💡 Earthquake forces are simplified and taken as base shear acting on the base of the structure for response to seismic action.

💡 Equivalent static force method is simplified way to compute the base shear which act on the structure in time of earthquake.

Base shear = Seismic weight * design horizontal seismic coefficient (Ah)

The Ah factor can be given as

Ah = Z /2 * I / R * Sa/g

and is dependent on following factors.

✔ Z : Zone Factor

➡ This signifies the seismicity of the location where the structural is located.

➡ India has been divided into four earthquake zones based on their past fault lines.

✔ I : Importance factor

➡ The importance factor signifies how critical or important the structure is which is being designed.

✔ R : Response reduction factor

➡ The seismic force acting to the structure is reduced by response reduction factor to account for structure’s capability to dissipate the energy.

➡ The energy dissipation capacity depends on redundancy & over strength of structure. It has direct correlation with the structural system used.

✔ Sa/g : Design Acceleration Coefficient

➡ The design acceleration coefficient or Sa/g is dependent on time period of the structure.

➡ The approximate time period as IS 1893 used to find the Sa/g values.

I have explained the calculation of base shear in the detailed video below. Please share your insights, any suggestions or comments are welcomed.