Project 1_Comparative study of different storey buildings for Seismic forces

PROJECT 1

Comparative Study of Different Story Buildings for Seismic Forces

AIM: TO USE ETABS SOFTWARE TO MODEL AND ANALYSE MULTIPLE BUILDINGS WITH VARYING FLOOR LEVELS AND DESIGN PARAMETERS AND TO COMPARE THE RESULTS IN ORDER TO UNDERSTAND THEIR OVERALL SEISMIC BEHAVIOUR.

INTRODUCTION:When it comes to assessing how the structure would react as a whole in the event of an earthquake, the structural arrangement of the building is a crucial factor. When identical seismic forces are applied to various building geometries, the resulting behaviours are very unpredictable. For this reason, it is of the utmost importance to have a thorough understanding of the reaction of buildings in relation to the structural configurations of such structures in order to ensure safer design. As a result, for this project, we have used ETABS to model many various types of structures, each of which has a unique geometry. In addition, the participants' replies are analysed with the aid of the software, and the findings are then compared in order to get a better comprehension of the structural behaviour of buildings that have a variety of storeys.

DETAILED PROCEDURE:

This project has been broken up into two stages so that it may be completed more easily. In the first step, we addressed the modelling aspect of the buildings, which entails drawing the buildings' structures from scratch in accordance with the geometrical and structural constraints that have been set in advance. In addition, the first phase itself takes care of carrying out the analysis and producing the structural output with relation to all of the buildings.

In addition, during the second phase of this project, a comparison examination of the structural findings that were achieved during the first half of the project is carried out. In addition, numerous structural attributes of the structures in relation to their seismic reaction are concluded based on the comparison of the data.

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

In order to reach a more all-encompassing finish with this project, a total of ten structures have been included into it. The buildings are shown here in alphabetical sequence, beginning with the letter A and on through the letter K. In the following, we will explain in further detail a comprehensive step-by-step approach that can be used to model all of the buildings and create the relevant structural outputs for each one:

1. BUILDING "A" (G+2)

• Open ETABS and Set up the Model

1. You may launch ETABS by double-clicking the shortcut on your desktop, or you can look for it in the start menu.
2. After the programme has been booted up and is ready to use, choose the "new model" option from the menu.
3. When you click on the new model, a dialogue box will appear. Inside of this box, you will be able to configure the codes and units that will be used in your project.
4. To begin modelling the first building for this project, make the necessary adjustments to the parameters, and then click the "ok" button.

• Setting up the Grid Data

1. To set up the grid data, you need to click on the custom grid option.
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, numbers of grids in both the orthogonal directions, 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. Also, you need to fix a master story and all the similar storeys in the same menu.
5. And for our project, story one is the master and all other remaining floors are similar storeys.
6. Once, you do this, your model is set to be executed in ETABS with the grid and storeys 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 practices, the M30 concrete and 450 steel is added by default into the project.

• Defining Section Properties

1. We are using the 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.

B300X400

C400X400

S150

• Execute the Modeling

1. Since we have just finished defining everything from the materials to the beams, columns, and slabs that will be required for our project, we are now able to proceed with drawing each structural element according to the requirements.
2. To do this, we may make use of the instructions in ETABS that are referred to as the rapid draw tool.
3. With the assistance of this tool, it is possible for you to sketch all of the beams, floors, and columns, as the name says.
4. To draw columns or rows, you only have to choose the appropriate tool, in this case fast draw columns or quick draw rows, respectively. Then make your selection from the corresponding dropdown list.
5. After that, go to the grid system in plan view and start clicking on the grid places where you wish to create these structural pieces. This should be done as quickly as possible.
6. Therefore, the same strategy was used, and the ETABS model was updated to include the beams, floors, and slabs that are listed below.

Quick Draw Columns

Quick Draw Beams

Quik Draw Floors

Complete Model in 3D

• Define the Load Patterns

1. After finishing the model, the next step is to make a list of the different kinds of loads that are likely to be placed on the structure throughout the time it is in service.
2. And to do so, go to define>load patterns>add loads.
3. You are able to input all of the loads in this section. For our project, we are considering the dead load, the super dead load (also known as the brick wall load), the live load, and the earthquake load in both the x and y directions.
4. You also need to specify the mass source by adding 100 percent of the dead load and the super dead load in addition to 25 percent of the load that is imposed or live.

• 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.

• 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 storeys 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.

Converted 5 Story Model

• 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 the show frame and shell loads one at a time.
4. This way the model will show if the corresponding load has been assigned till 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 like we did for the first model.

Check for Shell Loads

Check for the Frame Loads

Diaphragm Check

• 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 particular model, the only thing that has to be changed is the way in which the columns are arranged. The sizes of the columns will not change; however, the orientation of the columns will change from being in the X direction to being in the Y direction. Therefore, carry out the identical steps that we used for the earlier model, and save the result as the C file inside the D file. The next step is to choose the column segment, after which you will assign the local axes and reset the origin to 0. This manner, the Y-direction will be followed by each and every one of the columns. After that, follow standard operating procedure and do a load and diaphragm check before beginning the analysis. After then, feel free to make a note of the modal time period for constructing D.

Therefore, the model was carried out for the building D, and the natural time was recorded in the manner shown below (as was mentioned previously).

:

• 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.

• 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

• C600X600

• Select Bottom 5 Story Columns

• Assign C600x600

• Building Limit for Top 5 Stories

• Select All Columns and Assign them C400x400 Section

• Model After Assigning Different Columns for Different Floors

• 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

• Assign C600X600

• 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

• Select all the Columns and Assign C600x600 Section

• Set Building Limit 20-25

• 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

PHASE II: COMPARATIVE ANALYSIS OF BUILDING PROPERTIES BASED ON THEIR STRUCTURAL OUTPUTS

As we have completed the modeling and analysis of different story buildings in the previous phase of this project, now we are going to compare those results and try to understand the change in behavior of buildings based on their structural geometries. So, a comprehensive comparative analysis of structural properties of different story buildings is executed in the below sections on the basis of various criteria such as stiffness, mass, building height, etc.

• Effect of stiffness on T: Compare fundamental natural periods of buildings E & F as well as G & H. Why is there a marginal or significant difference in the fundamental natural periods?

Comparative Table

Conclusion:

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.

• Effect of mass on T: Compare fundamental natural periods of buildings H, J and K. Have the buildings become more flexible or stiff due to change in mass?

Comparative Table

Conclusion:

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

• Effect of Building Height on T: How do the fundamental natural periods of Buildings A, B, F, and H change with a change in building height?

Comparative Table

Conclusion:

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

• Effect of Column Orientation on T: How do the fundamental natural periods of Buildings B, C, and D change with a change in column orientation?

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.

• Effect of Flexural Stiffness of Structural Elements on mode shapes: Compare fundamental mode shape of Building B in two situations when the flexural stiffness of beams relative to that of adjoining columns is very small versus when it is large.

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.

• Effect of Axial Stiffness of Vertical Members on mode shapes: Compare the fundamental mode shape of Building H in two situations when the axial cross-sectional area of columns is very small versus when it is large.

Conclusion:

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

• Effect of Degree of Fixity at column bases on mode shape: Compare the fundamental mode shape of Building B in two situations when the base of columns is pinned versus when it is fixed.

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. As a result, it can be concluded that the fixed support will give you better control over the fundamental natural period of buildings.