Understanding the concepts on Degrees of Freedom

CHALLENGE 3

Aim: The main aim of this challenge is to gain the knowledge about the concept of mode shapes for a multi storey building and to calculate manually the base shear for a multi storey building in the modal analysis. From the challenge carried out the various terminologies that are involved in the modal analysis are also learnt.

Question 1:

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 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 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 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?

Ans:

Conclusion:

From the challenge carried out it can be concluded that the concept of modal analysis is learnt and the various terms that are used in the analysis are also learnt. The base shear calculation by SRSS method by manual method is also learnt.

Vijayalakshmi S H