Hi All,
I am designing an Isolated Footing and got confused in certain parameters under Cover & Soil - " Depth of water Table" and "Surcharge for Loading". I tried to found out the definition in Foundation Toolkit but it didn't specify in detailed matter. Kindly help me to answer the certain questions.
1. Loads being considered under surcharge?
2. Role of surcharge when depth of water table is below the soil and when depth of water table is above the soil ?
Modestas Kris Sathia Pls help here
Surcharge and buoyancy effects are completely independent effects and have no relationship to each other in SFA's calculations.
Didn't understand what you mean by loads under surcharge. Surcharge is a pressure, assumed to be derived from a source that is above the ground level, like a truck parked near the foundation. A positive value is treated the same way as the weight of the slab or soil, namely, acting downwards. It is considered a transient effect and so, its contribution is considered for the soil pressure calculations, but it isn't considered as contributing to the restoring effects for sliding or overturning.
Water table is treated as inducing an upward pressure on the footing if the water table level is above the base of the footing. It too is treated as a transient effect. Since it reduces the net downward force, its effect is considered for the stability calculations (since it reduces the restoring effects). By default, it is not considered for the soil pressure checks because it reduces the net downward pressure. However, there is a switch in the Global Settings if you wish to have its effect considered for soil pressure calculations.
Under the Surcharge for Loading parameter, it is not mentioned whether the programme will consider the depth above the base of the footing bottom or above base of the footing top.
As my understanding says 'It will depend on the Fixed Top / Fixed Bottom condition and programme will follow calculation (Unit weight of soil x Depth of soil above Footing).
Can we say, the 3 parameters ( Depth of soil above footing, Type of Depth and Surcharge for loading co-relate with each other). Please confirm here.
Surcharge is a pressure. Depth does not enter into the equations for calculating the forces and moments due to surcharge. Whether the Top of footing is fixed or Bottom of footing is fixed, the effect of surcharge is computed in the same manner. Take a look at the Indian isolated footing example 4 in the verification manual to find out how its effect is computed.