Hi
I have a question with regard to the Gross bearing capacity input. Here it is.
Let us assume that I have a footing which is critical in uplift. Let us also assume that Im using the “Gross Bearing Capacity Input” setting. If the depth of excavation is 2m then please clarify the following
The parameter “Depth of soil above footing” is “2m-footing thickness” for gravity load cases (and)
0m for the case where there is uplift. (I think this should be 0m because the bearing capacity has been increased already on account of additional soil).
Please clarify if my assumption above is correct. If it is correct, then is there a provision in staad foundation to have the soil weight act only for some load cases and not act for the remaining cases.
Regards
Sriprakash
Hi,
It is always suggested to use net bearing capacity instead of gross bearing capacity. The net bearing capacity is what you need to use directly to find out initial footing size from the bearing capacity concept. IS 6403 instructs to compute net bearing capacity for the foundation design. The effect of depth is being considered in net bearing capacity and which can directly be used for foundation design.
The first point you mentioned is correct. The depth of soil above footing would be 2m â€“ footing thickness but this is not just for gravity cases but for all cases including those which may cause uplift. In other words the effect due to the weight of soil and selfweight of the footing does not vary from case to case. For cases where the footing is subjected to overturning forces and moments, the weight of soil and selfweight of footing would counteract the uplift. If the overturning effect is stronger compared to the restoring effect due to soil weight + selfweight of footing, part of the footing would be subjected to uplift. There is no provision to make the soil weight act for some and not for some other cases.
Sye said:If the overturning effect is stronger compared to the restoring effect due to soil weight + selfweight of footing, part of the footing would be subjected to uplift.
...and in this case STAAD Foundation will do the maths by increasing the footing dimension to bring the uplift to the allowable zone (by satisfying the uplift factor of safety defined by the user) if 'Calculate Dimension' option was used, correct?
Slightly rephrasing your answer. Instead of "Uplift factor of safety", the phrase to use is "factor of safety against overturning". So, the answer is:
STAAD Foundation will do the math by increasing the footing dimension to bring the uplift to the allowable zone (by satisfying the factor of safety against overturning as defined by the user) if 'Calculate Dimension' option is used.
Sye said:The depth of soil above footing would be 2m â€“ footing thickness
Hi Sye,
There is confusion in the parameter mentioned under the soil and cover page - "Type of Depth above footing". As in the 2nd case where depth of footing is fixed, Type of Depth above footing should be ( 2m - Thickness of footing). It is secondary matter that thickness of "Type of Depth above footing" in this case (when footing base is fixed) will be less as compared to "when footing top is fixed"
As the difference in both the terms (Fixed Top and fixed Bottom) is only the way of calculation the footing thickness considering the one parameter fixed which is used for the reference as per user input ( Top / Bottom fixed).
Kris Sathia Pls help here
In recent versions of SFA,
a) if you choose the Type of Depth as "Fixed Top", the wording in the box for the soil depth becomes "Depth of Soil above footing", and stands for height of soil above the top surface of the footing.
b) if you choose the Type of Depth as "Fixed Bottom", the wording in the box for the soil depth becomes "Footing Embedment Depth", and stands for height of soil above the bottom surface of the footing.
An explanation is provided at the bottom of the table if you click on the individual rows of the table.
It is not showing in the version i am using. Which version you are talking about ?
I can see from the titlebar of the screenshot you just attached that you are running version 9.0.0.9 which was released in December 2018. The current version of the program is build 9.6.1.74 which is the 7th update since then. The announcement regarding the latest version can be found here.
Why I am getting the Weight of the soil above footing is so high as my calculation says ( unit weight of soil x Soil height) which should be 18 x 2.55 = 45.9 Kn / M^2
45.9 is the pressure due to 2.55 m of soil. You have to multiply it by the footing plan area to get the weight of soil.