Modelling of 25 storey building with the specified properties using ETABS

CHALLENGE.:– ETABS modelling of a 25 storey building.

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AIM.:-
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)

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: 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.

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PROCEDURE.:-

• Open ETABS
• Click the File menu and choose New Model from the list.
• A dialogue box will appear as Model Initialization.
• Choose (Use Built-in settings) with option and select the display unit as Metric SI and Indian codes.
• Steel Code:- IS 800:2007
• Concrete Code:- IS 456:2000

1. DEFINE GRIDS AND STORYS.:

• Now enter the grid and story inputs in the (New model quick templates) dialogue box.
• We can also choose to use custom grid spacing and custom story data for buidings with non uniform grid spacing.
• Here the building has uniform spacing along both X and Y directions and similar story heights except at bottom story. So, chosen uniform grid spacing.
• Enter the grid and story dimensions.
• Below are the grids created, shown in plan and storys in 3D.
• Right click and select (ADD/Modify Grids) for making changes in the grids and storys defined.
• Story above plinth, Story 2 has been labeled as master storey and all above levels are labeled similar to that.

2. DEFINE MATERIALS.:

• Select Material Properties under the Define Tab.
• Define the material of concrete M30 Grade.
• Define material of Steel of HYSD415.
• Under the Define Tab, Select Section Properties. Click on the Frame Section.
• Add new property as Beam and input the values as 300X400mm.
• Similarly add Column as 800X800mm and also Slab of 150mm Thickness.

3. MODELLING.:

• Select All Stories at the Bottom Corner.
• Now with Quick Draw Column tool draw the columns of size 800X800mm at the intersection of the horizontal and vertical grids.
• Now, using Quick Draw tool add Beams of the size 300X400mm between all columns.
• Beams are drawn keeping all the stories on.
• Similarly use the Quick Draw Floor option and draw all the slabs required by keeping Similar Stories ON.
• Select all the Joints at base.
• Select pinned support and assign.

4. LOADING.:

• Click on the Define tab and select Load Patterns.
• Define the below loads.
• 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/`m^2` (Let us assume the same for roof floor)
• Define earth quake load in x direction and positive x and negative x and same loads in the Y direction also.
• Select types as "Seismic", and IS 1893:2016 for all seismic loads defined.
• Click on modify lateral load, set range from base to roof and for seismic zone III, Z factor as 0.16.
• Soil type-II,importance factor I =1.25.
• Set time period 0.5 seconds.
• Select > Object Type > Floors.
• Assign > Shell Loads > Uniform.
• Click On Live Load and enter value as 3 KN/`m^2`.
• Click on Apply.
• Select the Beams from Select > object type > Beams.
• Assign > Frame Loads > Distributed.
• Assign Dead Load of 10KN/`m^2` on all the beams.

5. MASS SOURCE.:

• Select mass Source from the Define tab.
• Modify the mass source and check specified load patterns.
• For live loads >/ 3KN/`m^2`, modify Live Load multiplier as 0.25.
• Select Define > Load Combination.
• Click on the add default design combos.
• Check Concrete frame design.

6. DIAPHRAGM.:

• Now create the diaphragm by going to the assign tab then shell and select diaphragms.
• Now, Click on OK.

7. ANALYSIS.:

• Analyze the model using analyze option then click on check modeled structure is verified without errors and warnings.
• Using Analyze > Run Analysis. The Model is analyzed and deflection patterns, BMD/SFD patterns are cross checked logically. It is understood that the worked model provides reliable results.

8. RESULT.:

• Now click on Display > Show Tables > Analysis Results > Structure Output > Model information > Model periods and frequencies.
• The time period for fundamental mode-1Y direction and fundamental mode-1X direction are noted from the table and are as

i) Time Period Check: = 4.4 second = 4.2 second

• Display>show tables analysis results>structure output>modal information>modal participation mass ratios.
• From the table the natural time period for Mode-1,2 and torsional DOF are noted as follow:
• Tx1=4.416 seconds, Ty1=4.222 seconds, Txy1-3.311seconds

ii) Torsional irregularity: Table 5 of IS 1893 (part 1) - 2016

• From the results it is clear that the torsional time period Txy1-3.311seconds is less than two translational time period Tx1-4.416 seconds and Ty1=4.222 seconds as required by IS 1893 provision Table 5.

iii) Vertical irregularity: Clause 7 in Table 6 of IS 1893 (part 1) - 2016

• On adding, first three mode mass ratios we get 0.90 which means first three modes of seismic oscillation contribute to more than 65% of building weight as per ETABS analysis.
• Tx1=4.416 seconds and Ty1-4.222 seconds doesn't differ by 10%, hence this condition is not satisfied.

iv) Storey Drift

• Go to Display>Storey Response Plot.
• The storey drift for serviceability Limit State, Unfactored Seismic Base shear case(EQX or EQY) is graphically shown.
• Setting load case to EQY and switching to max story drift, the maximum drift value noted is 0.0017.
• This value is less than 0.004 times the storey height.
• Inter-storey drift limitations as in clause 7.11.1.1 satisfied.