The problem is by the high confidence of the various parties including residents, should not reduce the level of awareness of the possibility of the similar danger in the future. The location of the island of Java in particular and Indonesia in general is likely to get earthquake attack at any time. According to some sources up to 3000-5000 times earthquakes happen every year in Indonesia.
Reinforced concrete frame with steel bar
Reinforced Concrete Structure
From the view of the habitant community in the area such as Yogyakarta, the use of reinforced concrete materials has been seen as a guarantee of their safety from the dangers of earthquakes. This can be understood since most of the collapsed houses which only use masonry before, without the framework of reinforced concrete. Other options such as using steel or wooden houses are considered either too expensive or too low quality (e.g. the construction of bamboo house is always assumed as a poor house). The safety seems will be assured only by how good mix of concrete and large diameter of reinforcement steel they use. The assurance of salvation seems to be taken for granted.
House with reinforced concrete framework
Unfortunately, the safety factor of a building from the earthquake was not enough to be seen from the use of 'strong material' alone. Starting from the location, building design, structural systems design, precise construction, construction process, appropriate usage of materials, to use all of the building will greatly affect the safety level of the building and its users.
The concrete frame are popular both for reconstruction and new houses
Location Factors
Problem on location factor such as located in danger area, generally cannot be solved except by moving from the zone to the relatively more secure. Mass transmigration or bedol deso (village migration) is not easy solution to apply, although it has been done by local government of Bantul by sending approximately 400 families to Sumatera Island in 2009. But the problem is that the population in the area is still high. Other solutions would be highly desirable such as a proposal is to minimize the impact of earthquake shocks by adding a buffer or damping media such as sand base beneath the whole of building foundation.
Building Design
Ideal design for earthquake safe house should use a simple form. Usually, for the reconstruction of houses or new houses after the earthquake, the buildings are commonly in simple form. This is will become a problem later when the process of addition and enlargement of the houses done by the occupants afterward as their increasing needs. Simple buildings are changed to very complex both from the form and layout (plan). Though the buildings are already using reinforced concrete materials with wide diameter steel bar inside, a complex forms of building will very vulnerable to deal with earthquake shocks. Not to mention the quality of connections between the old and new buildings.
System Structure
The proper structural system would be in accordance with the nature of the material used. Wooden structure is using flexible structure principle, opposed to reinforced concrete structural system where have to be completely rigid. Applying new materials such as reinforced concrete by the old fashion (as same as using wood) should not be done. Unfortunately the people are proven to do this for example by replacing the wooden frame with casing concrete system exactly in the similar way without considering the material properties. Wooden truss-frame should be free to move by applying flexible joint (roll and hinge) while the reinforced concrete should be stiff, relating directly to the columns. Unfortunately, by forming the framework of concrete that seemed to be wood system is very popular in the area. This is very dangerous for residents because the concrete is very heavy and will be easily to fall when earthquake strike, if installed improper way.
Construction
The accuracy of construction method is also very important factor for the safety of building users. Many concrete frame houses in Bantul district were prooven to be failed and still took many casualties because of the weakness of the wall and column connection. Heavy brick walls will easily collapse if not fitted with adequate anchor to the columns, or the wall is too wide (rarely practical column).
Field Implementation
Construction process in the field can also affect the safety of the building even if buildings using good quality raw materials. Starting from the material mixing process to the installation will be very important to be considered. In the case of concrete material, although applying with more cement quantity mixture, without well stirred, or lack of water, concrete final produced will brittle and the strength will be lessen. Similarly, in the arrangement of reinforced steel connections without overlapping and bending would be extremely vulnerable. Applying all other aspects without tied field inspection just will put the building remains in danger.
Material Selection
The accuracy of the material also directly affects the safety of the building and its occupants. Back to one of the cases above example, the light wooden roof frame should not taken as precedence for applying heavy concrete structure, although the common people considers 'concrete is stronger than wood'. Because if there is a shock, heavy objects will potentially greater to fall following the Newton's second law F = MA. It will be worsen if the structural design and construction are not properly done.
Building Use
So is the use of the building. The arrangement of furnitures though not too significant for burdening the structures (such as a heavy chandelier or wall shelves) will still affect the safety in an earthquake. Perhaps the building remained standing but the lights or shelves may fall down hitting the inhabitants. Likewise, the placement of heavy cabinet that can be collapsed also will block resident evacuation. All of this will affect the security level of a building.
Conclusion
Many things should be considered to ensure the safety of the building and its occupants in the event of an earthquake. Assumptions that are less true, especially with the use of materials and structural systems still need to be reviewed again. Further study is needed to ensure the safety level of the building and its occupants against earthquake.
How do you think? Regards ....
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