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 intialization.

• Choose the built-in settings option and select the display units and codes.
• After model intialization select the grid and story inputs from the New model quick templates dialogue box.
• In this dialogue bchoose ctom grid spacing and custom story datata which is used to provide the inputs specified above.
• Enetr X grid data at a distance of 4 metreand Y grid data as 4 at a distance of 4 metre.

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

• Select material properties under the define tab.
• Define the materials of concrete M30 grade and steel of transverse reinforcement as 415 grade and longitudnal 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 ass 150mm

• Using the membrane property,slab size 150mm has been addded tothe 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 drawn using quick draw beam tool keeping all the stories on.

• Using the quick draw fllors 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 ta 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.

• 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 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.

Results:

• 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:

• 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 for mode-1 and torsional DOF are noted as follow:

• Tx1=3.581 seconds Ty1=3.3 seconds Txy1=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 ratios we get 0.9044 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 0.0020.

• 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.