Evaluasi Kinerja Seismik Fondasi Rakit Gedung Rumah Sakit Mempertimbangkan Efek Soil-Structure Interaction (SSI)

ADE CAHYANTO, Dr.Eng. Ir. Sito Ismanti, S.T., M.Eng., IPM.; Prof. Ir. Iman Satyarno, M.E., Ph.D.,IPU

2026 | Tesis | S2 Teknik Sipil

Daerah Istimewa Yogyakarta merupakan wilayah dengan tingkat kerawanan gempa yang tinggi akibat pengaruh subduksi Lempeng Indo-Australia terhadap Lempeng Eurasia serta keberadaan sesar aktif. Pembaruan Peta Sumber dan Bahaya Gempa Indonesia Tahun 2024 berpotensi mengubah parameter percepatan gempa sehingga evaluasi bangunan penting, seperti rumah sakit, perlu dilakukan. Namun, perencanaan ataupun evaluasi struktur umumnya masih menggunakan asumsi perletakan jepit sempurna (fixed base) yang belum sepenuhnya merepresentasikan kondisi lapangan. Oleh karena itu, penelitian ini menerapkan pendekatan Soil–Structure Interaction (SSI) untuk memperoleh respons struktur yang lebih realistis.

Analisis dilakukan menggunakan dukungan software MIDAS Gen dan MIDAS GTS-Nx dengan Free Field Boundary melalui Nonlinear Time History Analysis (NLTHA). Penelitian ini bertujuan menganalisis pengaruh SSI terhadap respons seismik gedung dengan membandingkan terhadap model fixed base. Parameter yang dievaluasi meliputi periode natural gedung, base shear, simpangan antar lantai, kinerja fondasi rakit, dan deformasi fondasi akibat pembebanan gempa. Seluruh hasil analisis dievaluasi berdasarkan SNI 1726:2019, SNI 8460:2017, serta referensi geoteknik yang relevan.

Hasil penelitian menunjukkan bahwa pemodelan Soil–Structure Interaction (SSI) memberikan respons struktur yang berbeda dibandingkan model fixed base. Karena SSI mempertimbangkan deformasi tanah, periode alami struktur menjadi lebih panjang sehingga respons dinamis meningkat. Hal ini ditunjukkan oleh kenaikan simpangan antar tingkat hingga 247?n Base Shear hingga 157%, meskipun begitu, nilai simpangan antar tingkat  masih memenuhi batas ijin. Dari aspek geoteknik, fondasi Raft tetap berada dalam kondisi aman dengan SF daya dukung sebesar 3,23, penurunan total sebesar 36,27 mm dan penurunan diferensial sebesar 6,3 mm, yang mana keduanya masih memenuhi batas izin penurunan. Analisis dinamik menunjukkan terjadinya Site Amplification sebesar 1,60–2,56. Sehingga pada beberapa skenario analisis gempa, terjadi perpindahan lateral permanen yang melampaui batas izin (100 mm), terutama akibat gempa Megathrust dan Benioff, disertai adanya kecenderungan rocking pada sistem fondasi raft akibat inersia struktur dan interaksi dinamis tanah fondasi selama pembebanan gempa.

The Special Region of Yogyakarta is one of the most seismically active regions in Indonesia due to the subduction of the Indo-Australian Plate beneath the Eurasian Plate and the presence of active faults. The 2024 Indonesian Seismic Hazard Map introduced revised seismic hazard parameters that may affect the seismic performance of existing buildings, particularly essential facilities such as hospitals. However, structural design and seismic evaluation are commonly performed using the conventional fixed-base assumption, which does not fully represent the interaction among the structure, foundation, and supporting soil. Therefore, this study incorporates a Soil–Structure Interaction (SSI) approach to provide a more realistic representation of structural seismic behavior.

This study investigates the influence of SSI on the seismic response of a hospital building by comparing an SSI model with a conventional fixed-base model. Structural analyses were conducted using MIDAS Gen and MIDAS GTS NX with free field boundary through Nonlinear Time History Analysis (NLTHA). The evaluated parameters included the natural period, base shear, interstory drift, raft foundation performance, settlement, and lateral foundation displacement under seismic loading. The analytical results were assessed in accordance with SNI 1726:2019, SNI 8460:2017, and relevant geotechnical references.

The results show that the SSI model results in significantly different structural responses compared with the fixed-base model. By accounting for soil deformation, the SSI model increases the natural period of the structure, resulting in a more flexible structural system. Consequently, interstory drift increased by up to 247%, while base shear increased by up to 157%. Nevertheless, the interstory drift remained within the allowable limits specified by the applicable design standards. From a geotechnical perspective, the raft foundation remained stable, with a bearing capacity safety factor of 3.23, a total settlement of 36.27 mm, and a differential settlement of 6.3 mm, all of which satisfied the allowable criteria. The analysis also identified site amplification factors ranging from 1.60 to 2.56. Under several 1,000-year return-period earthquake scenarios, permanent lateral displacement exceeded the allowable limit of 100 mm, particularly under Megathrust and Benioff ground motions. In addition, a rocking tendency of the raft foundation was observed due to the combined effects of structural inertia and the dynamic interaction between the soil and foundation during seismic loading.

Kata Kunci : Fondasi Raft, Fixed Base, Soil-Structure Interaction/SSI, Nonlinier time history Analysis, Gedung rumah sakit.