Use of GL parameter in IS893(Part-1)-2016 seismic analysis

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, A_h at the base is taken half the value of A_h 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 A_h. 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 A_h and 0.5A_h

For the portion of the structure, above the ground,

W_i= seismic weight of i^th floor above the ground.

h_i= height of i^th floor above the ground.

VBs= horizontal base shear

        = A_h.Ws

Ws= seismic weight of the portion which is above the ground

Q_i= design lateral force at i^th floor

For the portion of the structure, below the ground,

W_j= seismic weight of j^th floor below the ground.

h_j= height of j^th floor below the ground from base.

VBu= horizontal base shear

        = A_hR.Wu

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.