Braced frames are key players in earthquake-resistant structures. They use diagonal members to resist lateral forces, providing stiffness and strength. Concentrically Braced Frames (CBFs) and Eccentrically Braced Frames (EBFs) offer different approaches to energy dissipation and performance.

Seismic design of braced frames involves careful consideration of load paths, member sizing, and connection details. Ductility and energy dissipation are crucial for withstanding earthquakes. Critical components like braces, connections, and frame configuration all contribute to the overall system performance.

Seismic Design Principles for Braced Frames

Principles of seismic braced frames

• Concentrically Braced Frames (CBFs) resist lateral forces through axial load in braces intersecting at joints providing high initial stiffness (X-bracing, inverted V-bracing)
• Eccentrically Braced Frames (EBFs) combine moment frame and braced frame behavior with intentional eccentricity between brace connections utilizing link beam for energy dissipation
• Load path considerations involve lateral force distribution through diaphragms and vertical distribution of forces from upper to lower stories
• Seismic performance objectives guide design for different hazard levels: immediate occupancy allows quick reoccupation, life safety prevents collapse, collapse prevention maintains structural integrity

Seismic design for braced frames

• Seismic design parameters account for hazard level (PGA, spectral accelerations), site classification (soil type), and importance factor (building use)
• Load combinations incorporate dead loads (self-weight), live loads (occupancy), and seismic loads (ground motion effects)
• Member sizing involves selecting appropriate braces (HSS, wide flange), beams (floor framing), and columns (vertical support) to resist combined loads
• Connection design focuses on gusset plates (brace-to-frame), bolted connections (field assembly), and welded connections (shop fabrication) to ensure load transfer
• Capacity design principles emphasize strong column-weak beam concept and apply overstrength factors to critical elements ensuring desirable failure modes

Ductility of braced frames

• Ductility factors quantify global (entire structure) and local (individual components) ability to deform inelastically without significant strength loss
• Energy dissipation mechanisms rely on brace yielding and buckling in CBFs, link beam yielding in EBFs
• Hysteretic behavior describes force-displacement relationships under cyclic loading revealing energy dissipation capacity
• Performance-based design considerations include drift limits (story displacements) and damage control (repairable vs. irreparable)

Critical components in braced frames

• Braces serve as primary lateral force-resisting elements with tension and compression capacity influenced by slenderness ratio and potential buckling modes (flexural, torsional)
• Connections critical for load transfer: brace-to-gusset connections (pin or fixed), beam-to-column connections (moment or shear), column base plates (anchor bolt design)
• Beam and column members designed to prevent local buckling (width-to-thickness ratios) and lateral-torsional buckling (bracing requirements)
• Special seismic detailing includes protected zones (plastic hinge regions) and compact sections (ductile behavior)
• Frame configuration emphasizes symmetry and regularity in plan and elevation, redundancy in load paths to enhance overall system performance