Project 1_Comparative study of different storey buildings for Seismic forces

AIM:

To Model and Analyze multiple buildings with different floor levels and design parameters using Etabs software and compare the results to understand their overall Seismic behavior.

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:

• Open ETABS and Set up the Model

1. Once the software is up and running, click on the new model option.
2. Once you click on the new model, a dialogue box will pop up where you can set up the codes and units used in your project.
3. Thus, do the needful settings and click ok to start modeling the first building in this project.

• Setting up the Grid Data

1. You need to click on the custom grid option to set up the grid data.
2. Once you do that, a menu with all the grid settings will open up.
3. In the same menu, you can fix the length, the number of grids in both the orthogonal directions, the spacing between all the vertical and horizontal grids, etc.
4. 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

1. Similar to what we did while setting up the grids, click on the custom story data option.
2. Then, specify the base level, plinth level, height of each floor, and the number of floors to be included in the model.
3. Thus, we have arranged for a G+2 building with a 1.5m plinth height from the base
4. Once, you do this, your model is set to be executed in ETABS with the grid and stories as specified.

• Defining the Material Properties

1. It is important to set details of the materials you are going to use for the project in ETABS.
2. And for our model, we have to use the M30 grade concrete and Fe415 HYSD steel as per the specifications.
3. So, in order to define the same, go to define>material>properties>add new> concrete/steel> grade> ok.
4. However, since we have already specified the Indian Standard Codes of Practice, the M30 concrete and 415 steel are added by default into the project.

• Defining Section Properties

1. We are using rectangular concrete sections for all the structural elements like beams, columns, and slabs.
2. So, we need to define that in ETABS software so that it allows us to use these sections repeatedly throughout the project.
3. And to do so, go to define>section properties>frame sections>concrete rectangular.
4. 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.
5. So, you can define both columns and beams from the same menu.
6. Now, to define the slab section, again go to define>section>slab.
7. Again a menu will open up where you need to set the slab specifications required for the project.
8. Here, you need to specify the slab thickness as 150mm and the section as a membrane.

• Execute the Modeling

1. 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.
2. And to do that, we can use the commands in ETABS named quick draw tool.
3. As the name suggests, you can draw all the beams, floors, and columns with the help of this tool.
4. All you need to do is select the relative tool that is quick draw columns for drawing columns and vice versa. Then choose the section from the dropdown list.
5. 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.
6. So, the same approach was followed and the following beams, floors, and slabs were drawn in our ETABS model.

Quick Draw Columns

Quick Draw Beams

Quik Draw Floors

• Define the Load Patterns

1. After completing the model, now we need to list down the types of loads that are expected on the structure during its service period.
2. And to do so, go to define>load patterns>add loads.

• Generate Load Combinations

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

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• Assign Loads to the Structural Members

1. You can select a particular structural member from the model and assign it the load that is imposed on it.
2. For example, select the beam and assign a udl of super dead load due to the brick walls resting upon it.
3. 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/m^2 Live Load on Slabs

• Assign the Floor Diaphragm

1. Select the S150 slab section from the select option.
2. Go to assign>shell>diaphragm.
3. Select the default D1 diaphragm already incorporated in ETABS and press ok.
4. 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

1. Now that everything with regards to our first model is complete, we can proceed to run the analysis for this building.
2. But we'll have to first save our project before running the analysis so do it at the preferred location within your system.
3. 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 A

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. For example, we can increase the number of stories to 5 by editing the story data within the copied file. So, the complete procedure to do so in ETABS is discussed below:

• Make a Copy by "Save as"

1. From model A, go to files and click on the save as option.
2. Set the destination folder and name it "BUILDING B".
3. The model is set for changes as per the requirements of building b

• Edit Story Data

1. From the new file, go to edit>story and grid data.
2. Click on the modify story data option.
3. Then, right-click on the top story and select the add story option.
4. Set the height and the number of stories you want to add.
5. In our case, we need to add 3 more floors to convert our 2-story building into a 5-story one.
6. Also, make sure to make each story similar to the master story created in Model A.

• Check the Model before Running Analysis

1. Now that our model is ready we need to check for two to three things before running the analysis and noting the modal period.
2. The first check is for the load assignment.
3. From the bottom ribbon in ETABS, go to the show frame and shell loads one at a time.
4. This way the model will show if the corresponding load has been assigned to the top floor.
5. Similarly, you need to check if the diaphragm has been assigned to the new model or not.
6. You can check it by going to the view settings option and checking the diaphragm visibility.
7. If the diaphragm is not visible, you need to select all the floors and assign the diaphragm as we did for the first model.

Check for the Frame Loads

• Run the Analysis

1. Now that all the checks have been made and the structure is ok, we can run the analysis.
2. And to do that simply press the F5 button on your keyboard or click on the arrowhead available at the bottom ribbon in ETABS.
3. It will take a few seconds and the deflected shape of the building will appear indicating that the analysis has been completed.

Modal Response for Building B

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:

• Create a Copy Model by Save As

1. Like we did for building B, now do it for building C through building B.
2. Choose the appropriate location and save the file as building c.
3. Now, the model is set to be edited as needed.

• Edit the Column Size and Orientation

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

Required Column Section

• Checks Before Running the Analysis

1. Like we did for building B, checks for the frame and shell loads are to be made.
2. Also, make sure the floor diaphragm has also been assigned before going ahead with the analysis.

Check for the Frame loads

Check for the Shell Loads

Diaphragm Check

• Run the Analysis

1. Hit the F5 button on your keyboard and ETABS will automatically run the analysis for you.
2. You can display the modal table when the analysis is complete and the deflected shape of the model is visible on your screen.

Modal Response for Building C

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. So follow the same procedure as we did for the previous model and save as the C file into the D file. Then, select the column section and assign the local axes and bring the origin back to 0. This way, all the columns shall be oriented in the Y-direction. Then like usual, check for the loads and the diaphragm before running the analysis. Then, go ahead and note the modal period for building D.

So, as explained above, the model was executed for building D and the natural period was noted as inserted below:

• 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)

Required Column Section

Run the Analysis

Modal Response for Building D

5. BUILDING "E" (G+10)

Column sizes for top 5 storeys: 400x400

Column sizes for bottom 5 storeys: 600x600

PROCEDURE:

In this model, we need to add up 5 more storeys to our model. In addition, we also need to assign different column sizes on different floors. And to do that, we'll first save the ETABS file for building D as a new model for building E. Then, edit the story data as we have done for our previous models in ETABS. Then, we need to add two new column sections with the required directions and assign them at appropriate floors in our model as specified.

So, the stepwise procedure to create new columns and locate them on required floors is explained below:

• Add up 5 New Stories and Set the Model for Building E

• 

• Change the View Settings of the Model

1. 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.
2. The tool is called view option settings. It is easily accessible from the front tool ribbon of ETABS.
3. So, you can go to view option settings and uncheck the structural members that you want to hide currently for modeling purposes.
4. Since we need to assign different column sizes in our model, we don't need to view beams and slabs currently.
5. So, we go to the tool and uncheck beams and floors in order to make the only columns visible for editing purposes.
6. 

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

1. Now, since we need different columns for the top 5 floors and different columns for the bottom five.
2. Therefore, we need to edit the top 5 and bottom 5 stories one after the other.
3. So, we need first set the building limits up to the bottom five floors and then vice versa.
4. And to do that, you can go to edit>set building limits.
5. The set building limit option in ETABS allows you to view only the floors that you need to work on.
6. Hence, first set the limit to bottom 5>select all columns by holding the left-click and making the box around the entire model>go to assign and assign them the required column section.
7. Repeat the procedure for the remaining top columns and complete with the model.

• 

• Building Limits
• 

• C400X400

• Select Bottom 5 Story Columns

• Run the Analysis

• Modal Response for Building E

6. BUILDING "F" (G+10)

Consistent Column size throughout the structure: C600X600

PROCEDURE:

This building has the same columns for the entire structure. Therefore, we don't need to do much editing in terms of the sections. We just need to adjust the view settings>hide beams and floors>select all the columns>assign them the frame section properties of C600x600.

• Adjust View Option Settings

• Select all the Columns

• Modal Response for Building F

7. BUILDING G (G+25)

Columns for top 5 stories: C400X400

Columns for middle 10 stories: C600x600

Columns for bottom 10 stories: C800x800

PROCEDURE:

For this model, we need to add 15 new stories and assign different column sizes on different floors. We can follow the exact procedure that has already been explained for the previous project to create this model by editing building F. The same is discussed below:

• Edit the Story Data and Add up 15 New Stories

• View Settings

• Set Building Limit from 1-10

• Select all the Columns and Assign C800x800 Section

• Set Building Limit 11-20
• Set Building Limit 20-25

• Select all the Columns and Assign C600x600 Section
• Select all the Columns and Assign C400x400

• Run the Analysis

• Modal Response for Building G

8. BUILDING "H" (G+25)

Consistent column size throughout the building: C800X800

PROCEDURE:

This is the same model as the previous one. All we need to do is assign the column section as C800x800 for the entire structure which can be done by simply selecting all the columns>go to assign>select C800x800>click ok and done. Thus the same procedure is listed down below for better understanding.

• View Settings to Hide Beams and Floors

• Select All the Columns and Assign C800x800 Section

• Run the Analysis

• Modal Response for Building H

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. So, the procedure to do the same is listed down below:

• Select the S150 Slab Section

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

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

• Run the Analysis

• Modal Response for Building J

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%. So, the stepwise procedure has been listed down below:

• Select the S150 Slab section
• Assign 3.6 KN/m^2 distributed shell load for a 20% increase

• Run the Analysis

• Modal Response for Building K

RESULT:

• Effect of stiffness on time period on buildings.

Criteria	Model	Analysis result	Conclusion
Effect of stiffness on time period on buildings.	Building-E (10 storeys with varying column sizes)	1.755 sec	For low-rise buildings, changing the column size does not have much effect on the time period of the building. As you see in the analysis result, the time period is nearly the same.
	Building-F (10 storeys with uniform column sizes)	1.735 sec
Effect of stiffness on time period on buildings.	Building-G (25 storeys with varying column sizes)	4.17 sec	For High-rise buildings, changing the column size has many effects on the time period of the building. As you see in the analysis result, the time period of the two buildings has much different.
	Building-H (25 storeys with uniform column sizes)	4.416

• Effect of mass on time period on buildings.

Criteria	Model	Analysis result	Conclusion
Effect of mass on time period on buildings.	Building-H (25 storeys with 3KN/m2 imposed load)	4.416 sec
	Building-J (25 storeys with 3.3KN/m2 imposed load)	4.425 sec	imposed load is distributed in the building uniformly.
	Building-K (25 storeys with 3.6KN/m2 imposed load)	4.435 sec

• Effect of building height on time period on buildings.

Criteria	Model	Analysis result	Conclusion
Effect of building height on time period on buildings.	Building-A ( 2 storeys building)	0.5 sec	As per IS 1893: 2016we know the time period is directly proportional to the height of the building so we clearly see from the analysis result that the time period of buildings increases as the height of buildings increases.
	Building-B (5 storeys building)	1.036 sec
	Building-F (10 storeys building)	1.735 sec
	Building-H (25 storeys building)	4.416 sec

• Effect of column orientation on time period on buildings.

Criteria	Model	Analysis result	Conclusion
Effect of column orientation on time period on buildings.	Building-B ( column 400x400)	1.1 sec - x direction 1.036 sec- y direction	As column size is the same in both X and Y directions, there is not much difference in their time period. As stiffness is the same in both directions.
	Building-C ( column 550x300)	0.926 sec - x direction 1.145 sec- y direction	As column size is the not same in both X and Y directions, there is much difference in their time period. As stiffness is not the same in both directions.
	Building-D (column 300x550)	1.12 sec - x direction 0.953 sec- y direction

• Effect of Flexural Stiffness of Structural Elements on mode shapes

Criteria	Model	Analysis result	Conclusion
Effect of Flexural Stiffness of Structural Elements on mode shapes	Building-B (Flexural stiffness of beams close to zero takes 10% of it.)	2.627 sec
	Building-B ( full flexural stiffness of beams)	1.036 sec

• Effect of Axial Stiffness of Vertical Members on mode shapes

Criteria	Model	Analysis result	Conclusion
Effect of Axial Stiffness of Vertical Members on mode shapes	Building-H (Axial stiffness of column close to zero takes 10% of it.)	5.991 sec
	Building-H ( full axial stiffness of columns)	4.416 sec

• Effect of Degree of Fixity at column bases on mode shape

Criteria	Model	Analysis result	Conclusion
Effect of Degree of Fixity at column bases on mode shape	Building-B (pinned)	1.036 sec
	Building-B ( fixed base)	0.941 sec