Introduction
In earthquake-resistant building design, one of the most critical aspects is selecting a structural system that can effectively respond to lateral forces induced by seismic activity. One of the most widely used systems is the Moment Resisting Frame System (MRFS). This system has proven effective in dissipating and absorbing earthquake energy through controlled plastic deformation, making it the preferred choice for medium- to high-rise buildings in seismically active regions such as Indonesia. This article explores MRFS in detail, covering its definition, classification, working principles, and implementation according to national standards, particularly SNI 1726:2019 and SNI 2847:2019.
Definition of Moment Resisting Frame System (MRFS)
MRFS is a structural system composed of interconnected beams and columns where the joints are designed to resist bending moments, shear forces, and axial forces resulting from both gravity and lateral loads such as earthquakes. Unlike structural systems that rely on non-structural elements or shear walls, MRFS relies on rigid beam-column connections that can withstand deformation with high ductility.
A key characteristic of MRFS is its ability to undergo plastic deformation without significant failure, thanks to adequate detailing at the beam-column joints. This system is designed to ensure that earthquake-induced forces are distributed evenly and controlled in a way that any resulting damage is non-critical and repairable after a seismic event.
MRFS Classification Based on Ductility
According to SNI 1726:2019, MRFS is classified into three types based on ductility levels:
- Ordinary Moment Resisting Frame (OMRF): A low-ductility system used in areas with low seismic risk. OMRF is not recommended for essential structures or high-rise buildings. It requires minimal detailing compared to other types.
- Intermediate Moment Resisting Frame (IMRF): Offers a balance between seismic performance and construction cost. IMRF has moderate ductility and is suitable for mid-rise buildings in moderate seismic zones. Structural detailing and reinforcement must follow more stringent provisions than OMRF.
- Special Moment Resisting Frame (SMRF): A high-ductility system designed for high-risk seismic areas. SMRF requires rigorous joint and reinforcement detailing to ensure controlled plastic behavior and uniform damage distribution.
Below is a comparison table of the three MRFS types:
| Criteria | OMRF | IMRF | SMRF |
|---|---|---|---|
| Ductility | Low | Medium | High |
| Detailing requirements | Minimal | Moderate | Strict |
| Construction cost | Low | Medium | High |
| Application | General, low seismic areas | General buildings in moderate seismic zones | Critical buildings in high seismic zones |
Working Principle of MRFS
The fundamental principle of MRFS is the development of plastic hinges at beam ends near the beam-column joints. When subjected to lateral seismic forces, these joints undergo controlled plastic rotation, allowing the structure to remain standing despite large deformations.
In MRFS design, the strong column–weak beam concept is crucial. This principle requires the moment capacity of columns to be greater than that of the connecting beams, ensuring that plastic hinges form in the beams rather than the columns. This approach helps prevent soft-story collapse, which could otherwise lead to total structural failure.
The internal force distribution in MRFS is usually analyzed using either the equivalent static method or dynamic response spectrum analysis, depending on the complexity of the structure and the seismic zone classification.
Design Provisions Based on SNI Standards
Technical design requirements for MRFS refer to:
- SNI 2847:2019 (Structural Concrete Requirements) for reinforced concrete detailing.
- SNI 1726:2019 (Seismic Design Code) for determining seismic loads and lateral force design.
Key design provisions include:
- Transverse reinforcement detailing: In critical regions (plastic hinge zones), stirrup spacing should be no more than 100 mm or d/4.
- Moment capacity ratio for column-beam joints: The total nominal moment capacity of columns at a joint must exceed 1.2–1.4 times the total nominal moment of connecting beams.
- Anchorage and hook length: Longitudinal reinforcement in beams and columns must be adequately developed with proper anchorage length and standard hooks to ensure full tensile capacity.
- Mechanical splices: When used, mechanical splices must be capable of transferring full load without premature failure.
Advantages and Limitations of MRFS
Advantages:
- Architectural flexibility: No need for additional structural elements like shear walls or bracings.
- Excellent seismic performance: Especially for SMRFs with high ductility.
- Suitable for multi-story buildings: Rigid frames can resist both vertical and lateral loads simultaneously.
Limitations:
- Complex detailing: Requires high precision in reinforcement placement and joint construction.
- Higher cost: Due to extensive reinforcement and skilled labor.
- Longer construction time: Especially at beam-column joints, which require thorough inspection and quality control.
Case Study / Application Example
In a 10-story office building project in Padang, a SMRF system was selected due to the high seismic risk in the region. The main structure consisted of reinforced concrete frames with rigid joints. Joint detailing followed all provisions of SNI 2847:2019, particularly for transverse reinforcement, development lengths, and column-beam proportioning.
Structural analysis using software showed that plastic hinges formed at beam ends, while columns remained elastic, demonstrating successful application of design principles. A nonlinear pushover analysis confirmed that the capacity curve aligned well with expected ductile behavior.
Conclusion
The Moment Resisting Frame System is a reliable solution for designing earthquake-resistant structures, particularly in seismically active areas like Indonesia. By applying appropriate design principles, such as strong column–weak beam and proper reinforcement detailing, MRFS can maintain structural integrity during major earthquakes.
Selecting between OMRF, IMRF, or SMRF should consider seismic zone, building type, function, and project budget. In the future, advancements in materials and construction technology may improve MRFS implementation efficiency without compromising performance.
References
SNI 1726:2019, Earthquake Resistance Design Code for Building and Non-Building Structures
SNI 2847:2019, Structural Concrete Requirements for Buildings
ACI 318-19, Building Code Requirements for Structural Concrete
FEMA 356, Prestandard and Commentary for the Seismic Rehabilitation of Buildings
Chopra, A.K., Dynamics of Structures: Theory and Applications to Earthquake Engineering, Prentice Hall
Paulay, T. & Priestley, M.J.N. (1992), Seismic Design of Reinforced Concrete and Masonry Buildings, Wiley