Site Coefficient and Design Spectral Acceleration Evaluation of New Indonesian 2019 Website Response Spectra

Calculation of site coefficient and design response spectral acceleration are two important steps in the seismic design of buildings. According to Indonesian Seismic Code 2019, two information requirements for site coefficient calculations are the site soil class and Risk-targeted Maximum Considered Earthquake (MCE_R-S_S for short and MCE_R-S₁ for long period) spectral acceleration. Three different hard/SC, medium/SD and soft/SE are typically site soil classes used for building designs. Two different site coefficients (Fa for MCE_R-S_S and Fv for MCE_R-S₁ spectral acceleration) are used for surface and design response spectral acceleration calculations. The Indonesian Seismic Code provides two (Fa and Fv) tables for calculating site coefficients. If the MCE_R-S_S or MCE_R-S₁ values developed for a specific site are not exactly equal to the values in Fa or Fv tables, the site coefficients can then be predicted using straight-line interpolation between the two closest Fa or Fv values within the tables. When the straight-line interpolation is adjusted for Fa or Fv calculation, different results were observed in comparison to the values developed using website-based software (prepared by Ministry of Public Works and Human Settlements). This study evaluates site coefficients and design response spectral acceleration predictions in Semarang City, Indonesia, according to straight-line interpolation method and website software  calculations. The study was conducted at 203 soil boring positions in the study area. The site soil classes were predicted using average standard penetration test values (N-SPT) of the topmost 30 m soil deposit layer (N30). Three different site soil classes were observed in the study area. On average, the largest differences between the two analysis (linear interpolation and website) methods in the site coefficient values and design response spectral acceleration calculation were observed for the SD and SE classes. However, for the SC site soil class, the difference was small, with their values approximately similar.

Design response spectral acceleration; MCER; N-SPT; Site coefficient; Straight-line interpolation

    The new National Seismic Code of Indonesia (SNI 1726:2019, 2019) was announced in 2019. Some of the information introduced in this new seismic code was partially adopted from the American Standard Code for Seismic Design ASCE/SEI 7-16, specifically the site coefficient values and design response spectral acceleration calculation methods. Additional information for developing the site coefficients was adopted from Stewart and Seyhan (2013). Due to the improved methods described in ASCE/SEI 7-16 for developing site coefficients for site soil classes SD and SE, not all the information described in the American Code was adopted by SNI 1726:2019. Specifically, the site coefficients for the SD and SE classes presented in SNI 1726:2019 were completely adopted from Stewart and Seyhan (2013).

Following the SNI 1726:2019, the Ministry of Public Works and Human Settlements announced a new website software (online facility) for site coefficient and design response spectral acceleration calculation. Site or building position coordinates (in terms of longitude and latitude) and site soil class are two information requirements for design response spectral acceleration calculations. Risk-targeted Maximum Considered Earthquake (MCE_R) acceleration, MCE_R-S_S for short and MCE_R-S₁ for long periods, (Luco et al., 2007; Allen et al., 2015; Sengara et al., 2020), and two design response spectral acceleration, S_DS and S_D1, are four important values calculated by the website facility software. However, no information related to site coefficients F_a for short and F_v for long periods can be obtained from the new website. Thus, these values can be calculated using Equation 1 and Equation 2. All S_DS, S_S (MCE_R-S_S), S_D1, and S₁ (MCE_R-S₁) values can be obtained from the website.

                                                                                                                                                        (1)

                                                                                                                                                         (2)

To verify the F_a and F_v site coefficients estimated using Equations 1 and 2, straight-line interpolation can be conducted using the S_S and S₁ website calculations and applying site coefficient (F_a and F_v) table data provided by SNI 1726:2019. F_a and F_v are then estimated following the procedure described by SNI 1726:2019. Equation 3 shows a simple formula for F_a and F_v site coefficients calculation. Figure 1 shows a diagram of the straight-line interpolation of the F_a and F_v calculation. F and M_w represent the site coefficient to be estimated and the MCE_R value obtained from the website, respectively; M_1S and M_2S represent two boundary MCE_R values close to M_w; F_1S and F_2S represent the site coefficients for M_1S and M_2S, respectively; and M_1S, M_2S, F_1S, and F_2S are the four values obtained from the SNI 1726:2019 tables. F_a and F_v are estimated separately using Equation 3.

                                                                                                              (3)

This paper describes the site coefficients and design response spectral acceleration verification calculated using the website facility and the straight-line interpolation described in SNI 1726:2019. The objective of the study was to evaluate whether or not the website performed the analysis following the same procedures used by SNI 1726:2019. The study was performed in Semarang City, Indonesia, and conducted at 203 soil boring investigation positions. The study was performed as part of seismic microzonation research of the city. One of the important information requirements for seismic microzonation is the development of soil amplification or site coefficient distribution map at the study area. In this study, the standard penetration test (N-SPT) data observed during boring investigation were used for site class calculation.  All boring investigations in this study were conducted at a minimum depth of 30 m and a maximum depth 60 m. The average standard penetration test (N-SPT) of the topmost 30 m soil deposit layer (N₃₀) of every boring position was used for site soil class interpretation (Moghaddam, 2011; Partono et al., 2019; Syaifuddin et al., 2020). Figure 2a shows the 203 boring positions and the N₃₀ distribution within the study area. Figure 2b shows the distribution of the site soil classes developed based on the N₃₀ data (Partono et al., 2021). The maximum N-SPT data obtained from the boring investigation was 60. Following the procedure described by SNI 1726:2019, the N₃₀ value was estimated using Equation 4, where d_i and N_i represent the thickness and N-SPT value of any soil layer “i", respectively.

The parameter that can also be used for site interpretation is the average shear wave velocity (V_S) of the topmost 30 m soil deposit (V_S30) (Naji et al., 2020). The V_S30 value can be calculated using the same method as that shown in Equation 4 and replacing the N_i value with V_Si. The V_S value can be observed using seismic refraction multichannel analysis of surface waves (MASW) or seismometer array investigations. Prakoso et al. (2017) described a comparative study of V_S value obtained from MASW investigation and soil boring (N-SPT) data. The V_S value developed using MASW was more reliable compared to that developed based on the N-SPT data. Pramono et al. (2020) described the predominant frequency investigation at Lombok Island following the 2018 earthquake event. The greater the V_S30 value used, the greater the predominant frequency obtained from the wavelet analysis of the ground motion. Additionally, development of V_S30 and predominant frequency correlation was also conducted by Pramono et al. (2017) in the Palu area.

Figure 1 Straight-line interpolation for F_a and F_v calculations

                                                                                                                                      (4)

Evaluations of site coefficients estimated using the website and straight-line interpolation methods were performed for 203 boring positions in Semarang City. No significant differences were found in the F_a and F_v site coefficients between the two methods. The largest difference in the F_a site coefficient calculations was observed for the SD and SE site classes. The difference in site coefficients for the SD and SE site soil classes was less than 0.03, while, for the SC site soil class, the difference was less than 0.01. In terms of site coefficient F_v, the largest difference was observed for the SD and SE site soil classes with a maximum of 0.04. However, the difference in site coefficient F_v for site class SC was less than 0.02. When calculating F_a and F_v site coefficients, the linear interpolation method from SNI 1726:2019 is better compared to the calculated using MCE_R-S_S, MCE_R-S₁, S_DS, and S_D1 values obtained from the website.

No significant differences in the design response spectral acceleration S_DS and S_D1 values were found for any of the site classes. The largest design response spectral acceleration difference in SD between the two methods was less than 0.02 g, while, for the SC and SE site classes, the differences were less than 0.005 g.

    This study was funded by the Directorate Research and Community Service, Deputy of Research Empowerment and Development, Ministry of Research and Technology/National Research Council and Innovation, through its 2021 research grant (Contract Number: 187-12/UN7.6.1/PP/2021). The authors also appreciate the Centre for Housing and Settlement Research and Development for supporting data and information collection during the development of this study.

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