Lateral Load-Resisting Systems: Shear Walls, Braced Frames, and Moment Frames

Every multi-story building must resist horizontal forces: wind pushing on the facade and seismic inertia generated within the building mass itself. The lateral load-resisting system (LLRS) is the structural assembly responsible for this task, channeling horizontal forces from the floors and roof down to the foundation. Three families of system dominate modern construction: shear walls, braced frames, and moment-resisting frames. Each addresses the problem through a different structural mechanism, with distinct stiffness, ductility, and architectural trade-offs.

How Lateral Force Reaches the Vertical System

Floor and roof diaphragms collect lateral force from the building envelope and deliver it to the vertical elements of the LLRS. In a wind event, pressure on the windward face creates a story shear force at each level. These shears accumulate from the top down, so the base of the building sees the largest total horizontal force. In an earthquake, inertia forces at each floor are proportional to the mass and ground acceleration at that floor, again summing to a maximum base shear at the foundation. The LLRS must carry this base shear into the soil and simultaneously resist the overturning moment generated by the lever arm of the lateral forces above.

Shear Walls

A shear wall is an in-plane wall element that resists lateral force through its own in-plane flexural and shear stiffness. It acts as a vertical cantilever fixed at the base. The in-plane flexural stiffness of a solid wall is proportional to the cube of its length:

K_wall ∝ E t L³ / H³

where t is the wall thickness, L is the in-plane length, and H is the wall height. Doubling the wall length increases its stiffness by a factor of eight. This strong length dependence is why shear walls are most efficient when oriented parallel to the direction of loading and positioned as far from the building center as practical, maximizing torsional resistance.

Concrete shear walls designed to ACI 318 and ASCE 7 as Special Reinforced Concrete Shear Walls achieve high ductility through boundary elements — concentrations of closely tied reinforcement at the wall edges that confine the concrete and prevent crushing under cyclic loading. They are among the stiffest lateral systems available, limiting inter-story drift to small fractions of story height, which protects glass curtain walls and sensitive interior finishes.

Braced Frames

A braced frame adds diagonal steel members to a rectangular beam-column frame, creating truss action that carries lateral shear primarily through axial forces. Because axial stiffness (EA/L) is generally much greater than the bending stiffness of an equivalent member, braced frames are stiffer than moment frames for the same material quantity. Several configurations are standard practice:

Concentric braced frames (CBF) connect diagonals at beam-column joints, creating pure truss action. They are stiff and simple to design but have limited ductility because the diagonal members buckle in compression under large seismic displacements, losing much of their load-carrying capacity after the first cycle of buckling.

Eccentric braced frames (EBF) connect one end of each diagonal at a short link beam segment offset from the joint. Under seismic loading the link yields in shear, dissipating energy while the diagonal and columns remain elastic and the braces are protected from buckling. EBFs combine the stiffness of CBFs with seismic ductility approaching that of moment frames.

Buckling-restrained braced frames (BRBF) encase a steel core in a concrete-filled steel tube that prevents the core from buckling, allowing it to yield in both tension and compression with stable hysteretic behavior. BRBFs provide high stiffness and excellent ductility and have become the preferred braced frame system in high-seismic zones.

Moment-Resisting Frames

A moment-resisting frame (MRF) carries lateral load through the flexural stiffness of its beams and columns and the rigidity of the beam-column connections. Unlike braced frames, MRFs contain no diagonals, leaving full bay openings for architectural flexibility. This is the system of choice when the interior layout must remain unobstructed.

The trade-off is lateral flexibility. An MRF is significantly less stiff than a shear wall or braced frame built from equivalent material, and controlling inter-story drift typically governs the design of moment frames in taller buildings. Special Moment Frames (SMFs) designed for high seismic zones per AISC 341 are detailed to develop large inelastic rotations at beam plastic hinge zones near the column face, with strong-column weak-beam proportioning to ensure that columns remain elastic while beams yield — preventing story mechanisms that could cause global instability.

System Comparison

System	Lateral Stiffness	Seismic Ductility	Bay Openings	Typical Use
Concrete shear wall (special)	Very high	High	Limited	High-rise, core walls
Concentric braced frame	High	Moderate	Reduced by braces	Low-to-mid rise
Eccentric braced frame	High	High	Moderate	Mid-rise seismic zones
Buckling-restrained braced frame	High	High	Moderate	Mid-to-high seismic zones
Special moment frame	Low – moderate	Very high	Fully open	Open-plan seismic buildings

Dual Systems and Combining Approaches

Many buildings use hybrid systems: a stiff concrete core provides the primary lateral resistance while perimeter moment frames supply redundancy and an alternate load path. ASCE 7 defines dual system requirements, in which both the primary system and the backup moment frame must independently satisfy code requirements without the other present. The moment frame must be able to resist at least 25% of the design seismic force on its own. This ensures that the structure retains meaningful lateral resistance even if the primary system is significantly damaged — the core requirement of life-safety seismic performance.