Modelling of 25 storey building with the specified properties using ETABS

Q1).
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.

Answer:

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.

INTRODUCTION :

Given that:

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)

Carry out the following exercises:

• Check if the translational fundamental natural period of vibration in both the x and y direction is approximately: Tx1��1= 4.4 second Ty1��1= 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

PROCEDURE :

• Open the ETABS software.
• Click the file menu and choose to create a new model option.
• After choosing a new model a dialogue box will appear as model intialization.

• Choose the built-in settings option and select the display units and codes.
• New dialogue box will appear named New Model Quick Templates.
• Select Custom Grid spacing and custom story data.

• After models initialization, select grid and storey inputs from new model quick templates dialogue box.
• In the dialogue box choose grid spacing and custom story data which is used to provide the input specified above.
• Enter X Grid data at a distance of 4 m and Y Grid data as 4 at a distance of 4m

• Now edit stories by using the user-defined story input, 26 stories are incorporated (plinth at 1.5m and 25 story with 3m height each).
• Storey 1 has been labelled as a master storey and all above levels are labelled similar to that.
• Add Story height as 3m.
• Number of Added stories as 22.

• So, 25 storeys are successfully added in the peoject.
• Create alll the storeys as similar to storey 1.

• Select Material properties under define tab.

• New dialogue box will appear named Define Materials.
• Select Add new Material from the window.

• Define the materials of concrete M30 Grade and steel of reinforcement as 415 Grade and longitudinal reinforcement as 500 Grade.
• Now select Section Properties under Define Tab.
• Select Frame Sections.
• Select Add new properties in Frame Properties.

• Select Frame Sections.
• Select Add new properties in Frame Properties.

• Create a new Property as a beam and input values as 300X400.
• Select Modify/Show Rebar in Reinforcement option.
• Select M3 Design only (Beam).
• Select Reabar as HYSD415.

• Create a new Property as a column and input values as 800X800.
• Select Modify/Show Rebar in Reinforcement option.
• Select P-M2-M3 Design (Column).
• Select Reabar as HYSD415.

• Now select Section Properties under Define Tab.
• Select Slab Sections.
• Select Add new properties in Slab Properties.

• Using the membrane properties, slab size 150 mm has been added to the list.
• Membrane property will help to consider only slab weights and Floor weights in analysis of structure while no stiffness component of flange advantage are considered during calculation.

• Now draw columns of size 800X800mm at the intersection of the horizontal and vertical grids and beams with quick draw Column keeping all the stories on.

• Beams drawn using quick draw beam tool keeping all the stories on.

• Using the quick draw floors option,all the slabs have been modelled keeping"similar stories" ON.

• Select assign and click on joint select pinned support.
• Fix he pin supports at the base.
• Now define the load patterns: click on the define tab and click on load patterns.
• Define the loads like dead load,live load,brick load,earth quake load in x direction and positive x and negative x and same loads in the Y direction also.Mention IS codes of respective loads.

• Do the same for all the earthquake loads.
• Assign the loads and select similar stories.
• Click on shell load under the assign tab.
• Click on live load and give value as 3kn/m^2.
• Click on apply.

• For the roof we will take only 50% of the live load.
• Similarly add live load of 10Kn/m^2 on all the beams.
• For that select all the beams from select object type beams.
• Goto the frame load and click on the distributed load and enter the value as 10 Kn/m^2.

• Now go to mass source from the define tab.
• Modify the mass source and tick mark the required loads.
• Add load patterns and assign to the model.

• Define the load cases:
• Click on the load combinations.
• Click on the add default design combos.

Now create the diaphragm by going to the assign tab then shell and select diaphragms.

• CLICK ON OK.

• Analyze the model using analyze option then click on check model,the modelled strucrue is verified without errors and warnings
• Using analyze>run analysis,the model is analysed and deflection patterns,BMD/SFD patterns are cross checked logically.It is understood that the worked model provides reliable results.

RESULT:

• Now click on Display>show tables>Analysis results>Structure ouput>Modal information>modal periods and frequencies.
• The time periods for fundamental mode-1Y direction and findamental Mode -1X direction are noted from the table and are as follows:

• MODEL-1 TIME PERIOD = 4.383 SECOND
• MODEL-2 TIME PERIOD =4.222 SECOND
• Display>show tables>analysis results>structure output>modal information>modal participation mass ratios.
• From the table the natural time period for mode-1 and torsional DOF are noted as follow:

• Tx1=4.383 seconds Ty1=4.222 seconds Txy1=3.311 seconds
• So from the results it is clear that the torsional time period is less than two translational time period as required by IS 1893 provision table 5.
• When we add the three mode mass ratios we get 0.8042 which menas first three modes of seismic oscillation contribute more than 65% of building weight as per ETABS anaysis.
• The storey drift for serviceability Base shear case(EQX or EQY) is tabulated from the following tool path.
• Go to display>Storey response plot
• Keeping load case to EQY and switching to max story drift the maximum drift value noted is 0.001625
• Keeping load case to EQx and switching to max story drift the maximum drift value noted is 0.001492

• So as per the clause 7.11.1.1,the maximum inter storey drift is milited to 0.004 for each storey height.But our value is 0.001625 which is within the code limits.
• Conclusion: the modelling is completed and analysed by using the etabs software.