Earth Resistant Design PhilosophyRishabh Lala X SemCivil and Structural EngineeringIntegrated PG ProgramRajiv Gandhi Proudyogiki Vishwavidyalaya

Earthquake Design Philosophy• Shaking of earth is divided into 3 categories :

1. Minor Shaking Assumed to occur Frequently 2. Moderate Shaking Assumed to occur Occasionally3. Strong Shaking Assumed to occur Rarely

• Now, we know that, earthquake proof buildings are very expensive to construct, as they require additional provisions. Therefore, earthquake resistant design philosophy comes in existence. In this, we try to mitigate the earthquake after effects and hence structures are designed for forces much smaller than the actual earthquake forces.

Cases• CASE 1 : UNDER MINOR BUT FREQUENT SHAKING :

Beams and columns or any other main member (like shear walls) carries vertical and horizontal and horizontal forces successfully, however buildings may suffer repairable damage.

• CASE 2 : UNDER MODERATE BUT OCCASIONAL SHAKING :a. Main members may undergo repairable damage

b. Other members may be replaced

• CASE 3: UNDER STRONG BUT REPAIRABLE SHAKING : a. Main members may suffer severe and irreparable damage but the building should not collapse

Purpose• Building saves lives, even if building if it becomes dysfunctional

• Cracks in Building : does not mean that building is unsafe for use

• Economy of the construction project

• Key Strategy : Ductile Elements at key locations (load resisting locations) of the structure, protects non-ductile elements from getting over stressed. This inelastic response, tends to increase the effective period of the structure, resulting in reduction in the strength demand, to deal with wind and earthquake forces, as most of the earthquake forces go in overcoming the damped structure due to inelastic response.

Ductility• Main elements of Earthquake Resistant Building should be designed with property called Ductility

Example : Chalk on Breaking : gives a brittle FailureWhile, Steel Pins on applying force : gives a ductile failure, i.e. it bends

• Beams should be designed for such kind of ductility• Ductility enables structures, which do not have adequate elastic strength to

survive ground motions, through its inelastic response, i.e. ability of the structure to deform plastically without fracture.

Factors (ductility)

a.Architectural Plan : a. Vertically and Horizontally symmetrical plan, improves ductility

b.Soft Storey :a. Soft storeys reduce the overall ductility of the structure and should

be avoided, specially in the mid span of the vertical cantilever (tower)

c. Weak Storey :a. Aviod Weak Storey and provide strong diaphragm

d.Door Openings :a. To be provided at atleast 0.6m away from the column edges.

e.Location of Water tanks :a. Follow codal provisions, for determining water tank location and

swimming pool as it may affect the location of the center of mass and the shear center

Increase ductility

a.Increase transverse shear reinforcementb.Increase compressive strength of concretec.Increase percentage of compression steeld.Increase percentage of tension steel

Shear Walls• Provide large strength and stiffness to the structure in the

direction of their orientation, preventing it from lateral sways during earthquakes, as they are good transmitters of earthquake lateral forces that come along the direction of their orientation.

Shear walls (Location)

• Hence, shear walls should be provided along both directions. Door and window openings, may be provided in shear walls but their openings should be limited so that they offer least interruption to shear wall.

• Shear walls must be symmetrically located to avoid ill effects of torsion and twists. Hence, shear walls are more effective, when they are location in the exterior perimeters of the building and their unsymmetrical location should be avoided.

• Shear walls are easy to construct as detailing is pretty straight forward. Provisions from IS-13920 -1993, gives detailing concepts for shear walls.