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: Tx1">Tx1Tx1= 4.4 second Ty1">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

ANSWER :

AIM :

To model and anlysis the 25 storey building with given specifications and to cross check the parameters as per IS 1893 Provisions.

PROCEDURE :

• Open Etabs and select new model and make the settings

• An interface appears as shown , click the custom grid spacing and maitaining the spacing as 4m in both x andf y direction

• Click ok a box enters into the stories upto 25 maintaining height of 3m except for base to plinth which is 1.5m and 1st storey is entered as master storey

• Now go to define select material properties and enter the following

• M30 Grade Concrete, Steel 415

• Now go to section properties enter the size of the beam , column and slab

• Now go to load pattern enter the loads as shown ,

• Now select the quick draw column from the screen and place the columns at each grids

• Similarly place the beams and slabs

• Now assign the load to roof slab , select all the slabs go to assign option

• select the load as live and 1.5kn/m^2

• For other floors assign 3kn/m^2

• now assign the load to all the beams of 10kn/m^2
• now go to define option and select the mass source and assign the mass source as shown
• Go to load combination select modify , a list of combinations appears.
• go to shells from assign and then select diaphragam , assign as D1\

• Now go to analysis check the model

• since there are no errors , building looks like this after appllying the loads.
• go to display select the shown table and then model 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 three modes contributre more than 65% in the mass participating ratios of each plan direction.

RESULTS :

1. Now click on display>show tables>analysis results>structural output>model information>model periods and frequencies
2. The time periods for fundamental mode -1Y direction and fundamental mode -1X directionare noted from tables are as follows
3. From the table we can say that time period of mode of torsion is less than fundamental mode
4. First three modes contributre more than 65% in the mass participating ratios of each plan direction.
5. Modelling is completed and anlysis in Etabs , The results are within the given ranges.