I have designed isolated foundation in staad ,
The sizing of foundation is coming very high , eventhough my foundation pressure is very less ie) 40 kN/m2 , allowable is 200 kN/m2.
In-terms of sliding and overturning , i am getting minimum of 3.02 ratio.
Can u please suggestion on it , why i am getting more size.
I am attaching my staad file along with this Master.xmlMaster.sfa
Since you did not mention which footing your question pertains to, we tested it for just footing 1.
For this footing, there are load cases where the vertical load is small and overturning moment is large. Since you have specified the minimum area in contact for service and ultimate load cases to be at least 90%, the footing size needs to be large enough to meet that condition for every load case included in the job. However, large footing sizes will also result in small values of soil pressures, and large factors of safety in sliding and overturning.
If you do the following:
a) Change the design type to Calculate Dimensionb) Change the minimum footing size to 1.5 m X 1.5 mc) Reduce the minimum required contact area percentage to 50% or lowerd) Change the multiplier on soil bearing capacity for ultimate load cases from 1 to 1.7
you may get smaller footing sizes.
c) Reduce the minimum required contact area percentage to 50% or lower-
1. Since the ratio for uplift or overturning minimum value is 3.08 , allowable is 1.5 , almost coming double the time. If I want to minimize the value of 2 , automatically my size will decrease ? why it is not designing for those vales.
2. Also minimum base pressure value of 29 kn/m2 there is no uplift , hence the contact area will be 100 % ,
Hence why the sizing values are on higher side ?
a) Change the design type to Calculate Dimension- Noted but same result
d) Change the multiplier on soil bearing capacity for ultimate load cases from 1 to 1.7 -
In what way it is affecting my design ? sbc coming around in ultimate also 50kn/m2 only
A value greater than 1 for Restoring moment (RT) divided by overturning moment (OT) does not guarantee that there is no loss of contact (LOC). Even high values of factor of safety in OT can be accompanied by LOC.
Here is a calculation of the minimum soil stress for a 5 m X 5 m size for load case 101 for footing 1.
Height of pedestal = 0.95 mThickness of footing = 0.4 mTotal height of point of application of loads above base of footing = 1.35 m P = 66.102FX = -0.546FZ = -91.61 MX = -200.861MZ = 4.06 MZ due to FX = 0.7371MX due to FZ = -123.6735 Total MX = -324.5345 Total MZ = 4.7971
For a 5 m X 5 m footing, self weight of footing = 5 * 5 * 0.4 * 25 = 250
Height of soil above footing = 0.65 mm (negligible, are you sure the length unit is correct?)
Total P = 66.102 + 250 = 316.102 kN
For a 5m X 5m footing,AX = 25SX = SZ = 20.833
(P/AX) - ABS(MX)/SX - ABS(MZ)/SZ = 12.644 - 15.577 - 0.23 = -3.164 kn/m2 which means there is loss of contact, so the footing fails.
You will have to increase the size to 5.35 m X 5.35 m to get the minimum soil pressure to be nearly 0 which is what a 90% contact area requirement implies.
The condition for load case 485 is even worse than it is for 101. The footing has to be more than 6 m to ensure 95% contact area.
In this model, increasing the multiplying factor to 1.7 would have no implication. We suggested that only as a best practice.