Rigid Frame System or Moment Frame System

Rigid frames or moment frames consist of straight or curved members of the frame structures that are interconnected by rigid connections and resist vertical and lateral loads primarily through flexure. It is a type of frame structure based on the type of connection between the interconnected members.

Rigid connection implies that the rigid joints transfer the load coming on them by the action of bending the beam and column members.

Fig.1. Rigid Frame Connections for Steel and Concrete Frames

Rigid or moment frames are constructed either using steel or concrete. The stiffness and strength of rigid frames are provided by the bending rigidity of the beams and columns that have rigid connections. (Fig.1). 

The joints are hence designed to provide adequate strength and stiffness and negligible deformations. These frame arrangements do not allow any relative rotation to happen between the ends of the members.

This article clearly explains the behavior of rigid frames, their analysis methods, and the ways a moment frame can be designed; namely OMRF, IMRF, and SMRF.

Features of Rigid Frames

Fig. 2 shows the response of the rigid frame and the rigid joints under the action of lateral loads. Fig. 2(b) shows the deformation of the connected beam and the column due to the non-deformability of the rigid connection.

Fig.2

The amount of rotation and the distribution of the moment depends on the stiffness (EI/L) of the members in the joint. The end restraints on columns reduce the effective length making the column more slender. Due to the rigid joint connections, the deflections and moments in the beams are reduced as well.

The lateral loads coming on the rigid frame structure are resisted by rigid action i.e. the shear forces and bending moments are developed in the frame members and joints. They are typically statically indeterminate.

 

The depth of the frame members used in rigid frames is controlled by the stiffness rather than the strength, to reduce the story drifts in buildings due to lateral loads action.

Compared to other systems, the moment frames possess inherent flexibility that results in drift-induced non-structural damage. Based on the seismic zone and lateral load conditions of the site, the strength and the ductility of the connections between the beams and columns are considered.

Types of Rigid Frames

The rigid frames or moment frames are of two types: Fixed ended rigid frame systems and pin-ended rigid frame systems. 

1. Fixed-Ended Rigid Frames

These are rigid frames with fixed connections between the columns and beams which do not allow any rotation or displacement at the point.

When a load is applied on a fixed-ended rigid frame, the frame experiences bending moments and shear forces. The fixed connections between the columns and beams do not allow any rotation or displacement at the joint, which results in a moment-resisting structure. As a result, the entire frame is forced to bend as a single unit and transfer the load to the foundation.

The bending moments and shear forces that develop in the structure are transferred to the foundation through the columns, which are designed to resist these forces. These frame types are used where large moments are expected, like high-rise building frames, bridges, and towers.

2. Pin-Ended Rigid Frames

Pin-ended frames, also known as simply supported frames, have hinge connections between the columns and beams that allow for rotation and horizontal displacement at the joint.

When a load is applied to a pin-ended frame, the joints rotate and allow for horizontal displacement, causing the beams to bend and the columns to elongate or compress. The load is transferred through the members to the supports or foundation, and the frame system resists the deformation and maintains its stability.

This type of frame is commonly used in low-rise buildings, such as warehouses, residential buildings, and small bridges.

Pin-ended frames are not as rigid as fixed-ended frames, but they are more flexible and adaptable to different loading conditions.

Analysis of Rigid Frames

Structural analysis methods such as the portal method (approximate), the method of virtual work, Castigliano’s theorem, the force method, the slope-displacement method, the stiffness method, and matrix analysis, can be used to solve for internal forces and moments and support reactions. 

Rigid Frame Analysis

Shear and bending moment diagrams can be drawn for frame members by isolating the member from a joint and drawing a free-body diagram. The internal forces at the end will be equal and opposite, just like for connections in pinned frames. The direction of the “beam-like” member is usually drawn by looking from the “inside” of the frame.

Designs of Moment Resisting Frames (MRF)

Moment-resisting frames can be designed as:

1. Ordinary Moment Resisting Frames (OMRF) are frames that are not constructed with special detailing for ductile behavior. Hence, they are expected to withstand limited inelastic deformation in their members and connections under the action of lateral loads. So, they are used in low-seismic regions.

2. Intermediate Moment Frames (IMRF) are frames designed to release limited inelastic deformation under lateral load action.

3. Special Moment Resisting Frames (SMRF) are moment-resisting frames designed with special detailing to provide ductile behavior and are expected to withstand significant inelastic deformation in their members and connection under the action of lateral loads. SMRF is used for mid and high-seismic regions.

Construction of Rigid Frames

Rigid Frames can be either constructed: 

1. Steel Rigid Frames

Steel rigid frames provide flexibility in floor planning and are constructed fast. It is used efficiently for buildings with a maximum of 30 stories in height. The main disadvantages of steel rigid frames are that their connection is expensive and requires extensive fireproofing.

2. Concrete Rigid Frames

Concrete rigid frames provide flexibility in the floor planning. Even though they are easily mouldable, they require expensive formwork and slow down construction. The maximum number of stories that can be constructed by this method is 20.

Advantages of Rigid Frames

The main advantages of rigid frames are:

1. Moment redistribution
2. Small deflections
3. The effective column length is shorter
4. No structural walls or vertically oriented diagonal braces are employed for load action
5. The rigid frame design provides architectural freedom for the design.
6. The design imposes smaller forces on the foundation, compared to other structural systems.

Also Read: What are Frame Structures?