Please see the picture above and advice; does STAAD.Foundation consider the value in the input pane gross or net after clicking the apply button in the global settings? Because when I chose 'Net' then apply the description still says Gross.
We agree that the explanation requires a change.
If under Global settings, you have it set to Gross, the value you input is considered to be Gross.
If under Global settings, you have it set to Net, the value you input is considered to be Net.
Answer Verified By: dImeNsiOnFifTh
So regardless to the explanation, is the bearing capacity used in the behind the scenes calculations net if I chose 'Net' in the Global Settings or is it Gross?
Yes when Net option is chosen in the Global Settings, the bearing capacity input in the Data Input pane is considered internally as Net. As Kris mentioned, we need to change the description.
To add to what Sye wrote:
The design of the footing is always based on Gross bearing capacity. So, if under Global Settings-Rigid Foundation Settings-Bearing Capacity Settings, you choose the bearing capacity type as Net, and accordingly provide the value of the Net Bearing capacity in the Cover and Soils page, the program will internally calculate the gross bearing capacity as
Net bearing capacity + pressure due to soil above the level of the base of the footing
The calculation sheet will show you the net and gross bearing capacities.
Great add, thanks. Can I suggest that radio button to be pulled out from the global settings and placed beside/below the bearing capacity field?
Generally, it's easier to bring related parameters beside each other rather than scattered around in the software.
Since we talked about Net and Gross, there is another technical thing that I need clarified; are we dealing with ultimate bearing capacity or allowable? (Ultimate=Allowable x FOS). In your last picture at the bottom I see 'qmax=Respective Factored Bearing Capacity' but I'm unable to find any qmax in my calculation reports to see what it's value is.
We agree that the logical place to have the option to choose between gross and net is in the Cover & Soil page, and not the manner in which it is presently available. Remedying this is indeed in our plans for the future.
The Gross bearing capacity is first determined for the service conditions in the manner described earlier, after which, it is multiplied by the term denoted as "Multiplier on soil bearing capacity for ultimate loads" to arrive at the Gross bearing capacity for the Ultimate conditions.
Hi Kris,
I have a doubt regarding use of Gross / Net bearing capacity to design footing as footing dimension should be designed for "Safe Bearing Pressure / Allowable Bearing Pressure" as it is ( Lowest value in shear criteria & settlement criteria)
I also refer some textbooks & NPTEL lectures which also uses SBC for calculation of footing dimension.
(SBC = Net bearing capacity / FOS)
Kindly help me to know, how SFA's is calculating SBC to design foundation as it uses Gross Bearing capacity to find out dimension of footings.
See this section of the SFA Technical Manual
I have seen this section already as it doesn't mentioned anywhere about the use of SBC in calculating of footing dimension. Pls read and understand my question and help me to clear the doubt
SBC is the value against which the soil pressure due to loads is compared. If the actual pressure exceeds the allowable bearing pressure, the footing is deemed to have failed for the load case. Bending moments, shear forces, etc. are based on soil pressures due to the loads. SBC does not enter into any other calculations in SFA.
You asked:
Answer:
SFA does not calculate SBC. It is an input that you have to provide. Based on what you wrote, seems like SBC is the value you need to enter in the Cover and Soil page against the term
Base Value of Soil Bearing Capacity
Also, in Global Settings-Rigid Foundation Settings-Bearing Capacity Settings for input and report, you need to switch on "Net Bearing Capacity Input".
As described in the topic I referred to in the Technical manual, SFA will convert the SBC from the "Net" frame of reference to the "Gross" frame of reference. The soil pressures calculated for each load case are then compared against the Gross value. For reporting purposes, the soil pressures are converted back to the "Net" frame of reference.