KAJIAN EKSPERIMENTAL DAN NUMERIK KEGAGALAN TEKUK PADA STRUKTUR RANGKA PIPA BAJA BERLUBANG

Pipa baja langsing banyak digunakan pada elemen struktur rangka platform lepas pantai.Apabila elemen mendukung gaya desak, fenomena tekuk (buckling) menentukan kapasitasnya. Beban tekuk kritis (Pcr) merupakan indikator utama kegagalan tekuk elemen. Formula tekuk Euler untuk memprediksi Pcr tidak berlaku pada elemen pipa yang memiliki cacat, seperti lubang akibat pitting corrosion. Pengaruh posisi dan diameter lubang pada kapasitas tekuk elemen pipa baja langsing berlubang diteliti secara eksperimental dan secara numerik. Pengujian eksperimental dilakukan pada model terskala, dengan mengacu pada prototipe elemen platform lepas pantai di Indonesia. Variasi posisi lubang adalah pada 0,125L, 0,250L, dan 0,500L, dengan L adalah panjang elemen. Lubang berbentuk lingkaran dengan diameter 0,5 diameter pipa. Eksperimen dilakukan dengan 2 skala geometri, yaitu model kecil (skala 32:1) yang memiliki panjang 600 mm, diameter 12,7 mm, dan model besar (skala 9,5:1) yang memiliki panjang 2000 mm dan diameter 60,50 mm. Kedua model memiliki kelangsingan 126. Pembebanan aksial desak dilakukan secara bertahap sampai terjadinya kegagalan tekuk pada pipa, yang ditandai dengan meningkatnya displacement secara drastis. Load cell dan LVDT digunakan untuk memantau beban dan displacement. Strain gauges dipasang di sekitar lubang untuk memantau regangan. Hasil pengamatan disajikan dalam kurva beban-displacement, dan kurva hubungan antara posisi lubang dengan nilai faktor reduksi beban kritis. Model berupa truss 2D yang mengandung elemen desak berlubang juga dilakukan untuk mengetahui perbedaan antara element buckling dengan overall buckling dari struktur. Kajian numeris dilakukan dengan metode elemen hingga nonlinier 3-Dimensi melalui 2 pendekatan, yaitu : (a) eigenvalue (buckling analysis) berbasis solusi nonlinear incremental equilibrium equations yang memberikan nilai Pcr, dan (b) nonlinear geometric analysis yang merupakan solusi nonlinear equilibrium equations yang menghasilkan kurva beban-displacement. Terdapat 2 idealisasi yang ditempuh, yaitu menggunakan: (a) elemen 3D shell memanfaatkan software SAP-2000, dan (b) elemen 3D solid menggunakan software Abaqus. Berdasarkan hasil pengujian eksperimental terhadap 4 model kecil dan 12 model besar, dapat disimpulkan bahwa: (a) reduksi Pcr untuk posisi lubang pada 0,125L, 0,250L, dan 0,500L, adalah 3,75%, 6,34%, dan 11,33% untuk model kecil, (b) reduksi Pcr pada model besar adalah 2,71%, 5,38%, dan 12,15%, (c) untuk pipa yang lebih tebal diperoleh faktor reduksi serupa namun cenderung relatif lebih besar. Hasil yang diperoleh dari kajian numeris: (a) reduksi Pcr untuk posisi lubang pada 0,125L, 0,250L, dan 0,500L, adalah 4,73%, 5,46%, dan 6,3% dari eigen value analysis, (b) reduksi Pcr hasil nonlinear analysis adalah 3,6%, 5,82%, dan 10,61%, (c) dibanding hasil eksperimen, terdapat perbedaan faktor reduksi sebesar 1~5%, (d) semakin besar diameter lubang faktor reduksi akan semakin besar. Hasil eksperimental maupun numeris menunjukkan bahwa : (a) keberadaan lubang mengurangi kapasitas beban tekuk secara signifikan, (b) reduksi maksimum terjadi pada posisi lubang di tengah panjang pipa, (c) pada saat terjadi buckling, bentuk deformasi pipa adalah single curvature dengan arah kelengkungan berlawanan posisi lubang, (d) regangan yang terjadi menjelang buckling jauh lebih kecil dari regangan leleh, dan (e) pada model truss 2D untuk berbagai posisi lubang memberikan nilai Pcr overall buckling lebih kecil 2%~2,5% dibanding element buckling. Slender steel tubular members are widely used in jacket platform truss structures.When the member is in compression, buckling phenomenon governs its capacity. The critical load (Pcr) is a main indicator of buckling failure. Euler formula for predicting the capacity of slender compression member is no longer applicable when defect, such as a hole resulting from pitting corrosion, presence on the member.This research was conducted to study experimentally as well as numerically the effects of position and diameter of a circular hole on the buckling capacity of slender steel tubular member commonly found in typical platform structures. Experiments were performed on scaled models, with reference to a compression member prototype of offshore platform structure in Indonesia. Variations on hole position were at 0.125L, 0.250L, and 0.500L, where L was the length of the compression member. The hole was circular having diameter of 0.5 pipe diameter. Two geometric scales were chosen: (a) small scale models (32:1) with length of 600 mm, and 12.7 mm diameter, and (b) large scale models (9.5:1) with length of 2000 mm and 60.5 mm diameter. Both models had the same slenderness ratios of 126. Monotonic axial compressive loading was applied to the member up to buckling occur. Load cell was used to monitor the incremental loads, while the axial and lateral displacements at the center were observed by LVDTA¢i?½i?½s. Strain gauges were installed closed to the cutout position to monitor the strain. The results were presented in load-displacement curves, as well as relationship curves between hole positions and their critical load reduction factors. Models of 2D truss structure consisting compression member with circular hole were also tested to study the difference between element buckling and overall buckling of the structure. Numerical studies were carried outusing 3D nonlinear geometric finite element method through 2 approaches: (a) eigen value buckling analysis based on nonlinear incremental equilibrium equations that gave Pcr, and (b) nonlinear geometric analysis based on nonlinear equilibrium equations that produce load-displacement curves from which Pcr could be determined. Two idealizations were adopted, using: (a) 3D shell elements utilizing SAP2000, and (b) 3D solid elements utilizing Abaqus computer softwares. Based on the experimental test results upon 4 small and 12 large models, it was concluded that: (a) for small models the Pcr reduction factor were 3.75%, 6.34%, 11.33%, respectively, on hole positions of 0.125L, 0.250L, 0.500L, and (b) for large model the factors were 2.71% 5.38% and 12.15%, (c) for thicker pipes the reduction factors were similar but tend to increase. The results obtained from numerical studies: (a) reduction factors from eigen value analysis were 4.73%, 6.3% and 5.46%, (b) from nonlinear analysis were 5.82%, 3.6%, and 10.61%, (c) compared to experimental results, the difference in reduction factors were 1~5%, (d) the larger the hole diameter the reduction factor would also be larger. The experimental as well as numerical results showed that: (a) the existence of hole reduced the buckling load capacity significantly, (b) the maximum reduction occurred when the hole was in the middle of the pipe length, and (c) the buckling modes corresponded with single curvature toward opposite side of the cutout position, (d) strain intensity prior buckling was much smaller than the yield strain, and (e) in the 2D truss model with various hole positions, the overall buckling loads were about 2.5% lower than that of element buckling loads.