Project 1_Comparative study of different storey buildings for Seismic forces

PROJECT 1

Aim: The main aim of this challenge is to gain the knowledge about the concept of extracting the different seismic responses for a structure and to alter the properties of the members in the ETABS software. The factors that are influencing the dynamic characteristics of the building and the factors influencing the time period of the building and the factors influencing the mode shapes of the building are learnt in this project.

Structural Element Sizes:

• Beams : 300 × 400 mm
• Columns : 400 × 400 mm
• Slab : 150 mm thick

Material Properties:

• Grade of Concrete : M30
• Grade of Steel Reinforcement Bars : Fe 415

Loading:

• Dead Load on beams from infill wall : 10 kN/m (Let us assume the same for roof floor in view of parapet)
• Live load on floor : 3 kN/m2 (Let us assume the same for roof floor)

Buildings are assumed to be pinned at the base

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 modelled different buildings with varying geometries using ETABS. Furthermore, their responses are analysed with the help of the software and the results are then compared to grasp an understanding of the structural behaviour 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 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 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

PHASE ONE: Modelling 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:

1. BUILDING "A" (G+2)

• Open ETABS and Set up the Model
• 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.
• Thus, do the needful settings and click ok to start modelling the first building in this project.

2. 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 centre to centre spacing in-between
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3. 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.
• Also, you need to fix a master story and all the similar storeys in the same menu.
• 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
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• 3 - D VIEW OF MODEL A

4. Defining the Material Properties

• It is important to set details of the materials you are going to use for the project in ETABS.
• And for our model, 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
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• DEFINING CONCRETE PROPERTY
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• DEFINING REBAR PROPERTY

5. Defining Section Properties

• We are using the rectangular concrete sections for all the structural elements like beams, columns, and slabs.
• So, we need to define that in ETABS software so that it allows us to use these sections repeatedly throughout the project.
• 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.
• Again a menu will open up where you need to set the slab specifications required for the project.
• Here, you need to specify the slab thickness as 150mm and the section as a membrane
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• DEFINING BEAM PROPERTY
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• DEFINING COLUMN PROPERTY
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• DEFINING SLAB PROPER

6. Execute the Modelling

• 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.
• 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.
• 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.
• So, the same approach was followed and the following beams, floors, and slabs were drawn in our ETABS model.

7. Define the Load Patterns

• After completing the model, now we need to list down the types of loads that are expected on the structure during its service period.
• 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, and the earthquake load in x and y directions.
• 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
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• DEFINING OF LOADS
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• DEFINING EQX
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• DEFINING EQY

8. Generate Load Combinations

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

9. Assign Loads to the Structural Members

• You can select a particular structural member from the model and assign it the load that is imposed on it.
• For example, select the beam and assign a udl of super dead load due to the brick walls resting upon it.
• Thus, a similar technique is used and the loads on beams and slabs are assigned as follows:

10. 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.
• 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.
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• ASSININING OF FLOOR DIAPHRAGM

11. 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.
• But we'll have to first save our project before running the analysis so do it at the preferred location within your system.
• 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
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• TIME PERIOD TABLE FOR MODEL 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"

• From model A, go to files and click on the save as option.
• Set the destination folder and name it "BUILDING B".
• The model is set for changes as per the requirements of 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.
• Also, make sure to make each story similar to the master story created in Model A

Check the Model before Running Analysis

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

Run the Analysis

• Now that all the checks have been made and the structure is ok, we can run the analysis.
• And to do that simply press the F5 button on your keyboard or click on the arrowhead available at the bottom ribbon in ETABS.
• It will take a few seconds and the deflected shape of the building will appear indicating that the analysis has been completed.
• Repeat the same procedure for all the 10 models that are to be modelled and compare the results.
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• 3 - D VIEW OF MODEL B
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• TIME PERIOD TABLE FOR MODEL B
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• PLAN VIEW OF MODEL C
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• TIME PERIOD TABLE FOR MODEL C
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• PLAN VIEW OF MODEL D
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• TIME PERIOD TABLE FOR MODEL D
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• 3 - D VIEW OF MODEL E
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• TIME PERIOD TABLE FOR MODEL E
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• 3 - D VIEW OF MODEL F
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• TIME PERIOD TABLE FOR MODEL F
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• 3 - D VIEW OF MODEL G
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• TIME PERIOD TABLE FOR MODEL G
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• TIME PERIOD TABLE FOR MODEL H
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• TIME PERIOD TABLE FOR MODEL J
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• TIME PERIOD TABLE FOR MODEL K

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

As we have completed the modelling 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 behaviour 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.

1. 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?
2. 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?
3. 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?
4. 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?
5. 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.
6. 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.
7. 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.

CRITERIA	MODEL	ANALYSIS REDULT	CONCLUSION
Effect of stiffness on time period	Building – E (10 storey with varying column size)	1.73s	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.
	Building – F (10 storey with uniform column size)	1.735s
	Building – G (25 storey with varying column size)	4.17s
	Building – H (25 storey with uniform column size)	4.416s
Effect of mass on time period	Building – H (25 storey with 3kN/mm2 imposed load)	4.416s	As seen from the table, the natural period for the tall buildings is directly proportional to the imposed mass of the building
	Building – J (25 storey with 3.3kN/mm2 imposed load)	4.425s
	Building – K (25 storey with 3.6kN/mm2 imposed load)	4.435s
Effect of building height on time period	Building – A (2 Storey)	0.5s	As indicated in the above table, the natural period 'T' is also directly proportional to the building height
	Building – B (5 Storey)	1.036s
	Building – F (10 Storey)	1.735s
	Building – H (25 Storey)	4.416s
Effect of column orientation on time period	Building – B (All floors column sizes are 400*400)	1.036s	When the column cross-section is square, there's not much difference between the lateral displacements 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
	Building – C (All floors column sizes are 550*300)	0.874s
	Building – D (All floors column sizes are 300*550)	1.12s

CRITERIA	MODEL	CASE	ANALYSIS RESULT	CONCLUSION
Effect of flexural stiffness of structural elements on mode shapes	Building – B	Flexural stiffness of beams to nil (zero inertia) (10%)	2.627s	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.
		Full Flexural stiffness of beams to nil (regular) (100%)	1.03s
Effect of axial stiffness of vertical members on mode shapes	Building – H	Axial cross section for columns is close to nil (zero inertia) (10%)	5.99s	As the axial stiffness of columns decreases, the fundamental natural period of the building increases regardless of its structural geometry
		Full Axial cross section for columns (Regular) 100%	4.41s
Effect of degree of fixity of column base on mode shape	Building - B	Column support pinned	1.03s	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
		Column support fixed	0.941

CRITERIA	MODEL	MAXIMUM STOREY DRIFT DUE TO EQX	MAXIMUM STOREY DRIFT DUE TO EQY	CONCLUSION
Effect of column orientation on storey drift	Building – B (All floors column sizes are 400*400)	2.25	2.3	When the column cross-section is square, there's not much difference between the lateral displacements 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
	Building – C (All floors column sizes are 550*300)	2.1	2.5
	Building – D (All floors column sizes are 300*550)	2.45	2.15

Conclusion:

From the challenge carried out it can be concluded that the concept of extracting the different seismic responses for a structure and to alter the properties of the members in the ETABS software. The factors that are influencing the dynamic characteristics of the building and the factors influencing the time period of the building and the factors influencing the mode shapes of the building are learnt in this project.

Vijayalakshmi S H