Explain the principles of structural dynamics and their application in civil engineering. Discuss how structural dynamics theory can be used to analyze the behavior of various types of structures under dynamic loads such as earthquakes, wind, and machinery vibrations. Describe the key concepts such as natural frequencies, damping, and mode shapes, and how they influence the response of structures to dynamic forces. Additionally, elaborate on the importance of considering dynamic effects in the design and assessment of civil engineering structures, and the methods used to mitigate dynamic responses for ensuring structural safety and performance.