friday, November 16, 2012

uls sls ws

The relationship between ULS, SLS and working stress is a construct of the collective minds of structural engineers. It has become codified and is just caca el toro in soils terms, but we geotechnical engineers must live with the government decrees, no mater how stupid they are.

Consider the centre of this diagram a number line going from low to high value.
Above ground we start out with the structure loads, and up-scale those values by partial factors of safety, as found in the building code or bridge code. these become the factored loads.

The code says that the factored loads must be less than the factored resistance, which is all fine and good, but in soils terms, the factored soils value is largely theoretical. Here, and any time Standard Penetration is used, the bearing value is empirical working stress set from a bearing value that will give about 1 inch of differential settlement, 2 inches total settlement, aka SLS value. A similar exercise is used for Cone penetration, unconfined values from Shelby tubes, or pocket pen, whether in Qu or C.

Now what happens when we have a mix of values? we need to convert all to allowable design values, and compare, and then pick our report values. We can use a factor of safety to up scale the value to some theoretical unfactored ultimate, and then apply the CFEM factors to come to a factored resistance. But now both the SLS and ULS must be checked, and for most foundations the SLS should govern. But now we are doing something odd. Some use the approach of  up-scaling working stress value by 1.35 or some number between 1.25 and 1.5, the range in load factors, and use that value. 

My preference is to provide ULS and SLS values, and let the structural engineer do twice the work of previous. Some other appear to be calling the old allowable value the factored resistance, which demands larger foundations. The bearing value in some of soils reports of late from others firms seem to be odd in relationship to soil strength and traditional values, which is of concern. Not enough test holes and high factors of safety. Not good engineering. 

Enough said.           

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