Hi,
I am designing a large single storey building along with foundations in Seismic conditions. The load cases i am applying are:
1. EQ X Direction,
2. EQ -X Direction,
3. Eq Z Direction,
4. EQ -Z Direction,
5. Dead Load,
6. Live Load
For the purpose of foundation i am using serviceability combinations:
1. DL+0.8(LL+SL-Z),
2. DL+0.8(LL+SL+Z),
3. DL+0.8(LL+SL-X),
4. DL+0.8(LL+SL+X)
I am generating loads from Staad and designing the footing manually. The isolated footing size is coming in the order of 2.5 to 3m, which is very huge in comparison to the size of building. The loading combination is generating significantly high upliftment forces causing footing to fail.
I have tried using Staad foundation and it is given slightly higher values of footing than mentioned above.
Please guideBR at BVC.std
As we haven't heard from you, here are some suggestions.
In the SFA model, have you properly categorized your loads into service and ultimate types? Not doing so will result in the wrong loads being used in the service and strength checks.
In your hand calculations, are you increasing the allowable bearing capacity for load combinations involving the seismic cases? If so, in the SFA model, you need to do the same through the Pile/Soil Bearing Capacity Factors table for those load combinations. For example, to specify a 25% increase, set the multiplying factor to 1.25 in that table.
Is there a surcharge load in your hand calcs? Make sure the same pressure is specified in the SFA model too.
Specify small values for the increment for plan dimensions and thickness - say 50 mm for plan dimensions, and 10 mm for thickness. The program may take more time to arrive at a suitable size, but it will ensure the least size possible.
Is the bearing capacity type in your hand calculation gross or net? The same criteria needs to be used in SFA through Global Settings - Rigid Foundation Settings - Bearing Capacity Settings