Structural evaluation of Ikhwatun shelter building constructed on liquefaction potential area in Padang city, Indonesia

Indonesia has often experienced seismic natural disasters such as earthquakes and tsunamis especially Padang City, the capital city of West Sumatera, Indonesia. In order to face the future earthquake and tsunami disaster, the local government has built a number of vertical evacuation shelters. One of the shelters is Ikhwatun shelter building located in Koto Tangah Subdistrict of the Padang City. The shelter was built near to the coastal and expected has liquefaction potential. This study is conducted in order to evaluate the shelter to restrain the earthquake and tsunami loads. The building is made of the reinforced concrete structure with the floor area of 2680 m2 and the high of 22.78 m. Based on the result of the soil evaluation, it was found that the soil deposit in the shelter has high liquefaction potential. Therefore, the upper and lower structures are analyzed using special response spectrum of the earthquake loads for soil liquefaction, which is 1.5 higher than those on the non-soil liquefaction. The analysis result shows that the beams, columns, and foundations are all not able to resist the applied tsunami loads. It is suggested that the building to be strengthened before being used as a vertical evacuation shelter.


Introduction
Recently, Indonesia has experienced big seismic triggered disasters regarding earthquakes and tsunamis. The geographical location of Indonesia on the three main plates of the world is the main reason for the disasters. These plates become the resources of seismic activities that sometimes generate tsunamis.
In 2009, Padang, the capital city of the West Sumatra province, Indonesia, has experienced a big earthquake which caused damage to the buildings and infrastructures. The Padang City is also located on the West Coast of Sumatra Island, which wide open to the sea (Indian Ocean) where the active two-plate zone exists. This condition results in Padang City become one of the most earthquake and tsunami prone area. After the 2009 Sumatera earthquake, the government started establishing vertical evacuation shelters in Padang. One of the shelters is the Ikhwatun shelter building, located in Koto Tangah Subdistrict of Padang.
Generally, the sand sediments near to the coastal area in Padang have the liquefaction potential. During the earthquake of September 30 th , 2009, the liquefaction phenomena have occurred in Padang which caused the lateral movement, and sand boils as well as the landslides on the embankments [1].
Tsunami shelters are usually built in residential areas near the coastline, where the communities around the shelter can reach easily. The collapsed shelter clearly cannot be used as a post-earthquake shelter. Therefore, it is necessary to evaluate the existing shelter to ensure that the shelter can resist the earthquake, liquefaction, and tsunami.

Evaluation of soil liquefaction potential
Liquefaction is a natural phenomenon that occurs due to seismic shock. If the liquefaction happens, the increased water pressure in soil mass may reduce the soil strength. The liquefaction potential at Ikhwatun Shelter is assessed based on the relative density (Dr) and grain (D50) method [2]. The soil data was obtained from the samples, which is taken from the depth of 1.55-2.00 m with an N-SPT value of 23. The corrected N-SPT (N') value [3] is 19. The relative density value (Dr) is then estimated using Table 1. This value is obtained by interpolating the corrected N-SPT value gave the Dr of 0.485 (48.5%). Fig. 1 shows the sieve analysis data of the sample. Based on this figure, it is obtained the D50 of 0.36 mm.    The relative density (Dr) and the grain size distribution (D50) values are plotted into the liquefaction potential graph, as shown in Fig. 2. From the figure, it can be seen that the values mainly on the right of the 0.3 g lines and the left of the 0.6 g line. It means that the soil will experience liquefaction when the 0.6 g earthquake occurs. Table 2 shows the liquefaction assessment results of soil layers on this shelter. The ground of shelter building can be concluded as high liquefaction potential.

Structure data
The location of this shelter is close to the coastline, with a distance of 0.56 km from the Padang beach line. Based on the tsunami hazard map of Padang City, the depth of tsunami inundation in the location of the plan is about 4 to 5 m [5]. The tsunami inundation depth is based on the contours and tsunami wave prediction in Padang City.
The structural analysis of shelter building is modeled and analyzed using ETABS 9.7.1 software with linear analysis. The columns and beams of the building structure are shaped as frames. Meanwhile, the floor plates are modeled as slab elements. This 5-floor structure is made of reinforced concrete with the concrete compressive strength, f c ' of 30 MPa and steel yield strength, f y of 400 MPa. The building height is 7.82 m for the 1 st floor and 3.74 m for 2 nd , 3 rd , 4 th , and 5 th floors. The shelter area is 2680 m 2 with the thickness of the slab of 15 cm. Fig. 3 shows the 3D modeling of the shelter building.

Loads
The dead and live loads refer to Minimum loads for Building Design and Other Structures of SNI 1727-2013 [6]. The analysis of earthquake load using dynamic analysis (earthquake response spectrum) for Padang City based on SNI 03-1726-2012 and earthquake map of 2017 by making its design of response spectrum. According to Article 5.3, Table 3 SNI 03-1726-2012 or Table 53 SNI 8460-2017, the value of N-SPT of medium soil ranges from 15 to 50 [7,8]. The N-SPT value of the shelter in the middle ground (SD) is 20.5.
However, the soil at that location has the potential for liquefaction, so it is categorized as special soil (SF) according to Table 3 SNI 03-1726-2012 or Table 53 SNI 8460-2017. Furthermore, to calculate the response spectrum of a special soil (SF), it must follow the article 6.8 or 6.10.1 in SNI 03-1726-2012, which stated that if the spectrum of the maximum-risk-targeted seismic response (MCER) is required, the design response spectrum then should be multiplied by 1.5. Therefore, the design spectral acceleration parameter values for moderate soil, SD1 obtained is 0.6, and the SDS is one were increased by 1.5, so the value of SD1 and SDS will be 0.9 and 1.5, respectively [7,8]. The result of the response spectrum calculation is plotted in Fig. 4. Based on the FEMA P-646 [9], the refugee live load was taken 250 kg/m 2 on evacuation room. The tsunami loads are also calculated based on the FEMA P-646 Code 2012 [9]. The value of each load calculated based on the predicted wave height of the tsunami, the ground elevation of the shelter area, the distance from the shore and other assumptions used, which can be seen in Fig. 5. Table 3 shows the tsunami loads calculation applied to the shelter.

Column capacity
P-M interaction diagram is a diagram illustrating the ability or capacity of the column based on the relationship between the moment and axial load of the column.  Table 4.  Tables 5 and 6 show the results of bending and shear capacities of the beam. The bending beam capacity on the tie beam, first floor, and second floor cannot resist the applied loads. The shear capacity on all of the level is not strong enough to restrain the applied loads.

Foundation capacity
Based on SNI 8460-2017 of article 12.2.4.3, the friction resistance at the soil layer with liquefaction potential should be ignored for the pile foundation [8]. In the Ikhwatun shelter, the foundation type is pile with 350 mm diameter and 30 m depth and concrete strength, f c ' = 60 MPa. Fig. 10 shows the foundation plan of this shelter. The soil data was obtained from NSPT test results on the field and the results of the granular analysis in the laboratory [10]. Calculation of foundation capacity was carried out using the calculation of static axial pole capacity with SPT based on Mayerhof method [11]. Based on the results of the foundation bearing capacity in Table 7, it can be seen that foundation PC2, PC3 and PC4 has not enough capacity to resist the applied loads.

Conclusions
The Ikhwatun shelter building has been built in a high potential of the soil liquefaction. The structural analysis then must consider the spatial condition of the soil as stated by the standard code. Based on the result of the structural analysis, the bending columns capacities can resist the working loads, but shear capacities of the columns may not be able to withstand the applied loads. The beams also have not strong enough capabilities to resist the working loads. Further, the result of the foundation analysis obtains that the capacity of pile foundations is unable to resist the applied loads, especially when the case of earthquake and liquefaction.Then, it is suggested that the shelter must be strengthened before being used as a vertical evacuation building for the earthquake and tsunami.