Challenge

Project 1_Comparative study of different storey buildings for Seismic forces

NNeha Solanki19 Apr 2023

AIM : Comparative study of different storey buildings for Seismic forces

Factors influencing the dynamic characteristics of a building
Factors influencing the Natural Period of a building
Factors influencing the Mode shape of oscillations

INTRODUCTION : There are 10 project of different story building (A, B, C, D, E, F, G, H, J, K). These are compared for the different sesmic forces and further comparision of Natural period , Mode Shape to be done in ETABs software.

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 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 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.

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 period. But, in the mathematical modeling of 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 the 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 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.
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.
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.

PROCEDURE :

Material Properties assigned as :

Grade of Concrete : M30

Grade of Steel Reinforcement Bars : Fe 415

Loading :

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

Live load on floor : 3 KN/m2 for building A,B,C,D,E,F,G,H

3.3KN/m2 for building J

3.6KN/m2 for building K

Buildings are pinned at the base

Building is located on Soil type-II and in Zone-III.

For analysis of the structure IS 1893 : 2016 code is followed

Below table represents column sizes of different buildings

Building	Description	No.of storeys	No. of bays in X direction	No. of bays in Y direction	Column Dimension (mm)
A	2 storey building	2	4	3	400x400
B	Benchmark 5 storey building	5	4	3	400x400
C	Benchmark building with rectangular columns oriented along X direction	5	4	3	550x300
D	Benchmark building with rectangular columns oriented along Y direction	5	4	3	300x550
E	10 Storey building with varying column size along building height	10	4	3	Upper storey 400 x 400 Bottom storey 600 x 600
F	10 storey building	10	4	3	600 x 600
G	25 storey building with varying column size along building height	25	4	3	Upper 5 storey 400 x 400 Middle 10 storey 600 x 600 Bottom 10 storey 800 x 800
H	25 storey building	25	4	3	800 x 800
J	25 storey building with imposed mass 10% larger than building H	25	4	3	800 x 800
K	25 storey building with imposed mass 10% larger than building H	25	4	3	800 x 800

All beams in the building have same size of 300x400mm
All slabs in the building have same thickness 150mm

STEPS FOR ALL 10 BUILDING MENTIONED ABOVE :

Open Etabs
Enter units and select steel design (IS 800) and concrete design(IS456)
Create grids of 4 bays in X direction and 3 bays in Y direction at 4m distance each
Create story's according to the type of building in which each story height is 3m and plinth level is at 1.5m.

Define Material Properties
Click on Define tab and select Define material.
Choose concrete M30 and rebar of HYSD415.

Section Properties
Click on Define Tab and select Section Properties then go to Framed Properties for beam and column, go to Slab property for slab .
Define Beam, Column, Slab sections based on the given data.

Draw Columns and Beams
Now draw beams, columns and slabs using Draw Tool.

Define Load Pattern
Add new Load as Dead load, Live Load and Wall Load (Super Dead), seismic load as (EQX, EQY)
Define load patterns and modify seismic load by defining zone, soil type, timeperiod.

Load Combination, Add Default Design Combos From Load Combination Tab.
Mass Source, Define mass source from Define tab.
Modify mass source Live=0.25, Dead=1,Wall=1

Assign Loads
Select all the Beams and go to Assign Tab and then Frame load tab.
Assign Distributed load of 10kn/m^2 on all the beams.
Assign Loads on the floors by selecting all the floors.
Go to shell Loads and give load as 3kn/m^2 as live.

Assign Diagphrams
Select all Floors and go to Assign Tab and then Shell tab.
Assign Diaphragm (D1) on all the floors.

Finally Check All the Models and then Analyze.

Building A:

Two-story building
Beam : 300mm x 400 mm
Column : 400mm X 400 mm
Slab : 150mm
Grade M30, Fe 415,
Brick wall load = 10 Kn/m
Live load 3 KN/m^2

Building B:

5 -story building
Beam : 300mm x 400 mm
Column : 400mm X 400 mm
Slab : 150mm
Grade M30, Fe 415,
Brick wall load = 10 Kn/m
Live load 3 KN/m^2

Building C :

5 -story building
Beam : 300mm x 400 mm
Column : 550mm X 300 mm
Slab : 150mm
Grade M30, Fe 415,
Brick wall load = 10 Kn/m
Live load 3 KN/m^2

Building D:

5 -story building
Beam : 300mm x 400 mm
Column : 300mm X 550 mm
Slab : 150mm
Grade M30, Fe 415,
Brick wall load = 10 Kn/m
Live load 3 KN/m^2

Building E:

10 -story building
Beam : 300mm x 400 mm
Column : Bottom 5 ( 600mm X 600 mm ) & Upper 5 ( 400 mm X 400 mm )
Slab : 150mm
Grade M30, Fe 415,
Brick wall load = 10 Kn/m
Live load 3 KN/m^2

Building F:

10 -story building
Beam : 300mm x 400 mm
Column :600mm x 600 mm
Slab : 150mm
Grade M30, Fe 415,
Brick wall load = 10 Kn/m
Live load 3 KN/m^2

Building G:

25 -story building
Beam : 300mm x 400 mm
Column : Bottom 10 ( 800mm X 800 mm ); Upper 10 ( 600 mm X 600 mm ); top upper 5 ( 400 mm x 400 mm)
Slab : 150mm
Grade M30, Fe 415
Brick wall load = 10 Kn/m
Live load 3 KN/m^2

Buiding H:

25 -story building
Beam : 300mm x 400 mm
Column :800 x 800 mm
Slab : 150mm
Grade M30, Fe 415,
Brick wall load = 10 Kn/m
Live load 3 KN/m^2

Building J:

25 -story building
Beam : 300mm x 400 mm
Column :800 x 800 mm
Slab : 150mm
Grade M30, Fe 415,
Brick wall load = 10 Kn/m
Live load 3 KN/m^2 ( 10 percent )
with imposed mass 10 percent larger than h, therefore the imposed mass is 0.3

Building K :

25 -story building
Beam : 300mm x 400 mm
Column :800 x 800 mm
Slab : 150mm
Grade M30, Fe 415,
Brick wall load = 10 Kn/m
Live load 3 KN/m^2 ( 60 percent ) with imposed mass 20 percent larger than h, therefore the imposed mass is 0.45

RESULTS : COMPARISION

All the 10 buildings were modelled and analysed successfully.

Effect of stiffness on T: Compare fundamental nature periods of building E&F as well as G & H. There a marginal or significant difference in the fundamental natural periods

Model Type	No of Stories	Column configuration	Results Conclusion
Building E	10	Upper 5 stories = 400x400 mm Bottom 5 Stories = 600x600 mm	Changing the column sizes there will not be any effect on Natural period, it remains Same
Building F	10	Column Size = 600x600mm	Building H has uniform column section but it has more natural time period as compared to building F
Building G	25	Column Sizes Bottom 10 story (800x800 mm) Upper 10 story (600x600 mm) Top upper 5 story (400x400 mm)	Building G has large column at its base and reduces as we go higher level. But it has less natural time period as compared to H Providing larger size of column at base and reducing the size at higher level will reduces the Natural time period
Building H	25	Column size = 800x800 mm	Building H has uniform column section but it has more natural time period as compared to building F

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

Model Type	No of stories	Percentage increase in Load (Mass)	Conclusion
Building H	25	Load is taken as 3KN/m2	The time period is increased as we increase the load
Building J	25	Load Increase by 10% (3.3kn/m2)	The time period is directly proportional to the mass of the building
Building K	25	Load Increase by 20% (3.6kn/m2)	Time period is directly proportional to the mass of the building

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?

Model Type	No of stories	Height of Building	Conclusion
Building A	2	7.5 m	The height of the building increases the natural time period is also increase
Building B	5	16.5 m	As we could see that the height of the building increases the natural time period is also increase
Building F	10	31.5 m	Therefore, the natural period 'T' is also directly proportional to the building height
Building H	25	46.5 m	The natural period 'T' is also directly proportional to the building height

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

Model Type	No of stories	Column orientation	Conclusion
Building B	5	Equal in Both Direction 400x400 mm	When column is orientad in X and Y direction in Time period, Y direction Lateral Displacement is more
Building C	5	In X direction 550x300 mm	Y direction Lateral Displacement is more as compared to X direction
Building D	5	In Y direction 300x550 mm

Effect of flexural stiffness of structural elements on mode shapes:

Compare fundamental mod shape of building B in two situations when the flexural stiffness of beams relative to that of adjoining columns is very small versus it is large

Model Type	No of stories	Flexural stiffness of beam	Conclusion
Building B	5	100%	If you decrease the flexural stiffness of beams,the overall flexural strenght of the beam is also going to reduce which increase the Natural period of time

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.

Model Type	No of stories	Axial Stiffness of Columns	Conclusion
Building H	25	100%	As we axial stiffness of columns decrease, 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 dituations when the base of columns is pinned versus when it is fixed.

Model Type

No of stories

Base support

Conclusion

Building B

Pinned

As we can see that in pinned support natural time period is more as compared to fixed support.

Therefore fixed support will provide more resistance as compared to pinned support.

BUILDING A