Modelling of 25 storey building with the specified properties using ETABS

Aim:

To model and analyse a 25 storey building with a given specifications and to cross check the parameters as per IS 1893 provisions.

Procedure:

• Create a new model with built in settings as follow in the ETABS
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• After model initiations select the grid and storey unputs from the new model quick tempelates dialogue box.
• In this dialogue box choose custom grid spacing and custom storey data which is used to provide the inputs specified above.
• Enter X grif data at a distance of 4 m and Y grid data as 4 at a distance of 4 m.
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• Now edit the storeys by using the user defined storey input, 26 storeys are incorporated(plinth @ 1.5 m heigth + 25 storey with 3 m height each). Storey 1 has been labelled as meter storey and all above levels are similar to that.
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• Select material properties under the defined tab.
• Define the materials of concrete M30 grade and steel of transverse reinforcement as 415 grade and longitudinal reinforcement as 500 grade.
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• Select section properties under the define tab.
• Click on the frame section and create a new property as beam and input the values as 300x400 mm.
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• After the define properties for the column as 800x800 and slab thickness as 150 mm.
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• Using the membrane property, slab size 150mm 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 calculations.
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• Now draw a column of size 800x800 mm at the intersection of horizontal and vertical grid and beams with quick draw tool keeping all stories on.
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• Beams drawn using quick draw beam tool keeping all the storeys on.
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• Using the qiock draw floors option, all the slabs have been modelled keeping similar storeys ON.
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• Select assign and click on joint select pinned support.
• Fix the pin supports at the base.
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• Now define the load patterns. Click on the define to and click on the load patterns.
• Define the loads like dead, live, brick, and earthquake loads in X direction and negative x and same loads in the Y directions. Also mention IS codes of respective loads.
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• Click on modify lateral load, set range from base to 25th storey and seismic zone factor as 0.36, importance factor 1.25 and time period 0.5 seconds.
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• 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 set value as 3 KN/m^2.
• Click on Apply.
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• For the roof we will take only 50% of the live load.
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• Similarly live load of 10 KN/m^2 on all beams.
• For that select all the beams from select object type beams.
• Go to the frame load and click on the distributed load and enter 10 KN/m^2.
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• Now go to tha ,ass source from the define tab.
• Modify the mass source and tick mark the required loads.
• Add load patterns and assign to the model.
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• Define the load cases.
• Click on the load combinations.
• Click on the add default design combos.
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• Now create the diaphram by going to the assign tab then shell and select diaphrams.
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• Analyze the model using analyze options then clcik on the check model, the modelled structure is verified without errors and warnings.
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• Using analyse, run analyse, the model is analysed and deflection patterns, BMD/SFD patterns are cross checked logically. It is understood that the worked model provides reliable results.
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• Now click on Display>show tables>Analyse results>Structure Output>Model information>model periods and frequencies.
• The time periods for the fundamental mode -1 Y direction and fundamental mode -1 X direction are noted from the table as follows.
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• Tx1 - 3.746 seconds and Ty1 = 3.913 seconds.
• Display>Show tables>analyse results>structure output>modal information>model participation mass ratios.
• From the table the natural time period for mode -1 and torsional DOF are noted as follows.
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• Tx1 = 3.581 seconds and Ty1 = 3.3 seconds and Txy1 = 2.609 seconds.
• So from the results it is clear that the torsional time period is less than two transitional time period as required by IS 1893 provision table 5.
• When we add three mode mass ratios we get 0.9044 which means first three modes of seismic oscillations contribute more than 65% of building weight as per ETABS analysis.
• The storey drift for servicibiality base shear case (EQx and EQy) is tabulated from the following tool path.
• Go to display>Storey response plot.
• Keeping load case to EQy and swithching to max story drift the maximum drift calue noted is 0.0020.
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• So as per the clause 7.11.1.1 , the maximum inter storey drifts is milted to 0.004 for each storey height. But our value is 0.0020 which is within the code limits.

Result:

The moedlling is completed and analysed in etabs. The results are within the given ranges.