Understanding the concepts on Degrees of Freedom

UNDERSTANDING THE CONCEPTS OF DEGREE OF FREEDOM

AIM -

TO STUDY THE CONCEPTS OF THE DEGREE OF FREEDOM

OBJECTIVE -

• To understand modes and shapes of multi-story building
• Manually calculate the base shear for a multi-story building in model analysis
• Understand the terminologies involved in modal analysis.

Question :

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

L_n, M_n, and Γ_n for each of the five modes are to be computed from the Equations below.

Also, calculate the effective modal mass M_n* 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 the 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?

PROCEDURE -

CALCULATING ALL THE PARAMETERS OF MODE 1

CALCULATING ALL THE PARAMETERS OF MODE 2

CALCULATING ALL THE PARAMETERS OF THE MODE 3

CALCULATING ALL THE PARAMETERS OF THE MODE 4

CALCULATING ALL THE PARAMETERS OF THE MODE 5