Lateral Force Resisting Systems: Shear Walls, Frames & Braces

What Are Lateral Forces and Why Buildings Must Resist Them

Gravity pulls every structure straight down, and most introductory structural courses focus heavily on vertical load paths. Lateral forces, however, act horizontally—or at an angle that produces a significant horizontal component—and they can be far more destructive if a building is not designed to handle them. The two dominant sources of lateral loading in the United States are wind and seismic activity. Wind pressure varies with height, terrain roughness, and building geometry, creating a distributed load across the facade. Seismic forces arise from ground acceleration during an earthquake; the building's own mass, multiplied by that acceleration, generates inertial forces that must travel through the structure to the ground. Other lateral sources include soil pressure against basement walls, hydrostatic pressure, and, in certain industrial settings, blast or impact loads. Without a deliberate system to collect, transfer, and dissipate these forces, a structure can rack, overturn, or collapse. The lateral force resisting system (LFRS) is the set of structural elements specifically configured to perform that work. Understanding how each system type functions—and why engineers choose one over another—is foundational knowledge for anyone working in structural design.

How Shear Walls Transfer Lateral Loads to the Foundation

A shear wall is a vertical planar element—typically concrete, reinforced masonry, or wood structural panels—that acts like a deep cantilever beam fixed at its base. When a lateral force is applied at the roof or floor diaphragm level, the wall resists the tendency of the structure to rack by developing shear stress across its cross-section. The diaphragm (floor or roof deck) collects the lateral force and delivers it to the top of the wall; the wall then transfers that force down to the foundation through a combination of shear along its length and overturning moment at its base. Overturning is a critical design check: the wall wants to rotate about its compression toe, so hold-down anchors or sufficient dead load must counteract the tension that develops at the opposite end. Shear walls are most efficient when they are long relative to their height—a high aspect ratio wall behaves more like a flexure-dominated cantilever and requires more reinforcement to control cracking. Placement matters enormously. Walls should be distributed symmetrically in plan to minimize torsional eccentricity between the center of mass and the center of rigidity. Stacked walls—aligned vertically from floor to floor—provide the cleanest load path. Discontinuities, such as a wall that stops at a transfer slab, introduce complexity and demand careful detailing. In wood-frame construction, shear walls are typically plywood or oriented strand board panels with closely spaced nailing patterns; in concrete and masonry buildings, reinforced walls with boundary elements handle much larger demands.

Moment Frames: Resisting Lateral Forces Through Rigid Connections

A moment frame resists lateral loads through the bending stiffness of its beams and columns and, critically, through the rigidity of the connections between them. When a lateral force pushes the frame sideways, the columns deflect in double curvature and the beams develop moments at their ends. The rigid beam-column joint transfers those moments, allowing the entire frame to act as a single integrated unit rather than a collection of pin-connected members that would simply collapse sideways. Steel special moment frames (SMFs) are a common choice in high-seismic zones because they can undergo large inelastic deformations—plastic hinges form in the beams—while maintaining load-carrying capacity, a property called ductility. Concrete special moment frames achieve similar behavior through careful detailing of longitudinal and transverse reinforcement, particularly in the joint region where beam and column bars must be adequately developed. The primary advantage of moment frames is architectural flexibility: because lateral resistance comes from the connections rather than solid walls or diagonal braces, the frame leaves the bay open for windows, doors, and mechanical penetrations. The trade-off is drift. Moment frames are relatively flexible, and controlling story drift to code-allowable limits often governs member sizing more than strength alone. In tall buildings, this can lead to very large beam and column sections. Engineers frequently use moment frames in the perimeter of a building to maximize stiffness at the exterior while keeping the interior open.

Braced Frames: Diagonal Members and Their Load Paths

Braced frames introduce diagonal members into a bay to create a triangulated system that resists lateral loads primarily through axial forces—tension and compression—rather than bending. Because axial behavior is far stiffer than flexural behavior for a given member size, braced frames are significantly stiffer than moment frames, which helps control drift efficiently. The simplest configuration is the concentric braced frame (CBF), where all members meet at a single work point. Common brace layouts include single diagonal, X-brace, chevron (inverted-V), and V-brace patterns. Each geometry has implications for the load path and for how the brace behaves when it buckles under compression during a seismic event. In special concentrically braced frames (SCBFs), the detailing is designed so that braces yield in tension and buckle inelastically in compression in a controlled manner, providing ductility. Buckling-restrained braced frames (BRBFs) take this further: a steel core is encased in a concrete-filled steel tube that prevents the core from buckling, allowing it to yield in both tension and compression. This produces very stable, symmetric hysteretic behavior and is widely used in high-seismic applications. Eccentrically braced frames (EBFs) introduce a short link segment in the beam that is designed to yield in shear or flexure, combining the stiffness of a braced frame with the ductility of a moment frame. The primary architectural limitation of braced frames is that the diagonal members occupy the bay, restricting openings. Careful coordination with the building program is necessary to locate braced bays where they do not conflict with circulation or glazing.

Comparing Shear Walls, Moment Frames, and Braced Frames

Each LFRS type has a distinct stiffness profile, ductility characteristic, material preference, and architectural impact. Shear walls are generally the stiffest option for a given plan footprint and are highly effective at controlling drift, but they consume floor area and restrict openings. Moment frames offer the greatest architectural freedom but are the most flexible of the three, making drift control in taller buildings expensive in terms of member size. Braced frames occupy a middle ground: stiffer than moment frames, more open than solid shear walls, but constrained by the diagonal geometry. From a seismic performance standpoint, all three can be detailed for high ductility in the appropriate material and configuration, but the mechanisms differ. Shear walls dissipate energy through distributed cracking and yielding of reinforcement. Moment frames rely on plastic hinging at beam ends. Braced frames depend on brace yielding or link yielding. Construction cost and schedule also vary. Concrete shear walls require formwork and curing time but are often integrated into the core of a building with minimal additional framing. Steel moment frames require precise fabrication of the connections, which historically have been a source of concern—the 1994 Northridge earthquake revealed widespread weld fractures in pre-Northridge SMF connections, leading to significant changes in connection design and inspection requirements. The table below summarizes key comparative attributes.

Combining Systems: Dual and Hybrid Lateral Force Resisting Strategies

Real buildings rarely rely on a single LFRS type. Combining systems allows engineers to exploit the strengths of each while mitigating their weaknesses. A dual system, as defined in ASCE 7, pairs a moment frame with either shear walls or braced frames. The moment frame must be capable of resisting at least 25 percent of the design seismic forces independently, while the primary system (walls or braces) handles the remainder. This redundancy is valuable: if the primary system is damaged, the moment frame provides a secondary load path and prevents progressive collapse. The interaction between systems also produces a favorable behavior under lateral load. A shear wall or braced core tends to deflect in a cantilever mode—more drift at the top—while a moment frame deflects in a shear mode—more drift at the lower stories. When coupled through the diaphragm, the two systems restrain each other, producing a more uniform drift profile over the building height. Hybrid strategies extend beyond the dual system definition. Podium structures use a stiff concrete base—often a parking garage or retail podium—to anchor a lighter wood or steel superstructure above. The podium acts as a massive shear wall system that effectively fixes the base of the upper structure. Transfer diaphragms and outrigger systems in tall buildings are further examples of hybrid thinking, where specialized elements redistribute lateral forces to take advantage of the full building geometry. The key design challenge in any combined system is ensuring that the load distribution between systems is accurately modeled and that the connections between different system types are detailed to transfer the required forces.

Code Requirements and Design Considerations for LFRS Selection

In the United States, LFRS design is governed primarily by ASCE 7 for load determination and by material-specific standards—ACI 318 for concrete, AISC 341 for steel seismic systems, and the Special Design Provisions for Wind and Seismic (SDPWS) for wood—for member and connection design. ASCE 7 assigns each building a Seismic Design Category (SDC) from A through F based on the site's mapped spectral accelerations and the building's occupancy. Higher SDCs restrict which system types are permitted and impose stricter detailing requirements. An ordinary moment frame, for example, is not permitted in SDC D, E, or F for most building types; only special moment frames with full ductile detailing are allowed. The Response Modification Coefficient (R) is a central parameter in seismic design. It represents the system's ability to absorb and dissipate energy beyond the elastic range, effectively reducing the design force. A higher R value means the engineer can design for a lower force, but only if the system is detailed to deliver the assumed ductility. Using a high R value without the corresponding detailing is a dangerous shortcut that codes explicitly prohibit. Wind design under ASCE 7 Chapter 27 or 28 requires calculating design wind pressures based on the basic wind speed for the site, exposure category, and building geometry. Unlike seismic design, wind forces are treated as essentially elastic loads with no ductility reduction factor, so LFRS elements must remain within elastic limits under design wind. Drift limits under both wind and seismic loading are checked against story drift ratios; typical allowable values range from H/400 to H/600 for wind-sensitive structures and are defined by table in ASCE 7 for seismic. Early in the design process, the structural engineer must evaluate the building's height, occupancy, site hazard, program requirements, and budget to select an appropriate LFRS. That decision shapes the entire structural system and has lasting implications for constructability, cost, and performance.

Example

Comparative Overview of Primary Lateral Force Resisting System Types

FAQ

What is the difference between a shear wall and a moment frame in terms of how they resist lateral loads? A shear wall resists lateral loads as a deep vertical cantilever, developing in-plane shear and bending stresses across its solid cross-section. A moment frame resists lateral loads through the bending stiffness of beams and columns connected by rigid joints; there is no solid panel, so resistance comes entirely from the members and their connections. Shear walls are stiffer and better at controlling drift; moment frames leave the bay open for architectural use but deflect more under the same load.

Why does the Response Modification Coefficient (R) matter so much in seismic design? The R factor reduces the elastic design force to account for a system's ability to yield and dissipate energy without collapsing. A higher R means a lower design force, which reduces member sizes and cost—but only if the system is detailed to actually achieve that ductility. Codes tie specific R values to specific detailing requirements. If a designer uses a high R without the required detailing, the structure will not perform as assumed and could fail in a design-level earthquake.

Can a building use more than one type of lateral force resisting system? Yes, and it is common practice. A dual system combines a moment frame with shear walls or braced frames so that each handles a portion of the lateral load and the moment frame provides a backup load path. Hybrid strategies—such as a concrete podium supporting a steel superstructure, or an outrigger system in a tall building—also mix system types to optimize stiffness, ductility, and architectural flexibility across the building height.

What is a buckling-restrained braced frame and when would an engineer choose it over a standard braced frame? A buckling-restrained braced frame (BRBF) uses a steel core encased in a concrete-filled tube that prevents the core from buckling laterally. This allows the brace to yield in both tension and compression, producing stable and symmetric energy dissipation. Standard concentric braced frames buckle in compression, which creates asymmetric behavior and can degrade performance over repeated cycles. Engineers choose BRBFs in high-seismic zones when they need the stiffness of a braced frame combined with the ductility and predictable behavior more typical of a moment frame.

How does building height affect the choice of lateral force resisting system? As buildings get taller, drift control and overturning become increasingly dominant design concerns. Moment frames become very expensive to stiffen because controlling drift requires large member sections. Shear walls and braced cores are more efficient at limiting drift in taller buildings, which is why most high-rise structures use a stiff concrete core—essentially a tube of shear walls—as the primary LFRS. Outrigger systems that connect the core to perimeter columns are added in supertall buildings to further reduce overturning demands on the core.

What role does the floor diaphragm play in a lateral force resisting system? The diaphragm—the floor or roof deck—acts as a horizontal structural element that collects lateral forces from the building's mass and facade and delivers them to the vertical LFRS elements (walls, frames, or braces). Without a properly designed diaphragm, lateral forces cannot reach the vertical system efficiently. Diaphragms can be rigid, semi-rigid, or flexible depending on their construction, and the assumption made about diaphragm behavior affects how forces are distributed to the vertical elements in the structural model.

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