How To Earthquake-Proof Our Buildings

Most of Indonesia territory is located in areas with middle-high to high earthquake risk levels. In 2018 alone, there have been a series of destructive earthquakes, namely in Lombok in July and August 2018, with a magnitude of up to 7 Mw, and in Palu, Sulawesi, in September 2018, with a magnitude of 7.4 Mw.

These earthquakes have caused the collapse of various physical facilities and infrastructure—such as residences, school buildings, houses of worship, and health centres or hospitals—as well as massive casualties and material losses.

Therefore, it is imperative for each building structure in Indonesia to be earthquake-resistant, especially those located in areas with medium to high seismic risk levels, so that in the event of an earthquake, the building structure can survive and protect its inhabitants. Generally, damage of building structures due to earthquake are due to the following reasons:

1.Building systems used are not in accordance with the level of seismic risk
2. Structural components and reinforcement details are inadequately designed
3. Material quality and construction practices are not good
4. Supervision and quality control during construction are not implemented properly

Impact of severe damage during the Lombok earthquake in 2018

The regulation on earthquake-resistant structural design for buildings and non-buildings, SNI 1726 has undergone several revisions along with the increasing numbers of earthquake data and the advancement in the analysis and design.

The seismic regulations must be used as a reference in design, along with other regulations, such as the Requirements for Building Code Requirements for Structural Concrete (SNI 2847-2013) and Seismic Provisions for Steel Structures (SNI 7860-2015). If implemented consistently, a fatal collapse of the building structure can be prevented in the event of strong earthquake.

Buildings must be designed to withstand a 2,500 annual seismic load, in accordance with SNI Earthquake. The design earthquake (DE) level is two-thirds of maximum earthquake level (MCER). In principle, earthquake-resistant building structures can be designed against earthquake loads that are reduced by a structural response modification factors (R-factor), which is a representation of the level of ductility possessed by the structure.

With the application of this concept, selected structural elements of the building that are ductile and would not easily collapse are allowed to experience plastification (damage) as a means of dissipating earthquake energy received by the structure. Other structural elements that are not expected to experience damage must remain elastic during an earthquake.

Earthquake’s impact in North Lombok

The hierarchy or sequence of plastification in structural elements must be designed using a capacity design concept. Not all structural elements are made equally strong with respect to the required internal forces, but there are structural elements or parts on the structure that are made weaker than the others. This is done so that only these elements or parts of the structure will experience damage under design seismic loads.

To ensure the plastification only occurs in selected structural elements, the structural elements that are expected to remain elastic at the time of a strong earthquake must be designed to be stronger than the selected elements with the implementation of the overstrength factor.

To ensure building structures do not collapse in the event of an MCER earthquake, the structural elements that are expected to experience damage must be given proper and adequate reinforcement detailing, so that their behaviour remains stable even though they have experienced large inelastic deformations. The detailing provisions stipulated in Indonesian Concrete Code Article 21.1 for reinforced concrete structures are basically differentiated based on the level of seismic risk in the area where the structure is located. The higher the risk of seismicity in an area, the more stringent the detailing requirements for the structural elements.

Therefore, building damage due to earthquakes is basically unavoidable. However, the damage can be controlled according to the desired objective. Based on the recommendations of Structural Engineers Association of California (SEAOC) in 1997, the performance of building structures at the time of the earthquake can be divided into three categories to ensure safety in accordance with the functions and interests of the structure, namely:structure:

  1. Basic Objective category: for the design of houses and office buildings, etc, which may be severely damaged but may not be collapsed by a strong earthquake
  2. Essential or Hazardous Objective category: for schools, hospitals, health centres, emergency shelters, fire and police stations and chemical plants, which may be mildly to moderately damaged due to a strong earthquake
  3. Safety Critical Objectives category: for nuclear reactors or armouries that must remain fully operational despite a strong earthquake

Seismic activities in Indonesia (PuSGeN, 2017)

The concrete material and reinforcing steel’s characteristics for earthquake-resistant reinforced concrete structures will greatly influence the behaviour of the resulting structure’s plastification. One of the most influential parameters of concrete material in this case is the compressive strength value.

Based on Indonesian Concrete Code Article, compressive strength (fc’) for concrete materials used in earthquake-resistant building structures should be no less than 21 MPa. With that much strength, the building will have good resistance, and its performance will not easily change with the building ages.

For reinforcing steel, the steel surface conditions are one of the most influential parameters for the behaviour of plastification. Reinforcing steel can be divided into two types: plain and deformed reinforcing bars. The current Indonesian Concrete Code limits the use of plain reinforcing bars only for spiral reinforcement.

Indonesian Concrete Code limits the specified yield strength value for reinforcing steel materials to be not more than 400 MPa. The use of higher strength reinforcing steel materials can cause a higher bond demand between the reinforcing steel and concrete, which can generate brittle failure when the structural element develops its maximum flexural capacity.

Another parameter is the value of overstrength factors. The value of the overstrength factor of the reinforcing steel material is the ratio of the actual yield strength and the specified yield strength. The value of this parameter should be limited and should not be excessive. The over-strength parameters are needed for structural elements designed based on the capacity design concept and are used to design structural elements that are expected to remain elastic when plastification are formed in the structural elements that are directly attached to it.

The same principle is also applied to the design of beam-column joints, which are according to Indonesian Concrete Code must meet the `strong column – weak beam’ principles. Thus, the column elements framing on a beam -column joint must have flexural strength that is 1.2 times greater than the flexural strength of the beam elements framing into the same joint.

Recommended building performance of SEAOC (1997)

Examples of earthquake damage to the structure of reinforced concrete buildings generally show that the damage is caused by inadequate aspects of design and construction. In the design aspect, there are many seismic detailing requirements for structural elements that have not been fulfilled. In addition, inadequate material quality and improper construction also influence the performance of buildings during an earthquake.

Here are some recommendations that need to be considered to prevent or minimise damage:

  • The implementation of structural system must be in accordance with the level of seismic risks in the area where the building structure is located.
  • The building’s continuity and structural integrity aspects need to be considered; in the details of reinforcement and connections, the structural elements of the building must be effectively bound together to enhance overall structural integrity.
  • The consistency between structural system developed in the design with that implemented in the construction must be maintained.
  • Concrete and reinforcing steel materials used must meet the construction material requirements for earthquake-resistant building structures.
  • Architectural elements that have large masses must be firmly tied to the main structural system and must be taken into account in the design of the main structural system.
  • The implementation methods, quality control systems and quality assurances at the construction stage must be carried out properly, in accordance with applicable rules.
  • Issues on worker skills, application procedures, the use of appropriate construction materials, quality control systems and quality assurances at the construction stage need to be well managed to avoid failure in well-design structures.
  • Skills of workers and supervisors for construction activities need to be improved through socialisation and training programmes to ensure proper planning and implementation of regulations.
  • National and regional governments must develop policies to improve licensing systems and supervision on earthquake-resistant building construction. In addition, a solid policy is needed for the post-earthquake rehabilitation of building structures, both for short-term and long-term actions. This policy can later be used as a reference in repairing or strengthening and reconstructing damaged or undamaged building structures after an earthquake.

Director of Research Center for Disaster Mitigation, Institut Teknologi Bandung (ITB)

Iswandi is also a professor in Concrete Materials and Structures at the Faculty of Civil and Environmental Engineering, ITB. He is also a registered professional engineer of HAKI. He graduated with a degree in Civil Engineering in 1987 from ITB, and obtained a Master of Applied Science in Civil Engineering (Structural Engineering) from the University of Toronto, Canada in 1990, and a Doctor of Philosophy in Civil Engineering (Structural/Material Engineering) in 1994.

His research interests are in concrete materials and structures, earthquake-resistant reinforced concrete structures, corrosion and durability of concrete structures and structural assessment, as well as repair and retrofit of bridge structures. He has published many papers in international and national journals and served as keynote speaker and editor in international and national conferences. He also wrote several technical books in his field, including “Structural Design of Earthquake Resistant Reinforced Concrete Buildings”, which was co-written with his colleague, Hendrik F. وان ایکس بت بت فوروارد