Project 1_Comparative study of different storey buildings for Seismic forces

AIM: 

TO model and analyse multiple buildings with  different floor levels and design parameters using etabs software and compare the result to understand their overall seismic behaviour. 

PROCEDURE: 

For an easier approach, this project is divided into two phases. In the first phase, we have covered 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 in the first phase itself. 

PHASE ONE: Modeling the Buildings and Generating their Respective Structural Outputs using ETABS

There are a total of 10 buildings included in this project to achieve a more comprehensive conclusion. The buildings are listed in alphabetical order starting from A and ending at K. A complete stepwise procedure to model all the buildings and generate their respective structural outputs is discussed in detail as follows. 

BUILDING A (G+2) 

• Open ETABS from the desktop shortcut or by searching for it from the start menu.
• Once the software is up and 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 set up 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 between all the vertical and horizontal grids, etc.
• So, by following the same approach, we have set up a 4 x 3 bay grid with 4m center to center spacing in-between. 

Fixing the Story Data

• Similar to what we did while setting up the grids, click on the custom story data option.
• Then, specify the base level, plinth level, height of each floor, and the number of floors to be included in the model.
• Thus, we have arranged for a G+2 building with a 1.5m plinth height from the base.
• And for our project, story one is the master and all other remaining floors are similar storeys.
• Once, you do this, your model is set to be executed in ETABS with the grid and storeys as specified. 

Defining the Material Properties 

• we have to use the 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.
• And to do so, 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 (HYSD415) 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.
• Here, you need to specify the slab thickness as 150mm and the section as a membrane. 

Execute the Modeling

• Now that we have defined everything from materials to beams, columns, and slabs to be needed for our structure, we can go ahead and draw each structural element as per the specifications.
• And to do that, we can use the commands in ETABS named quick draw tool.
• As the name suggests, you can draw all the beams, floors, and columns with the help of this tool. 

Define the Load Patterns

• After completing the model, now we need to list down the types loads 
• And to do so, 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 
• Also, you need to define the mass source by adding 100% of dead and super dead load as well as 25% of the imposed or live load. 

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.

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. 

2. Building B (G+5) 

Now to model building B, we need not create a new ETABS file from scratch. Rather we will save a copy of the building A file and do the alterations needed for building B. 

Make a Copy by Save as

• From model A, go to files and click on the save as option.
• Set the destination folder and name it Building B

Edit Story Data

• From the new file, go to edit>story and grid data.
• Click on the modify story data option.
• Then, right-click on the top story and select the add story option.
• Set the height and the number of storeys you want to add.
• In our case, we need to add 3 more floors to convert our 2 story building into a 5 story one. 

Run  Analysis

• Now that all the checks have been made and the structure is ok, we can run the analysis.
• It will take a few seconds and the deflected shape of the building will appear indicating that the analysis has been completed. 

3.BUILDING C (G+5 with column orientation in X-direction)

The story and grid data for building c will remain still. However, we need to change the column size and orient the same in the X-direction. So, the complete stepwise procedure for the same is discussed in detail below 

Edit the Column Size and Orientation

• To edit the column section, go to define>section properties>frame section.
• Now, select the previously created column for building A and B.
• Click on the modify/show properties option.
• Rename the column as needed which in our case is 550x300.
• Then, edit the length and breadth by entering the values in the box.
• Now, we need to orient the 550mm in the X-direction.
• So, select the column section then go to assign>local axis>rotate>90 degrees.
• This way, all the columns shall be oriented in the X-direction meaning 550mm laying down in the x direction. 

Checks Before Running the Analysis

Like we did for building B, checks for the frame and shell loads are to be made. 

 Run the Analysis

• Hit the F5 button on your keyboard and ETABS will automatically run the analysis for you. 

4.Building D (G+5 WITH COLUMN ORIENTATION IN THE Y-DIRECTION) 

In this model, we don't need to alter anything other than the column orientation. The sizes of the column will remain the same however, we have to shift the column orientation from X to Y direction 

Select the Column Section and Bring the Orientation Back to Zero

The detailed procedure to do it has already been discussed for the previous model 

5.Building E (G+10) 

Column sizes for top 5 storeys: 400x400

Column sizes for bottom 5 storeys: 600x600 

Change the View Settings of the Model

• There's an option in ETABS that allows users to adjust the model and view the only objects that are needed to be worked on currently.
• The tool is called view option settings. It is easily accessible from the front tool ribbon of ETABS. 

Set the Building Limit, Select Columns, and Assign them the Required Sections

• Now, since we need different columns for the top 5 floors and different columns for the bottom five.
• Therefore, we need to edit the top 5 and bottom 5 stories one after the other. 

6.Building F(G+10) 

Consistent Column size throughout the structure: C600X600 

7.Building G(G+25) 

Columns for top 5 stories: C400X400

Columns for middle 10 stories: C600x600

Columns for bottom 10 stories: C800x800 

8.Building H (G+25) 

Consistent column size throughout the building: C800X800

9.Building J(G+25) 

10% increment in the imposed loads

PROCEDURE: 

Building J is the exact copy of the model H and we don't need to alter anything related to the modeling. However, we need to increase the imposed load on the building by 10% which is assigned as 3KN/m^2 on all slabs of the building. 

Go to Assign>Shell loads>Uniform>increase it by 10%

• We have assigned a 3KN/m^2 imposed load on all slabs for all of our previous buildings.
• So, we need to increase that by 10% meaning we need to replace the existing imposed load with 3.3 KN/m^2 

10.Building K(G+25) 

20% increment in the imposed loads 

PROCEDURE: 

For this model, we can follow the exact procedure as we did for building j. However this time, we need to increase the live load by 20% as opposed to 10%.

 Comparative Table conclusion for (E and F) (G and H) 

For buildings E and F, varying column design doesn't make a big difference as there are only five stories. However, we can see that for buildings G and H, the consistent column size throughout the structure causes a more natural period. Therefore, it can be concluded that providing larger columns at the base and then reducing the size on top floors helps control the natural period 'T' for tall buildings. 

Comparative table T for H,J and k Conclusion: 

As seen from the table, the natural period for the tall buildings is directly proportional to the imposed mass of the building. 

Comparative table T for A,B,F AND H Conclusion :

As indicated in the above table, the natural period 'T' is also directly proportional to the building height.

Comparatibe table orientation T for B,C AND D Conclusion : 

When the column cross-section is square, there's not much difference between the lateral displacement in both the orthogonal directions. However, when you orient the column in a particular direction be it X or Y, the lateral displacement in that particular direction will be less than that in the other direction which has a lesser column length. 

Flexural stiffness of structural elements on mode shape B ,comparative Table conclusion : 

It is pretty obvious that if you decrease the flexural stiffness of beams, the overall flexural strength of the beam is also going to reduce, and consequently, the fundamental natural period will shoot up. 

Axial stiffness of vertical members on shape T ,comparative Table conclusion : 

As the axial stiffness of columns decreases, the fundamental natural period of the building increases regardless of its structural geometry 

Effective of degree of fixity ar column base on mode shape B ,comparative Table conclusion : 

It is evident that pinned support offers fewer restraints compared to the fixed one which leads to more movement and thus increased natural period