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 BEHAVIOUR.
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 modeling the first building in this project.
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 center to center spacing in-between.
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.
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 then click 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.
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.
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 slab section.
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.
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.
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.
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 pattern.
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.
Go to define load combination.
A list of load combinations will appear which you can look at and modify anytime during the project if needed.
You can select a particular structural member from the model and assign it the load that is imposed on it.
10 KN/m brick Wall Load on Beams
3 KN/m^2 Live Load on Slabs
Go to assign 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.
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.
2) BUILDING "B" (G+5)
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
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.
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.
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.
3) BUILDING "C" (G+5 with column orientation in X-direction)
Like we did for building B, now do it for building C through building B.
Choose the appropriate location and save the file as building c.
Now, the model is set to be edited as needed.
Edit the Column Size and Orientation.
To edit the column section, go to define section properties.
Now, select the previously created column for building A and B.
Click on the modify/show properties option.
Rename the column as needed which in our case is 550x300.
Then, edit the length and breadth by entering the values in the box.
Now, we need to orient the 550mm in the X-direction.
So, select the column section then go to assign>local axis>rotate>90 degrees.
This way, all the columns shall be oriented in the X-direction meaning 550mm laying down in the x direction.
Like we did for building B, checks for the frame and shell loads are to be made.
Also, make sure the floor diaphragm has also been assigned before going ahead with the analysis.
Hit the F5 button on your keyboard and ETABS will automatically run the analysis for you.
You can display the modal table when the analysis is complete and the deflected shape of the model is visible on your screen.
4. BUILDING "D" (G+5 WITH COLUMN ORIENTATION IN THE Y-DIRECTION)
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)
5. BUILDING "E" (G+10)
Add up 5 New Stories and Set the Model for Building E
Change the View Settings of the Model
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.
The tool is called view option settings. It is easily accessible from the front tool ribbon of ETABS.
So, you can go to view option settings and uncheck the structural members that you want to hide currently for modeling purposes.
Since we need to assign different column sizes in our model, we don't need to view beams and slabs currently.
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
Now, since we need different columns for the top 5 floors and different columns for the bottom five.
Therefore, we need to edit the top 5 and bottom 5 stories one after the other.
So, we need first set the building limits up to the bottom five floors and then vice versa.
And to do that, you can go to edit>set building limits.
The set building limit option in ETABS allows you to view only the floors that you need to work on.
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.
Repeat the procedure for the remaining top columns and complete with the model.
6. BUILDING "F" (G+10)
Consistent Column size throughout the structure: C600X600
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.
7. BUILDING G (G+25)
Columns for top 5 stories: C400X400
Columns for middle 10 stories: C600x600
Columns for bottom 10 stories: C800x800
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:
8. BUILDING "H" (G+25)
Consistent column size throughout the building: C800X800
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.
9. BUILDING "J" (G+25)
10% increment in the imposed loads
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:
Go to Assign>Shell loads>Uniform>increase it by 10%
We have assigned a 3KN/m^2 imposed load on all slabs for all of our previous buildings.
So, we need to increase that by 10% meaning we need to replace the existing imposed load with 3.3 KN/m^2
10. BUILDING "K" (G+25)
20% increment in the imposed loads
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:
Assign 3.6 KN/m^2 distributed shell load for a 20% increase.
RESULT:
EFFCT OF STIFFNESS ON T:COMPARE FUNDAMENTAL NATURAL PERIOD OF BUILDING E & F AS WELL AS G & H.WHY IS THERE A MARGINAL OR SIGNIFICANT DIFFERENCE IN THE FUNDAMENTAL NATURAL PERIODS?
| MODEL NAME | NO.OF SOREY |
COLUMN DESIGN SPECIFICATION |
NATURAL PERIOD (SECOND) |
| BUILDING E | 10 |
UPPER 5 STOREY: 400X400 BOTTOM 5STOREY: 600X600 |
1.731 |
| BUILDING F | 10 |
CONSISTS COLUMN SIZE THROUGHOUT:600X600 |
1.735 |
| BUILDING G | 25 |
UPPER 5 STOREY:400X400 MIDDLE 10 STOREY:600X600 BOTTOM 10 STOREY: 800X800 |
4.17 |
| BUILDING H | 25 |
CONSISTS COLUMN SIZE THROUGHOUT:800X800 |
4.416 |
EFFCT OF MASS ON T:COMPARE FUNDAMENTAL NATURAL PERIOD OF BUILDING H,J AND K. HAVE THE BUILDING BECOME MORE FLEXIBLE OR STIFF DUE TO CHANGE IN MASS?
| MODEL NAME | NO.OF SOREY |
PERCENTAGE INCREASE IN MASS |
NATURAL PERIOD (SECOND) |
| 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 |
EFFCT OF BUILDING HEIGHT ON T: HOW DO THE FUNDAMENTAL NATURAL PERIOD OF BUILDING A,B,F AND H CHANGE WITH A CHANGE IN BUILDING HEIGHT?
| MODEL NAME | NO.OF SOREY |
TOTAL BUILDING HEIGHT(m) |
NATURAL PERIOD (SECOND) |
| BUILDING A | 2 | 7.5 | 0.5 |
| BUILDING B | 5 | 16.5 | 1.036 |
| BUILDING F | 10 | 31.5 | 1.735 |
| BUILDING H | 25 | 76.5 | 4.416 |
EFFCT OF COLUMN ORIENTATION ON T: HOW DO THE FUNDAMENTAL NATURAL PERIOD OF BUILDING B,C AND D CHANGE WITH A CHANGE WITH A CHANGE IN COLUMN ORIENTATION?
| MODEL NAME | NO.OF SOREY |
COLUMN ORIENTATION |
MAXIMUM STORY DRIFT (mm) DUE TO DUE TO EQx EQy |
| BUILDING B | 5 |
EQUAL IN BOTH DIRECTION |
2.25 2.30 |
| BUILDING C | 5 | X-DIRECTION | 2.10 2.50 |
| BUILDING D | 5 | Y-DIRECTION | 2.45 2.15 |
EFFCT 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 OD ADJOINING COLUMNS IS VERY SMALL VERSUS WHEN IT IS LARGE.
| MODEL NAME |
FLEXURAL STIFFNESS OF BEAMS |
NATURAL PERIOD |
| BUILDING B |
100% 10% |
1.036 2.627 |
EFFCT OF FLEXURAL STIFFNESS OF VERTICAL MEMBER ON MODE SHAPES:COMPARE 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.
| MODEL NAME |
FLEXURAL STIFFNESS OF BEAMS |
NATURAL PERIOD |
| BUILDING H |
100% 10% |
4.416 5.991 |
EFFCT OF FIXITY AT COLUMN BASES ON MODE SHAPES:COMPARE FUNDAMENTAL MODE SHAPE OF BUILDING B IN TWO SITUATIONS WHEN THE BASE OF COLUMN IS PINNED VERSUS WHEN IT IS FIXED.
| MODEL NAME | NO.OF SOREY | FIXITY AT BASE | NATURAL PERIOD |
| BUILDING B | 5 |
PINNED FIXED |
1.036 0.941 |