Project 1_Comparative study of different storey buildings for Seismic forces

1.AIM:

Factors influencing the dynamic characteristics of a building.

Factors influencing the Natural Period of a building.

Factors influencing the Mode shape of oscillations.

INTRODUCTION:

Factors influencing the dynamic characteristics of a building

In many practical circumstances, the vibration characteristics of a dynamic structure require to be understood and, subsequently, an accurate mathematical model needs to be derived. Such a model is needed for response and load prediction, stability analysis, system design, structural coupling etc. Since a dynamic structure is a continuous system rather than a discrete one, theoretically, an infinite number of coordinates are necessary to specify the position of every point on the structure and hence the structure can be said to have an infinite number of degrees of freedom. Its vibration characteristics should then include an infinite number of vibration modes and cover the active frequency range from zero to infinity. However, for most practical applications, only a certain frequency range is of major interest and only those vibration characteristics which fall in this range will be investigated. In this case, only a certain number of vibration modes are to be sought and it becomes feasible to represent the continuous system by an approximate, discrete, one. For a discrete linear dynamic system with lumped masses and massless elastic components, theory has been well developed to study such vibration characteristics. This is because the differential equations of a discrete linear dynamic system are generally available, and hence mathematics can be introduced directly to solve the equations of motion and the vibration characteristics can then be defined accurately. For a truly continuous system, such as a practical structure, such advantages do not exist. However, like many other sciences to achieve good approximation by discretization, the strategy of investigating the vibration characteristics of a practical structure relies basically on the hypothesis of discretizing the structure so that the theory for discrete systems can then apply and the mathematical model for the structure can then be built. It is evident that as
the number of coordinates employed in the discretization approaches infinity, the discrete system will approach the continuous one. Basically, there are two ways of achieving a mathematical model for a dynamic structure
with the help of the discretization concept, they being by theoretical prediction and by experimental measurement respectively. Both approaches effectively assume that the vibration characteristics of a continuous system within certain frequency range can be described approximately by a limited number of coordinates. In the following, both approaches are reviewed briefly.

Factors influencing the Natural Period of a building

As the height of building increases, its mass increases but its overall stiffness decreases. Hence, the natural period of a building increases with increase in height. Buildings A, B, F and H have same plan size, but are of different heights.

Factors influencing the Mode shape of oscillation

1.Effect of Flexural Stiffness of Structural Elements

The overall lateral translational mode shapes depend on the flexural stiffness of beams relative to that of adjoining columns. The fundamental mode shape of buildings changes from flexural-type to shear-type as beam flexural stiffness increases relative to that of column.

2.Effect of Axial Stiffness of Vertical Members

Mode shapes depend on axial stiffness of vertical members in a building (i.e., of columns or structural walls).

Small Axial Stiffness causes significant axial compressive and tensile deformation in columns in addition to single or double curvature flexural deformations.

Additional Axial Deformation changes the fundamental mode shape from shear-type to flexural type, particularly in tall buildings.

Pure Flexural Response is not desirable because of large lateral sway, particularly at higher floors. Hence, designers ensure that the axial areas are large of building columns and structural walls.

3.Effect of Degree of Fixity at Member Ends

Highly Flexible soils make column bases as good as hinged, and rocky layers below as good as fixed.

Lack of Rotational Fixity at the Column Base (hinged condition) increases the lateral sway in the lower storey than in the higher storey, and the overall response of the building is more of shear-type.

On the other hand, full rotational fixity at the column base restricts the lateral sway at the first storey and thus, induces initial flexural behaviour near the base.

The overall response of the building is still of shear-type due to flexural stiffness of beams.

4. Effect of Building Height

In well-designed Low Height moment frame buildings, the fundamental translational mode of oscillation is of shear-type.

Buildings become laterally flexible as their height increases. As a result, the natural period of buildings increases with an increase in height. However, the fundamental mode shape does not change significantly (from shear type to flexure type).

Flexural type behaviour is exhibited only near the lower storeys where the axial deformation in the columns could be significant, particularly in tall buildings. However, at higher floor levels, the response changes to shear type as the axial load level lowers.

5. Effect of Unreinforced Masonry Infill Walls in RC Frames

URM Infill Walls are not considered in Analysis and Design of RC Frame Buildings in current design practice in many countries.

They are assumed to not carry any vertical or lateral forces, and hence, declared as non-structural elements insofar as the transfer of forces is concerned between structural elements (e.g., beams and columns) that are generated in the building during earthquake shaking.

Model-"A"

PROCEDURE:

• Set the Units as SI and select the indian standard code book for steel and concrete.

• Set the Grids orientation and how much you want a space between the orientation.
• I just taken for 4m for both length and width of the room.

• There are story on our project ,so the plinth level is 1.5m and the story 1 and 2 is 3m height.
• Hear i choosed the master story is Story1.

• This is the procedure to define all the material properties.
• Define>Material properties,Then i added the M30 concrete.
• After that i added the 415HYSD and 500HYSD rebars.

• To add the slab and change the sizes is Define > Section properties>Slab section.
• Then chance the size of the slab i changed 150mm thickness for the slab.

• To add load patterns go to Define > Load Patterns.
• Then add Live load,Dead load,Brick wall load,Eart quack in X direction and Eart quack in Y direction.

• Now we have to add Mass Source so go to Define> Mass Source.
• Modify the mass source1 and select the specfied load patterns and add live,dead,eartquack x and earthquack y load.

Result:

Model-"B"

RESULT:

Model-"C"

RESULT:

MODEL-"D"

RESULT:

MODEL-"E"

RESULT:

Model-"F"

RESULT:

Model-"G"

RESULT:

Model-"H"

RESULT:

Model-"J"

RESULT:

Model-"K"

RESULT:

TIME PERIODS:

MODAL-A

MODEL-B

MODEL-C

MODEL-D

MODEL-E

MODEL-F

MODEL-G

MODEL-H

MODEL-J

MODEL-K

DISPLACEMENT:

MODEL-A(X)

MODEL-A(Y)

MODEL-B(X)

MODEL-B(Y)

MODEL-C(X)

MODEL-C(Y)

MODEL-D(X)

MODEL-D(Y)

MODEL-E(X)

MODEL-E(Y)

MODEL-F(X)

MODEL-F(Y)

MODEL-G(X)

MODEL-G(Y)

MODEL-H(X)

MODEL-H(Y)

MODEL-J(X)

MODEL-J(Y)

MODEL-K(X)

MODEL-K(Y)

Flexural stiffness:

Fixed support:

Column distribution loads:

Building E (10 storey with various column size load)=1.73s

Building F (10 storey with various column size load)=0.97s

Building G (25 storey with various column size load)=2.442s

Building H (25 storey with various column size load)=2.144s

Building H (25 storey with 3kM/sq.m imposed load)= 4.57s

Building j (25 storey with 3.3kM/sq.m imposed load)=2.147s

Building k (25 storey with 3.6kM/sq.m imposed load)= 2.15s

Effect of building height on time period:

Building A (2 Storey)= 0.5s

Building B (5 Storey)= 1.036s

Building F (10 Storey)=0.97s

Building H (25 Storey)=2.144s

Effect of column orientation on time period:

Building B (All floor column size 400*400)= 1.08s

Building C (All floor column size 550*300)oriented in x direction= 1.03s

Building D (All floor column size 300*550)oriented in y direction= 1.14s