TDMRC-EEFIT team in Petobo, one of the liquefaction sites

Central Sulawesi Reconnaissance Survey: Impacts on Structures

The Survey

A masonry school building brought almost entirely down by the earthquake
A masonry school building brought almost entirely down by the earthquake

On September 28, 2018, a shallow earthquake with a magnitude of 7.4 hit the Central Sulawesi. This earthquake triggered landslide and tsunami in many parts of Palu and Donggala. TDMRC-EEFIT team conducted an observation on the performance of buildings against the extreme events in Palu and Donggala, including schools and residential houses.  This particular survey was carried out by a joint team from TDMRC and Epicenter UCL, consisting of Prof. Tiziana Rossett0, Dr. Yunita Idris, and Mr. Rohit Adhikara.

On the first day, the team conducted structural assessments at Universitas Tadulako, a local state university. The university was not hit by the tsunami, but was significantly affected by the earthquake. The assessments included visually observing the structures, determining level of damage, and investigating causes of structural failures.

TDMRC-EEFIT team assessing a surviving hotel building near the Palu bay
TDMRC-EEFIT team assessing a surviving hotel building near the Palu bay

The next day, the team visited the west coast of Palu to assess structural damage caused by tsunami. The third day saw the team taking a trip to Donggala, along with other teams. The structure team worked to observe bayside structures. Some buildings were found to have been wiped by the tsunami. Some others appeared to have been destructed earlier due to land cracks and subsidence.

TDMRC-EEFIT team surveying a school building that received shear cracks around the window frames due to earthquake force
TDMRC-EEFIT team surveying a school building that received shear cracks around the window frames due to earthquake force

On the last day, the team visited some schools in Palu to observe the buildings’ earthquake damage. The damage pattern found in Palu schools could be used as a reference for assessment of structural vulnerability in other parts of Indonesia because school buildings in the country typically use the same type of structures and plan.

The Findings

Most buildings in the Palu and Donggala region were found to be low-rise structures. The team found three general types of low-rise constructions: stilt timber houses, timber frame with the mortar infill and confined masonry (clay brick and concrete block). All three types showed different level of non-engineered construction practice in the city.

The stilt houses constructions was mainly the timber constructions that were raised on top of the piles with the concrete foundation. In Central Sulawesi, these houses were built either on land or in the body of bay water. These houses proved to be remarkably resistant to earthquake, standing unharmed in areas unaffected by tsunami. However, in tsunami affected areas, these houses were mostly wiped away, with only their concrete foundation blocks left.

The remains of concrete foundation of the timber stilt houses in the Labuan Bajo Fisherman Village
The remains of the concrete foundations of the timber stilt houses in Labuan Bajo fisherman village
Out of plane failure on timber-frame wire-mortar structure due to earthquake
Out of plane failure on timber-frame wire-mortar structure due to earthquake

Another typical low-rise construction in Donggala and Palu was timber frame structures with either mortar or masonry infill. The timber frames were from hard Ebony timber called Ulin. Some of the infill walls were wire-reinforced mortar, while some others were masonry (red clay brick or concrete block). During the earthquake, these buildings mostly underwent out of plane failure, where the infill walls detached from the frame.

Confined masonry wall tumbled due to confinement shortage
Confined masonry wall tumbling due to confinement shortage

The other common type of non-engineered construction in the area was confined masonry. This construction are usually comprised of clay or mortar brick walls with small confining concrete columns and stone mortar foundation. The typical damage for this type of construction during the September 2018 earthquake includes wall tumbles due to insufficiency of confinement, cracks around window frames, and total collapse.