STRUCTURE MODAL PAREMETER by HILBERT - HUANG TRANSFORM Mohammad A. Rizal 1 , Amrinsyah Nasution 2 1 Civil Enginneer ST, Master of Structure Engineer MT., Institut Teknologi Bandung Email: rizalalhas@yahoo.com 2 Professor of Civil Engineering, Faculty of Civil Engineering & Enviromental, Institut Teknologi Bandung Email: amrinsyah@si.itb.ac.id Abstract. Natural frequency of structure is acquired from structure responses of acceleration, velocity, and displacement in an interval time domain. Since time domain structure responses are difficult to examine, one may transform the responses into frequency domain; of which an irregular signal such as seismic accelerograph is represented in a combination of pure frequencies. Fast Fourier Transform (FFT) is one of common method to convert data from time domain to frequency domain, but the method had to be modified for analysis of non-stationary vibration by Short Time Fourier Transform (STFT). This paper introduces Hilbert Huang Transform (HHT) for data analysis (non-linear and non- stationary) based on Empirical Mode Decomposition, EMD. Hilbert spectrum by HHT shows better result and ore accurate compared with wavelet spectrum. Keywords: structure response, time domain, frequency, Fast Fourier Transform, Hilbert Huang Transfornm. 1. Introduction Structural safety and serviceability are main concerns for structural system under external loads. The structure response due to dynamic loading such as vibration is depended on the modal parameter of structure. Airplane, ship, car, bridge, and building have the structure systems that are subjected to static or dynamic loads. The dynamic response of a structure to arbitrary loading is solved from the dynamic equilibrium equations          (t) P x k x c x m       (1) where  k is the stiffness matrix;  c is the damping matrix;   m is the diagonal mass matrix;   x x x and , ,    are accelerations, velocities, and the displacements of the structure; and   (t) P is the applied load. Behavior of structures or structure responses to the dynamic load such vibration mode shapes and frequencies of the system are determined by eigen vector analysis. The eigen value is the square of the circular frequency, w, f is the natural frequency, period T and ξ is damping ratio for related shape mode. Natural Frequency The frequency or frequencies at which a structure tends to vibrate with when hit, struck, plucked, strummed or somehow disturbed is known as the natural frequency of the structure system. The actual frequency is dependent upon the properties of the material the structure is made. It affects the speed of the wave and the length of the material also effects the wavelength of the wave.