Understanding the concepts on Degrees of Freedom

AIM : To calculate the,

1. Modal property Ln for each of the five modes
2. Modal mass Mn for each of the five modes
3. Modal participation factor Γn for each of the five modes
4. Effective modal mass Mn* and
5. Modal mass participating ratio for the 5 modes
6. Obtain the base shear for each of the 5 modes of vibration
7. The combined base shear as per SRSS rule

Of a five story structure with 5 translational DOF . Each story has mass ‘m’. The eigenvalue problem has been solved and the 5 periods of vibration Tn and their corresponding mode shapes, φn has been derived.

QUESTIONS : A five storey structure with 5 translational DOF is shown here. Each storey has mass ‘m’. The eigenvalue problem has been solved and the 5 periods of vibration Tn and their corresponding mode shapes, φn has been derived.

The fundamental period of vibration (T1 = 2 seconds) and its associated mode shape:

The second mode of vibration (T2 = 0.6852 seconds) and its associated mode shape:

The third mode of vibration (T3 = 0.4346 seconds) and its associated mode shape:

The fourth mode of vibration (T4 = 0.3383 seconds) and its associated mode shape:

The fifth mode of vibration (T4 = 0.2966 seconds) and its associated mode shape:

• Ln , Mn and Γn for each of the five modes are to be computed from Equations below.
• Also, calculate the effective modal mass Mn* and the modal mass participating ratio for the 5 modes. The mass matrix will be required to evaluate these parameters.

• Finally, obtain the base shear for each of the 5 modes of vibration and the combined base shear as per SRSS rule. The pseudo acceleration design spectrum to obtain the base shear for each mode is provided below.

• How many modes are to be considered such that a modal mass participating ratio of 90% is obtained?

SOLUTION -