Projefct 1_Comparative study of different storey buildings for Seismic forces

•  Aim :- To model and analyse the building  from A to K with the given structural data and compare the analysis between the models based on the different factors.

• Introduction :-

• Every building has a number of natural frequencies (how many?), at which it offers minimum resistance to shaking induced by external effects (like earthquakes and wind) and internal effects (like motors fixed on it).
• Each of these natural frequencies and the associated deformation shape of a building constitute a Natural Mode of Oscillation. 
• The mode of oscillation with the smallest natural frequency (and largest natural period) is called the Fundamental Mode; the associated natural period T1 is called the Fundamental Natural Period.
• Regular buildings held at their base from translation in the three directions, have two fundamental translational natural periods, Tx1 and Ty1, associated with its horizontal translational oscillation along X and Y directions, respectively, and one fundamental rotational natural period Tθ1 associated with its rotation about an axis parallel to Z axis.

• The data required for modeling the building is as shown below

•  Answer :-

•  Step 1 : Model for Building A

  • Open the ETAB software from the start menu
  • Click > New Model
  • One dialogue box will open
  • Select model initializasation option as > Use Build-in setting with
  • Select Display unit > Metric SI
  • Steel section database > Indian
  • Steel design code > IS 800:2007
  • Concrete design code > IS 456:2000

  

  Step 2

  • To set the grids and story
  • Select > Custom grid setting
  • Click > Edit grid data
  • One dialogue box will open
  • Select > Display grid data as ordinates
  • Fill the data in X-Grid data and in Y-Grid data as per shown below

  

  • Select > Custom story data
  • Click > Edit storey data
  • one dialogue box will open
  • Double click > Story 1 and rename it as > Plinth Level
  • Select plinth level height > 1.5
  • Double click > Story 2 and rename it as > Story 1 and keep height > 3
  • Double click > Story 3 and rename it as > Roof Storey and keep height > 3
  • Select master story > Yes only to > Story 1
  • Keep all other remaining story > No
  • Keep all other storeys similar to > Story 1
  • Keep Plinth > Similar to > None

  

  • It will shows the following results

  

   

  Step 3

  • Keep all the unnecessary viewports close
  • Select Plan view > Story 1
  • To select the material
  • Go to define tab
  • Select > Material Properties

  

  • Click > Add new material
  • Select Region > India
  • Material Type > Concrete
  • Standard > Indian
  • Grade > M30

  

  • Click > Ok
  • One dialogue box will open
  • Fill the data as per shown below

  

   

  • Click > OK
  • The concrete material is added
  • Again go to add material
  • Add the rebar for longitudinal reinforcement

  

  • Click > Ok

  

  • Click > Ok
  • By this similar way, add the rebar for shear reinforcement

  

  • Click > Ok

  

  • Click > Ok
  • All the three materials are added

  

   

  Step 4

  • To add the sizes of beam, column and slab
  • Go to Define tab
  • Select > Section properties
  • Select > Frame sections

  

  • Click > Add new property

  

  • Select > Rectangular concrete
  • Select Property name > B 300 X 400
  • Material > M30
  • Select section dimension Depth > 400 and Width > 300

  

  • Select > Modify rebar
  • Fill the data as per shown below

  

   

  • Click > Ok
  • The beam is added
  • Now, add another element > Column
  • Click > Add new property

  

  • Select > Rectangular concrete
  • Select Property name > C 400 X 400
  • Material > M30
  • Select section dimension Depth > 400 and Width > 400

  

  • Select > Modify rebar
  • Fill the data as per shown below

  

  • Click > Ok
  • The column is added
  • The beam and column is added

  

  • To add the structural element > Slab
  • Go to define tab
  • Select > Section properties
  • Select > Slab section
  • Click > Add new property
  • Propert name > Slab 150
  • Slab material > M30
  • Modeling type > Membrane
  • All the slabs are two way slab, so no need to keep one way slab on

  

  • Click > Ok
  • The slab is added

   Step 5

  • To start the drawing, start with columns
  • Keep > All story
  • Go to > Plan view > Story 1
  • Take Quick draw column command
  • Select the appropriate column
  • Provide the column, by clicking to all the intersection point of grids
  • It will shows the following results

  

  

  Step 6

  • To draw the beam
  • Open first story plan view
  • Keep > All story
  • Go to quick draw beam command
  • Select appropriate property
  • Click on all grids to provide the beams
  • It will shows the following results

  

  

  Step 7

  • To draw the slab
  • Open first story plan view
  • Keep > Similar stories
  • Go to quick draw floor command
  • Select appropriate property
  • Click in between grids to provide the slab
  • It will shows the following results

  

  

  Step 8

  • To assign the base of building as > Pinned
  • Go to plan view > Base plan
  • Select > One story
  • Select all the joints
  • Go to assign
  • Select > Joints
  • Select > Restraints
  • Select the base as pinned 

  

  • Click > Apply
  • Click > Ok

  Step 9

  • To add the load pattern
  • Go to define
  • Select > Load patterns
  • Add the loads as per shown below

  

  • Select Eqx load
  • Click > Modify lateral load
  • Set all the things as per shown below

  

   

  • Similarly modify lateral load for all other eq loads

  Step 10

  • To assign the live load
  • Keep > Similar stories
  • Select all the shells
  • Go to assign tab
  • Select > Shell loads
  • Select > Uniform
  • Set as per shown below

  

  • It will shows the following results

  

  Step 11

  • To assign the loads of infill walls
  • Go to select tab > Select > Object type > Beams > Select > Close
  • All the beams are selected
  • Go to assign > Frame loads > Distributed
  • Select load > Brickwall
  • Set as per shown below

  

  • Click > Apply
  • Click > Close
  • The infillwall load is applied

  

  Step 12

  • To assign the mass source
  • Go to define tab
  • Select > Mass source 
  • Select > Modify existing mass source

  

  • Click > Ok

  Step 13

  • To generate the load combination
  • go to define tab
  • Select > Load combination
  • Click > Add default design combos
  • Switch on > Concrete frame design
  • Switch on > Convert to user combinations
  • The load combinations are generated

  

  Step 14

  • To assign the diphragms
  • Go to Select > Select > Object type > floor > Select > Close
  • Go to assign
  • Select > Shell > Diphragms
  • one dialogue box will open
  • Click > D1
  • Click > Apply
  • Click > Ok
  • The diphragms are applied

  

  

  Step 15

  • To check the model
  • Go to analyze
  • Select > Check the model

  

  • The model has been checked

  

  Step 16

  • The model is ready for analysis
  • Go to run analysis
  • It will shows the following results

  

  Step 17

  • After analysis, we need to check deflection
  • Go to show deformed shapes
  • One dialogue box will open
  • Select the load for which, we want to see the deflection

  

   

   

  

  

  Step 18

  • To see the base reactions
  • Go to display
  • Select > Show tables

  

  • Click > Ok
  • It will shows the following results
  • Modal mass participating ratio for A building is ready

  

  Step 19 : Model for Building B

  • Copy the model of building A
  • Open the copied model of building A to prepare the model of building B
  • Unlock the model
  • Go to edit tab
  • Select > Edit stories and grid system
  • Click > Modify story data
  • Add the stories and set as per shown below
• 
•  
  • To check the model
  • Go to analyze tab
  • Click > Check model
  • It will shows the following result
•  
  • The model is ready for analysis
  • Go to run model
  • It will shows the following result
•  
  • The model mass participating ratio for building B is ready
•  

   Step 20 : Model for Building C

  • Copy the model of building B
  • Open the copied model of building B to prepare the model of building C
  • Unlock the model
  • Go to define tab
  • Select > Section properties
  • Select > Frame section
  • Select C 400 X 400 section 
  • Add copy to property
  • Set as per shown below
•  
  • Go to select tab
  • Select > Select
  • Select > Object type
  • Select > Column
  • All the columns are selected
  • Go to assign tab
  • Select > Section properties
  • Select C 550 X 300
  • Click > Apply
  • All the columns are applied
•  
  • To check the model
  • Go to analyze tab
  • Click > Check model
  • It will shows the following result
•  
  • The model is ready for analysis
  • Go to run model
  • It will shows the following result
•  
•  
  • The model mass participating ratio for building C is ready
•  

•  Step 21 : Model for Building D

  • Copy the model of building C
  • Open the copied model of building C to prepare the model of building D
  • Unlock the model
  • Go to select > Select > Object type > Column > Close
  • Go to assign > Frame > Local axes
   
  • Apply > Ok
  • All the columns are oriented as per shown below
•  
  • To check the model
  • Go to analyze tab
  • Click > Check model
  • It will shows the following result
•  
  • The model is ready for analysis
  • Go to run model
  • It will shows the following result
•  
•  
  • The model mass participating ratio for building D is ready
•  

•  Step 22 : Model for Building E

  • Copy the model of building A
  • Open the copied model of building A to prepare the model of building E
  • Unlock the model
  • Go to edit tab
  • Select > Edit stories and grid system
  • Click > Modify story data
  • Add the stories and set as per shown below
• 
•  
  • Go to define tab
  • Select > Section properties
  • Select > Frame section
  • Select C 400 X 400 section 
  • Add copy to property
  • Set as per shown below
•  
  • Go to view tab
  • Select > Building views limit
  • Set as per shown below
•  
  • It will shows the following results
•  
  • Go to set display option
  • Switch off the beams, joints and floors
  • It will shows only columns on the screen
•  
  • Select > Column
  • All the columns are selected
  • Go to assign tab
  • Select > Section properties
  • Select C 600 X 600
  • Click > Apply
  • The column C 600 X 600 is applied from Base upto the 5th floor
•  
  • Go to view tab
  • Select > Building views limit
  • Click > Show all
  • Go to set display option
  • Switch on the beams, joints and floors
•  
  • To check the model
  • Go to analyze tab
  • Click > Check model
  • It will shows the following result
•  
  • The model is ready for analysis
  • Do to run model
  • It will shows the following result
•  
•  
  • The model mass participating ratio for building E is ready
•  

•  Step 23 : Model for Building F

  • Copy the model of building E
  • Open the copied model of building E to prepare the model of building F
  • Unlock the model
  • Go to edit tab
  • Go to view tab
  • Select > Building views limit
  • Set as per shown below
•  
  • Select > Column
  • All the columns are selected
  • Go to assign tab
  • Select > Section properties
  • Select C 600 X 600
  • Click > Apply
  • The column C 600 X 600 is applied from 6th to roof story
  • Go to view tab
  • Select > Building views limit
  • Click > Show all
  • Go to set display option
  • Switch on the beams, joints and floors
•  
  • To check the model
  • Go to analyze tab
  • Click > Check model
  • It will shows the following result
•  
  • The model is ready for analysis
  • Do to run model
  • It will shows the following result
•  
•  
  • The model mass participating ratio for building F is ready
•  

•  Step 24 : Model for Building G

  • Copy the model of building F
  • Open the copied model of building F to prepare the model of building G
  • Unlock the model
  • Go to edit tab
  • Select > Edit stories and grid system
  • Click > Modify story data
  • Add the stories and set as per shown below
• 
• 
•  
  • Go to define tab
  • Select > Section properties
  • Select > Frame section
  • Select C 800 X 800 section 
  • Add copy to property
  • Set as per shown below
•  
  • Go to view tab
  • Select > Building views limit
  • Set as per shown below
•  
•  
  • Select > Column
  • All the columns are selected
  • Go to assign tab
  • Select > Section properties
  • Select C 800 X 800
  • Click > Apply
  • The column C 800 X 800 is applied from Base upto the 10th floor
•  
  • Go to view tab
  • Select > Building views limit
  • Set as per shown below
•  
•  
  • Select > Column
  • All the columns are selected
  • Go to assign tab
  • Select > Section properties
  • Select C 400 X  400
  • Click > Apply
  • The column C 400 X 400 is applied from 21st story upto the roof story
• 
  • Go to view tab
  • Select > Building views limit
  • Click > Show all
  • Go to set display option
  • Switch on the beams, joints and floors

• 
•  
  • To check the model
  • Go to analyze tab
  • Click > Check model
  • It will shows the following result
•  
  • The model is ready for analysis
  • Go to run model
  • It will shows the following result
•   
  • The model mass participating ratio for building G is ready
•  

•  Step 25 : Model for Building H

  • Copy the model of building G
  • Open the copied model of building G to prepare the model of building H
  • Unlock the model
  • Go to view tab
  • Select > Building views limit
  • Set as per shown below
•  
•  
  • Select > Column
  • All the columns are selected
  • Go to assign tab
  • Select > Section properties
  • Select C 800 X 800
  • Click > Apply
  • The column C 800 X 800 is applied from 11th story upto the roof story
  • Go to view tab
  • Select > Building views limit
  • Set as > Show all
  • It will shows the following results
•  
  • To check the model
  • Go to analyze tab
  • Click > Check model
•  
  • The model is ready for analysis
  • Go to run model
  • It will shows the following result
•  
•  
  • The model mass participating ratio for building H is ready
•  

•  Step 26 : Model for Building J

•  

  • Copy the model of building H
  • Open the copied model of building H to prepare the model of building J
  • Unlock the model
  • Set > All stories
  • Go to select
  • Select > Object type
  • Select > Floors
  • Click > Close
  • All the slabs are selected
  • Now as per instructions, we have to add imposed mass 10% larger than the building H
  • Go to assign
  • Select > Shell loads
  • Select > Uniform
  • Set as per shown below
• 
  
  • Click > Apply > Ok
  • The imposed mass in increased by 10%
•  
  • To check the model
  • Go to analyze tab
  • Click > Check model
  • It will shows the following result
•  
  • The model is ready for analysis
  • Go to run model
•  
  • The model mass participating ratio for building J is ready
• 

•  Step 27 : Model for Building  K

• • Copy the model of building H
  • Open the copied model of building H to prepare the model of building K
  • Unlock the model
  • Set > All stories
  • Go to select
  • Select > Object type
  • Select > Floors
  • Click > Close
  • All the slabs are selected
  • Now as per instructions, we have to add imposed mass 20% larger than the building H
  • Go to assign
  • Select > Shell loads
  • Select > Uniform
  • Set as per shown below
• 
  
  • Click > Apply > Ok
  • The imposed mass in increased by 20%
•  
  • To check the model
  • Go to analyze tab
  • Click > Check model
  • It will shows the following result
•  
  • The model is ready for analysis
  • Go to run model
•  
  • The model mass participating ratio for building K is ready
• 

• Result :-

• The modal mass participating ratio of building A :

• The modal mass participating ratio of building B :

• The modal mass participating ratio of building C :

• The modal mass participating ratio of building D :

• The modal mass participating ratio of building E :

• The modal mass participating ratio of building F :

• The modal mass participating ratio of building G :

• The modal mass participating ratio of building H :

• The modal mass participating ratio of building J :

• The modal mass participating ratio of building K :

Factors influencing the Natural Period of a building

• 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?

•  Here,The time period of Building F and building H is marginally greater than the building E and building F because of uniformity in column sizes of building F and building H.
• The time period is directly proportional to the mass of the building, so here, in building E and building F the mass is increased and thats whye it results in increase in the time period.
• The arrangement will not effect much more on the stiffness

• 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?

• Here, All the above buildings are 25 story building only difference is that, the Building J and Building K has 10 % and 20 % more mass than the mass of building H.
• The time period is directly proportional to the mass of the building, so here, in building J and building K the mass is increased and thats whye it results in increase in the time period.
• More the time period of a building results in the more flexible and so the building K and building J is more flexible than the building H.

• Effect of Building Height on T: How does the fundamental natural periods of Buildings A, B, F and H change with change in building height?

• Here, we can see that the funamental natural period of building is increases with the increasing in the building height
• As the height of the building increases, its mass increases but the overall stiffness will be reduces

• Effect of Column Orientation on T: How does the fundamental natural periods of Buildings B, C and D change with change in column orientation?

• Here,The building B has the square columns, so the building B has nearly same natural period along the X axis and along the Y axis.
• In Building C, the columns are oriented along the X-axis so thats why, the building B has greater natural time period in X axis compared to the Y axis

Factors influencing the Mode shape of oscillations

• Effect of Flexural Stiffness of Structural Elements on mode shapes: Compare fundamental mode shape of Building B in two situations when flexural stiffness of beams relative to that of adjoining columns is very small versus when it is large.

• The time period of original building B is kept as it is, the building B model is copied
• Opened that copied model B and name it as B1 building
• Change the moment of inerta of beam as per shown below

• and the time period is noted for B1 building

• Here, the time period is increased for modified building B1.
• It means, as the flexural stiffness of the building is reduced, the building becomes more flexible and hence it results in increases in the natural time period.

• Effect of Axial Stiffness of Vertical Members on mode shapes: Compare fundamental mode shape of Building H in two situations when axial cross-sectional area of columns is very small versus when it is large.

• The time period of original building H is kept as it is, the building H model is copied
• Opened that copied model H and name it as H1 building
• Change the Cross-Section (axial) Area of column as per shown below

• and the time period is noted for H1 building

• Here, the reduction in the axial stiffness will results in the increment in the natural time period.

• Effect of Degree of Fixity at column bases on mode shape: Compare fundamental mode shape of Building B in two situations when base of columns is pinned versus when it is fixed.

• The time period of original building B is kept as it is, the building B model is copied
• Opened that copied model B and name it as B2 building
• Prepare the model B2 by keeping fixed condition at base
• And the time period is noted for B1 building

• Here, after the application of fixed support to the building B it results in reducing the natural time period.
• We know that, fixed support is more rigid than the pinned support and hence the column rigidity will be higher and due to this the natural time period is reduced.