Concept on positioning of Columns

1 Ans:

• The given layout of the framing plan is symmetric. An asymmetrical, well-distributed in-plan layout of structural elements is appropriate to achieve uniformity. Symmetric should be sought both in what concerns the plan-wise distribution of stiffness and strength, but also that of mass distribution, to avoid eccentricities that entail ordinal response, which tends to increase the horizontal displacements in some parts of the periphery of the building plan and hence is unfavorable.
• Horizontal seismic motion is by nature a bi-directional phenomenon and thus the building structure must be able to resist horizontal actions in any direction. Hence, the structural elements should be arranged in an in-plan orthogonal structural pattern, so that the building structure is able to direct the seismic action in any direction.
• Having similar resistance and stiffness characteristics in both main directions is also a desirable feature, enabling essentially in-plane seismic response with little orthogonal response, that is, for a given direction of the seismic action, the response of the structure will be in that same direction; the structure responds in an uncoupled manner.

2 Ans:

• The mass irregularity shall be considered to exist when the seismic weight of any floor is more than 150% of the floor above. The seismic response of buildings with such irregularity must be done using dynamic analysis.
• A building is said to be tensional irregular when the maximum horizontal displacements of any floor in the direction of the lateral force at one end of the floor is more than 1.5 times its minimum horizontal displacement at the far end of the same floor in that direction. In such a case the building configuration must be revised.
• The distribution in the plan of the lateral stiffness and mass must be approximately symmetrical with respect to two orthogonal horizontal axes.
• If there is an irregularity in elevation, it is unlikely to have a dominant response from the f first mode shape that varies almost linearly from the base of the building to the roof. The overall seismic response will have significant contributions from higher modes of vibration.
• As such the lateral-force method of analysis is not considered applicable to buildings characterized as irregular in elevation.
• The modal response spectrum method, by contrast, capable of capturing well the effects of structural irregularity in elevation on the linear elastic response. .·. the application of modal response spectrum analysis becomes necessary for buildings that are irregular in elevation.
• Out of the many possible irregularities in building structures, the final structural design should, in absolute terms, avoid the two most dangerous irregular situations:
  • Soft story’s and
  • Extremely flexible structures in torsion.

Soft story effect due to mass irregularity

• From the above image, the mass of the lower floors is much higher than the upper floors due to the termination of part terraces in the building. Prima facie, the ratio of the floor mass between successive floors may be higher than 5
• This effect can be minimized by adopting any lightweight construction materials on lower floors so that the total mass of the Lower floor becomes nearly equal to the upper floor

Soft story effect due to absence of in-fill walls

• As we imagine the side view of the given building, the fifth floor is intentionally ignored of any walls for architectural purposes (or) Part terrace view. This non-structural arrangement will lead to differences in story stiffness between successive levels
• The soft-story effect shall be minimized by providing at least masonry partitions in the particular view wherever possible with architect concern.

Both cases correspond to extremely irregular stiffness distributions: the former regarding the distribution in height/elevation with a very weak story and the latter regarding the distribution in the plan, with the concentration of the stiffness at the center or at a corner.

The concept of building regularity is presented separately for regularity in plan and regularity in elevation.

3 Ans:

• In buildings, floors play a very important role in the overall seismic response of the structure. They act as horizontal diaphragms that collect and transmit the inertial forces to the vertical structural systems and ensure that those systems act together in resisting the horizontal seismic action.
• A soft-story at ground level is certainly the most serve situation because they're in the inter-story shear force is at its maximum and that story supports the whole building.
• However, a soft-story at any other height of a building, for instance, due to interruption of a shear wall at an intermediate height, may be extremely detrimental to the seismic response as well.
• In this building, there is no proper placement of Masonry infill walls.
• In this building, Redundancy in the vertical direction is not provided properly.