What GPT said. First you look at the ultimate capacity of the bearing soil (i.e., do a bearing capacity analysis). Then you look at the tendency for settlement under the factored bearing pressure. If the total settlement is too great, you adjust the bearing pressure to get the settlement under control. Once that's done, the rule of thumb is that differential settlement will account for up to 1/2 the total settlement. So, if you calculate 1-in of settlement, the differental settlement may be 1/2 in.
Thanks alot guys
" the rule of thumb is that differential settlement will account for up to 1/2 the total settlement. So, if you calculate 1-in of settlement, the differental settlement may be 1/2 in".
Where this has come from
You have to base your reported differential settelement on a scientific background.If someone asks you to explain how did you bring this value,, You can not tell the rule of thumb .....
technically, the structure engineer (or designer) should provide the allowable settlement of a structure. to provide an estimated total and differential, just run the calculations. from all the calcs i've run on "typical" sites (meaning nothing completely wacky), 1/2 is pretty darn consistent although not absolute. if you have one column footing on goo and another on rock, differential may be jacked up. if you're not sure, run the calcs on the best case and worst case (of nearby columns or maybe not) and see what the difference is. if you run the calcs, it will not be rule of thumb although it may very well follow it. keep in mind too that "1 inch of settlement" may very well be 1/2 inch or it could be 1 1/2 inches. so 1/2 inch may be 1/4 inch or it may be 1 inch. consider the structure, structure type, expected performance, variability in subsurface conditions (as well as how well you think you know those conditions), etc etc and apply good sound engineering judgement with a "reasonable" safety factor.
Suggest that you might want to refer to Tomlinson's book Foundation Design and Construction. He spends a lot of time discussing foundation movement, differential settlements (actually relative rotations) and the like. Table 2.7 is "Limiting values of distortion and deflexion of structures." Great book - great insights.
I just wanted to point out yhat you have to consider two sources of differential settlements in a structure ( it is obvious but maybe not for everybody ) :
1) differential loads ( ie center and periphery of a tank will not settle the same amount because loading is different )
2) heterogeneity of soils where I would apply half of the total settlement value which is a maximum. if you run pressuremeter tests you have a formula to calculate differential settlements.
