Modelling of 25 storey building with the specified properties using ETABS

Question :-

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 a 25 storey building on ETABS with structural properties as specified and 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 :-

Etabs :-

• ETABS is one of the most powerful software tools for structural analysis.
• 3D modeling, visualization, and automatic code-based learning are some of the unique features of this software.
• ETABS also supports several analytical models like response spectrum analysis, time-history analysis, and line direct integration time-history analysis. 

Advantages :-

• Built-in drawing utilities: To aid the engineers in modeling, ETABS comes with a built-in feature for drawing and drafting. Some other packages also have this feature, but the quality is much better in ETABS.
• Extensive reports: ETABS generates detailed and comprehensive reports for every project or task you perform, be it calculation of stresses, deformation or failure analysis, and design summary.
• Design of concrete and steel frames: Among all the materials available to build structures, concrete and steel are by far utilized the most in terms of volume. ETABS has specialized modules that deal with concrete and steel frames to optimize your calculations and offer capacity checks for frame elements.

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 Initialization.

              

• 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.

• 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.

• Now Go to Replicate tool in Edit Menu.
• In Linear Option, Select dx as 4 and dy as 0.
• inser the Number as 4.

• Columns are Successfully added to the structure.

• Now draw the Beams.
• Draw beams of size 300X400mm at the intersection of the horizontal and vertical grids and beams with quick draw beam keeping all the stories on.

• Beams are Successfully added to the structure.

• Now select the PLAN option from menu bar.
• Select Story 1 in Plan View.

• Now draw the Slab.
• Draw slab of size 150mm thickness, Draw slabs with quick draw floor/wall keeping all the stories on.

• Slabs are Successfully added to the structure.

• 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, earthquake load in X direction and same loads in Y direction also.
• Mention IS codes of respective loads.

• Click on modified lateral load.
• Set range from base to 25th story and siesmic Zone factor as 0.36, important factor 1.25 and time perod 0.5 S.
• Now go to Mars Source from define tab.
• Modify the mars source and tick mark the required loads.
• Add load patterns and assign the model.
• Insert the dead load as multiplier to 1, live load as multiplier to 0.25 and wall load as multiplier to 1.

• Mars source is been added successfully.

• Now go to Load cases in define Tab.

• Now go to Load combinations in define Tab.

• Select add default design combos.
• Select concrete frame Design.

• Now go to Select option in select Menu.
• Slect Properties.
• Select Frame Sections.
• Select B400X300 option.

• Now assign the loads.
• Frame loads -> Distributed.

• Now go to Select option in select Menu.
• Slect Properties.
• Select slab Sections.
• Select All the Slab150.

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

 

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

• Analyse the model using analyse option then click on check model the modelled structure is verified without errors and warnings.

• Using analyse -> Run analysis, the model is analysed and deflection pattersn are cross checked it logically it is understood that the V model provides reliable results

 

Result :-

• Now click on display -> show tables -> Analysis result structure output -> model information -> model periods and frequencies.
• Time periods Fundamental model 1 Y direction and fundamental mode 1X direction are noted from the tables and are as follows.

• Tx1= 3.733 s and Ty1= 3.589 s.
• Display -> show tables -> Analysis result structure output -> model information -> model participation pass mass ratio.
• From the table the natural time period for Mode 1 and torsional DOF are noted as below.

• Tx1= 3.733 s and Ty1= 3.589 s and Txy1 = 2.981 s.
• So from the result it is clear that the torsional time period is less than two translational time period as requiered by IS 1893 provision Table:5.

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

      

• When we add the three mode mass ratios we get 0.944 which means first three modes of seismic oscillation contribute more than 65 % of building weight as per ETabs analysis.

• 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

• The story drift for serviceability Base shear case is tabuated from the tool Path.
• go to display -> story response Plot.
• keeping load case to EQY and switching to make story drift the maximum drift value noted is 0.020.

• So as per the clause7.11.1.1, the maximum inter storey drift is milited to 0.004for each storey height.
• But our value is 0.002 which is within the core limit.

• Conclusion:- The modelling is completed and analysed in ETabs. The results are within given range.