Project 1_Comparative study of different storey buildings for Seismic forces

PROJECT

Comparative Study of Different Story 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.

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.

PHASE ONE: Modeling 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

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.

• Setting up the Grid Data

1. To set up the grid data, you need to click on the custom grid option.
2. Once you do that, a menu with all the grid settings will open up.
3. 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.
4. So, by following the same approach, we have set up a 4 x 3 bay grid with 4m center to center spacing in-between.

• Fixing the Story Data

• Modal Response for Building G

8. BUILDING "H" (G+25)

Consistent column size throughout the building: C800X800

PROCEDURE:

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.

• View Settings to Hide Beams and Floors

• Select All the Columns and Assign C800x800 Section

• Run the Analysis

• Modal Response for Building H

9. BUILDING "J" (G+25)

10% increment in the imposed loads

PROCEDURE:

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:

• Select the S150 Slab Section

• Go to Assign>Shell loads>Uniform>increase it by 10%

1. We have assigned a 3KN/m^2 imposed load on all slabs for all of our previous buildings.
2. So, we need to increase that by 10% meaning we need to replace the existing imposed load with 3.3 KN/m^2

• Run the Analysis

• Modal Response for Building J

10. BUILDING "K" (G+25)

20% increment in the imposed loads

PROCEDURE:

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:

• Select the S150 Slab section

• Assign 3.6 KN/m^2 distributed shell load for a 20% increase

• Run the Analysis

• Modal Response for Building K

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

Conclusion:

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 in mass?

Comparative Table

Conclusion:

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?

Comparative Table

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 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.

Comparative Table

Conclusion:

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.

Conclusion:

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.

Conclusion:

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.