Project 1_Comparative study of different storey buildings for Seismic forces

Factors influencing the dynamic characteristics of a building :-

• Buildings oscillate during earthquake shaking. The oscillation causes inertia force to be induced in the building.
• The intensity and duration of oscillation, and the amount of inertia force induced in a building depend on features of buildings, called their dynamic characteristics, in addition to the characteristics of the earthquake shaking (beyond the control of an engineer) itself.
• The important dynamic characteristics of buildings are modes of oscillation and damping (assumed constant in most practical cases). A mode of oscillation of a building is defined by the associated Natural Period and Deformed Shape in which it oscillates.
• 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 constitutes 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 their 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 the Z axis.

Factors influencing the Natural Period of a building :-

• Structural Element Sizes 

               - Beams : 300x400mm

               - Columns : 400x400mm

               - Slab : 150mm thick

• Material Properties

               - Grade of concrete : M30

               - Grade of steel Reinforcement Bars : Fe415

• Loading 

               - Dead Load on beams from infill wall : 10kN/m

               - Live load on Floor : 3kN/m2

                  (what is the seismic mass?)

• Buildings are assumed tobe pinned at base

            Five storey benchmark Building: Elevation and plan of benchmark building showing the structural moment frame grid(All dimensions are in mm)

Note 

1.Bay length in each plan direction is 4m(center to center).

2.All columns at each storey are of the same size.

3.All beams in all buildings are of the same size(300mmx400mm).

      Buildings considered to illustrate concept of natural period: Details of 10buildings considered.

* Effect of stiffness on T : Compare fundamental natural periods of buildings E and F as well as G and H. Why is there a marginal or significant difference in the fundamental natural periods?

* 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?

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

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

Factors influencing the Mode shape of oscillations:-

* Mode shape of oscillation associated with a natural period of a building is the deformed shape of the building when shaken at the natural period. Hence, a building has as many mode shapes as the number of natural periods.

* For a building, there are infinite numbers of natural periods. But, in the mathematical modeling of a building, usually, the building is discretized into a number of elements. The junctions of these elements are called nodes. Each node is free to translate in all three Cartesian directions and rotate about the three Cartesian axes. Hence, if the number of nodes of discretization is N, then there would be 6N modes of oscillation, and associated with these are 6N natural periods and mode shapes of oscillation.

* 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.

* 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.

* 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.

Data Given:-

 Structural Element Sizes 

               - Beams: 300x400mm

               - Columns: 400x400mm

               - Slab: 150mm thick

Material Properties

               - Grade of concrete: M30

               - Grade of steel Reinforcement Bars: Fe415

Loading 

               - Dead Load on beams from infill wall: 10kN/m

               - Live load on Floor: 3kN/m2

Buildings are assumed to be pinned at base.

Procedure:-

• Open ETABS software.
• Click New model.
  • 
• Give model initialization setting as follow
  • 
• Click ok
• In the new model quick template dialog box , select custom grid spacing and click Edit grid data
  • 
• Input the following tables accordingly as per the requirement in grid data as coordinate.
  • 
• Click ok.
• Select custom story data under story dimension and click edit story data.
  • 
• The plan view and 3D model view of the structure.
  • 
  • Go to define --> Material Property
    • 
  • Then click --> Add new material --> Define concrete of M30 grade
    • 
  • Then ok 
    • 
  • Ok
  • Then again click on Add new material to define rebars.
    • 
  • Ok
    • 
  • Ok --> Ok

• Again go to Define --> Section Property --> Frame Sections --> Add New Property

  • 

  Select concrete rectangular.

  • Then give respective values for the concrete beam section by defining the beam size and grade of concrete.
    • 
  • Ok
  • Go to Modify/show rebars--> change the required fields as per the requirement. Then Ok.
    • 
  • Again for defining column size click on Add new property -->
    • Ok-->Ok
    • The property for the column has been defined.
    • Then Click--> Section properties--> Slab section
      • 
      • Select Add new property and give the following settings.
      • 
    • Ok--> Ok
    • Define Load patterns.
      • 
    • 
    • Ok-->Ok
    • Check load cases by clicking Define-->Load cases.
      • 

  • Load Combinations

    • Define--> Load combinations --> Add Default Design Combos.
      • 

Draw Section

• Now we will add columns to the structure.
• Go to the bottom right corner and select ALL STORIES
• Click on "Quickdraw columns" of left side toolbars->a new tab will open
• select the C 400x400 from the property
• select all the columns -> All the columns will be applied to the grids
  • 
• Now beam will be added->Select quick draw beams (keep the selection to ALL stories)
• a tab will open->select the property to 300x400
• select all the beams section->the beams will be applied
  • 
• Then Go to Story 1 Plan--> Select Similar Stories --> Draw --> Draw floor and wall objects --> Quick draw floor/wall 
  • 
• Then brick loads of 10Kn/m2 need to be applied on all the beams.
• So first for selecting the beams go to --> select --> Properties--> Frame sections --> select the beam type 
  • 

• Then the live loads in the slabs need to be assigned.

  • Select Similar story --> Go to --> Assign --> Shell Load --> Uniform
  • 
• Then diaphragms property,select all the slabs from select option--> assign diaphragms
  • 
• Run Analysis
  • 

B. 5 story building number of bay X direction 4, Y direction 3 size of beam 400X400mm.

Ans:

• Then save the file with a different name --> Go to Edit --> Edit story and grid system data --> Modify story data --> Right click on stroy 2 row --> Keep existing story height 
  • 
  • Ok
  • 
• Check whether the loads shall be applied to all the floors or not.
  • 
• Checking for frame loads.
  • 
• Assigning Diaphragm.
  • 
• Run Analysis
  • 

C. 5 story building number of bay X direction 4, Y direction 3, size of column 550X300mm.

Ans:

• Go to Define --> Section Properties --> Frame Section--> Select C400 x 400 --> Modify/Show property
  • Change the required as per the new section size.
  • 
• Ok
  • 
• Assigning Diaphragm.
  • 
  • Run Analysis
    • 

D. 5 story building number of bay X direction 4, Y direction 3, size of column 300X550mm.

Ans:

• Go to Select  --> Select -->Properties --> Frame Section--> Select C400 x 400 -->Close
  • 
  • Go to  Assign --> Frame --> Local Axes-->change the angle as 90
    • 
  • Then click Ok
  • 
• Assigning Diaphragm.
  • 
  • Run Analysis
    • 

E. 10-story building with a number of bay X direction 4, Y direction 3, size of column 400x400mm for top 5 story & 600 x 600 m for bottom 5 stories.

Ans:

• Go to Edit --> Edit Story and grid system data -->Modify story data 
  • 
  • Then right click on top row -->Add story --> Keep existing storey heights-->Change number of added stories as 5
    • 
  • Ok
  • Change all similar to story 1
    • 
• Go to Define --> Section Properties --> Frame Section--> Select C600 x 600 --> Modify/Show property
  • Change the required as per the new section size.
  • 
• Ok
  • 
• Then again go to define --> Section properties --> Frame sections --> Add new Property --> Select rectangular
  • 
• Go to Modify Rebar--> Change rebar material to HYSD 415 for both
  • 
  • Ok --> Ok
• Then Go to view --> Set building view limits --> Select story range for the bottom story as "story 6" 
  • 
• Go to Set view option in the tool --> Deselct beams & Floors
  • 
• Ok
• Then select all columns --> Assign --> Frame --> Section property --> Select C 400 x 400 --> apply -->Ok
  • 
• Then again Go to view --> Set building view limits --> Select story range for the bottom story as "Base"-->Ok
  • 
• Then again go to set display options --> select beam & floors --> Apply -->Ok
  • 
• Assigning Diaphragm.
  • 
  • Run Analysis
    • 

F. A 10-story building with a number of bay X direction 4, Y direction 3, size of column 600x600mm for all the stories.

Ans:

• Open the previous model.
• Go to view -->Set building view limits --> Select range from story 6  to story 10
  • 
• Go to Set view option in the tool --> Deselct beams & Floors --. Apply
  • 
• Then Select all columns -->Assign --> Frame --> Section Properties --> Select C 600x600 --> Apply
  • 
• Then again go to set display options --> select beam & floors --> Apply -->Ok
  • 
• Assigning Diaphragm.
  • 
•  Run Analysis
  • 

G. A 25-story building with a number of bay X direction 4, Y direction 3, size of column 400x400mm for top 5 story, 600 x 600 m for 10 stories & 600 x 600 m for the bottom 10 stories.

Ans:

• Then save the file with a different name --> Go to Edit --> Edit story and grid system data --> Modify story data --> Right click on stroy 2 row --> Keep existing story height 
  • 
  • Ok
  • Change all similar to story 1
  • 
  • 
• Check whether the loads shall be applied to all the floors or not.
  • 
• Checking for frame loads.
  • 
• Go to view -->Set building view limits --> Select range from story 21  to story 25
  • 
• Go to Set view option in the tool --> Deselct beams & Floors --. Apply
  • 
• Then Select all columns -->Assign --> Frame --> Section Properties --> Select C 400x400 --> Apply
  • 
• Go to view -->Set building view limits --> Select range from Base to story 10
  • 
• Then again go to define --> Section properties --> Frame sections --> Add new Property --> Select rectangular
  • 
• Go to Modify Rebar--> Change rebar material to HYSD 415 for both
  • 
  • Ok --> Ok
• Then Select all columns -->Assign --> Frame --> Section Properties --> Select C 800x800 --> Apply
  • 
• Go to Set view option in the tool --> Deselct beams & Floors --. Apply
  • 
• Then again go to set display options --> select beam & floors --> Apply -->Ok
  • 
• Go to view -->Set building view limits --> Select range from Base to story 25
  • 
• Assigning Diaphragm.
  • 
• Run Analysis
  • 

H. A 25-story building with a number of bay X direction 4, Y direction 3, size of column 800x800mm for all the stories.

Ans:

• Open the previous model.
• Go to view -->Set building view limits --> Select range from story 11  to story 25
  • 
• Go to Set view option in the tool --> Deselct beams & Floors --> Apply
  • 
• Then Select all columns -->Assign --> Frame --> Section Properties --> Select C 800x800 --> Apply
  • 
• Go to view -->Set building view limits --> Select range from Base to story 25
  • 
  • 
• Then again go to set display options --> select beam & floors --> Apply -->Ok
  • 
• Assigning Diaphragm.
  • 
•  Run Analysis
  • 

J. A 25-story building with a number of bay X direction 4, Y direction 3, size of column 800x800mm for all the stories. With increase of 10% Live Load

Ans:

• Go to Select --> Select --> Properties --> Slab Section --> Slab 150 
  • 
• Go to --> Assign --> Shell Load --> Uniform --> Change Load Pattern as Live Load  --> Input load as 3.3 --> Replace existing loads-->Apply
  • 
• Ok
• Go to Select --> Select --> Properties --> Slab Section --> Slab 150
  • Assigning Diaphragm
  • 
• Run Analysis
  • 

J. A 25-story building with a number of bay X direction 4, Y direction 3, size of column 800x800mm for all the stories. With increase of 20% Live Load

Ans:

• Go to Select --> Select --> Properties --> Slab Section --> Slab 150 
  • 
• Go to --> Assign --> Shell Load --> Uniform --> Change Load Pattern as Live Load  --> Input load as 3.6 --> Replace existing loads-->Apply
  • 
• Ok
• Go to Select --> Select --> Properties --> Slab Section --> Slab 150
  • Assigning Diaphragm
  • 
• Run Analysis
  • 

Comparison:-

1. 

Criteria	Model	Analysis Of Result	Conclusion
Effect of Stiffness On time	Building-E	1.731 Sec
	(10 Story with Varrying column size)
	Building-F	1.735 Sec
	(10 Story with Uniform column size)
Effect of Stiffness On time	Building-G	4.17 Sec
	(25 Story with Varrying column size)
	Building-H	4.416 Sec
	(25 Story with Uniform column size)

2. 

Criteria	Model	Analysis Of Result	Conclusion
Effect of Mass On time period	Building-H	4.416 Sec	Mass  is directly propertional to the lateral force. So the lateral displacement is higher where the mass is higher.
	(25 Story with 3 KN/Sq.m Imposed Load)
	Building-J	4.425 Sec
	(25 Story with 3.3 KN/Sq.m Imposed Load)
	Building-K	4.435 Sec
	(25 Story with 3.6 KN/Sq.m Imposed Load)

3.

Criteria	Model	Analysis Of Result	Conclusion
Effect of Building Height On time period	Building-A	0.5 Sec	Ht. of building is directly propertional to the Time period. So as the height of building increses the time period also increases.
	(2 Story )
	Building-B	1.036 Sec
	(5 Story)
	Building-F	1.735 Sec
	(10 Story)
	Building-H	4.416 Sec
	(25 Story)

4.

Criteria	Model	Analysis Of Result	Conclusion
Effect of column orientation On time period	Building-B	1.036 Sec
	(All floors column size 400x400)
	Building-C	1.145 Sec
	(All floor column size 550x300) Oriented in X-direction
	Building-D	1.12 Sec
	(All floor column size 300x550) Oriented in Y-direction

5.

Criteria	Model	Case	Analysis Of Result	Conclusion
Effect of Flexural Stiffness of structural elements on mode shapes	Building-B	Flexural Stiffness of beams close to nil (Zero Inertia)	2.627 Sec	For zero inertia beam the time period is aprrox double of the regular stiffness
		Flexural Stiffness of beams close to nil (Regular)	1.036 Sec

6.

Criteria	Model	Case	Analysis Of Result	Conclusion
Effect of Axial Stiffness of vertical members on mode shapes	Building-H	Axial cross-section for columns is close to nil (Zero Inertia)	5.991 Sec	Significantly Affect
		Full axial cross section for columns (Regular)	4.416 Sec

7.

Criteria	Model	Case	Analysis Of Result	Conclusion
Effect of Degree of fixity at column base on mode shapes	Building-B	Column support pinned (Regular)	1.036 Sec	More the fixity Lesser the time period.
		Column support fixed	0.941 Sec