Seismic Design and Optimization Solutions for Steel Frame and Steel Braced Frame Structures

Core Seismic Characteristics of Steel Structural Systems

Steel structures rely on steel's high ductility and plastic deformation to dissipate seismic energy, differing fundamentally from brittle masonry structures and stiffness-controlled concrete structures. Steel moment frames resist lateral loads through rigid beam-column joints and beam flexural deformation, forming plastic hinges to consume earthquake energy. Steel braced frames adopt axial-stress braces to provide high lateral stiffness, effectively controlling structural displacement under frequent earthquakes.

The main seismic weaknesses of steel systems include excessive elastic drift of pure moment frames, compression buckling failure of ordinary steel braces, and residual deformation after strong earthquakes. Without targeted optimization, steel buildings may experience large vibration acceleration, non-structural component damage, or even local structural failure under rare earthquakes. The core design principle is to establish a clear seismic fuse mechanism, ensuring auxiliary components yield first and protect main frame members.

1. Seismic Setup for Steel Moment Frame Structures

Steel moment frames (SMF) are widely used in buildings requiring flexible interior space, with no diagonal braces restricting architectural layout. Their seismic design focuses on ductility control and plastic hinge rational arrangement, strictly following the strong-column weak-beam criterion.

Designers shall ensure plastic hinges form at beam ends rather than column ends or column bases, avoiding overall structural instability. Beam-column connections are the key seismic details; rigid welded or bolted connections guarantee reliable moment transmission. Continuity plates and doubler plates must be installed at joint panels to prevent local shear failure and panel buckling under cyclic seismic loads.

To control excessive inter-story drift, strict limits on story displacement angles are required per AISC specifications. For mid-rise steel moment frames in medium seismic zones, standard joint reinforcement and member section optimization can meet fortification requirements. For high-seismic areas, additional damping devices are recommended to reduce structural vibration and improve seismic redundancy.

2. Seismic Optimization for Steel Braced Frame Systems

Steel braced frames are categorized into Concentrically Braced Frames (CBF) and Eccentrically Braced Frames (EBF), serving as high-stiffness seismic systems for steel buildings. Traditional ordinary concentric braces are prone to repeated buckling under cyclic seismic loads, resulting in degraded energy dissipation capacity, making them unsuitable for high-intensity zones.

Special Concentrically Braced Frames (SCBF) are the upgraded solution for high-seismic applications. Designed based on capacity design principles, braces act as seismic fuses that yield and dissipate energy first, while frame beams, columns, and gusset plates are reinforced to withstand the maximum brace capacity without failure. V-type and inverted V-type braces are commonly used, while K-type braces are prohibited in seismic design due to unbalanced vertical force risks.

Eccentrically Braced Frames (EBF) balance the high stiffness of concentric braces and the superior ductility of moment frames. By setting short link beams between braces and columns, EBFs concentrate seismic damage on replaceable link beams. The links yield in shear or flexure during earthquakes, stably dissipating energy and protecting main frame components, which is ideal for mid-to-high rise steel buildings in high-seismic regions.

3. Advanced Energy Dissipation Seismic Scheme (BRB & Dampers)

To solve the buckling defect of traditional braces and further improve seismic performance, Buckling Restrained Braces (BRB) have become the optimal seismic upgrade for steel braced frames. Composed of a steel core and outer restraining casing, BRBs eliminate compression buckling failure, achieving consistent tensile and compressive bearing capacity and stable hysteretic energy dissipation.

BRBs serve as controllable seismic fuses, concentrating all inelastic deformation on the replaceable brace core. The main steel frame remains elastic during moderate and strong earthquakes, minimizing residual deformation and post-earthquake repair costs. For steel buildings requiring high seismic resilience, viscous fluid dampers can be installed collaboratively with BRBs. These velocity-dependent dampers effectively suppress structural vibration, reduce floor acceleration and inter-story drift, and protect non-structural facilities.

4. Standard Layout and Detailing Principles for Seismic Steel Structures

Regular structural layout is the foundation of steel seismic design. Symmetric plane and vertical arrangement eliminates torsion effects caused by asymmetric stiffness and mass distribution. Sudden stiffness changes and vertical component discontinuities must be avoided to prevent concentrated seismic damage on weak floors.

Gusset plate design is a critical seismic detail for braced frames. Following the standard 8tp clear distance rule ensures sufficient deformation space for brace rotation and yielding, avoiding brittle connection failure. All seismic welds and bolt connections shall comply with AISC 341 seismic detailing requirements, with strict quality control on field fabrication and installation.

5. Seismic Scheme Selection Guide for Steel Structures

For low-rise steel buildings in low-to-medium seismic zones, ordinary concentric braced frames or standard moment frames are sufficient to meet basic seismic needs with cost advantages. For mid-rise and high-rise steel structures in high-intensity zones, SCBF or EBF systems are preferred for balanced stiffness and ductility.

For important public buildings, industrial key facilities, and structures requiring post-earthquake rapid recovery, BRB energy dissipation systems or hybrid damping seismic schemes are the best choice. This solution realizes damage concentration on replaceable energy-dissipating components, achieving low damage and easy repair after earthquakes.