Modelling of 25 storey building with the specified properties using ETABS

MODELLING OF 25 STOREY BUILDING WITH THE SPECIFIED PROPERTIES USING ETABS

Question:

Model a 25-storey building on ETABS 2018 with structural properties as specified below. Ground floor or plinth floor level can be taken at a height of 1.5 m from the base. Each of the successive 25 storeys has a storey height of 3 m.

Structural element size

• Beams: 300 x 400 mm
• Columns: 800 x 800 mm
• Slab: 150 mm thick

Material properties

• Grade of concrete: M30
• Grade of steel reinforcement bars: Fe415

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/m^2 (let us assume the same for roof floor).

Buildings are assumed to be pinned at the base.

All columns in all storey are of the same size.

All beams in all storey are of the same size.

Building is located on soil type-2 and in zone-3. 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 = 4.4 second Ty1 = 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 1873 (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 provisional 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 structure is subjected to unfactored seismic base shear.

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 initialization.

• Choose the built-in settings option and select the display units and codes.
• After model initialization select the grid and storey inputs from the new model quick templates dialogue box.
• In this dialogue choose grid spacing and custom storey data which is used to provide the inputs specified above.
• Enter X grid data at 4 metre and Y grid data as 4 at 4 metres.

• Now edit the storey by using the user defined storey input, 26 storeys are incorporated (plinth at 1.5 metre height + 25 storey with 3 metre height each). Storey 1 has been 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 reinforcement as 415 grade and longitudinal reinforcement as 500 grades.

• Select section properties under the define tab.
• Click in the frame section and create a new property as beam and input the values as 300 x 400 mm.

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

• Using the membrane property 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 a column of size 800 x 800 mm 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 storey on.

• Using the quick draw floor option, all the slabs have been modelled keeping similar storey ON.

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

• 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 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 25^th 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 storey.
• Click on shell load under the assign tab.
• Click on live load and give value as 3 kN/m^2.
• Click on apply.

• For the roof we will take only 50% of the live load.

• Similarly add live load of 10 kN/m^2 on all the beams.
• For that select all the beams from select object type beams
• Go to the frame load and click on the distribution 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 combinations.

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

• Click on ok.

• 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 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 > model information > model periods and frequencies.
• The time periods for fundamental mode – Y direction and fundamental mode – X 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 > model information > model participation mass ratios.
• From the table the natural time period for mode-1 and torsional DOF are noted as follows:

• Tx1 = 3.581 seconds and Ty1 = 3.3 seconds and Txy1 = 2.609 seconds.
• So, from the results 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 means first three modes of seismic oscillation contribute more than 65 % of building 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 storey drift the maximum drift value noted is 0.0020.

• So as per the clause 7.11.1.1, the maximum inter-storey drift 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.