Modelling of 25 storey building with the specified properties using ETABS

                                                                  *  Challenge 6 – ETABS modelling of a 25 storey building   *

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.

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

            

Answer :- 

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 E-TABS 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 300 X 400mm.

              

• After the define properties for the  column as 800 X 800 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 & 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 & 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 & 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 & 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 E-TABS 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 E- Tabs.The results are within given ranges.