Moment-Resisting Frames

Moment-Resisting Frames (MRFs) are most commonly used in mid-rise and high-rise buildings, since architectural freedom is a priority in these buildings. They are made of steel or reinforced concrete.

WHAT THEY ARE AND HOW THEY WORK

An MRF is a type of “structural skeleton” where the bones (vertical columns and horizontal beams) are connected by rigid joints called moment connections. These connections allow the frame to act as a single, unified unit when resisting earthquake forces.

Instead of relying on diagonal braces (like a braced frame) or solid walls (like a shear wall), an MRF uses the strength and flexibility of its beam-column joints to absorb and dissipate energy. Think of bending a stiff wire back and forth—it takes effort, and the wire absorbs energy as it flexes.

KEY PRINCIPLE: STIFFNESS vs. STRENGTH

Term	Definition
Strength	How much force a member can handle before it breaks or permanently bends
Stiffness	How much a member resists bending in the first place (a stiff member doesn’t deflect much under load)

Engineers control stiffness to meet design goals. Making a beam deeper makes it stiffer, which helps control building sway (drift). However, a stiffer building has a shorter period, which can cause it to experience higher forces from earthquakes that emit more frequent waves.

HOW AN MRF BEDS: CANTILEVER vs. RACKING

When a lateral force pushes on an MRF, the building sways through two actions:

CANTILEVER BENDING

Imagine the building as a giant flagpole stuck in the ground. When the wind pushes it, the pole bends, and the maximum stress is at the base. In an MRF, columns rigidly connected to the foundation resist overturning by bending, concentrating stress at their base. This is most noticeable in upper stories.

FRAME RACKING

Push a rectangular picture frame sideways—the corners distort into a parallelogram. In an MRF, this is “racking.” Beams and columns bend in opposite directions, creating a racking shape. This action is constant from bottom to top. If rotation is too extreme, connections can fracture.

Beam Racking (Left) Column Racking (Right)

THE COMBINED EFFECT

For most MRF buildings, frame racking accounts for about 70% of total sway. The remaining 30% comes from cantilever bending and beam flexure.

In a medium-rise building (which is our design criteria):

15–20% from cantilever action (columns stretching/compressing)
50–60% from shear racking (beams bending)
15–20% from shear racking (columns bending)

TYPES OF MRFs

The three types differ by their level of inelastic deformation (ductility):

Type	Ductility	Best For
OMF (Ordinary)	Low	Low seismic zones, wind-governed designs
IMF (Intermediate)	Moderate	Moderate seismic zones (with height limits)
SMF (Special)	High	High seismic zones, unlimited height

During an earthquake, a structure can resist with brute strength (elastic behavior) or absorb energy by bending in controlled ways (inelastic behavior). SMFs do the latter extensively; OMFs rely on the former.

OMF (ORDINARY MOMENT FRAME)

The moment connections (beam-column connections) are designed for gravity, wind, and low-level seismic activity, not necessarily major earthquakes.

Pros:

They have simpler behavior, so they’re easier to model and predict
They are often used in wind-governed designs
They are cheaper and simpler to construct than IMFs or SMFs

Cons:

They cannot form plastic hinges, which means that they will deform elastically (break immediately and suddenly without bending)
Since the connections remain elastic, they require stronger, stiffer members, making it less efficient in moderate seismic zones

Limitations: ASCE 7 explicitly prohibits OMFs as the primary seismic system in Seismic Design Categories D, E, and F (high seismicity). They can only be used for secondary elements or low-rise buildings in SDC C.

Common use: Low to moderate seismic activity; low-rise and mid-rise steel buildings.

IMF (INTERMEDIATE MOMENT FRAME)

Key Features:

Designed for moderate inelastic deformation (limited plastic rotation without fracturing)
Requires continuity plates (steel plates welded to the column web to stiffen it and prevent buckling)
Stricter lateral bracing to prevent beam twisting (lateral-torsional buckling)
Must sustain code-mandated interstory drift (e.g., 0.04 radians or 4% story drift)
Must use AISC 358 prequalified connections approved for IMF use

Pros: In moderate seismic zones (SDC C and D), IMFs offer more seismic capacity than OMF without the full complexity and cost of SMF.

Cons: In high seismic zones (SDC D, E, F), IMFs are severely restricted to a height of 35 feet (essentially one to two stories), making them impractical for mid- or high-rise construction in places like California or Japan.

Common use: Areas with moderate seismic activity.

SMF (SPECIAL MOMENT FRAME)

Key Features:

Designed for significant inelastic deformation (high ductility) through controlled plastic hinges in beams
Reduced Beam Section (RBS) — the “dogbone” connection — strategically removes material from beam flanges near the column, creating a weak point where the beam bends during an earthquake, protecting the column weld
Strong Column-Weak Beam (SCWB) — a mandatory code-enforced ratio ensuring plastic hinges form in beams (easier to inspect/replace), not columns (which could cause story collapse)
Panel zone yielding — the column web area at the beam-column joint is intentionally designed to yield before the RBS hinge, providing additional energy dissipation
Protected zones — specific areas where welding, cutting, and non-structural attachments are restricted to prevent defects that could initiate fractures

Pros: Safest of the three — even after major earthquakes that cause visible beam bending, columns and gravity load capacity remain intact, preventing collapse.

Cons: Requires rigorous third-party special inspection, precision CNC fabrication, more continuity plates, stricter column splices, and demanding bracing requirements — all adding to material and labor costs.

Common use: Severe seismic conditions.

ORIGINS

MRFs originated in Chicago and New York in the 1880s with the first steel-framed skyscrapers. Early riveted connections performed unexpectedly well in the 1906 San Francisco earthquake, leading engineers to believe steel frames were inherently earthquake-resistant — a belief that went largely unchallenged for nearly 90 years.

The 1994 Northridge earthquake shattered this assumption. Widespread brittle fractures were discovered in welded beam-column connections across hundreds of buildings. The 1995 Kobe earthquake revealed identical failures in Japan, proving the problem was global.

This forced a complete redesign. By 2000, new standards emerged — AISC 341 (Seismic Provisions) and AISC 358 (Prequalified Connections) — centered on the Reduced Beam Section (RBS) “dogbone” connection.

PERFORMANCE EVIDENCE

Period	Event	Outcome
Pre-1994	Early steel frames	Performed well enough to create false confidence
1994	Kobe Earthquake (Japan)	Revealed widespread brittle fractures; triggered complete redesign
1995	Kobe Earthquake (Japan)	Confirmed problem was global; spurred international collaboration
Post-2000	AISC 341/358, RBS adoption	Modern SMFs designed for ductility and collapse prevention

Examples of brittle failure of welded beam-column connections during the 1994 Northridge earthquake

Ongoing risk: Hundreds of pre-Northridge buildings with obsolete connections still exist today and remain vulnerable.

CULTURAL ASPECTS

Japan’s five-story wooden pagodas have survived countless earthquakes, including the 1995 Kobe earthquake that flattened modern buildings around them.

The secret is the shinbashira — a thick central column running the full height of the pagoda. It is not rigidly connected to the floors, allowing it to move independently. During an earthquake, when the outer building sways one way, the central pillar swings the opposite way, canceling the sway and stabilizing the structure.

This design is uniquely Japanese — it does not appear in Chinese or Korean pagodas — and represents indigenous earthquake engineering developed over centuries.

In 2012, engineer Steven Tipping adapted this concept to retrofit a vulnerable steel moment frame building in San Francisco (680 Folsom Street).

The retrofit saved $4 million in construction costs, reduced the schedule by 10 weeks, and met building code requirements. Tipping was named one of Engineering News-Record’s Top 25 Newsmakers for this innovation.

Original Shinbashira (Left) 680 Folsom Street Retrofit (Right)

WINNER OF THIS CATEGORY

SMFs perform the best for the design criteria of this project:

Mid-Rise Building: Suitable for any height; widely used in mid-rise and high-rise construction.
San Francisco Clay Soil: Flexible enough to accommodate soft soil amplification without sudden failure; deep foundations (piles or mat slabs) mitigate soil effects.
Immediate Occupancy: Achievable with strict drift limits and careful detailing of non-structural components; requires supplemental damping.

Key Trade-Off: SMFs are flexible. While this flexibility provides excellent energy dissipation, it can result in higher drifts that damage non-structural elements (pipes, elevators, glass) unless drift is strictly controlled through increased member sizes or supplemental dampers.

For detailed scoring comparisons, see the Lateral Force Resisting Systems: MRFs Spreadsheet below: