SEISMIC ANALYSIS :AN ADVANCE ANALYSIS
Elastic analysis is performed in elastic range whereas SEISMIC analysis is performed to account for excited moment and induced accidental torsion.
Elastic analysis is performed in elastic range whereas SEISMIC analysis is performed to account for excited moment and induced accidental torsion.
A close comparison of (only column ) reinforcement between elastic analysis and seismic analysis is shown here.These required amount of reinforcement depends on many factors such as,
1) the drift due to irregularity of the building is a controlling criteria, 2) for tall building,
3) Special consideration of load resisting system etc...
Translation of top floor in Y-direction for ELASTIC analysis lies almost zero but for seismic analysis it gives a very significant translation due to induced accidental torsion.
Performing SEISMIC analysis requirements for building is as per provision of NEHRP -2009 .
Some Assumption & procedure for Design of Earthquake resistant structure :
A) The characteristics of seismic ground vibrations at any locations depends upon the magnitude of earthquake,its depth of focus,distance from epicentre ,characteristics of the path through which the wave travels, and the soil strata on which the structure stands,Ground motions are predominant in horizontal directions.
B) Earthquake generated vertical forcesif significant,asin large spans where differential settlement is not allowed must be considered.
C) The response of a structure to the ground motions is a functions of the nature foundation soil,materials size and mode of construction of structures,and the duration and characteristic of ground motions.
D)The design approach is to ensure that structures possess at least a minimum strength to withstand minor earthquake(DBE),which occur frequently without damage,resist moderate earthquake without significant damage through some non-structural damage may occur and aims that structures withstand major earthquake(MCE) without collapse.Actual forces that appeared on structures are much greater than the design forces specified here,but ductility arising due to inelastic material behavior and detailing,and over strength arising from the additional reserve strength in structures over and above the design strength are relied upon to account for this difference in actual and design lateral forces.
E) Reinforced and prestressed members shall be suitably designed to ensure that premature failure due to shear or bond does not occur.
F)In steel structures,members and their connections should be so proportioned that high ductility is obtained.
G) The soil structure interactions refers to the effect of the supporting foundation medium on the motion of structure,the structure interactions may not be considered in the seismic analysis for structures supporting on the rocks.
H) The design lateral forces shall be considered in two orthogonal horizontal directions of the structures.For structures which have lateral force resisting elements in two orthogonal directions only,design lateral force must be considered in one direction at a time.Structures having lateral resisting elements in two directions other than orthogonal shall be analyzed.
I) When a change in occupancy results in a structure being re-classified to a higher importance factor (I).The structure shall be confirm to the seismic requirements of the new structure with high importance factor.
Some Assumption & procedure for Design of Earthquake resistant structure :
A) The characteristics of seismic ground vibrations at any locations depends upon the magnitude of earthquake,its depth of focus,distance from epicentre ,characteristics of the path through which the wave travels, and the soil strata on which the structure stands,Ground motions are predominant in horizontal directions.
B) Earthquake generated vertical forcesif significant,asin large spans where differential settlement is not allowed must be considered.
C) The response of a structure to the ground motions is a functions of the nature foundation soil,materials size and mode of construction of structures,and the duration and characteristic of ground motions.
D)The design approach is to ensure that structures possess at least a minimum strength to withstand minor earthquake(DBE),which occur frequently without damage,resist moderate earthquake without significant damage through some non-structural damage may occur and aims that structures withstand major earthquake(MCE) without collapse.Actual forces that appeared on structures are much greater than the design forces specified here,but ductility arising due to inelastic material behavior and detailing,and over strength arising from the additional reserve strength in structures over and above the design strength are relied upon to account for this difference in actual and design lateral forces.
E) Reinforced and prestressed members shall be suitably designed to ensure that premature failure due to shear or bond does not occur.
F)In steel structures,members and their connections should be so proportioned that high ductility is obtained.
G) The soil structure interactions refers to the effect of the supporting foundation medium on the motion of structure,the structure interactions may not be considered in the seismic analysis for structures supporting on the rocks.
H) The design lateral forces shall be considered in two orthogonal horizontal directions of the structures.For structures which have lateral force resisting elements in two orthogonal directions only,design lateral force must be considered in one direction at a time.Structures having lateral resisting elements in two directions other than orthogonal shall be analyzed.
I) When a change in occupancy results in a structure being re-classified to a higher importance factor (I).The structure shall be confirm to the seismic requirements of the new structure with high importance factor.
(THANKS)
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