Modelling of 25 storey building with the specified properties using ETABS

To proceed with the modeling and analysis of the 25-storey building on ETABS, let's break down the steps needed to perform the checks outlined in your request. Based on the given structural properties, material specifications, and loading conditions, here's how to approach the tasks:

1. Check the Translational Fundamental Natural Period of Vibration

According to IS 1893 (Part 1) - 2016, the natural period of vibration for a building in the translational modes in the X and Y directions is influenced by the building's mass distribution, stiffness distribution, and overall geometry. The formula used to estimate the fundamental period of vibration for a building is typically:

$$T = 0.075 \times H^{3/4}$$

where:

• T = Natural period of vibration (in seconds)

• H = Height of the building (in meters)

Steps to perform in ETABS:

• Model the building with the specified dimensions and structural properties in ETABS.

  • Set up the beam dimensions (300 × 400 mm), column dimensions (800 × 800 mm), and slab thickness (150 mm).

  • Define the material properties: Concrete grade M30 and steel reinforcement grade Fe 415.

  • Apply the specified dead loads (10 kN/m on beams, 3 kN/m² live load on floors, including roof).

  • Assume pinned boundary conditions at the base of the building.

• Perform a dynamic analysis in ETABS to calculate the fundamental period of vibration in the X and Y directions.

  • Use the modal analysis option to extract the fundamental natural period of vibration in each direction.

• Comparison:

  • The fundamental periods should be approximately T_x = 4.4 seconds and T_y = 4.2 seconds, within a ±2% tolerance.

  • If there's a major deviation, ensure that all modeling assumptions (boundary conditions, material properties, loads, and geometry) are correctly defined. Recheck the mass distribution and stiffness properties of the model.

2. Check for Torsional Irregularity (IS 1893, Clause 5)

According to IS 1893 (Part 1) - 2016, a building exhibits torsional irregularity if its torsional mode has a natural period greater than the two fundamental translational modes of vibration in both principal directions. The specific requirement is that the natural period of the torsional mode should be less than both translational modes.

Steps to check torsional irregularity:

• After modeling the building and performing the dynamic analysis, ETABS will provide the natural frequencies and mode shapes for the building.

• Identify the torsional mode and compare its period with the two translational modes along the X and Y directions.

• Torsional Irregularity Criteria:

  • The torsional period should be less than both T_x1 and T_y1. In this case:

    • T_x1 = 4.4 seconds

    • T_y1 = 4.2 seconds

  • If the torsional mode period is greater than either of these values, it indicates torsional irregularity, which would violate the provisions of IS 1893.

3. Check for Vertical Irregularity (IS 1893, Clause 7)

According to IS 1893 (Part 1) - 2016, vertical irregularity can occur in the building if there are significant discontinuities in the stiffness or mass distribution along the height of the structure. Table 6 of IS 1893 lists two conditions related to vertical irregularity that need to be satisfied:

Conditions for Vertical Irregularity:

1. Soft Storey Condition: If any storey has stiffness significantly less than the adjacent storeys, it can be considered a soft storey. This leads to excessive displacement and vulnerability during seismic events.

2. Mass and Stiffness Discontinuity: If there are significant discontinuities in the mass or stiffness (e.g., the building has rigid floors at some levels and flexible floors at others), it can result in torsional response and affect seismic performance.

Steps to check vertical irregularity:

• Inspect the floor-by-floor stiffness distribution in ETABS to ensure there are no soft storey conditions (i.e., floors with very low stiffness compared to adjacent floors).

• Review the mass distribution across the stories. If a floor has a significantly different mass from the surrounding floors (due to changes in partition walls, floor layouts, or mechanical equipment), it could lead to mass discontinuity.

• Verify that the stiffness and mass distribution is reasonably uniform across the building height.

• Comparison with IS 1893:

  • If any irregularities are observed (e.g., large variation in stiffness or mass), take corrective actions by adjusting the structural design, such as adding shear walls, bracing, or strengthening beams and columns in the irregular floors.

Note: Vertical irregularity checks may also require a detailed dynamic analysis to ensure the building performs well under seismic forces.

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Conclusion:

• Model the building correctly in ETABS with all provided details.

• Perform a dynamic analysis to check the fundamental natural periods in both X and Y directions to ensure they are close to the expected values (T_x ≈ 4.4 s, T_y ≈ 4.2 s).

• Verify the torsional mode and ensure its period is less than the translational periods in both directions to satisfy torsional irregularity provisions.

• Check for vertical irregularity by reviewing the distribution of stiffness and mass across the stories and ensuring no soft storey or discontinuity occurs.

By following these steps, you will be able to confirm if the model satisfies the provisions of IS 1893 (Part 1) – 2016 for seismic design.