Modelling of 25 storey building with the specified properties using ETABS
Q. Challenge 6 – ETABS modelling of a 25 storey building
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">Tx1��1= 4.4 second Ty1">Ty1��1= 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
Ans:
Aim of the Challenge:-
- The aim of the challenge is to model a 25-storey building using ETABS software and verify the seismic code requirement from ETABS analysis.
Procedure:
- Open ETABS software.
- Click New model.
- Give model initialization setting as follow
- Click ok
- In the new model quick template dialog box , select custom grid spacing and click Edit grid data
- Input the following tables accordingly as per the requirement in grid data as coordinate.
- Click ok.
- Select custom story data under story dimension and click edit story data.
- Then in edit story data different stories as a plinth story, story 1, story 2 & story 3 with respective heights are defined & created by keeping story 1 as a master story.
- Then go to story--> Right Click --> Add story --> keep existing story height --> Keep story height as 3m
-> Number of Added stories --> 22 --> Ok
- The plan view and 3D model view of the structure.
- Go to define --> Material Property
- Then click --> Add new material --> Define concrete of M30 grade
- Then ok
- Ok
- Then again click on Add new material to define rebars.
- Ok
- Ok --> Ok
Again go to Define --> Section Property --> Frame Sections --> Add New Property
Select concrete rectangular.
- Then give respective values for the concrete beam section by defining the beam size and grade of concrete.
- Ok
- Go to Modify/show rebars--> change the required fields as per the requirement. Then Ok.
- Again for defining column size click on Add new property -->
- Ok
- Go to Modify/show rebars--> change the required fields as per the requirement. Then Ok.
- Ok-->Ok
- The property for the column has been defined.
- Then Click--> Section properties--> Slab section
- Select Add new property and give the following settings.
- Ok--> Ok
- Define Load patterns.
- Ok-->Ok
- Check load cases by clicking Define-->Load cases.
Load Combinations
- Define--> Load combinations --> Add Default Design Combos.
Draw Section
- Now we will add columns to the structure.
- Go to the bottom right corner and select ALL STORIES
- Click on "Quickdraw columns" of left side toolbars->a new tab will open
- select the C800x800 from the property
- select all the columns ->all the columns will be applied to the grids
- Now beam will be added->Select quick draw beams (keep the selection to ALL stories)
- a tab will open->select the property to 300x400
- select all the beams section->the beams will be applied
- Then Go to Story 1 Plan--> Select Similar Stories --> Draw --> Draw floor and wall objects --> Quick draw floor/wall
- Then brick loads of 10Kn/m2 need to be applied on all the beams.
- So first for selecting the beams go to --> select --> Properties--> Frame sections --> select the beam type
- Then go to Assign --> Frame loads --> Distributed --> Select wall from the drop-down menu --> assign load as 10 --> ok
Then the live loads in the slabs need to be assigned.
- Select Similar story --> Go to --> Assign --> Shell Load --> Uniform
- Then select story 25 -->assign load of 1.5 kn/m2
- Now create the diaphragm by going to the assign tab then shell and select diaphragms.
- Click on ok.
-
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 fundamental Mode -1X direction are noted from the table and are as follows:
- Using analyze>run analysis, the model is analyzed and deflection patterns and BMD/SFD patterns are cross-checked.
- So from the results, it is clear that the torsional time period is less than two translational time periods as required by IS 1893 provision table 5.
- The first three modes of seismic oscillation contribute more than 65% of building weight as per ETABS analysis.
- The story drift for serviceability Base shear case(EQX or EQY) is tabulated from the following tool path.
- Keeping the load case to EQY and switching to max story drift the maximum drift value noted is 0.0020.
- So as per clause 7.11.1.1, the maximum inter-story drift is limited to 0.004 for each story height. But our value is 0.0020 which is within the code limits.
