Building for the Unsteady Earth

Braced Frames

Braced frames are most commonly used in high-rise buildings where stiffness is a priority.

WHAT ARE THEY AND HOW DO THEY WORK

Braced frames resist lateral forces using diagonal members (braces) that form a truss system. When wind or an earthquake pushes on the building, the braces act like push-and-pull rods. One brace goes into tension (pulling), while another goes into compression (pushing). These axial forces travel through the braces into the beams and columns, then down to the foundation.

Unlike moment frames, which rely on the bending of beams and columns, braced frames channel forces directly through the braces. This makes them significantly stiffer—they sway much less under load.

The trade-off: Braced frames are stiffer and often more cost-effective, but traditional braces can buckle suddenly under compression. Moment frames are more flexible but can absorb more energy through bending.

CONCENTRICALLY BRACED FRAMES (CBF)

In CBFs, the centerlines of braces, beams, and columns all meet at common points. This creates a simple truss where forces travel straight through connections without bending. There are different types of these braces.

Materials: Steel is most common, though reinforced concrete CBFs exist

Pros:

• Stiffer than MRFs, reducing sway and non-structural damage
• More cost-effective than MRFs for mid-rise buildings
• Easier to analyze and design than shear walls

Cons:

• Less ductile than MRFs; braces can buckle and lose strength suddenly
• Braces block architectural openings (doors, windows, hallways)
• Poor energy dissipation compared to MRFs

Best for: Low to mid-rise buildings in low to moderate seismic zones, or as part of a dual system in high seismic zones.

X-BRACING (CROSS BRACING)

In this type of CBF, two diagonal braces cross in an “X” pattern. Under lateral load, one brace pulls (tension) while the other pushes (compression).

Materials: Steel sections (angles, tubes, wide flanges).

How forces transmit: Lateral force enters the frame → tension brace carries load across bay → compression brace also engages → forces converge at beam-column joints → transferred to foundation.

Pros:

• Highest stiffness of any braced frame configuration
• Reduces lateral displacement by approximately 73% compared to unbraced frames
• Reduces column shear forces by approximately 67%
• Symmetrical performance in both directions

Cons:

• Blocks large portions of the building facade
• Creates congestion at the center crossing point
• Compression brace can buckle under heavy seismic loading

Best for: High-rise buildings where stiffness is the priority.

V BRACING/CHEVRON BRACING

In this type of CBF, two braces form an inverted “V” shape, meeting at a point on the beam above.

Materials: Steel sections (angles, tubes, wide flanges).

How forces transmit: Lateral force enters frame → braces converge on beam midpoint → beam experiences vertical unbalanced force when one brace buckles → beam transfers combined forces to columns.

Pros:

• Allows for openings below the chevron (doorways, corridors)
• More architectural flexibility than X-bracing
• Distributes forces to two columns rather than one

Cons:

• Beam must be heavily reinforced to handle unbalanced forces after brace buckling
• Requires special design for post-buckling behavior
• More complex detailing than X-bracing

Design requirement: The beam must be designed to resist the full tension capacity of one brace plus 30% of the compression capacity of the opposite brace.

Best for: Mid-rise buildings where some architectural openness is needed below the brace.

K-BRACING

In this type of CBF, two braces connect from the column midpoint to a common point on the beam, forming a “K” shape.

Materials: Steel sections.

How forces transmit: Lateral force enters frame → braces converge at column mid-height → column experiences concentrated horizontal force at a vulnerable location → forces transfer to foundation.

Pros:

• Allows for openings in the center of the bay
• Simple connection detailing

Cons:

• Concentrates stress at column mid-height — a critical failure point
• Poor ductility can cause column buckling under seismic loading
• Severely restricted or prohibited in high seismic zones
• Seismic performance factor of only 2 (much lower than other systems)

Best for: Low-rise buildings in low-seismicity zones only.

SINGLE DIAGONAL BRACING

In this type of CBF, a single diagonal brace is placed in a bay, typically oriented to resist tension.

Materials: Steel sections (often angles or rods for tension-only systems).

How forces transmit: Lateral force enters frame → diagonal brace carries load in tension → asymmetric force creates lateral deflection component → orientation alternates on opposite sides to balance forces.

Pros:

• Minimal obstruction of the building facade
• Simple connection details
• Can be optimized to reduce overall sway by approximately 2% compared to X-bracing in tall buildings

Cons:

• Asymmetric; requires alternating orientation to balance forces
• Under vertical loads, the brace creates unwanted lateral movement
• Typically designed as tension-only, meaning compression brace is ignored

Best for: Tall buildings where strategic orientation can improve performance. Often used in perimeter frames where X-bracing would block views.

ECENTRICALLY BRACED FRAMES (EBF)

In EBFs, braces are intentionally offset from beam-column joints, creating a short beam segment called the seismic link.

Materials: Steel.

How forces transmit: Lateral force enters frame → braces transfer force to seismic link → link yields in shear or bending, absorbing energy → remaining frame stays elastic → forces continue to foundation.

Pros:

• Combines high stiffness of CBFs with high ductility of MRFs
• Seismic link acts as a replaceable fuse
• Excellent energy dissipation
• Allows for openings in the frame

Cons:

• More complex design and detailing than CBFs
• Requires careful proportioning of link length
• Higher cost than CBFs
• Link may require replacement after major earthquake

Best for: Moderate to high seismic regions where both stiffness and ductility are required. Ideal for mid-rise buildings where architectural openness is needed.

BUCKLING-RESTRAINED BRACED FRAMES (BRBF)

A steel core encased in a steel tube filled with concrete or mortar. The casing prevents the core from buckling, allowing it to yield in both tension and compression.

Materials: Steel core, concrete or mortar infill, steel casing.

How forces transmit: Lateral force enters frame → core yields in tension or compression without buckling → casing prevents core from bending → stable, predictable force transfer to foundation.

Pros:

• Eliminates buckling failure entirely
• Stable, symmetrical performance in tension and compression
• Superior energy dissipation — the best of all braced frames
• Allows for more slender, architecturally flexible designs

Cons:

• Higher cost than traditional CBFs
• Requires specialized fabrication and quality control
• Larger member sizes can impact architectural space
• Limited number of manufacturers

Best for: High seismic regions requiring maximum performance. Ideal for buildings where traditional braces would be too large or where superior energy dissipation is critical.

SUMMARY

System	Stiffness	Ductility	Cost
X-Bracing	Very High	Moderate	Low
V/Chevron	High	Moderate	Moderate
K-Bracing	Moderate	Poor	Low
Single Diagonal	Moderate	Low	Low
EBF	Moderate	High	High
BRBF	High	Very High	High

ORIGINS

Cast Iron Bridges (1790s–1840s): Braced frames trace their origins to bridge building. As early as 1796, cast-iron trussed arches were being constructed in England. The first straight metal trusses appeared in 1840 on a bridge spanning the Erie Canal, using cast iron for compression members and wrought iron for tension members.

First Buildings: Crystal Palace (1851): The technology moved from bridges to buildings. The Crystal Palace in London featured discrete bracing rods visible in its cast iron frame — one of the first major buildings using cross-bracing for stability.

Northridge and Kobe Wake-Up Calls (1994–1995): For over a century, braced frames were designed primarily for wind loads. The 1994 Northridge and 1995 Kobe earthquakes shattered this complacency. A four-story chevron-braced building in North Hollywood suffered major damage, with braces fracturing under cyclic loading. Ordinary concentrically braced frames (OCBFs) were found to be not ductile.

The Response — New Systems Emerge:

• EBF: Professor Igor Popov initiated research on EBFs in the 1970s at UC Berkeley, developing the “capacity design” approach. EBF design provisions first appeared in Canadian standards in 1989.
• BRB: Developed in Japan by Nippon Steel at the end of the 1980s under the trademark “Unbonded Brace.” BRBs first appeared in the US in 1999 at UC Davis. By 2004, Japan had over 250 buildings with BRBFs.

CULTURAL ASPECTS

The Yi Minority — Braced Arch Structures in China: In the Liangshan region of China, the Yi minority has long used a traditional wooden structural system called “shan-jia” (braced arch structure) for their tile board houses. Liangshan is located in a high-intensity earthquake zone, and modern engineers are now studying these traditional structures using computer simulations to identify weaknesses and propose optimization measures.

Japanese Wood-Framed Architecture: Japanese traditional architecture developed unique seismic adaptations. Traditional houses used wood-framing with cardboard and paper rather than stone or brick. The elaborate system of cornice bracketing that crowns Japanese buildings creates a flexible, energy-dissipating connection between the roof and walls.

Connection to Braced Frames:

Traditional Principle	Modern Braced Frame Application
Wood-framing with flexible joints	Braces with pinned or bolted connections
Lightweight materials (paper, wood)	Steel braces with controlled yielding
Diagonal bracing in roof trusses	Concentric and eccentric bracing systems
Energy dissipation through joint movement	Seismic links and buckling-restrained cores

PERFORMANCE EVIDENCE

Pre-1994 (False Confidence Era): Braced frames were considered efficient and reliable for wind resistance, but had not been tested in major seismic events with modern instrumentation.

Northridge Earthquake (1994, M6.7): Exposed critical weaknesses:

• Chevron-braced buildings suffered extensive damage
• Main failure types: brace fracture, brace buckling, and gusset plate tearing
• Key lesson: Ordinary braced frames (OCBFs) were not ductile

Kobe Earthquake (1995, M6.9): Confirmed the problem was global and accelerated research into BRBFs.

Modern Era — The BRBF Solution:

• Energy dissipation and ductility significantly increased compared to CBFs
• BRBFs can achieve a seismic response modification factor (R) of 8, comparable to Special Moment Frames
• Studies show BRBFs reduce member sizes, simplify connections, and decrease foundation demands

Ongoing Concerns (New Zealand): The Canterbury earthquakes of 2010/2011 drove significant uptake of BRBFs. However, a 2020 study warns that New Zealand lacks documented guidance for BRBF design, and physical testing requirements are sometimes bypassed: “There may be designs that pose risk and that issues are being overlooked.”

WINNER OF THIS CATEGORY

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

• Mid-Rise Building: Suitable for mid-rise and high-rise; widely adopted in buildings up to 20+ stories.
• San Francisco Clay Soil: High stiffness controls drift on soft soil; proven at 181 Fremont (54 stories) with integrated dampers.
• Immediate Occupancy: Achieves it with supplemental dampers; no brace buckling means minimal residual drift.

Key trade-off: BRBFs are more expensive than traditional CBFs and require specialized fabrication. However, studies show they can produce savings of up to $5 per square foot compared to other seismic systems when factoring in reduced foundation demands and simplified connections.

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