# DOMV No 3 RESPONSE OF LINEAR SDOF SYSTEMS TO GENERAL LOADING (1).pdf

30 May 2023
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### DOMV No 3 RESPONSE OF LINEAR SDOF SYSTEMS TO GENERAL LOADING (1).pdf

• 1. In considering the general response y(t) of a SDOF system to arbitrary forcing, we have previously defined three special functions: The Unit Amplitude Free Decay Function 𝑦UAFD 𝑡 The Unit Velocity Free Decay Function 𝑦UVFD 𝑡 , and The Impulse Response Function h(t) But h(t) is identical to yUVFD t so really we only have two unique functions. We will see in this lecture that only these functions are needed to generate the general response y(t) of a SDOF system excited by arbitrary forcing from any set of initial conditions. The general solution of a SDOF system to arbitrary forcing
• 2. Reminder of what a unit impulse is
• 3. A reminder of what the Impulse Response function h(t) typically looks like
• 4. Four Scenarios involving the Impulse response function h(t) To build-up the solution to general forcing, I am going to consider four scenarios i) – iv) involving the Impulse response function. Note: Time-delayed ‘firing’ of the impulse function When h(t) is an impulse response function, and we consider h(T-t), then the ‘spike’ of the impulse occurs when the argument of h(t) is zero, i.e. when T-t=0 or t=T. This is effectively a time-delay of magnitude T. h(T-t) is therefore an impulse that is delayed until t=T rather than occurring at t=0.
• 5. Scenario i) involving the Impulse response function h(t)
• 6. Scenario ii) involving the Impulse response function h(t)
• 7. Four Scenarios involving the Impulse respone function - Scenario iii): Scenario iii) involving the Impulse response function h(t)
• 8. Scenario iv) involving the Impulse response function h(t)
• 9. The General time domain solution for arbitrary excitation
• 10. Frequency Domain Solution Method
• 11. Relationship between the Impulse Response Function h(t) and the Frequency Response Function H(jω) Note: for nonzero initial conditions we have to add the complementary solution
• 12. Some general comments about Time and Frequency Domain analysis of Linear Systems We can generate the response of a SDOF system to any input function. Inverse Fourier Transform is usually difficult analytically since requires contour integration in the complex plane. Practical approach involves use of numerical discrete Fourier Transform algorithms i.e. the FFT (Fast Fourier Transform) to convert input to frequency domain, and the use of the IFFT (Fast Inverse Fourier Transform) to obtain the response Y(t). Time and frequency domain approaches can be extended to MDOF systems, in fact, for general types of damping, frequency domain methods are usually used.