Modelling of 25 storey building with the specified properties using ETABS
A 25 storey building is modelled in ETABS software with specifications given as:
>Ground floor or plinth floor level can be taken at a height of 1.5 metres from the base.
>Each of the successive 25 storeys has a storey height of 3 metres. Total height of the building 76.5m.
>Beams : 300 × 400 mm
>Columns : 800 × 800 mm
>Slab : 150 mm thick
>Grade of Concrete : M30
>Grade of Steel Reinforcement Bars : Fe 415
>Dead Load on beams from infill wall : 10 kN/m
>Live load on floor : 3 kN/m2
>Buildings are assumed to be pinned at the base
>All columns in all storey are of the same size
>All beams in all storeys are of the same size
>Building is located on Soil type-II and in Zone-III
With these specifications we need to verify the seismic code requirements from ETABS analysis.
a) The translational fundamental natural period of vibration in both the x and y direction according to modal period and frequencies report table:
Tx1= 4.854 second
Ty1= 4.641 second
The approximate time periods assumed in the question can have a deviation of -+2% . Therefore , check is accepted. Hence modelling done correctly.
b) The provision in Table 5 of IS 1893 (part 1) – 2016, on torsional irregularity says:
The natural period corresponding to the torsional mode of oscillation must be less than the two fundamental translational modes of oscillation along each principal plan directions.
Yes , it is less.
c) The two provisions of clause 7 in Table 6 of IS 1893 (part 1) – 2016, regarding vertical irregularity says :
The first three modes contribute atleast sixty % - The first mode alone satisfies the clause.
The fundamental lateral natural period are away by atleast 10% - No, it does not satisfy the condition , the time period of 2nd should be 4.3 second but we got 4.6 second.
d)The provision on inter-storey drift limitations in clause 7.11.1.1 says :
Storey drify in any storey shall not exceed 0.004 times the storey height - Yes , the clause is satisfied.
The etabs file of the model and report analysis table is attached below.