Project 1_Comparative study of different storey buildings for Seismic forces
Aim:
⦁ To Compare fundamental natural periods of buildings H, J and K. using ETABS
Procedure:
BUILDING - A:
⦁ 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.
⦁ After we have to input the story data.
⦁ AndIt is important to set details of the materials you are going to use for the project in ETABS.
⦁ 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.
⦁ 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
⦁ 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
⦁ 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
⦁ 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:
⦁ 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.
BUILDING-B:
⦁ 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.
BUILDING-C:
⦁ To edit the column section, go to define>section properties>frame section.
⦁ 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.
BUILDING-D:
⦁ To change the orientation of column, use the local axes and change it to 90 degree.
BUILDING-E:
⦁ 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
⦁ Run the analysis after changing the column sizes.
BUILDING-F:
BUILDING-G:
BUILDING-J:
BUILDING-K:
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?
⦁ 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 imass?
⦁ 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?
⦁ 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?
⦁ 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.
⦁ 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
⦁ 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.
⦁ 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.