Project 1_Comparative study of different storey buildings for Seismic forces

PROJECT 1

Comparative study of different storey buildings for Seismic forces :-

Aim:-

• To model building from A to K as per the deatils given in below table :-

• To analyse compartive study of building for all the building for Sesimic forces.

Introduction:-

Every building has a number of natural frequencies at which it offers minimum resistance to shaking induced by external & internal effects.

Each of these natural frequencies and its associated deformation shape of the building constitute a natural mode of oscillation.

The mode of oscillation with smallest natural frequency and largest natural period is called fundamental mode and the associated natural period, T1 is called fundamental natural period.

In this project we need to find out the effects of various parameters on Natural period of the building as well as on the mode shapes of oscillation details of which are given below:

• Factors affecting natural period of building :

• Effect of stiffness on T :

Compare fundamental natural period of buildings E & F as well as G & H. Why is there a marginal or significant differenece in the fundamental natural period.

• Effect of mass on T :

Compare fundamental natural period of buildings H, J & K. Have the buildings become more flexible or stiff due to change in mass.

• Effect of building Height on T :

How does fundamental natural period of buildings A, B, F & H chnage with change in building height.

• Effect of Column orientation on T :

How does fundamental natural period of buildings B, C & D chnage with change in column orientation.

• Factors influencing the mode shapes of oscillations :

• Effect of Flexural Stiffness of Structural elements on mode shape :

Compare fundamental mode shape of buildings 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 shape :

Compare fundamental mode shape of buildings H in two situtaions 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:-

• Initially open ETABS software and provide units, storey and grid system

• Now define material property and section properties.

• Now, develop column, beams and slabs.
• Frame component has to be assigned as given below

• Now we will assign loads to floor members and frame.
• After assigning all loads, check model and run anaylsis.
• Building A

• Similarly, carry out modelling for other building also.
• Building B

• Building C

• Building D

• Building E

• Building F

• Building G

• Building H

• Building J

--- Imposed mass 10% larger i.e. live load =3.3 KN/sq.m.

• Building K

--- Imposed 20% larger i.e. live load =3.6 KN/sq.m.

• Hence modelling of building has completed.
• Now comparision needs to carried out.

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

• For building E & F :

Building E Building F

Tx = 1.59s Tx = 1.635s

Ty = 1.59s Ty = 1.635s

Tz = 1.434s Tz = 1.495s

• For building 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 the building E wheras in building F the column size is uniform. similiarly, in building G & H also both the building are 25 storey building but their masses varies due to the variatio in column sizes.

Building E & G has cpmpatitively lesser mass compared to the building F & H resp. and as such the time period of the E & G is also less than of F & H resp.

Now, considering the stiffness of the structure, in both the structure the columns are arranged in regaular manner and hence there wont be much shift in the center of stiffness from center of mass. hence the small differeme in time period of the two building are mainly due to differenece in their masses.

• Effect of mass on T : (Compareing 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 three building have same storey height column size but mass imposed on it varies.

Here, the variation of time period in three building is very minimal in range 0.008 and this variation is due to variation mass imposed on it which adds to the total mass of the buiding. Since the time period is directly propportional to mass the variation in time period is given as K > J> H.

Thus from above we get to know that K > J> H.

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

Building A(2 storey) Building B(5 storey) Building F(10 storey) Building F(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 Tx = 3.773s

Thus from above table it is clear that with increase in height the time period of building increases.

This is because as the height increase the stiffness of the building decreses as stiffness is inversely proportional to height of the structure and as the stiffness decreases the time period of the building increases as

• Effect of column orientation on T : (Comparing building B, C and D)

In building B, all column are square i.e. 400X400 mm , hence the entire stiffness of the buikding will be eqaula in both X and y -direction.

Building B ( Column 400X 400)

Tx = 0.988s

Ty = 1.009s

Tz = 0.89s

In building C the longer edge of column is arranged in X-direction as shown in the figure below. hence the stiffness will be more in X-direction compared to y direction. So, the time period will be lesser in X-direction and it will be more in y-direction.that is the reason that for this building the time period in X-direction is that least.

Building C ( Column 550X 300)

Tx = 0.837s

Ty = 1.041s

Tz = 0.757s

In building D the longer edge of column is arranged in X-direction as shown in the figure below. hence the stiffness will be more in X-direction compared to y direction. So, the time period will be lesser in X-direction and it will be more in y-direction.that is the reason that for this building the time period in X-direction is that least.

Building D ( Column 300 X 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 original building B.

2. For the building B with flexural stiffness very small compared to the original building.

So, far 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 proportionally between time period and stiffness.

Effect of axial stiffness of vertical members on mode shapes:

Comparing the fundamental mode shapes of the building H in two situations:

1. For original building H.

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

So, far this we need to develop another model H with smaller axial cross sectional 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 axial stiffness of column is reduced, the time period of the building is increased.

Effect of Degree od fixidity at column bases on mode shapes:

Comparing the fundamental mode shapes of the building B in two situations:

1. For original building B i.e. when the base is pinned.

2. Another model of the building B, when the base of 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.

Result:-

All the 10 model from A to K has been analysed and comparision study is also carried out with regarding to time period of the structure.