Modelling of 25 storey building with the specified properties using ETABS

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 the floor: 3 kN/m² (Let us assume the same for the roof floor)

1. Buildings are assumed to be pinned at the base
2. All columns in all storey are of the same size
3. All beams in all storeys are of the same size
4. 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: T_x1= 4.4 second T_y1= 4.2 second

This check will help confirm if modelling is done correctly. Some deviations from the above values are 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 is 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 direction.

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

AIM

To model a 25-storey building on ETABS with given structural properties. 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.

PROCEDURE:

Open E-Tabs ->new file

Use built-in settings

Change the units and codes as per Indian standard, click Ok

Click on the custom grid spacing

Edit grid data

Enter the grids ordinates in X direction And Y direction

Click on Add to make more grids as per the requirement

Click on custom storey data

Right-click on and select "add new story"

Enter the number of stories

Enter the distance between them

Select 1st storey as the master story and all others as No

Select similar to "1st Story"

Click on Define Tab>material properties

Click on add new material

Add Concrete M30 Grade and Rebars of HYSD415 and HYSD500

Go to define>section properties>frame sections

Click on Add new property

Select the rectangular concrete and click ok

Add new beam and column frames

Change the property name as per given data B300x400> change material to M30

Enter the depth and width of the sections

Click on Modify/show rebar>to enter the rebars types and numbers

Click on M3 design only

Change longitudinal bars and Tie bar types to the defined materials

Go to define>section properties>frame sections

Select the rectangular concrete section> click on OK

Rename the property as C800x800

Enter the width and depth of the column

Click on modify/show rebar> to enter the bars type of columns

Click on P-M2-M3 design> Enter longitudinal and tie bars type

Reinforcement to be designed> Enter the number of bars required in 2&3 direction

Select slab section for the slab of 150mm

Go to define> section properties> slab sections

Click on slab1> modify/Show property

Rename the slab as Slab150

Select the slab material as M30> Modelling type as "membrane"

Enter the thickness of the slab as 150

Add columns to the structure.

Go to the bottom right corner and select ALL STORIES

Click on "Quickdraw columns" on the left side toolbars

Select the C800x800 from the property

Select all the columns ->all the columns will be applied to the grids

Select quick draw beams (keep the selection to ALL stories)

Select the property to B300x400

Select all the beams section> the beams will be applied

Now add slabs to the section> Select quick draw floors/Slabs

Change the property to slab 150

Change the selection to similar stories

Select all the slabs of any one floor

Now add joints to the base

Go to Assign bar> Joints> Restraints

Select the pin joint

Now we need to define load patterns

Go to Define Bar> Load patterns

Add dead load, live load, brick wall load and earthquake loads

Click on earthquake loads "EQx" > Click on Modify lateral load

Select the Load direction (X dir-eccentricity)

Select Site III as suggested in the question & time period as the program calculated

Do this for all earthquake loads in both directions

Assign the live load to the slabs and brick wall loads to the frames

Select all the slabs of any floor> Change the selection to similar stories

Go to Assign> Shell loads> Uniform

Type live load of 3 kN/m²

Now the live load will be applied to the roof

Go to the roof plan> Change the selection to One story

Select all the slabs of the roof

Go to assign> shell loads> Uniform

Type live load of 1.5kN/m²

Assign the brick wall load to the beams

Change the selection to all stories

Go to select menu> object types> select beams (all beams will be selected)

Go to Assign menu> Frame load> distributed

Select the brick wall from the selection

Enter the value of 10kN/m

Define mass source

Go to Assign Menu> Mass source

Click on Modify/show mass source

Select specified load patterns

Input dead=1, brick wall1, live=0.25

Go to define menu> load combination

Click on Add default design combos

Select concrete frame design

Making the diaphragm of the structure

Go to select menu> object types> select the floors

Assign> Shell> Diaphragm

select D1 and apply

Check the model for errors

Go to analyse menu> check model

Go to analyse menu> run analysis

Go to Display> Deformed shape or F6>Tab will open and select any case(live/dead, earthquake) to view the deformed shape

Click on Display menu> Show tables> Tables display will open

Select analysis results> structure output> modal information> Modal participation mass ratios

CONCLUSION:

The analysis is done and the result is within the range

• From the table, the Natural time period and mass participation factor are noted
• T_x1=4.326sec, T_y1=4.137sec and T_xy=3.254sec
• So it is clear from the table that the torsional time period is less than the two translational time periods as required by IS 1893 provision table 5
• The building is located in seismic zone-III and the first three modes together contribute more than 65% mass participation factor in each principal plan direction.

• Provision on inter-storey drift limitations in clause 7.11.1.1 is satisfied. Storey shear does not exceed 0.004 times the storey height under the action of design base shear with no load factors