Modelling of 25 storey building with the specified properties using ETABS

Modelling of 25 storey building with the specified properties using ETABS

Challenge 6 – ETABS modelling of a 25 storey building

Model a 25 storey building on ETABS with structural properties as specified below. 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.

Structural Element Sizes

• Beams : 300 × 400 mm
• Columns : 800 × 800 mm
• Slab : 150 mm thick

Material Properties

• Grade of Concrete : M30
• Grade of Steel Reinforcement Bars : Fe 415

Loading

• Dead Load on beams from infill wall : 10 kN/m (Let us assume the same for roof floor in view of parapet)
• Live load on floor : 3 kN/m2 (Let us assume the same for roof floor)

a) Buildings are assumed to be pinned at the base

b) All columns in all storey are of the same size

c) All beams in all storeys are of the same size

d) Building is located on Soil type-II and in Zone-III. Assume other factors if required.

Carry out the following exercises:

• Check if the translational fundamental natural period of vibration in both the x and y direction is approximately: Tx1Tx1= 4.4 second Ty1Ty1= 4.2 second

This check will help confirm if modelling is done correctly. Some deviations from the above values is possible (± 2%). But any major deviation will suggest an error in modelling.

• Check if the following provision in Table 5 of IS 1893 (part 1) – 2016, on torsional irregularity satisfied:

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.

• Check if the two provisions of clause 7 in Table 6 (shown below) of IS 1893 (part 1) – 2016, regarding vertical irregularity is satisfied

• Check if provision on inter-storey drift limitations in clause 7.11.1.1 satisfied. Please note that this check is for Serviceability Limit State and hence is for situation when stucture is subjected to unfactored seismic base shear

STEP 1:

open ETABS and select new model and make the following settings.

an interface appears as shown ,click the custom grid spacing and maintain the spacing as 4m

in both x and y direction.

click ok, a box appears,enter the stories upto 25 maintaing height of 3m except for base to plinth

which is 1.5m and 1st storey is entered as master story.

now go to define and select material properties, and enter the following.

M30 grade concrete and steel 415.

now go to section properties and enter the size of beam column and slab.

now go to the load pattern,and enter the loads as shown.

now select the quick draw column from the screen and place the columns at each grid.

similarly place the beams and slabs.

now assign the load to roof slab,select all the slabs and go to assign option.

select the load as live and 1.5kn/m2.

for other floors assign 3kn/m2.

now assign the load to all the beams of 10kn/m2.

now go to define option and select mass source and assign the mass source as shown.

go to the load combination and select modify ,a list of combination appears.

go to the shells from assign and then select diaphragm, assign as D1.

now go to the analyse and check model.

since there are no errors, building looks like this after applying loads.

go to the display and select shown tables and then modal mass participating ratios.

table is obtained.

from the table we can say that time period of mode of torsion is less than fundamental mode.

first 3 modes contributre more than 65% in the mass participating ratios of each plan direction.