Seismic analysis of a structures having structural components below the ground level
For underground structures and buildings whose base is located at depths of 30m or more, Ah at the base is taken half the value of Ah as mentioned in clause 6.4.2, IS 1983(Part I):2016. This reduced value shall be used only for estimating inertia effects due to masses at the corresponding levels below the ground; the inertia effects for the above ground portion of the building shall be estimated based on the unreduced value of Ah. For estimating inertia effects due to masses of structures and foundations placed between the ground level and 30m depth, the design horizontal acceleration spectrum value shall be linearly interpolated between Ah and 0.5Ah
For the portion of the structure, above the ground,
Wi= seismic weight of ith floor above the ground.
hi= height of ith floor above the ground.
VBs= horizontal base shear
Ws= seismic weight of the portion which is above the ground
Qi= design lateral force at ith floor
For the portion of the structure, below the ground,
Wj= seismic weight of jth floor below the ground.
hj= height of jth floor below the ground from base.
VBu= horizontal base shear
Wu= seismic weight of the portion which is above the ground
For this computation, the program needs the ground level Y coordinate in the the model and it has to be defined using GL parameter in IS1893(Part-1)-2016 seismic defination.