Dual Systems

Dual systems are most commonly used in mid to high-rise buildings where both stiffness and ductility are required.

WHAT THEY ARE AND HOW THEY WORK

A dual system is a structural system that combines two different lateral force-resisting systems to work together. Specifically, it pairs moment-resisting frames with either braced frames or shear walls.

The official definition (ASCE 7-22, Section 12.2.5.1): A dual system consists of a moment-resisting frame that provides support for vertical loads, while seismic force resistance is provided by a combination of the moment frames AND shear walls or braced frames.

Why they are used: In mid- to high-rise structures, dual systems enable structural designers to satisfy stringent drift limitations of building codes without compromising ductility. Essentially, you get the best of both worlds — the stiffness of braces or walls to control sway, and the ductility of moment frames to absorb earthquake energy.

WHY MRFs MUST BE ONE OF THE PAIR

The defining feature of a dual system is that the moment frames must be capable of resisting at least 25% of the design seismic forces. This is a codified requirement in ASCE 7.

Why does this 25% rule exist?

Reason	Explanation
Redundancy	If the primary system (braces or walls) is damaged or degrades during an earthquake, the moment frames serve as a backup, preventing collapse
Ductility Backup	Braced frames and shear walls are stiff but can be brittle. Moment frames provide ductility — the ability to bend without breaking — when the stiff system reaches its limit
Load Path	The moment frames and the braced frames/walls must be capable of resisting the entire seismic forces in proportion to their relative rigidities

WHY BRACED FRAMES CANNOT BE PAIRED WITH SHEAR WALLS

Code Definitions Are Explicit

ASCE 7 defines dual systems specifically as moment frames + braced frames OR moment frames + shear walls. There is no category for “braced frames + shear walls.” If such a system were built, it would not be classified as a dual system under building codes and would not receive the associated design benefits (such as reduced redundancy factors or increased height allowances).

Stiffness Conflict Creates Unpredictable Load Distribution

Both braced frames and shear walls are stiff systems. When you put two stiff systems together, they compete to attract seismic forces based on their relative stiffness — but both degrade differently under cyclic loading. This creates:

Unpredictable force distribution: It becomes unclear which system is carrying what load as the building sways back and forth
No ductile backup: If both systems are stiff and potentially brittle, there is no “flexible” system to provide ductility when the stiff systems fail

Dual Systems Require One Flexible and One Stiff Component

The entire philosophy of dual systems is based on complementary behavior:

Component	Role
Stiff component (braces or walls)	Controls drift, limits sway, carries most of the lateral load
Flexible component (moment frames)	Provides ductility, acts as a backup, dissipates energy

Braced frames and shear walls both occupy the “stiff” role. Pairing them gives you two stiff systems and no ductile backup — defeating the purpose of a dual system.

Shear Wall-Frame Interactive Systems Are Different

There is a code category called “Shear Wall-Frame Interactive System” (ASCE 7, Category F). However, this is distinct from a dual system. In this configuration:

Shear walls must resist at least 75% of the design story shear at each story
Frames (which can be ordinary moment frames) must resist at least 25% of the design story shear

This is a specialized system with different requirements, and the frames used are typically ordinary moment frames — not the special or intermediate moment frames required for dual systems.

HOW DUAL SYSTEMS ARE CONSTRUCTED

Most dual systems fall into two categories:

Category	System Type	MRFs Used	Braces/Walls Used
D	Dual with Special Moment Frames	Steel SMF or Special RC MF	Steel EBF, SCBF, BRBF; Special RC shear walls
E	Dual with Intermediate Moment Frames	Steel IMF or Intermediate RC MF	Steel EBF, SCBF, BRBF; Special RC shear walls

HOW DUAL SYSTEMS ARE CONSTRUCTED

Increased Height Limits

When dual systems use eccentrically braced frames (EBF), special concentrically braced frames (SCBF), buckling-restrained braced frames (BRBF), or special RC shear walls, the structural height limit increases from:

160 ft to 240 ft for Seismic Design Categories D or E
100 ft to 160 ft for Seismic Design Category F

This makes dual systems ideal for mid-rise and high-rise buildings in high seismic zones.

Redundancy Factor Reduction

In codes like IBC and UBC, specifying a building as a dual system enables using a 0.8 reduction factor on redundancy factors. This translates to cost savings and less stringent detailing requirements.

Improved Drift Control Without Ductility Loss

Dual systems combine the drift control of stiff systems with the energy dissipation of ductile frames — allowing buildings to meet strict drift limits while maintaining seismic resilience.

ORIGINS

Origins: 1960s-1970s

The concept emerged from research at the University of California, Berkeley, and other institutions studying how different lateral systems interact. Early dual systems combined reinforced concrete shear walls with steel moment frames — a hybrid approach that gained popularity in high-rise construction.

Code Recognition: 1990s-2000s

Dual systems were formally recognized in building codes beginning with UBC 1997 and continuing through IBC 2000/2003 and ASCE 7. The 25% moment frame requirement was established during this period.

Post-Northridge Refinement (1994-2005)

After the 1994 Northridge earthquake, dual systems gained renewed attention. The earthquake revealed that buildings with redundant lateral systems performed better than those relying on a single system. Research following Northridge refined the understanding of how moment frames and braced frames interact under cyclic loading.

Modern Era: Performance-Based Design

Today, dual systems are a standard solution for tall buildings in high seismic zones. Research continues on optimizing the relative strength between the two systems, with some studies suggesting the 25% moment frame requirement may be conservative.

PERFORMANCE EVIDENCE

Northridge Earthquake (1994)

Steel moment frames paired with concrete shear walls generally performed well, while buildings relying solely on moment frames or solely on braced frames suffered damage. This reinforced the value of redundancy.

Laboratory Testing (2013 Study)

A study comparing dual systems in 30-story buildings found:

Steel moment frames + RC shear walls: Higher ductility and response modification factor
Steel moment frames + concentrically braced frames: Lower ductility

Conclusion: The RC shear wall system provided better seismic performance overall.

Recent Research (2025 Study)

A 2025 study on dual systems with eccentric braced frames found that inelastic beam rotations in moment frames were close to 0.02 radians at lower stories, and overall building strength improved with dual systems in taller structures.

WINNER OF THIS CATEGORY

SMF + BRBF is the best dual system for the design criteria of this project.

Criterion	How SMF + BRBF Delivers
Mid-Rise Building	Unlimited height under ASCE 7 for Seismic Design Categories D, E, F
San Francisco Clay Soil	BRBFs provide high stiffness to control drift on soft clay
Immediate Occupancy	Proven at 181 Fremont (San Francisco)
Ductility	Both systems are highly ductile (SMF beams hinge; BRBF cores yield without buckling)
Redundancy	If one system degrades, the other carries the load

Why not SMF + RC shear walls? Mixing steel and concrete adds complexity, concentrated foundation forces, and blocks floor plans.

Key Trade-Off: SMF + BRBF is more expensive than traditional systems and requires specialized BRBF fabrication. However, for immediate occupancy on San Francisco clay, the added stiffness, ductility, and proven performance justify the cost.

For detailed scoring comparisons, see the Lateral Force Resisting Systems: Dual Systems spreadsheet below.