 Open Access
 Total Downloads : 4082
 Authors : Mr. Mohammed Siraj
 Paper ID : IJERTV1IS7442
 Volume & Issue : Volume 01, Issue 07 (September 2012)
 Published (First Online): 29092012
 ISSN (Online) : 22780181
 Publisher Name : IJERT
 License: This work is licensed under a Creative Commons Attribution 4.0 International License
Modal Analysis Of Plane Frames
Mr. Mohammed Siraj
Professor, Department of Civil Engineering, Deogiri Institute of Engineering and Management Studies Aurangabad, M.S, India.
ABSTRACT
In the modal analysis of plane frames the main aim is to determine the natural mode shapes and frequencies of an object or structure during free vibration. The dynamic analysis of frames requires inclusion of axial effect in the stiffness and mass matrices. It also requires a coordinate transformation of the nodal or local coordinates to global coordinates. The analysis is performed using Modal Analysis. The physical interpretation of the eigenvalues and eigenvectors which come from solving the system are that they represent the frequencies and corresponding mode shapes. The objective of this paper is to study the vibration, frequency and mode shape of plane frames. Formulation of stiffness matrix and mass matrix are to be done using direct stiffness method. Programs are to be developed using ANSYS and MATLAB codes.

Introduction
Modal analysis is the study of the dynamic properties of structures under vibrational excitation. When a structure undergoes an external excitation, its dynamic responses are measured and analysed. This field of measuring and analysing is called modal analysis. In structural engineering, modal analysis is applied to find the various periods that the structure will naturally resonate at, by using the structure's overall mass and stiffness. The modal analysis is very important in earthquake engineering, because the periods of vibration evaluated helps in checking that a building's natural frequency does not coincide with the frequency of earthquakes prone region where the building is to be constructed. In case a structure's natural frequency coincidently equals an earthquake's frequency, the structure suffers severe structural
damage due to resonance. The frequency and mode shape of a model is determined by modal analysis. When the models are subjected to cyclic or vibration loads, the dynamic response of structures due to these external loads acting, which include resonance frequencies (natural frequencies), mode shape and damping, are estimated.
1.1 Formation of Stiffness Matrix
In Fig. 1a, we assume a member of a plane frame in the member coordinate system x'y'z' whose global orthogonal set of axes are xyz. Some of the assumptions to be considered are:

The plane in which the frame lies is xy plane.

All the members of the plane frame should have uniform flexural rigidity EI and uniform axial rigidity EA.
This modal analysis takes the axial deformation of member into consideration. The possible displacements at each node of the member are: translation in x' and y' direction and rotation about z' axis.
x'
stiffness matrix at the coordinates of each node of the frame member is obtained. Thus, the local coordinate axes for the plane frame can be represented by the loaddisplacement relation as shown in Fig a, b as, (eq. 1a)
u5'
u4'
K
= (1a)
y' u6'
u2'
J
u1' u3'
z'

Member displacement
x'
q5' q4'
K
y' q6'
q2'
J
q1' q3'
z'

Member Forces

This may be compactly written as (eq. 1b), = . (1b)
Where is the member stiffness matrix. Another method of obtaining the member stiffness matrix is calculating resultant restraint actions by applying one at a time unit displacement along each possible displacement degree of freedom.


ILLUSTRATIVE EXAMPLE
A twostorey steel rigid frame is to be analyzed whose weights of the floors and walls inclusive of the structural weights are indicated in the figure. The frames of the building are 15ft apart. The structural properties along the length of the structure are assumed to be uniform.
W1 = 150 lb/ft
u1
W 10 x 21
20
W1 = 150 lb/ft2
From the above Fig. 1a we can see that the
frame members have six (6) degrees of freedom. u2
Fig. 1b shows the member being subjected to forces at nodes j and k. By merging stiffness matrix for axial effects and the stiffness matrix for flexural effects into a single matrix, the
W 10 x 45
20
40
Natural Frequencies and Modal Shapes: According to the lumped mass system, the concentrated weights are the total of floor weight and the weight of tributary walls.
W1 = 150x40x15 + 20x20x15x2 = 102,000 lb
m1 = 265 /in
W2 = 75x40x15 + 20x10x15x2= 51,000 lb
m2 = 132 lb.sec2/in
For each storey the stiffness coefficient is calculated as,
Using dynamic equilibrium on every element of the system which is free from external vibration, the equations of motion are obtained as already discussed which when solved give the natural frequencies as,
1 = 7.542 rad/sec
2 = 20.334 rad/sec
in cycles per second f1 = 1/2 = 1.2 cps
f2 = 2/2 = 3.236 cps
The time periods are, T1 = 1/ f1 = 0.833 sec T2 = 1/ f2 = 0.309 sec
Substituting 1 = 7.542 rad/sec in the matrix equation,
3526 a11 5600 a21 = 0
By assigning a unit value to one of the amplitudes,
a11 = 1
a21 = 0.63
Likewise putting 2 = 20.334 rad/sec, we get the second normal modes,
a12 = 1
a22 = 6.42
Normalized Modal Shapes of Vibration
Normalized modes, Ã˜ij
By putting the values of amplitudes already calculated and the masses,
=
Normalized modes are,
Ã˜11 = = 0.056; Ã˜12 = = 0.0132
Ã˜21 = = 0.0354; Ã˜22 = = 0.0852
To satisfy the orthogonality equation,
[M] [Ã˜] = [I] [Ã˜] =
X
=
MATLAB Results:
These values are also confirmed by the MATLAB results which gave the following values:
Figures below shows the modal shapes for the two modes of the shear building
For fourstorey shear building
W4=100 lb/ft W3=100 lb/ft W2=100 lb/ft W1=100 lb/ft2
10
10
10
30 15
A
s s o c i a
t
In the similar way as in the case of twostorey shear building, the natural angular frequencies of the considered system computed by MATLAB were
Figures below shows the modal shapes for the four modes of the shear building

CONCLUSION
After obtaining the natural frequencies and the Eigen vectors for twostorey shear building manually, we compared it with the results of the MATLAB code and the natural frequencies in both cases were found out to be 7.542 rad/sec and 20.334 rad/sec.
After getting the same results, we created a generalized MATLAB code for nstorey shear building and obtained the natural frequencies for fourstorey building to be 5.218 rad/sec, 15.0245 rad/sec, 23.0189 rad/sec and 28.236 rad/sec. With the same MATLAB code we obtained the mode shape for each degree of freedom for specific natural frequency already calculated.
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