Bagaimana Gempa Bumi Merusak Bangunan?

Referensi :  Slide kuliah Prof. Masyhur Irsyam dan Dr. Ir Drajat Hoedajanto.

Indonesia merupakan negara kepulauan dengan resiko kegempa bumian yang tinggi. Nurhasanah dan Jamil (2009) menyebutkan bahwa selama selang waktu 1897 – 2000, tercatat sekitar 837 kejadian gempa bumi dengan magnitude diatas 5 skala richter. Terletak pada 6^o LU hingga 11^o LS serta 95^o BT hingga 141^o BT, kejadian-kejadian gempa bumi di Indonesia sering terjadi karena letak Indonesia yang berada di pertemuan tiga pelat tektonik besar dan Sembilan pelat tektonik kecil (Bird et al., 2003). Pelat-pelat tektonik dengan berbagai jenis pergerakan menciptakan zona subduksi dan sesar yang selalu aktif. Saat suatu lapisan tidak dapat menahan pergerakan lempeng yang bersinggungannya, akumulasi energi yang tercipta akan terlepas menjadi gempa bumi bumi.

Gambar 1 Peta Lempeng Tektonik di Indonesia

Gempa bumi dapat menimbulkan kerusakan pada bangunan sebagaimana yang kita ketahui dari gempa bumi padang, aceh, papua, dan gempa bumi-gempa bumi besar lainnya. Secara garis besar, besarnya kerusakan pada bangunan karena gempa bumi bergantung pada kekuatan bangunan dalam menahan gaya gempa bumi yang terjadi. Lalu, bagaimana cara gempa bumi menimbulkan kerusakan pada bangunan?

Untuk mengetahuinya, mari kita analogikan sebuah bangunan sebagai seseorang yang tengah berdiri diatas mobil bak terbuka. Kondisi awal adalah mobil sedang dalam keadaan diam dan orang itu pun dalam keadaan diam (stabil), tidak bergerak dan berat badannya ditopang langsung oleh bagian dasar mobil searah gravitasi. Berat orang tersebut merupakan sebuah gaya (force) searah gravitasi yang besarnya :

Berat (Newton)  = massa (kg)  * gravitasi (m/s2)

Dari hal ini, gravitasi merupakan sebuah satuan percepatan yang arahnya menuju pusat bumi. Dengan kondisi mobil yang diam, tidak ada percepatan lain kecuali percepatan gravitasi tersebut.

Sekarang kita liat ketika mobil mulai bergerak untuk mencapai kecepatan 100 KM/jam. Adanya perubahan kecepatan dari 0 hingga 100 KM/jam membuat mobil mengalami percepatan sesuai arah mobil bergerak. Adanya percepatan ini membuat orang yang berada diatas mobil terdorong ke arah belakang (Gambar 2). Besarnya gaya dorong sesuai dengan persamaan  :

Gaya dorong (Newton)  = massa (kg)  * percepatan mobil (m/s2)

Semakin besar massa seseorang atau semakin besar percepatan mobil tersebut, semakin besar pula gaya dorong ke belakang yang timbul.

Gambar 2

Selanjutnya, ketika mobil dengan kecepatan 100 KM/jam hendak menghentikan lajunya sehingga kecepatannya menjadi 0 KM/jam, terjadi perlambatan yang juga adalah percepatan dengan besaran negatif.  Dengan demikian, orang diatas mobil akan terhempas kearah depan (Gambar 3). Sama dengan kejadian sebelumnya, Besarnya gaya dorong sesuai dengan  persamaan:

Gaya dorong (Newton)  = massa (kg)  * percepatan mobil (m/s2)

Gambar 3

Ketika gempa bumi bumi terjadi, permukaan tanah akan bergerak dengan percepatan tertentu. Dengan demikian, bangunan yang mengalami gempa bumi dapat dianalogikan sebagai seseorang yang sedang berdiri diatas mobil yang mengalami perubahan kecepatan. Selanjutnya, gaya dorong yang berpotensi merusakan bangunan akan timbul karena perubahan kecepatan itu.

Dampak dari gaya yang timbul akibat gempa pada bangunan yang tidak cukup kuat menahan gayanya dapat dilihat pada gambar-gambar dibawah ini :

The SIBE 2009

The first International Conference on Sustainable Infrastructure and Built Environment in Developing Countries (SIBE) was held in Sasana Budaya Ganesha, Bandung, 2-3^rd November 2009. The conference states that civil and environmental engineers are at the epicenter in seeking means to enhance human life through modernization of infrastructure as evidenced by provision of shelter, water, and transport, amongst others. Those engineers are to meet multifaceted challenges that are being encountered by the environment of the earth. The civil and environmental engineer must solve the problem through multidisciplinary and holistic approach. The global momentum for sustainable development must now lead to practical applications of the engineering and science of sustainability.

The conference was organized by Faculty of Civil and Environmental Engineering (FCEE), Institut Teknologi Bandung (ITB) in collaboration with:  Hokkaido University, Japan and National Taiwan University, Taiwan. It was also supported by AUN/SEED-net, JICA, and ASEAN Foundation.

The objectives of this conference as stated in their official website are :

• To provide a platform for exchange of ideas and information among academics, researchers, consultants, engineers, manufacturers and post graduate scholars in civil and environmental engineering.
• To discuss and evaluate the latest approaches, innovative technologies, policies and new directions in infrastructure development, pollution prevention and eco-friendly technologies adapted to developing countries.
• To promote cooperation and networking amongst practitioners and researchers involved in addressing infrastructure and built environment issues.

There were eight topics related to civil and environmental engineering fields discussed in the conference which were:

• Structure and material
• Transportation system and engineering
• Water engineering and management
• Waste engineering and management
• Ocean engineering
• Construction management
• Geotechnical engineering
• Environmental protection and management

It was my second time in attending big scientific conference. The first time was the JSCE conference in Fukuoka, Japan. Although I was at documentation division where I used my friend’s Nikon D40 for taking pictures of the conference, I still could watch the presentations presented during the conference.

On the first day of the conference, the opening ceremony was held. Conference participants, ITB faculty, and guests were attending the ceremony. Current Rector of ITB, Prof. Dr. Djoko Santoso gave his opening remark in the opening ceremony. The conference was officially opened as the officials shake the symbolic Angklung. The opening ceremony was then continued by keynote speech from two invited speaker.

After the keynote speech session ended, the participants went for having coffee break. In this coffee break session, participants and guest were served Indonesian snack. While enjoying their snack, participant could also explore the exhibition booths.

Then, the parallel session was held. In this session participants presented their paper according to their topic. Each participant had 15 minutes for presentation and 5 minutes for discussion time. The presentations were brought in English. The number of question was depended on how good the presenter presenting their paper. However, the topic that was popular enough usually got many questions even though the presenter was not good enough in presenting their paper because usually most people understand the basic concept of the topic.

On the night of the first day, the cultural night was held. The cultural night was focusing on the Angklung Performance (Sundanese traditional musical instrument). That night, Angklung attraction presented by Saung Angklung Ujo Group impressed the audience. The group is a famous Angklung performer from Bandung. At the end of the performance, every audience was given a piece of Angklung. It was my second times watching their performance.

The activity on the second day was not different from the first day except there was no opening ceremony and cultural night. The parallel session was held with coffee break in almost every 2 hours. On that day I also served as documentation staff, still, using my friend’s Nikon D40.

On that precious occasion, I was photographed together with Indonesian Earthquake expert, Ir. Masyhur Irsyam, MSE., Ph.D. I am very admiring him because his way of giving lecture and also because he is a famous earthquake engineer in Indonesia. I am so lucky to be able to take his class this year.

The conference was closed by a closing ceremony. The head of faculty of civil and environmental engineering, Dr.Ir. Puti Farida Marzuki, gave her  closing remarks and invited main committee to the main stage.

To be able to attend a conference is a very pleasant experience. Moreover, it increases our academic point when we are pursuing our degree. However, making a good paper is not easy. It requires hard work and strong motivation. I just remember when I was writing my JSCE conference paper several months ago. I had many experiments, spending hours in the lab. I had many discussions about my experiment’s result. I also had many writing revisions of my paper. It was not happened to me alone, all member of the lab who wrote a conference paper were also working hard to present their paper.  What a nice experience.

My First Technical Writing

For this three weeks I put all my brain on writing a paper for 2009 JSCE annual meeting. This paper should be submitted to JSCE committee by April 10th 2009. Before submitting to the committee I have to submit it to my lab’s Assistant professor to get a comment for revision and finally to my lab’s professor for the last and concluding comment for revision. And then I can send the paper to JSCE committee. It is quite a hard work to write a conference paper especially for the first-time-writing-paper-person like me.

The paper is only consisted of pages. It is a short story and conclusion of my research; what I was working on, what the result was and what the outcome was. The paper should be written in a given standard from JSCE. Although it is only two pages, it is not easy. We can’t do it carelessly, every flow of thinking should be written clearly. Every procedure should be written briefly. Every figure and table should be presented correctly. I should write a good English language paper. That why I have through many times of paper revision.

As for my research, I did 10 time concrete casts and 10 time concrete test to get enough data for this paper. My research is about identifying the fracture properties of high strength concrete reinforced with various fibers. I did 10 times of experiment involving 5 types of fibers and 1 type of hybrid fiber. In this paper, I analyze 4 properties of FRC. There are one is compressive strength, tensile strength, fracture energy, and tension softening curve. Though from these properties could lead me to more advance discussion, I just wrote general discussion in this conference paper.

My lab considers this paper as the first step in entering “paper” world. Laboratory member in bachelor and new master 1 grade should write this paper. The materials were taken from final project research for bachelor fourth year (B4) student and new master first year (M1) student. After submitting this paper, we have to submit paper to Japan Concrete Institute for the next steps. In the end, for Master second year (M2) student, they have to write a final graduation thesis.