Irawan, Candra (2019) Kontribusi Beton Pengisi Terhadap Peningkatan Kinerja Tiang Pancang Spun Pile Akibat Beban Lateral Siklik Dan Aksial Tekan. Doctoral thesis, Institut Teknologi Sepuluh Nopember.
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Abstract
Salah satu tipe tiang pancang umum digunakan untuk pondasi dalam adalah spun pile. Tidak hanya digunakan sebagai struktur bangunan bawah, namun spun pile juga diaplikasikan sebagai pondasi sekaligus kolom. Peneliti menyebutkan bahwa spun pile terbukti mengalami kegagalan plastis akibat gaya geser, momen lentur dan aksial saat gempa Kobe 1995. Kegagalan tersebut dipicu spun pile berperilaku getas (brittle) saat berdeformasi akibat gempa. Oleh karena itu Japan code 1995 dan NEHRP 2000 mensyaratkan batasan daktilitas perpindahan () yang harus dipenuhi oleh elemen struktur pemikul momen. Untuk memenuhi kebutuhan daktilitas tersebut maka peraturan gempa mengatur syarat rasio tulangan pengekang tiang pancang yang harus dipenuhi. Penelitian ini mengkaji kinerja lentur spun pile melalui pengujian eksperimen dan pemodelan numerik. Tiga variasi benda uji yang dikaji adalah spun pile eksisting, spun pile eksisting dengan beton pengisi, dan spun pile usulan dengan beton pengisi. Spun pile eksisting memiliki tulangan spiral diameter 3,2 mm spasi 100 mm (ρ_s = 0,0016). Sedangkan spun pile usulan menggunakan tulangan pengekang dengan rasio volumetrik sesuai syarat peraturan ρ_s = 0,013, yaitu diameter 8 mm spasi 50 mm. Beton pengisi dicor ke dalam lubang spun pile sebagai beton inti agar spun pile memiliki penampang pejal sehingga syarat rasio tulangan pengekang dapat digunakan. Pengujian tiang pancang eksisting dengan dan tanpa beton pengisi dilakukan dengan pembebanan lentur monotonik dan juga lentur siklik dengan aksial tekan P_0=0,08f_c^' A_g dan P_0=0,16f_c^' A_g. Model numerik finite element menggunakan Abaqus dibuat untuk memvalidasi hasil pengujian spun pile eksisting dengan beton pengisi dan untuk memprediksi kinerja spun pile usulan dengan beton pengisi. Hasil uji beban lentur monotonik menunjukkan keruntuhan spun pile dipicu oleh PC bar yang putus tanpa kehancuran beton serat tekan. Beban aksial pada uji beban lentur siklik menyebabkan spun pile runtuh dipicu oleh beton serat tekan yang hancur. Beton pengisi dapat meningkatkan daktilitas spun pile. Pada pengujian lentur monotonik daktilitas meningkat dari µ = 3,9 menjadi 5,2. Dibandingkan dengan spun pile tanpa beton pengisi, pada uji beban lentur siklik dengan aksial tekan terjadi peningkatan daktilitas perpindahan spun pile eksisting dengan beton pengisi sebesar 48% (dari µ = 4,0 menjadi 5,9) untuk P_0=0,08f_c^' A_g dan 36% (dari µ = 2,6 menjadi 3,6) untuk P_0=0,16f_c^' A_g. Dengan penampang berlubang, kinerja spun pile tanpa beton pengisi tidak terpengaruh oleh tulangan spiral. Rasio volumetrik tulangan spiral kurang dari yang disyaratkan oleh codes tidak mampu mengekang dengan efektif penampang spun pile dengan beton pengisi. Spun pile yang diisi dengan beton pengisi dengan tulangan pengekang sesuai codes diameter 8 mm spasi 50 mm (ρ_s = 0,013) dapat mempertahankan kekuatan lenturnya hingga rasio simpangan 3,5% dengan daktilitas perpindahan μ_∆≥8. Beton pengisi menjadikan proses kegagalan spun pile menjadi lebih stabil yang ditunjukkan dengan tidak terjadinya peristiwa meledaknya dinding beton spun pile ke arah dalam. Beton pengisi berperan sebagai pemikul beban aksial setelah crushing beton terjadi di kedua sisi spun pile. Sehingga spun pile tidak meledak (explosion) seperti yang terjadi pada spun pile tanpa beton pengisi. Dengan daktilitas perpindahan yang dicapai kurang dari 8, spun pile eksisting dengan beton pengisi dapat digunakan untuk struktur dengan kategori desain seismik (KDS) A, B, dan C. Sedangkan spun pile usulan dengan beton pengisi berpotensi dapat digunakan untuk struktur dengan KDS D, E, dan F.
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Precast concrete member with a circular hollow section, otherwise termed as the spun pile, is a class of deep foundation which is made of high strength concrete and prestressed bars. Although it normally serves as a foundation, in specific types of the structure such as slab on pile of the bridge, a group of the spun pile is also functioned both as foundations and pillars. With respect to the structural responses under high unforeseen loading conditions such as 1995 Kobe earthquake, the post-failure state was known to be triggered by brittle localized damage, implying the reinforcement detailing was not deemed to resist high combined internal forces. Therefore, design specifications such as Japan Code 1995 and NEHRP 2000 specify limitations for the minimum displacement ductility as well as other deemed-to-comply requirements.
In this research, the behavior of spun pile was studied experimentally and numerically. A series of spun pile specimens used in this study comprised of three types of the spun pile: existing spun pile, existing spun pile with infilled concrete, and proposed spun pile with infilled concrete. The existing spun pile specimen had a spiral reinforcement diameter of 3,2 mm with the spacing of 100 mm thereby giving the steel-to-concrete ratio of 0,0016. The proposed specimen, on the other hand, had confining reinforcement with a volumetric ratio of 0,013 obtained from the use of spiral reinforcement diameter and spacing of 8 mm and 50 mm respectively. The infilled concrete is cast inside the spun pile hollow and also served as the concrete core. This infilled concrete will provide a solid cross-section of spun pile such that the confinement requirements can, therefore, be used. The laboratory testing of existing spun pile with and without infilled concrete was performed under monotonic four-point bending and cyclic bending with constant axial loads being provided at both ends. Two different magnitudes of axial loads were 0,08fc’Ag and 0,16fc’Ag. The numerical analysis using Abaqus was also performed by means of corroborating the results from experimental work and to foresee the behavior of the proposed spun pile with infilled concrete. The experimental results from the monotonic four-point bending test exhibited that failure occurred once the PC bar broke into two pieces prior to compressive damage of concrete. However, it was found that the infilled concrete played an important role in increasing the ductility of the spun pile with the ductility factor ranging from 3,9 to 5,2. As for the cyclic bending test, it was found that the crushing failure of concrete was indeed caused by axial loads. The displacement ductility of spun pile existing with infilled concrete obtained from the cyclic loading test 48% (from µ = 4,0 into 5,9) and 36% (from µ = 2,6 into 3,6) higher than existing spun pile without infilled concrete for 0,08fc’Ag and 0,16fc’Ag, respectively. With hollow sections, the performance of spun pile without infilled concrete was not affected by the presence of spiral reinforcement. Furthermore, the volumetric ratio of spiral reinforcement which does not comply with the requirement from design codes was found to be ineffective to confine the section of the spun pile with infilled concrete. In contrast, the proposed spun pile with infilled concrete comprising the volumetric ratio of 0,013 was capable of maintaining its flexural strength to 3,5% drift ratio with displacement ductility greater than 8. The infilled concrete marked the failure of the spun pile to be more stable, and the manifestation of the explosive crushing of concrete inside the wall of the spun pile was ceased. From this study, it can be concluded that the infilled concrete serves as a restrainer of axial load after the concrete crushing occurs thereby preventing the explosive crushing. For the design of foundation or column, the proposed specimen can be potentially used in seismic design category (SDC) D, E, and F, whereas the existing spun pile can only be used in SDC A, B, and C.
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