Modelling of 25 storey building with the specified properties using ETABS

Modelling of 25 storey building with the specified properties using ETABS:

Q). 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 the following: `T_X1 = 2.98s` and `T_Y1 = 3.14s`

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

Aim:

To Model the 25 storey buildings with specified properties for different floor levels and design parameters using Etabs software:

Introduction:

• The structural configuration of the building plays a major role in determining the overall response during an earthquake. Different building geometries behave differently when induced by the same seismic forces. Hence, it is very important to properly understand the response of buildings related to their structural configuration for a safer design approach, therefore in this project, we have modeled different buildings with varying geometries using ETABS, furthermore their response are analyzed with the help of the software and the results are then compared to grasp an understanding of the structural behavior of building with different storeys.

Procedure:

• For an easier approach, in this project we have covering the modeling part of the buildings where the structures are drawn from scratch as per the pre-specified geometrical and structural conditions, moreover, running the analysis and generating the structural output with regard to all the buildings is also covered.

Open ETABS & setup the model.

• Once the software is up & running, click on the new model option.
• Once you click on the new model, a dialogue box will pop up where you can set up the codes and units to be used in your project.

Setting up the Grid data.

• To setup the grid data, you need to click on the custom grid option.
• Once you do that, a menu with all the grid settings will open up.
• In the same menu, you can fix the length, numbers of grids in both the orthogonal directions, spacing b/w all the vertical & horizontal grids etc,
• So, by following the same approach, we have setup a 4 x 3 bay grid with 4m center spacing in b/w.

Fixing the storey data.

• Similarly, to what we did while setting up the grids, click on the custom storey data option.
• Then, specify the base level, plinth level, height of each floor, & the no:of floors to be included in the model.
• We have arranged for a G+3 building with a 1.5m plinth ht. from bae.
• Also, you need to fix a master storey and all the similar storeys in the same menu.
• Storey 1 is master & all other remaining floors are similar storeys.
• Once, you do this, model is set to be executed in ETABS with the grid & storeys as specified.

Defining the Material Properties.

• It is important to set details of the materials you are going to use for the project in ETABS.
• We have to use M30 grade concrete and Fe415 HYSD steel as per the specifications.
• So, in order to define the same, go to define > material properties > add new > concrete/steel > grade > ok.
• However, since we have already specified the Indian standard codes of practices, the M30 concrete and 450 steel is added by default into the project.

Defining section properties.

• We are using the rectangular concrete sections for all the structural elements like beams, columns, and slabs.
• So, we need to define that in ETABS software so that it allows us to use these sections repeatedly throughout the project.
• Go to define > section properties > frame sections > concrete rectangular.
• Then, a dialogue box will appear on your ETABS screen where you can specify the column/beam dimensions and the type of reinforcement (HYSD 415) to be used in the model.
• So, you can define both columns and beams from the same menu.
• Now, to define the slab section, again go to define > section > slab.
• Again, a menu will open up where you need to set the slab specifications required for the project.
• Here, you need to specify the slab thickness as 150mm & the section as a membrane.

BEAM 300 x 400 mm

COLUMN 800x800 mm

SLAB 150 mm

Execute the Modeling.

• Now that we have defined everything from materials to beams, columns & slabs to be needed for our structure, we can go ahead and draw each structural element as per the specifications.
• We can use the commands in ETABS named quick draw tool.
• As the name suggests, you can draw all the beams, floors & columns with the help of this tool.
• Select the relative tool that is quick draw columns for drawing columns and vice versa, then choose the section from the dropdown list.
• Then, simply go to the grid system in plan view and start clicking on the grid locations where you want to build these structural elements.
• So, the same approach was followed and the following beams, floors & slabs were drawn in our ETABS model.

Quick draw columns

Quick draw beams

Quick draw floors

Complete Model in 3D

Define the load patterns.

• After completing the model, now we need to list down the types of loads that are expected on the structure during its service period.
• Go to define > load patterns > add loads.
• Here, you can add all the loads and for our project, we are having the dead load, super dead load (Brick wall load), live load & the earthquake load in x & y directions.
• Also, you need to define the mass source by adding 100% of dead & super dead load as well as 25% of the imposed or live load.

Generate load combinations.

• Go to define > load combinations > check the convertible option.
• A list of load combinations will appear which you can look at and modify anytime during the project if needed.

Assign loads to the structural members.

• You can select a particular structural member from the model and assign it the load that is imposed on it.
• For example, select the beam and assign a UDL of super dead load due to the brick wall resting upon it.
• Thus, a similar technique is used and the loads on beams and slabs are assigned as follows.

10 KN/m brick wall load on beams

3 KN/m² Live load on slabs

Load on structure.

Assign the floor diaphragm

• Select the S150 slab section from the select option.
• Go to assign > shell > diaphragm.
• Select the default D1 diaphragm already incorporated in ETABS and press ok.
• It is now done, and you can see the center of mass for our model has been automatically calculated as we assigned the floor diaphragm.

Save the Model and Run Analysis.

• Now that everything with regards to our first model is complete, we can proceed to run the analysis for this building.
• But we’ll have to first save our project before running the analysis so do it at the preferred location within your system.
• Then, you can simply press F5 on your keyboard or click on the arrowhead available on the ETABS screen to run the analysis for the current model.

Modal Response for Building.

Conclusion:

* Tx1 = 4.416 sec & Ty1 = 4.222 sec & Txy1 = 3.311 sec

* From the table we can say that time period of mode of torsion is less than fundamental mode by IS 1893 provision table 5.

* First 3 modes contribute more than 65% in the mass participating ratios of each plan direction.

* The Modelling is completed and analyzed in Etabs; the results are within given ranges.