AIM

To model and analyse different buildings with varying parameters and study the influence of these parameters on building characteristics by comparing the models.

INTRODUCTION:

The overall response to an earthquake is strongly affected by the structural design of the building. When the same seismic forces are applied, different building geometries respond differently. For a safer design approach, it is crucial to understand how buildings react to changes in their structural configuration. For this reason, in this project, we used ETABS to model various buildings with different geometries. We also used the software to analyse the responses of the buildings, and the results were compared to gain an understanding of how buildings with different storeys behave structurally.

Material Properties:

Grade of Concrete: M30

Grade of Steel Reinforcement Bars: Fe 415

Structural Element Sizes:

Beams: 300 x 400 mm

Columns: (GIVEN IN TABLE) for different models

Slab: 150 mm thick

Loads:

Live load on the floor: 3 kN/m²

Brick wall Load on beams: 10 kN/m

Buildings are assumed to be pinned at the base

PROCEDURE:

Modelling of Building A (G+2)

Open E-Tabs ->new file

Use built-in settings

Change the units and codes as per Indian standard, click Ok

Click on the custom grid spacing

Edit grid data

Enter the grids ordinates in X direction And Y direction

Click on Add to make more grids as per the requirement

Click on custom storey data

Right-click on and select "add new story"

Enter the number of stories

Enter the distance between them

Select story 1 as the master story and all others as “No”

Select similar to "Story 1"

Click on Define Tab>material properties

Click on add new material

Add M3 grade concrete

Check material property data

Add rebar Fe415

Check material property data of the HYSD bar

Go to define>section properties>frame sections

Click on Add new property

Select the rectangular concrete and click ok

Add new beam and column frames

Change the property name as per given data B300x400> change material to M30

Enter the depth and width of the sections

Click on Modify/show rebar>to enter the rebars types and numbers

Click on M3 design only

Change longitudinal bars and Tie bar types to the previously defined

Again on frame sections> Add new property

Select the rectangular concrete section> click on OK

Rename the property as C400x400

Enter the width and depth of the column

Click on modify/show rebar> to enter the bars type of columns

Click on P-M2-M3 design> Enter longitudinal and tie bars type

Reinforcement to be designed> Enter the number of bars required in 2&3 direction

Select slab section for the slab of 150mm

Go to define> section properties> slab sections

Click on slab1> modify/Show property

Rename the slab as Slab150

Select the slab material as M30> Modelling type as "membrane"

Enter the thickness of the slab

Add columns to the structure.

Go to the bottom right corner and select ALL STORIES

Click on "Quickdraw columns" on the left side toolbars

Select the C400x400 from the property

Select all the columns ->all the columns will be applied to the grids

Select quick draw beams (keep the selection to ALL stories)

Select the property to B300x400

Select all the beams section> the beams will be applied

Now add slabs to the section> Select quick draw floors/Slabs

Change the property to slab 150

Change the selection to similar stories

Select all the slabs of any one floor

Now add joints to the base

Go to Assign bar> Joints> Restraints

Select the pin joint

Now define load patterns

Go to Define Bar> Load patterns

Add dead load, live load, wall load and earthquake load

Click on earthquake loads "EQx-" > Click on Modify lateral load

Select the Load direction (X dir.-eccentricity)

Select Site III as suggested in the question & time period as the program calculated

Do this for all the earthquake directions, just change the eccentricity and direction in the above

Assign the live load to the slabs and brick wall loads

Select all the slabs of any floor> Change the selection to similar stories

Go to Assign> Shell loads> Uniform

Type live load as 3 kN/m²

Assign the wall load to the beams

Go to select menu> object types> select beams (all beams will be selected)

Go to Assign menu> Frame load> distributed

Enter the value of 10kN/m

Define mass source

Go to Assign Menu> Mass source

Click on Modify/show mass source

Select specified load patterns

Input dead=1 wall=1, live=0.25

Go to define menu> load combination

Click on Add default design combos

Select concrete frame design

Making the diaphragm of the structure

Go to select menu> object types> select the floors

Assign> Shell> Diaphragm

Select D1 and apply

Check the model for errors

Go to analyse menu> check model

Go to analyse menu> run analysis

Go to Display> Deformed shape or F6>Tab will open and select any case (live/dead, earthquake) to view the deformed shape

Click on Display menu> Show tables> Tables display will open

Select analysis results> structure output> modal information> Modal participation mass ratios

Modelling of Building B (G+5)

From model A, go to files and click on the “save as” option

Save the file as Building B

Go to Edit> Edit stories and grids system.

Click on the modify story data option.

Then, right-click on the top story and select the add story option.

Set the height as 3 and the number of storeys to add as 3.

Make each story similar to the master story created in Model A.

Now check the load assignment on the frames and shell

Display> Load assigns> Frames> Wall load

Display> Load assigns> Shell> Live load

Check the diaphragm assigned to each floor

Go to analyse menu> check model

Go to analyse menu> run analysis

Modelling of Building C (G+5)

From model B, go to files and click on the “save as” option

Save the file as Building C

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 C550x300.

Then, edit the length and breadth by entering the values in the box.

To orient the 550mm side in the X-direction select the column section

Then go to Assign> local axis> rotate> 90 degrees.

Now check the load assignment on the frames and shell

Display> Load assigns> Frames> Wall load

Display> Load assigns> Shell> Live load

Check the diaphragm assigned to each floor

Go to analyse menu> check model

Go to analyse menu> run analysis

Modelling of Building D (G+5)

From model C, go to files and click on the “save as” option

Save the file as Building D

Orient the 300mm side in the X-direction and select the column section

Then go to Assign> local axis> rotate> 0 degrees.

Now check the load assignment on the frames and shell

Display> Load assigns> Frames> Wall load

Display> Load assigns> Shell> Live load

Check the diaphragm assigned to each floor

Go to analyse menu> check model

Go to analyse menu> run analysis

Modelling of Building E (G+10)

From model D, go to files and click on the “save as” option

Save the file as Building E

Go to Edit> Edit stories and grids system.

Click on the modify story data option.

Then, right-click on the top story and select the add story option.

Set the height as 3 and the number of storeys to add as 5.

Make each story similar to the master story

Go to define>section properties>frame section.

Now, select the previously created column

Click on the modify/show properties option.

Rename the column as C400x400.

Then, edit the length and breadth by entering the values in the box.

Go to define>section properties>frame section> Add new property

Select the rectangular concrete section> click on OK

Rename the property as C600x600

Enter the width and depth of the column

Click on modify/show rebar> to enter the bars type of columns

Click on P-M2-M3 design> Enter longitudinal and tie bars type

Reinforcement to be designed> Enter the number of bars required in 2&3 direction

Go to Select> Object Types> Columns

Go to Assign> Frame> Section property> C600x600

C600x600 is assigned to the whole structure

Go to View> Set building view limits

Select story 6 to story 10 to view

Go to the Set display option (Ctrl+W) on the top bar

Hide all Beams and Floor elements

Select all the columns

Go to Assign> Frame> Section property> C400x400

C400x400 is assigned to Storey 6 to storey 10

Now check the load assignment on the frames and shell

Display> Load assigns> Frames> Wall load

Display> Load assigns> Shell> Live load

Check the diaphragm assigned to each floor

Go to analyse menu> check model

Go to analyse menu> run analysis

Modelling of Building F (G+10)

From model E, go to files and click on the “save as” option

Save the file as Building F

Go to Select> Object Types> Columns

Go to Assign> Frame> Section property> C600x600

C600x600 is assigned to the whole structure

Now check the load assignment on the frames and shell

Display> Load assigns> Frames> Wall load

Display> Load assigns> Shell> Live load

Check the diaphragm assigned to each floor

Go to analyse menu> check model

Go to analyse menu> run analysis

Modelling of Building G (G+25)

From model F, go to files and click on the “save as” option

Save the file as Building G

Go to Edit> Edit stories and grids system.

Click on the modify story data option.

Then, right-click on the top story and select the add story option.

Set the height as 3 and the number of storeys to add as 15.

Make each story similar to the master story

Go to define>section properties>frame section> Add new property

Select the rectangular concrete section> click on OK

Rename the property as C800x800

Enter the width and depth of the column

Click on modify/show rebar> to enter the bars type of columns

Click on P-M2-M3 design> Enter longitudinal and tie bars type

Reinforcement to be designed> Enter the number of bars required in 2&3 direction

Go to View> Set building view limits

Select story 1 to story 10 to view

Go to the Set display option (Ctrl+W) on the top bar

Hide all Beams and Floor elements

Select all the columns

Go to Assign> Frame> Section property> C800x800

C800x800 is assigned to Storey 1 to storey 10

Go to View> Set building view limits

Select story 21 to story 25 to view

Go to the Set display option (Ctrl+W) on the top bar

Hide all Beams and Floor elements

Select all the columns

Go to Assign> Frame> Section property> C400x400

C400x400 is assigned from Storey 21 to storey 25

Now check the load assignment on the frames and shell

Display> Load assigns> Frames> Wall load

Display> Load assigns> Shell> Live load

Check the diaphragm assigned to each floor

Go to analyse menu> check model

Go to analyse menu> run analysis

Modelling of Building H (G+25)

From model G, go to files and click on the “save as” option

Save the file as Building H

Go to Select> Object Types> Columns

Go to Assign> Frame> Section property> C800x800

C800x800 is assigned to the whole structure

Now check the load assignment on the frames and shell

Display> Load assigns> Frames> Wall load

Display> Load assigns> Shell> Live load

Check the diaphragm assigned to each floor

Go to analyse menu> check model

Go to analyse menu> run analysis

Modelling of Building J (G+25)

From model H, go to files and click on the “save as” option

Save the file as Building J

Go to Select> Object Types> Floors

Go to Assign> Shell loads> Uniform

Enter live load as 3.3 kN/m²

Now check the load assignment on the frames

Display> Load assigns> Frames> Wall load

Check the diaphragm assigned to each floor

Go to analyse menu> check model

Go to analyse menu> run analysis

Modelling of Building K (G+25)

From model J, go to files and click on the “save as” option

Save the file as Building K

Go to Select> Object Types> Floors

Go to Assign> Shell loads> Uniform

Enter live load as 3.6 kN/m²

Now check the load assignment on the frames

Display> Load assigns> Frames> Wall load

Check the diaphragm assigned to each floor

Go to analyse menu> check model

Go to analyse menu> run analysis

COMPARATIVE ANALYSIS OF BUILDING PROPERTIES BASED ON THEIR STRUCTURAL OUTPUTS

Now we compare the results and try to understand the change in behaviour of buildings based on their structural geometries. A comprehensive comparative analysis of structural properties of different story buildings is performed below based on 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

Building	No. of stories	Column Dimension	T (sec)
E	10	Upper 5: 400x400 Lower 5: 600x600	1.731
F	10	600x600	1.735
G	25	Upper 5: 400x400 Middle 10: 600x600 Lower 10: 800x800	4.17
H	25	800x800	4.416

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 larger 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

Building	No. of stories	Imposed Mass kN/m2	Column Dimension	T (sec)
H	25	3	800x800	4.416
J	25	3.3	800x800	4.425
K	25	3.6	800x800	4.435

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

Building	No. of stories	Column Dimension	T (sec)
A	2	400x400	0.5
B	5	400x400	1.036
F	10	600x600	1.735
H	25	800x800	4.416

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?

Comparative Table

Building	No. of stories	Column Dimension	T (sec)
B	5	400x400	1.036
C	5	550x300	1.145
D	5	300x550	1.12

Conclusion:

When the column cross-section is square, there's not much difference between the lateral displacements in both the orthogonal directions. However, when we orient the column in a particular direction, 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 the 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

Building	Flexural Stiffness of Beam (%)	Column Dimension	T (sec) Mode 1	T (sec) Mode 2	T (sec) Mode 3
B	100	400x400	1.036	1.01	0.912
B	10	400x400	2.627	2.549	2.014

Conclusion:

It is obvious that if we decrease the flexural stiffness of beams, the overall flexural strength of the beam would also reduce, and consequently, the fundamental natural period will significantly increase.

• Mode shapes of Building B with 100% flexural stiffness

• Mode shapes of Building B with 10% flexural stiffness

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.

Comparative Table

Building	Axial stiffness of Column (%)	Column Dimension	T (sec) Mode 1	T (sec) Mode 2	T (sec) Mode 3
H	100	800x800	4.416	4.222	3.263
H	10	800x800	5.991	5.312	3.289

Conclusion:

As the axial stiffness of columns decreases, the fundamental natural period of the building increases regardless of its structural geometry.

• Mode shapes of Building H with 100% axial stiffness oof vertical members

• Mode shapes of Building H with 10% axial stiffness oof vertical members

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.

Comparative Table

Building	Support Condition	Column Dimension	T (sec) Mode 1	T (sec) Mode 2	T (sec) Mode 3
B	Pinned	400x400	1.036	1.01	0.912
B	Fixed	400x400	0.941	0.919	0.828

Conclusion:

It is evident that pinned support offers fewer restraints compared to the fixed one, leading 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.

• Mode shapes of Building B with pinned support

• Mode shapes of Building B with fixed support