Moment Resisting Frame | What is Moment Resisting Frame

What is Moment Resisting Frame?

Important Point

The Moment-resisting frame is a rectilinear gathering of beams and columns, in which the beams are inflexibly joined to the columns. Protection from parallel forces is given chiefly by unbending frame activity by the advancement of bending moment and shear force in the frame individuals and joints.

Under the unbending bar segment associations, a moment frame can’t dislodge horizontally without bending the beams or columns relying upon the math of the affiliation.

The bending rigidity and strength of the frame individuals are the essential wellsprings of sidelong firmness and force for the whole frame. In moment resisting frames, the joints or associations among columns and beams are intended to be unbending.

This makes the columns and beams twist during a seismic tremor, so these primary individuals are intended to be solid in bending. These frames are trusses without corner to corner supporting, so the shear forces following up on the construction should be opposed as moments in the joints among beams and columns.

The subsequent design is more generous than a truss with askew propping yet embraced where such supporting burdens its utilization, i.e., at divider openings. These moment-resisting frames are called special moment frames due to these extra necessities, which improve the inelastic reaction attributes of these frames.

Primary tests have shown that steel moment-resisting frames may give fantastic malleability and inelastic conduct under extreme seismic stacking. Since these frames are much of the time very adaptable, float restricts frequently control the plan.

Also, read: What Is Rolling Margin | The Procedure of Rolling Margin

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Types of Moment Resisting Frame

There are three types of Moment Resisting Frame:

IMRF: Intermediate Moment Resisting Frame.
SMRF: Special Moment Resisting Frame.
OMRF: Ordinary Moment Resisting Frame.

1. Intermediate Moment Resisting Frame ( IMRF)

When subjected to the stresses resulting from the design earthquake’s motions, intermediate moment frames ( IMRF ) are expected to tolerate restricted inelastic deformations in their members and connections. The standards in this section must be met by the IMRF.

1.1. Beam-To-Column Connections

The seismic load resisting system’s (SLRS) beam-to-column connections must meet the following requirements:
An interstory drift angle of at least 0.02 radians must be maintained by the link.
At an interstory drift angle of 0.04 radians, the measured flexural resistance of the connection, measured at the column face, must equal at least 0.80Mp of the linked beam.
For the earthquake load impact E, the required shear strength of the connection shall be derived using the following quantity:

E = 2[1.1RyMp]/Lh.

- The required shear strength must not be more than the shear resulting from the amplified seismic load applied in accordance with the applicable building code.

1.2. Beam Flanges

In plastic hinge zones, abrupt changes in beam flange area are not permitted.
If testing or qualification shows that the resulting design may create stable plastic hinges, drilling of flange holes or trimming of beam flange width is allowed.

1.3. Lateral Bracing of Beams

Both flanges must be supported laterally in some way, either directly or indirectly. Between the lateral braces, the unbraced length must not exceed 0.17ryE/Fy.
Where research shows that a plastic hinge will form during inelastic deformations of the IMF, lateral braces should be positioned near concentrated loads, changes in cross-section, and other areas.
The required strength of lateral bracing close to plastic hinges is

Pu = 0.06 Mu/ho (LRFD) .

Pa = 0.06Ma/ho (ASD).

- Where, ho is the distance between flange centroids.

Also, read: IS 516:1959 Most Important Point (Method of Tests For Strength of Concrete)

2. Special Moment Resisting Frame ( SMRF )

Whenever applied to the stresses arising from the design earthquake’s motions, special moment frames ( SMRF ) are supposed to survive considerable inelastic deformations. The conditions in this section must be met by SMRF.

2.1. Beam-To-Column Connections

The following three conditions must be met by beam-to-column connections used during seismic load resisting systems (SLRS):

An interstory drift angle with at minimum 0.04 radians must be maintained by the link.
At an interstory drift angle of 0.04 radians, the observed flexural resistance of the connection, evaluated at the column face, must approximate at minimum 0.80Mp of the linked beam.
For the earthquake load impact E, the required shear strength of the connection shall be derived using the following quantity:

E = 2 [1.1 Ry Mp] / Lh.

- Where,
- Ry is the ratio of the expected yield stress to the minimum yield stress specified.
- Fy Mp = kip-in nominal plastic flexural strength (in N-mm)
- Lh is the distance between the sites of the plastic hinges in inches. (in mm)

2.2. Conformance Demonstration

The following conditions must be met by beam-to-column connections utilized in the SLRS:

The use of SMF connections has been planned.
Using a connection that has been pre-qualified for SMF.
Results of qualifying cyclic tests are provided. At least two cyclic connection tests must be performed and the results must be supplied.

Also, read: 23 Different Types Cements Available in India and As Per IS Standards

2.3. Panel Zone of Beam-To-Column Connections

The required panel zone thickness must be calculated using the same approach that was used to measure the panel zone of the verified or preapproved connection.
The necessary shear strength of the panel zone shall be established at a least by adding the predicted moments at the plastic hinge points to the column faces and calculating the total of the moments at the column faces.
The design shear strength shall be φv Rv .
The allowable shear strength shall be Rv / Ωv.
- Where,

φv = 1.0 ( LRFD ) Ωv = 1.50 ( ASD ).

- The nominal shear strength, Rv, according to the limit state of shear yielding.

2.4. Panel Zone Thickness

The thicknesses of column webs and doubler plates, if used, shall conform to the following requirement:

t ( dz + wz ) / 90

Where,
- t = thickness of column web or doubler plate, in. (mm)
- dz = panel zone depth between continuity plates, in. (mm)
- wz = panel zone width between column flanges, in. (mm)

2.5. Panel Zone Doubler Plates

Doubler plates must be welded to the column flanges employing a completejoint-penetration groove-welded or fillet-welded joint that creates the whole doubler plate thickness’s potential shear strength.
When doubler plates are welded against the column web, the top and bottom edges must be welded together to develop the proportion of total force transmitted to the doubler plate.
When doubler plates are installed away from the column web, they must be symmetrically arranged in pairs and welded to continuity plates in order to develop the proportion of total force transmitted to the doubler plate.

Also, read: IS 516:1959 Most Important Point (Method of Tests For Strength of Concrete)

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3. Ordinary Moment Resisting Frame (OMRF)

Whenever applied to the stresses arising from the designed earthquake’s motions, ordinary moment frames (OMF) are intended to endure negligible inelastic deformations in its members including connections. The criteria of this Part must be met by OMF.

3.1. Beam-To-Column Connections

Welds and/or high-strength bolts are required for beam-to-column connections. As seen below, connections can be fully restrained (FR) or partially restrained (PR) moment connections.
The required flexural strength of FR moment connections that are part of the seismic load resisting system (SLRS) shall be equivalent to

1.1RyMp (LRFD) or (1.1/1.5)RyMp (ASD)

As appropriate, of the beam or girder, or the maximum moment that the system can create, whichever is smaller.

Also, read: What Is Rolling Margin | The Procedure of Rolling Margin

3.2. FR Connections Shall Meet the Following Requirements.

Steel backing and tabs must be removed from connections with complete-joint-penetration (CJP) beam flange groove welds, with the exception of top-flange backing joined to the column by a continuous fillet weld on the edge below the CJP groove weld.
Where weld access holes are required, they must be positioned. The surface roughness of the weld access hole must not exceed 500 in. (13 m), and it must be free of notches and gouges.
Notches and gouges must be corrected according to the engineer of record’s specifications. In bolted moment end-plate connections, weld access holes in the beam web adjacent to the end-plate are prohibited.
The necessary strength of double-sided partial-joint-penetration groove welds and double-sided fillet welds in connections that withstand tensile forces is 1.1RyFyAg (LRFD) or (1.1/1.5)RyFyAg (ASD) of the connected element or part, as appropriate. To resist tensile stress in the connections, single-sided partial-joint-penetration groove welds and single-sided fillet welds must not be employed.
For FR moment connections, the needed shear strength, Vu or Va, is obtained by multiplying the earthquake load impact E by

E = 2[1.1RyMp]/Lh.

3.3. Requirements for PR Moment Connections

When the following conditions are met, PR moment connections are permitted:
such connections must be designed for the requisite strength.
The connection’s notional flexural strength, Mn, must be at least 50% of the connected beam or column’s Mp, whichever is lower.
The stiffness and strength of the PR moment connections, as well as the effect on overall frame stability, must be considered in the design.
For PR moment connections, Vu or Va should be calculated using the load combination above plus the shear caused by the maximum end moment that the connection can withstand.

FAQ

Moment Frames

A moment frame is a special type of frame that uses rigid connections between each of its constituent members.

Structural Moment Frame

This configuration is able to resist lateral and overturning forces because of the bending moment and shear strength that is inherent in its members and the connecting joints.

Steel Moment Frame

A steel moment-frame is an assembly of beams and columns, rigidly joined together to resist both vertical and lateral forces. Construction of a modern steel frame building in which the ends of beams are rigidly joined to columns by welded connections.

Special Moment Frame

Structural steel special moment frames (smf) are typically comprised of wide-flange beams, columns, and beam-column connections. Connections are proportioned and detailed to resist internal forces (flexural, axial, and shear) that result from imposed displacement as a result of wind or earthquake ground shaking.

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Moment Resisting Frame | What is Moment Resisting Frame | Types of Moment Resisting Frame