Benarkah Tacoma Bridge Runtuh Akibat Resonansi?

Aslinya ini merupakan thread yang saya lontarkan di milis civeng, sekaligus email pribadi saya kepada Prof. Bambang Suhendro, dosen Teknik Sipil UGM di bidang struktur, “dewa”nya metode elemen hingga dan tetek bengeknya seperti matriks, analisis dinamika, dan lain sebagainya. Sembari menunggu jawaban dari peraih Magna Cumlaude Michigan University pada jenjang S2 dan S3 tersebut, itupun kalau saya tidak salah kirim email , sementara tanggapan dari milis civeng juga masih sedikit, okelah saya publish saja pertanyaan saya. Barangkali pembaca sekalian bisa menanggapi.

Apakah benar Tacoma Bridge runtuh karena resonansi?Selama ini penyebab itulah yang selalu diutarakan kepada kami di Teknik Sipil UGM. Namun, saya membaca di Wikipedia English beberapa paragraf berikut :

“The bridge’s collapse had a lasting effect on science and engineering. In many physics textbooks the event is presented as an example of elementary forced resonance with the wind providing an external periodic frequency that matched the natural structural frequency, even though its real cause of failure was aeroelastic flutter“

“In the case of the Tacoma Narrows Bridge, there was no resonance. According to Professor Frederick Burt Farquharson, an engineering professor at the University of Washington and one of the main researchers about the cause of the bridge collapse, the wind was steady at 42 miles per hour (68 km/h) and the frequency of the destructive mode was 12 cycles/minute (0.2Hz).^[16] This frequency, was neither a natural mode of the isolated structure nor the frequency of blunt-body vortex shedding of the bridge at that wind speed (which was approximately 1 Hz). It can be concluded therefore that the vortex shedding was not the cause of the bridge collapse. The event can be understood only while considering the coupled aerodynamic and structural system that requires rigorous mathematical analysis to reveal all the degrees of freedom of the particular structure and the set of design loads imposed.”

Saya search tentang flutter di situs yang sama. Kesimpulan singkat saya adalah keduanya sama2 berkaitan erat dengan vibrasi kecil, akan tetapi flutter tidak harus memiliki frekuensi sama dengan frekuensi alami struktur. Nah untuk yang ini saya terus terang ndak mudheng.

“Flutter is a self-feeding and potentially destructive vibration where aerodynamic forces on an object couple with a structure’s natural mode of vibration to produce rapid periodic motion. Flutter can occur in any object within a strong fluid flow, under the conditions that a positive feedback occurs between the structure’s natural vibration and the aerodynamic forces. That is, that the vibrational movement of the object increases an aerodynamic load which in turn drives the object to move further. If the energy during the period of aerodynamic excitation is larger than the natural damping of the system, the level of vibration will increase, resulting in self-exciting oscillation. The vibration levels can thus build up and are only limited when the aerodynamic or mechanical damping of the object match the energy input, this often results in large amplitudes and can lead to rapid failure. Because of this, structures exposed to aerodynamic forces – including wings, aerofoils, but also chimneys and bridges – are designed carefully within known parameters to avoid flutter. It is however not always a destructive force; recent progress has been made in small scale (table top) wind generators for underserved communities in developing countries, designed specifically to take advantage of this effect.^[1][2]

In complex structures where both the aerodynamics and the mechanical properties of the structure are not fully understood flutter can only be discounted through detailed testing. Even changing the mass distribution of an aircraft or the stiffness of one component can induce flutter in an apparently unrelated aerodynamic component. At its mildest this can appear as a “buzz” in the aircraft structure, but at its most violent it can develop uncontrollably with great speed and cause serious damage to or the destruction of the aircraft.^[3]

In some cases, automatic control systems have been demonstrated to help prevent or limit flutter related structural vibration.

Flutter can also occur on structures other than aircraft. One famous example of flutter phenomena is the collapse of the original Tacoma Narrows Bridge.”

Begitulah. Berikut saya sertakan linknya.

http://en.wikipedia.org/wiki/Tacoma_Narrows_Bridge_(1940)

http://en.wikipedia.org/wiki/Aeroelasticity#Flutter

Ada yang bisa membantu ?