Project 1_Comparative study of different storey buildings for Seismic forces

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 configurations for a safer design approach. Therefore in this project, we have modeled different buildings with varying geometries using ETABS. Furthermore, their responses are analyzed with the help of the software and the results are then compared to grasp an understanding of the structural behavior of buildings with different storeys.

DETAILED 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.

Moreover, a comparative analysis of the structural results obtained in the first half is executed in the second phase of this project. And based on the comparative results, various structural properties of the buildings pertaining to their seismic response are concluded.

1. BUILDING "A" (G+2)

• Open ETABS and Set up the Model

1. Open ETABS from the desktop shortcut or by searching for it from the start menu.
2. Once the software is up and running, click on the new model option.
3. 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.
4. Thus, do the needful settings and click ok to start modeling the first building in this project.

• 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

S150

• 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.

. 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.

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.

• 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.

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.

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 5

• 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.

RESULTS:

i)COMPARISON:

(a) Effect of stiffness on the time period between building E and building F:

Building E (This is a 10-story building with different column sizes):

Modal frequency and analysis: 2.74 s


Building F (this is a 10-story building with the same size columns):

Modal frequency and analysis: 2.79 s.


(b) Effect of stiffness on the time period between building G and building H:

Building G (25-story building with different column sizes):

Modal periods and frequencies: 6.66 s.


Building H (25 story building with the same column size):

Modal periods and frequencies: 7.07 s.


- If we compare the above buildings with different column sizes and different column sizes of 10-story buildings, we can see that there are large differences between their modal periods and frequencies.

- But if we see building G and building H with different column sizes and with the same column sizes as the 25-story building, we can see that building G with different column sizes is more stable than building H with the same column sizes.

- Because with the same column sizes, the mass is concentrated in the upper floors. And the weight is directly proportional to the lateral load acting on the building, which causes more lateral displacement of the building, resulting in a longer period of time.

(ii) COMPARISON:

(a) Effect of MASS on the time period between building H, building K, and building J:

Building H (25-story building with the same size columns with a useful load of 3Kn/m2):

Modal periods and frequencies: 7.07 s.


Building J (25-storey building with a useful load of 3.3 kn/m2):

Modal periods and frequencies: 7.09 s.


Building K (25-storey building with a useful load of 3.6 kn/m2):

Modal periods and frequencies: 7.10 s.


- As we can see after observing their modal periods and frequencies of these three buildings, we observe that there is not much difference between the time periods of these buildings.

- We have an even load, so there is a difference, but not that big.

(iii) COMPARISON:

(a) Effect of building height on the time period between Building A, Building B, Building F, and Building H:

Building A (this is a 2-story building):

Modal periods and frequencies: 0.68 s.


Building B (it is a 5-story building):

Modal periods and frequencies: 0.78 s.


Building F (This is a 10-story building):

Modal periods and frequencies: 2.79 s


Building H (it is, among other things, a 25-story building):

Modal periods and frequencies: 7.07 s.


- After observing the modal periods and frequencies of the given buildings, we can say that the time period increases with the increasing height of the building.

- It is obvious that if the height of the building increases, the time periods of the building also increase. It can be said that the height of the building is directly proportional to the time period according to the IS code, so the construction time increases with the height.

(iv) COMPARISON:

(a) Effect of column orientation on the time period between Building B, Building C, and Building D:

Building B (This is a 5-story building with column dimensions of 400X400 mm):

Modal periods and frequencies: 0.78 s.


Building C (This is a 5-story building with column dimensions of 550X300 mm, which is oriented in the X direction):

Modal periods and frequencies: 0.87 s.


Building D (This is a 5-story building with column dimensions of 300X550 mm, which is oriented in the Y direction):

Modal periods and frequencies: 2.74 s.


- For building B, we have the same 400X400mm column size throughout the building, so the response of the building in each direction should be similar. Due to the same size of the columns in both directions i.e. X and Y direction.

- For building C we have larger size of columns in X direction i.e. 550X300mm and for building D we provide larger size of columns in Y direction i.e. 300X550mm, that means floor deflection will be higher in building D than in building C.

(v) COMPARISON:

(a) Effect of flexural stiffness of structural elements on the shapes of building B shape:

Flexural stiffness of structural elements on mode shapes:

Bending stiffness of beams close to zero (zero inertia): 2.74 s.

Full bending stiffness of the beam: 0.78 s.
- From the above observation, we can see that if we do not consider the stiffness of the beam, we get more than double the actual time.

(vi) COMPARISON:

(a) Effect of axial stiffness of vertical members on building shape shapes H:

The axial cross-section for columns is close to NIL:
Modal periods and frequencies: 8.14 s.




Full axial section for columns:
Modal periods and frequencies: 7.07 s.


- After observing from the above, we can say that the axial stiffness plays an important role in the time period.

(vii) COMPARISON:

(a) Effect of degree of strength on column based on building mode shapes B:

Column supports are attached:
Modal periods and frequencies: 0.78 s.


Column supports are fixed:
Modal periods and frequencies: 0.75 s.


- From the above observation, we can conclude that the greater the column fixation, the shorter the time period and the less it will deviate.

- So the time period is shortened if the condition changes from pinned to fixed.