Introduction
In earthquake-resistant building design, the selection of an appropriate structural system is critical to ensure the safety and performance of the structure during seismic events. One of the primary structural systems for buildings is the Reinforced Concrete Moment-Resisting Frame System (MRFS). However, not all MRFS types are the same; they include Ordinary Moment-Resisting Frame (OMRF), Intermediate Moment-Resisting Frame (IMRF), and Special Moment-Resisting Frame (SMRF). The selection of the MRFS type depends on the building’s risk category, seismic design category, and other factors such as site class and building occupancy.
This article will discuss the fundamental theory behind MRFS selection, followed by a real project case study to clarify the step-by-step selection process based on SNI 1726:2019, which is adapted from the International Building Codes (IBC) 2018.
1. Determine the Risk Category
The selection process begins by identifying the risk category of the building or nonbuilding structure for seismic loads. According to SNI 1726:2019 Table 3, the risk category is determined by the building’s use or occupancy function. For example, school buildings and other educational facilities are under Risk Category IV.
2. Determine the Site Classification
In seismic design, ground conditions at the project site significantly influence the magnitude of seismic loads imposed on the structure. Therefore, it is essential to determine the site class based on geotechnical parameters such as the average Standard Penetration Test (SPT) N-value, average shear wave velocity (Vs), or Cone Penetration Test (CPT) results. According to SNI 1726:2019 Table 5, soils with an average SPT N-value of 10 are classified as Site Class SE (Soft Soil). More specifically, using SPT data, the site classification is determined as follows:
- Site Class SC (Hard Soil): N-value > 50
- Site Class SD (Medium Soil): N-value between 15 and 50
- Site Class SE (Soft Soil): N-value < 15
3. Determine the Seismic Design Category (SDC)
The Seismic Design Category (SDC) classifies the seismic hazard level at a building site, ranging from Category A to F. The criteria for determining SDC are provided in SNI 1726:2019 Tables 8 and 9. Besides the risk category, the SDC is also determined by seismic design parameters:
- SDS: Design spectral acceleration for short periods (0.2 seconds), adjusted for site conditions.
- SD1: Design spectral acceleration for a 1-second period, also adjusted for site conditions.
Currently, these parameters can easily be obtained using a web-based application developed by the National Earthquake Study Center (PuSGeN), Research and Development Center for Housing and Settlements, Ministry of Public Works and Housing.
4. Select the Appropriate MRFS
The MRFS selection follows SNI 1726:2019 Table 12, which provides R-factors, Cd values, and system applicability for seismic force-resisting systems. The table uses two codes:
- TB: Permitted
- TI: Not permitted
Guidelines:
- Categories A and B: May use OMRF, IMRF, or SMRF.
- Category C: OMRF not permitted; must use either IMRF or SMRF.
- Categories D, E, and F: Must use SMRF (stricter requirements).
Case Study: MRFS Selection
Project Data:
- Building Function: Office
- Location: Makassar, Indonesia
- Average Soil SPT N-value (from soil investigation): 40 (N)
- Seismic design values based on RSA 2021:
- SDS = 0.24
- SD1 = 0.17
Let’s walk through the step-by-step selection process:
Step 1: Determine the Risk Category
Since the building is a regular office, it falls under Risk Category II according to SNI 1726:2019.
Step 2: Determine the Site Class
Ground conditions heavily influence seismic loads. In this case, the available average SPT N-value is 40. Based on SNI 1726:2019:
- Site Class SC: N > 50
- Site Class SD: 15 < N ≤ 50
- Site Class SE: N < 15
Since N = 40, the site class is SD (Medium Soil).
Site Class SD indicates moderately stiff soil — not as hard as SC but not as soft as SE. This soil condition is typical in urban areas and provides moderate seismic amplification.
Step 3: Determine the Seismic Design Category
Based on SNI 1726:2019 Tables 8 and 9:
- SDS = 0.24 (between 0.167 and 0.33)
- SD1 = 0.17 (between 0.133 and 0.20)
For Risk Category II:
- SDS leads to Seismic Design Category B
- SD1 leads to Seismic Design Category C
Using the more stringent (conservative) category, the project falls into Seismic Design Category C.
Step 4: Determine the Allowed MRFS
For Seismic Design Category C, per SNI 1726:2019:
- OMRF: Not permitted
- IMRF: Permitted
- SMRF: Permitted
Thus, the choice must be between IMRF and SMRF.
Step 5: Decide Between IMRF and SMRF
Both systems are allowed. The decision can be based on:
- IMRF:
- Simpler detailing requirements
- Lower construction costs
- Suitable for standard office buildings with moderate seismic performance requirements
- SMRF:
- Very stringent detailing requirements (including strong column-weak beam philosophy, strict reinforcement ratios)
- Higher construction costs
- More appropriate for critical facilities or tall structures
In this case, since the building is a standard office without post-earthquake operational requirements, IMRF is sufficient.
Step 6: Conclusion
After considering all factors:
- Building Function: Office (Risk Category II)
- Location: Makassar
- Site Class: SD
- Seismic Design Category: C
- Permitted systems: IMRF or SMRF
From both technical and economic perspectives, the Intermediate Moment-Resisting Frame System (IMRF) is the most appropriate choice.
Closing
The selection of a moment-resisting frame system impacts not only the building’s seismic safety but also the project’s overall economy. Therefore, following the correct procedure — from identifying the risk category, determining the site class, calculating seismic parameters, to selecting the structural system — is crucial for successful structural design.