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.

Introduction: ETABS offers is the recognition of story levels, allowing for the input of building data in a logical and convenient manner. You can define your models on a floor-by-floor, story-by-story basis, analogous to the way a designer works when laying out building drawings. ETABS dynamic analysis capabilities include the calculation of vibration modes using Ritz or eigen vectors, response-spectrum analysis, and time-history analysis for both linear and nonlinear behaviour.

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 initialize.

Choose the built-in settings option and select the display units and codes.

After model initialize select the grid and story inputs from the New model quick templates dialogue box.

In this dialogue choose custom grid spacing and custom story data which is used to provide the inputs specified

Enter X grid data at a distance of 4 metre and 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 incorporated( plinth at 1.5 meter height+ 25 storey with 3 meter 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 as 415 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 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 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 floors option, all the slabs have been modelled keeping "similar stories" ON.

• 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 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 0.16 (zone 3), importance factor 1, site-type II , Response Reduction Factor as 5 and time period program calculated.

• 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.
• For that select all the beams from select object type beams.
• Go to the frame load and click on the distributed load and enter the value as 10 Kn/m^2.

• 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.

• We add mass source

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>Modal information>modal periods and frequencies.
• The time periods for fundamental mode-1Y direction and fundamental Mode -1X direction are noted from the table and are as follows:

• So as per the clause 7.11.1.1,the maximum inter storey drift is to 0.004 for each storey height .

Conclusion:

The modelling is completed and analysed in ETABS .The results are within given ranges.