Design of seismic forces


Design lateral force:

The total design lateral force acting on the structure is equal to the product of the design horizontal coefficient (Ah) and the seismic weight of the structure. The seismic weight if a building is equal to the sum of the seismic weights of all the floors for the building. The seismic weight of each floor is equal to the full dead load plus an appropriate fraction of the imposed load, as specified in code. With computing seismic weight of any floor, the dead weight of columns and walls in any storey is equally distributed tot eh floors above and below them.

The designed seismic base shear acting on the structure is equal to the total design lateral force ad is given by the expression;

Vb= Ab.W

Where, Ah- design horizontal seismic coefficient

W- Seismic weight of the building

VB- design base shear

The design base shear is distributed to different floor levels as specified in the code. The horizontal force acting on the foundation is equal to the design base shear.

Design vertical force:

Since the design vertical coefficient is equal to the two-thirds of the design horizontal coefficient (Ab). The design vertical force is given by,

Fv= 2/3 Ab.W

Where, W is the seismic weight of the building and Fv= design vertical force.

The design vertical force is transferred from the structure to its foundations.

Design of structure:

Buildings and portions thereof should be designed and constructed to resist the effect of design seismic forces. Since the seismic forces occur suddenly and without any warning, so it is important to avoid construction practices that leads to brittle failure. The members should be designed to behave in a ductile manner so that the complete collapse of the structure is avoided even during severe instantaneous earthquakes. Actual design is beyond the scope of this text.

Design of foundation:

Since the earthquake-resistant designs are generally performed by pseudo-static analysis, the seismic loads on the foundation are considered as static loads. These loads are capable of producing settlements like other loads.

The code principally allows the increase in the allowable bearing pressure depending upon the soil foundation system. The increase in allowable bearing pressure is generally 25-50 percent. In soil foundations where small settlements are likely to occur during earthquake, the increase in allowable bearing pressure shall be larger and vice-versa. For instance; for all sorts of foundations on rocks or hard soil the allowable increment is about 50%. For soft soils the allowable increase for piles resting on hard rocks or hard soils and the raft foundations is 50% whereas that for other types of foundations it is only 25%.


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