Modelling of 25 storey building with the specified properties using ETABS

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

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 instialization.

• Choose the built-in setting option and select the display units and codes.
• After model intialization select the great and story inputs from the new model quick templates dialouge box.
• In this dialouge box choose grid spacing and custom story data which is used to provide the inputs specified above.
• Enter X grid data a distance of 4 meter and Y grid data as 4 at a distance of 4 meter.

• Now edit stories by using the user defined storeys input, 26 storey are incorporated( plinth at 1.5 meter height +25 with 3 meter height each.) Storey 1 has been labeled as master storey and all above levels are labeled similar to that.

• Select material properties used the define tab.
• Define the materials of concrete M30 grades and steel of transverse reinforcement as 415 grade and longitudinal reinforcement as 500 grade.

• Select section properties under the define tab.
• Click on the frame section and create a new property as beam and input the values as 300X400mm.

• After the define properties for the column as 800X800 and slab thickness as 150mm

• Using the membrane property, slab size 150mm has been added to the list. Mebrane 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 a column of size 800X800mm at the intersection of the horizontal and vertical grid and beams with quick draw tool keeping all stories on.

• Beams drwan using quick draw beam tool keeping all the storeis on.

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

• Select assign and cilck on the select pinned support.
• Fix the pin at the base.

• Now define the load patterns: click on the define and click on load patterns.
• Define the loads like dead load, live load, brick load, eathquake load in x direction and positive x and negative x and same loads in the Y direction also. Mention IS codes of respective loads.

• 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.

• 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 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 enetr 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 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 structure is verified without errors and warnigs.

• 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.

Results:

• Now click on display >show tables >Analysis results >Structure output >Modal information >modal periods and frequencies.
• The time periods for fundamental mode - 1Y direction and fundamental mode -1X direction are noted from the table and are as follows.
• Tx1= 3.746 seconds and Ty1= 3.913 seconds.
• Display>show tables>analysis results>structure output>modal information>modal participation mass ratios.
• From the table the natural time period of mode -1 and torsinal DOF are noted as follows:

• Tx1=3.581 seconds Ty1=2.609 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 ratio we get 0.9044 which means first three modes of seismic oscillation contribute more than 65% of buliding weight as per ETABS analysis.
• 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 0.0020.

• 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 0.0020 which is within the code limits.
• Conclusion: the modelling is completed and analysed in ETABS. The results are within given ranges.