Challenge

Project 1_Comparative study of different storey buildings for Seismic forces

SSamuel Hembrom29 Jul 2024

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 pays 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 modelled 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 and understanding of the structural behavior of buildings with different story.

Detailed Procedure:-

For and easier approach, this project is divided into two phases. In the first phase, we have covered the modelling 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 outputs 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.

Phase One:- Modeling the buildings and generating their respective structural outputs using ETABS:-

There are 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 details as follows:

1.Building "A"(G+2)

Procedure:-

Open the ETABS software and open a new model.

Specify the units as shown in image below.

Grids and Story:-

Provide the grid line dimensions as shown in image below.

Specify the story data as shown in image below.
Since the first building has two floors therefore provide data for plinth, Story 1 & Story 2.

Following is the result for above provided data for grids and story.

Material Properties:-

Next for providing material properties go to Define -> Material Properties.
Add concrete properties having M30 grade and rebar properties having HYSD415 specifications.

Section Properties:-

For Beams section properties follow the following steps.
Go to Define -> Frame Sections.
In "Frame Properties" tab click on "Add New Property" button.

In "Frame Property Shape Type" tab select rectangular section in "Concrete" section.
In "Frame Section Property Data" tab provide property name as "B300X400", material as "M30".
Provide Section Dimension's depth and width as 400mm and 300mm respectively.
Next click on "Modify/Show Rebar" button.

In "Frame Section Property Reinforcement Data" tab in Design Type section select "M3 Design Only (Beam)".
In the Rebar Material section select "HYSDA415" for Longitudinal Bars and Confinement Bars (Ties).

Again in "Frame Properties" tab click on "Add New Property" button for adding column properties.
In "Frame Property Shape Type" tab Select Rectangular Section from "Concrete Section".
Provide property name as C400X400, material as M30.
Provide Section Dimension's depth and width as 400mm and 400mm.
Click on the "Modify/Show Rebar" button.
In the "Frame Section Property Reinforcement Data" tab select "P-M2-M3 Design(Column)" from "Design Type" section.
Select Rebar Materials as "HYSD415" for Longitudinal bars and confinement bars. Leave the other properties as it is.

For slab section go to Define -> Section Properties -> Slab Sections.
In "Slab Properties" tab click on "Add New Property" button.
In "Slab Property Data" tab provide Property Name as "S150", slab material as "M30" and slab thickness as "150mm".

Load Patterns:-

Go to Define -> Load Patterns.
In the "Define Load Patterns" provide the wall loads and seismic loads as shown in image below.

Click on "Modify Lateral Load" button for adding seismic load parameters.
In the next tab select the parameters as shown in the image below.

Mass Source:-

Go to Define -> Mass Source.
Click on "Modify/Show Mass Source" button in Mass Source tab.
In the "Mass Source Data" tab provide 100% of dead and wall load and 25% of live load.

Check the load cases by going to Define -> Load Cases.

Load Combinations:-

Go to Define -> Load Combinations.
In the "Load Combinations" tab click on "Add Default Design Combos" button.
Tick "Concrete Frame Design" box and "Convert to User Combinations" box.

Drawing Columns:-

Select the "Quick Draw Columns" in the left side of the screen.
In the property browser select the property of "C400X400" & make sure to select "All stories" from the drop down menu.
Select the intersecting points of the grids.

Select the columns drawn and go to Edit -> Replicate.
In the "Replicate" menu select "dx" value as 4 and "numbers" as 4.

Drawing Beams:-

Click on "Quick Draw Beams/Columns" icon in left sided of the screen.
In the properties browser select "B300X400", and make sure to select "All Stories" from the drop down menu.

Select the grids for placing the beams.

Select the 1st story following the steps as shown in the image below.

Click on "Quick Draw Floor/Wall(Plan, Elev, 3D)" icon from the menu on the left side of the sceen.
From the properties browser select S150 type and select "Similar Stories" from the drop down menu as shown in image below.

For Assigning Loads:-

Select the beams by going to Select -> Select -> Properties -> Frame Sections.
In the "Select by Frame Property" tab select the "B300X400" type beam.
Next go to Assign -> Frame Loads -> Distributed.
Next in the "Frame Load Assignment - Distributed" tab select "Wall" for the "Load Pattern Name" and set load to 10kN/m.

Next go to Select -> Select -> Properties -> Slab Sections.
In the "Select by Slab Property" click on "S150" type of slab.
Next go to Assign -> Shell Loads -> Uniform.
In the "Shell Load Assignment - Uniform" tab select "Live" as the "Load Pattern Name" and set the load magnitude to 3kN/m^2.

For calculating the center of mass in each floor "diaphragms" has to be assigned in each floor.
For that go to Slab -> Properties -> Slab Sections, and select all the "S150" type of slabs.
Next go to Assign -> Shell -> Diaphragms & select "D1" from the "Shell Assignment - Diaphragms" tab.

For running analysis click on the play button in the icon ribbon under the menu bar as shown in image below.

Once the analysis is done we can see the deformed shape of the building.

2.Building "B"(G+5):-

Now to model building B, we need not to 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:

Go to File -> Save As.
In the "Save Model File As" tab save the file as "Building B (G+5)".

Editing Story Data:-

Go to Edit -> Edit Stories and Grid Systems.
In the "Edit Story and Grid System Data" tab click on "Modify/Show Story Data" button.
Right click on the top story column and go for Add Story -> Keep Existing Story Heights.
In the "Add Story - Keep Existing Story Heights" tab add 3 number of added stories.
Make added stories similar to Story 1.

Check the model before running analysis:-

Now that our model is ready we need to check for live loads and wall loads.
Click on "Display Shell Load Assigns" icon in the icon tab under the menu tab under as shown in image below.

In the "Show Shell Loads" tab choose "Live" as the "Load Pattern".

Again click on "Display Frame Load Assigns.." icon in the icon tab under the menu tab as shown in image below.

In the "Show Frame Loads" tab select "Wall" as the "Load Pattern".

For calculating the center of mass in each floor "diaphragms" has to be assigned in each floor.

For that go to Slab -> Properties -> Slab Sections, and select all the "S150" type of slabs.

Next go to Assign -> Shell -> Diaphragms & select "D1" from the "Shell Assignment - Diaphragms" tab.

For running analysis click on the play button in the icon ribbon under the menu bar as shown in image below.
After the Analysis is done we can see the deformed shape of the building.

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.

Creating copy model by by Save As:-

Like we did for building B, repeat the same for building "C".
Choose the appropriate location and save the file as building "C".
Now the model is set to be edited as needed.

Changing the column direction in x-direction:-

Go to Define -> Section Properties -> Frame Sections.
In the "Frame Properties" tab select the previously used column and click on "Modify/Show Property" button.
Change the "Property Name" to C550X300 and change "Depth" and "Width" to 300mm to 550mm.

For rotating columns along x-axis:-

Go to Select -> Properties -> Frame Sections.
Select the "C550X300" from the next tab.
Next go to Assign -> Frame -> Local Axes.
In the next tab change the angle from 0 degrees to 90 degrees.

Assigning Diaphragms:-

Go to Select -> Properties -> Slab Sections.
In the next section select "S150" type of slab.
Next go to Assign -> Shell -> Diaphragms.
In the next section select "D1" type.

Click on "Run Analysis" button as shown in image below.

We can see the deformed structure.
After the analysis is done we can see the deformed shape of the building.

4. Building "D" (G+5 with column orientation in 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 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 original back to 0. This way, all the column shall be oriented in Y direction.

Creating a new file:-

Go to File -> Save As.
In the next tab save the file as "Building D (G+5)".

Changing Orientation:-

Go to Select -> Properties -> Frame Sections.
In the next tab select "C550X300".
As all the columns are selected go to Assign -> Frame -> Local Axes, in the following tab change the angle to 0 degrees.

Assigning Diaphragms:-

Go to Assign -> Shell -> Diaphragms.
In the next tab select the "D1" type.

Use the "Run Analysis" icon as shown in image above to run an analysis for the structure created.
After the analysis is complete we can see the deformed shape of the structure.

5. Building E(G+10)

Column sizes for top 5 storeys:- 400X400

Column sizes for bottom 5 storeys:- 600X600

Creating a new file:-

Go to File -> Save As.
In the next tab save the file as "Building E (G+10)".

Adding storey:-

Go to Edit -> Edit Stories & Grid Systems.
In the next tab click on the "Modify/Show Story Data".
In the "Story data" tab add more storeys as shown in image below.

Check the model before running analysis:-

Now that our model is ready we need to check for live loads and wall loads.
Click on "Display Shell Load Assigns" icon in the icon tab under the menu tab under as shown in image below.

In the "Show Shell Loads" tab choose "Live" as the "Load Pattern".

Again click on "Display Frame Load Assigns.." icon in the icon tab under the menu tab as shown in image below.

In the "Show Frame Loads" tab select "Wall" as the "Load Pattern".

Creating two different columns(C600X600) & (C400X400):-

For creating columns go to Define -> Section Properties -> Frame Sections.
In the next tab click on "Modify/Show Property" button and modify previous column i.e. C550X300.
In the next tab provide the property name as C600X600, having depth and with of 600mm and 600mm.
In the "Frame Properties" tab click on "Add New Property" button and add a new column with property name as "C400X400" having depth and width as 400 and 400.
Click on "Modify/Show Rebar" button and provide suitable design type and rebar materials. Leave the other specifications as it is.

Assigning Column Properties to different levels:-

First of all we need to hide the slabs and beams in the building.
So in order to do so, click on the "Set Display Options" button in the icon bar as shown in image below.

From the next tab uncheck the beam, floors and layers option box and hit apply and ok.
After doing so we shall not see floors and beams in the structure.

For assigning columns sizes to 6th storey to 10th storey. Go to View -> Set Building View Limits.
In the next tab keep the top story as it is and change the bottom story to story 6 and hit "OK" button.
Set the view to 3-D view and select the columns.

After selecting the columns go to Assign -> Frame -> Section Property.
In the Frame Assignment - Section Property tab select "C400X400" and hit apply.

Set the building view limit from base to top floor again.
We shall see the different column sizes have been assigned to different floor levels.

Make the floors and beams visible once again.

Go to the plan view and assign the diaphragm to the structure and generate a deformed view of the structure.

6.Building F(G+10):-

Consistent Column Size throughout the structure: C600X600.

Save the previous project as "Building F (G+10)".
Go to File -> Save As.

Next go to View -> Set Display Options. From the next tab uncheck the "Beams" & "Floors" box and hit apply and ok button.
Select all the Columns and go to Assign -> Frame -> Section Property.
From the next tab Select "C600X600" type column.

Next go to View -> Set Display Options. From the next tab uncheck the "Beams" & "Floors" box and hit apply and ok button.

In the next tab check the "Beams" & "Floors" box and hit apply and ok.

Click on "Plan" icon in the icon tab as shown in image below.

In order to assigning diaphragm we need to select all the slabs.
For that go to Select -> Properties -> Slab Sections.
In the next tab select "S150" type slab.

Next go to Assign -> Shell -> Diaphragms.
In the next tab select "D1" type diaphragm.

In the icon toolbar click on "3D" button & in the icon toolbar itself click on "Run Analysis" icon.
After the analysis is complete we can see the deformed form of the structure.

7.Building G(G+25):-

Save the previous project as "Building G (G+25)".

For adding more story go to Edit -> Edit Stories and Grid Systems.
In the next tab click on "Modify/Show Story Data" button.
In the "Story Data" tab right click on the vacant area and go to Add Story -> Keep Existing Story Heights.
Provide 15 number of stories and make all the added stories similar to story 1.

Check for live loads and wall loads on the structure.

In this structure we have three type of columns, so we need to add one more type of column.
For that go to Define -> Section Properties -> Frame Section.
In the "Frame Properties" tab click on "Add New Property".
Provide "Property Name" with C800X800, provide depth and width of 800 and 800mm.

Next in order to change the column sizes in different story, we need to hide the beams and slabs.
Go to View -> Set Display Options, in the next tab uncheck the beams and floors box and hit apply and ok.
Next go to View -> Set Building View Limits.
In the "Set Building View Limits" change the Top Story and Bottom Story to Story25 and Story21 respectively.
Next go to Assign -> Frame -> Section Property.
Select the column C400X400.
Repeat the above process for other stories as well.

Assigning Diaphragm property:-

Go for the plan view.

Next go to Select -> Properties -> Slab Sections. In the next tab select "Slab 150".
Go to Assign -> Shell -> Diaphragms. In the next tab select "D1".

Hit the "Run Analysis" button as shown in below image and wait for the deformed shape of the building.

8. Building H (G+25) having column size 800x800 in all floors:-

Save the previous project as Building H(G+25).

Click on "Set Display Option" icon as shown in below image.

In the next tab uncheck the Beams and Floors box.
Select all the columns.

Go to Assign -> Frame -> Section Property. In the next tab select "C800X800".

Again go to View -> Set Display Options. In the next tab check the beam and floors box.

In the icon toolbar click on "Plan" icon as shown in image below.

In order to assigning diaphragm we need to select all the slabs.
For that go to select -> Properties -> Slab Sections.
In the next tab select "S150" type slab.
Next go to Assign -> Shell -> Diaphragms.
In the next tab select "D1" type diaphragm.
In the icon toolbar click on "3D" icon.

In the icon toolbar click on the "Run Analysis" icon.

9.Building J(G+25) with imposed mass 10% larger than building H:-

Save the previous project as Building J(G+25).

For assigning 10% larger imposed mass than model G.
Go to Select -> Properties -> Slab Sections. In the next tab select "S150".
Once all the slabs are selected go to Assign -> Shell Loads -> Uniform.
In the "Shell Load Assignment - Uniform" tab change the load magnitude to 3.3 for live load. Make sure to select the replace existing loads radio button.

To see the plan view of the model click on the "Plan" icon in the icon toolbar. From the next tab select the floor level and click ok and apply,

In order to assign diaphragm. Go to Select -> Properties -> Slab Sections. In the next tab select "S150" type of slab.
Next go to Assign -> Shell -> Diaphragms. In the next tab select D1 type of diaphragm.

In the icon toolbar click on Run Analysis icon. After analysis is complete we can see the deformed form of the model.

10. Building K ( G+25) with imposed mass 20% larger than building H:-

Save the previous project as Building K(G+25).

For assigning the loads first we need to select the slabs.
For that go to Select -> Properties -> Slab Sections. In the next tab select S150 type of slab.
Next go to Assign -> Shell Loads -> Uniform.
In the "Shell Load Assignment - Uniform" tab change the load magnitude to 3.6 for live load. Make sure to select the replace existing loads radio button.
Set the model to plan view for viewing live loads assignment.

For assigning diaphragm. Go to Select -> Properties -> Slab Sections.
Next go to Assign -> Shell -> Diaphragms. In the next tab select D1 type of diaphragm.
In the icon toolbar click on the run analysis button.
After the analysis is complete we can see the deformed model.

Modal Response for Building A:-

Open ETABS file of Building A (G+2).
In the Model Explorer tab go to Tables -> Analysis Results -> Structural Output -> Modal Information -> Table: Modal Period and Frequencies.
Right click on the Table: Modal Period and Frequencies and click on Show Table.
A tab will appear providing the information about the modal periods and frequencies.

Repeat the process for all the buildings.
Modal Response for Building B:-

Modal Response for Building C:-

Modal Response for Building D:-

Modal Response for Building E:-

Modal Response for Building F:-

Modal Response for Building G:-

Modal Response for Building H:-

Modal Response for Building I:-

Modal Response for Building J:-

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:-

MODEL NAME	NO. OF STOREYS	COLUMN DESIGN SPECIFICATIONS	NATURAL PERIOD (SECONDS)
BUILDING E	10	UPPER 5 STOREYS: 400X400 BOTTOM 5 STOREYS: 600 X 600	1.73
BUILDING F	10	CONSISTANT COLUMN SIZE THROUGHOUT: 600X600	1.735
BUILDING G	25	UPPER 5 STOREYS: 400X400 MIDLLE 10 STOREYS: 600X600 BOTTOM 10 STOREYS: 800X800	4.17
BUILDING H	25	CONSISTANT COLUMN SIZE THROUGHOUT: 800X800	4.416

Conclusion:-

For buildings E and F, varying column design doesn't make a big difference as there are 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 help, 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 becomes more flexible or stiff due to change in mass?

Comparative table

MODEL NAME	NO. OF STOREYS	PERCENTAGE INCREASE IN MASS	NATURAL PERIOD (SECONDS)
BUILDING H	25	Regular Mass with no increment	4.416
BUILDING J	25	10% mass increment	4.819
BUILDING K	25	20% mass increment	4.854

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 the both 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 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 building increases regardless of its structural geometry.

Effect of Degree of Fixity at column base 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 time period. As a result, it can be concluded that the fixed support will give you better control over the fundamental natural period of buildings.

Project 1_Comparative study of different storey buildings for Seismic forces

Ship work like this
in your own course.

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Ship work like thisin your own course.

Talk to a career counsellor

Ship work like this
in your own course.