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.

• Edit the stories now by adding 26 storeys (plinth at 1.5 meter height plus 25 storey at 3 meter height each), using the user-defined storey input.
• All levels above Storey 1 are labeled similarly, and Storey 1 has been designated as the master storey.

• Select the define tab and then choose material properties.
• Define the components of concrete as being of M30 grade, and the steel used for longitudinal and transverse reinforcement as being of grades 415 and 500, respectively.

• Section properties can be found under the define tab.
  Create a new property called beam by clicking on the frame section and entering the values 300 x 400 mm.

• 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.

• Click on define TA and then load patterns to define the load patterns.
• Define the loads in the x direction, including positive and negative x values as well as the same loads in the y direction.
• Mention the IS codes for the relevant 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.

• Add a live load of 10Kn/m2 on each beam in a similar manner.
• Select all of the beams from the select object type beams for that.
• Enter a value of 10 Kn/m2 in the distributed load field under the frame load.

• Now select the define tab and then mass source.
• Change the mass source and check the necessary loads.
• Assign load patterns to the model after adding them.

• Define the load cases
• Click on the load combinations.
• Click on the add default design combos.

• Go to the assign tab, click shell, and then choose diaphragms to create the diaphragm.

• Click on ok.

• Use the analyze option to analyze the model, then click on check model to ensure that the modelled structure is verified to be free of errors and warnings.

• The model is analyzed using analyze > run analysis, and deflection patterns, BMD/SFD patterns are logically cross-checked.
• It is acknowledged that the worked model yields accurate outcomes.

Results :-

• To view the analysis results, structure output, modal information, and modal periods and frequencies, click Display > Show Tables.
• The table indicates the following time periods for fundamental Mode-1Y direction and fundamental Mode-1X direction:

• 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.
• According to E-TABS analysis, the three first modes of seismic oscillation account for more than 65% of building weight when the three mode mass ratios are added together, giving us a result of 0.9044.
• The serviceability of the storey drift The following tool path is used to tabulate the base shear case (EQX or EQY).
• To display, click.Map of the Storey response
• When switching to maximum story drift while keeping the load case at EQY, the maximum drift value is 0.0020.

• Accordingly, the maximum inter-storage drift is limited to 0.004 for each storey height under clause 7.11.1.1.But our value, 0.0020, is within the permitted range for the code.
• In summary, the modeling is finished and has been analyzed in E- Tabs.Results fall within specified bounds.