Project 1_Comparative study of different storey buildings for Seismic forces

introduction -

 

ETABS OR ETABS  they meAn the same  its 

full form is extended 3D Analysis  of 

buillding system . its engineering software 

product thats caters  to muiltistorey 

building design and analysis .

-ETABS offer single user interface 

analysis  to perform  modelling ., analysis ,

design ,and , reporting .

there is no limit to the number of 

modelling windows model manipulation 

 

PROCEDURE-

INTIAlly open  ETABS SOFTWARE 

and provides units ,grids ,and storey information 

 

-and define material property and section property .

 

-and now deploped column ,beam and slabs .

-and frame components have been assigned 

as given below .

 

-now we need to assign load to frame member

and floors .

-after assign all loads , check model run and 

 analysis.

 

EFFECT OF STIFFNESS ON  T 

[CONSIDERING FRIST 3 MODES ]

 

FOR BUILDING  E AND  F :

 

BUILDING E                  BUILDING F 

 

TX  1.59s                      Tx  1.635

 

Ty   1.638s                  Ty 1.683s

 

 Tz    1.434   s                Tz 1.495s

 

 

 

 

 

Procedure:

• Initially open the ETABS software and
• provide the units, grid and storey informations.

• Now, define the material property and section properties.

 

 

 

 

 

 

 

• Now, we need to assign loads to the
• frame members and floors.
• After assigning all the loads,
• check the model and run the analysis.
• Building A:

 

 

 

 

 

• Building C:

• Building D:

 

 

 

• Building F:

• Building G:

 

 

• Building F:

 

 

 

 

• Building J:

- Imposed mass 10% larger i.e,

Live load = 3.3kN/

• Building K:

- Imposed mass 20% larger i.e, Live load = 3.6kN/

• Hence, modelling of the buildings have been completed.
• Now, comparison needs to be carried out.

- Effect of stiffness on T: ( considering first three modes)

• For buildings E & F:

  • Building E	Building F
    Tx = 1.59s	Tx = 1.635s
    Ty = 1.638s	Ty = 1.683s
    Tz = 1.434s	Tz = 1.495s

• For buildings G & H:

  • Building G	Building H
    Tx = 3.982s	Tx = 4.158s
    Ty = 4.154s	Ty = 4.338s
    Tz = 3.552s	Tz = 3.773s

In both E & F and G & H, the difference in time period is very less.

In building E & F, the total mass of the

structure varies as the column size

varies in Building E whereas in building F

the column size is uniform.

Similarly, in building G & H also both

the buildings are 25 storey

buildings but their masses varies due

to the variation in column sizes.

Building E and G  has comparatively

lesser mass compared to the

building F and H respectively, and

as such the time period

of the E and G is also less than

that of F and H respectively.

Now, considering the stiffness

of the structures, in both the

structures the columns are arranged in a regular manner

and hence the there wont be much shift in the centre of

stiffness from centre of mass.  Hence, the small

difference in time period of the two buildings

are mainly due to the difference in their masses.

 T = $\left(2\pi \right)\cdot \sqrt{\frac{\mathrm{m/}}{k}}$. Hence, 

 

 and 

$T\propto \frac{1}{\sqrt{k}}$

 

- Effect of mass on T : ( comparing H, J and K buildings)

Building H	Building J	Building K
Tx = 4.158s	Tx = 4.166s	Tx = 4.174s
Ty = 4.338s	Ty = 4.346s	Ty = 4.354s
Tz = 3.773s	Tz = 3.778s	Tz = 3.784s

All the three buildings have same storey

height and column sizes, but the mass imposed on it varies.

Here, the variation of time period in the three

buildings is very minimal in the range 0.008 and this variation is

due to the variation in the mass imposed on

it which adds to the total mass of the building. Since the time period

is directly proportional to mass, the

variation in time period is given as K>J>H.

- Effect of building height on T: ( Comparing buildings A,B,F and H)

Building A (2 storey)	Building B (5 Storey)	Building F (10 Storey)	Building H (25 Storey)
Tx = 0.439s	Tx = 0.988s	Tx = 1.635s	Tx = 4.158s
Ty = 0.447s	Ty = 1.009s	Ty = 1.683s	Ty = 4.338s
Tz = 0.397s	Tz = 0.89s	Tz = 1.495s	Tz = 3.773s

From the above table it is clear that, with the increase

in height the time period of the building increases.

This is because as the height increases the stiffness

of the building decreases, as stiffness

is inversely proportional to height of the

structure and as the stiffness decreases

the time perios of the building increases

 

as     $T\propto \frac{1}{\sqrt{k}}$

 

- Effect of column orientation on T: ( Comparing buildings B, C and D)

• In Building B, all the columns are square i.e 400mmx400mm, he

 

• In Building B, all the columns are square i.e 400mmx400mm, he

    hence the entire stiffness of building is equal 

in both in x and  Y  direction .

 

BUILDING  B [COLUMN  400X400]

 

Tx   =.988s

 

Ty=1.009s 

 

Tz=.899s

 in building c longer edege of column arranged in  X 

direction and lesser edges of column in  Y direction 

so time period is lesser in x direction  wrt. to  y 

direction , that is reason the time period is lesser 

in  x direction .

and stiffness is more where column edges  are more 

so stiffness is more in  x  direction .

 

BUILDING  C [COLUMN 550X300]

 

Tx  =.83S

 

Ty= 1.04 S 

 

Tz= .75 S 

 

in building D ,longer edege of column arranged in  Y

direction and lesser edges of column in  X direction 

so time period is lesser in Y direction  wrt. to  X

direction , that is reason the time period is lesser 

in  Y direction .

and stiffness is more where column edges  are more 

so stiffness is more in  Y direction .

 

 

-BUILDING  D  [COLUMN 300X 550]

 

 

Tx = 0.916s

Ty = 1.109s

Tz = 0.869s

-Effect of flexural stiffness of structural

elements on mode shapes: 

Comparing the fundamental mode shapes of

the building B in two situations:

1. For the original building B.
2. For the building B with flexural stiffness very

small compared to the original building.

So, for this we need to develop another model

B with lesser flexural stiffness of beam.

In this model the moment of inertia about

the 2 and 3 axis is changed to 0.1.

 

Original building B	Modified building B1
Tx = 0.988s	Tx = 1.768s
Ty = 1.009s	Ty = 1.791s
Tz = 0.89s	Tz = 1.696s

From table, it is clear that the time period of the building has been increased.

As the flexural stiffness of the beam is reduced, the building becomes more

flexible and the time period is increased hence maintaining the inverse

proportionality between time period and stiffness.

- Effect of Axial stiffness of

vertical members on mode shapes:

Comparing the mode shape of building H in two situation:

1. Original building H
2. When axial cross sectional area of the column is reduced by a factor of 0.1. 

So, for this we need to develop

another model H with smaller axial cross section of column.

 

 

 

Original building H	Modified building H1
Tx = 4.158s	Tx = 5.412s
Ty = 4.338s	Ty = 6.145s
Tz = 3.773s	Tz = 3.82s

Here also, as the axial stiffness of column is reduced,

the time period of the building is increased.

- Effect of Degree of fixidity at column bases on mode shape:

Comparing the mode shape of building B in two situation:

1. Original building B i.e when the base is pinned 
2. Another model of the building B,

when the base of the column is fixed. 

Original building B Modified building B2  Tx = 0.988s  Tx = 0.967s  Ty = 1.009s  Ty = 0.987s  Tz = 0.89s  Tz = 0.87s From the table we can understand that, as the support is changed to fixed support the time period of the building is reduced.

 

 

Results:

All the 10 model from A to K has been

analysed and comparison study is also

carried out with regarding to the time

period of the structure.