Lateral Force Resisting Systems

Every building must resist not only gravity loads (which push straight down) but also lateral loads — horizontal forces from wind and earthquakes that try to push the building sideways, twist it, and overturn it. Without a lateral force resisting system (LFRS), even a building that can easily support its gravity loads would topple like a house of cards in a strong wind.

This lesson examines the three primary lateral force resisting systems used in building construction — braced frames, shear walls, and moment frames — and the horizontal elements (diaphragms, collectors, and drag struts) that connect them.

Wind loads produce pressure on the windward face and suction on the leeward face, creating a net horizontal force that pushes the building. The wind force increases with height and with the building's surface area perpendicular to the wind.

Seismic loads are inertial forces generated when the ground moves and the building's mass resists the motion. Seismic forces act at each floor level, proportional to the floor weight and height.

In both cases, the lateral loads must be transferred through the structure to the foundation and into the ground. The path is:

1. Lateral load acts on the building exterior (wind on walls) or is generated at each floor (seismic inertia)
2. Walls and floors transfer the load to the diaphragm (horizontal floor/roof deck)
3. Diaphragm distributes the load to vertical LFRS elements (shear walls, braced frames, or moment frames)
4. Vertical LFRS elements transfer the load down to the foundation
5. Foundation transfers the load to the soil through friction and passive earth pressure

A braced frame uses diagonal members (braces) to provide lateral stiffness and strength. The braces triangulate the frame — just like a truss — so that lateral loads are resisted by axial forces (tension and compression) in the braces rather than by bending.

Types of braced frames:

Concentrically Braced Frame (CBF): All members (beams, columns, braces) meet at a common point (the work point). The frame behaves like a vertical truss. CBF types include:

• X-bracing: Diagonals cross in an X pattern, each covering the full bay. One diagonal is in tension, the other in compression.
• Single diagonal: One diagonal per bay, alternating direction in adjacent bays or stories.
• V-bracing (Chevron): Two braces meet at the mid-point of the beam above, forming a V or inverted V. The beam must be designed for the unbalanced vertical force when one brace buckles.
• K-bracing: Two braces meet at the mid-height of the column. K-bracing is prohibited in seismic zones because the unbalanced force from brace buckling is applied directly to the column, which can cause column failure.

Eccentrically Braced Frame (EBF): Braces are offset from the beam-column joint, creating a short segment of beam called the link. The link is designed to yield in shear (absorbing earthquake energy) before the braces or columns fail. EBFs combine the stiffness of braced frames with the ductility of moment frames, making them excellent for seismic design.

Buckling-Restrained Braced Frame (BRBF): A special brace containing a steel core within a concrete-filled tube. The tube prevents the core from buckling, so the brace can yield in both tension and compression. BRBFs have excellent ductility and energy absorption, making them superior for seismic applications.

A shear wall is a wall element designed to resist in-plane lateral forces. Shear walls act as vertical cantilever beams, fixed at the base and free at the top, resisting lateral loads through in-plane shear and bending.

Concrete shear walls are typically 8–24 inches thick, reinforced with horizontal and vertical rebar. They are extremely stiff and strong. In high-rise buildings, concrete shear walls are often arranged around elevator shafts and stairwells to form a core — a structural box that provides the primary lateral resistance. Concrete shear walls can be either bearing walls (carrying gravity loads as well as lateral loads) or non-bearing walls (carrying lateral loads only).

Masonry shear walls are reinforced concrete masonry (CMU) walls with vertical rebar in grouted cells and horizontal rebar in bond beams. Common in low-rise commercial and institutional buildings. Masonry shear walls must have adequate reinforcement to provide ductility in seismic zones.

Wood shear walls consist of wood framing (studs, plates) sheathed with structural panels (plywood or OSB) nailed to the framing with a specific nailing pattern. The sheathing provides the shear resistance. In high-wind and seismic zones, the nailing pattern is critical — closer nail spacing increases the wall's shear capacity. Hold-down connectors at the ends of the wall prevent overturning (uplift).

Critical construction details for wood shear walls:

• Sheathing must be the correct thickness and type (structural rated)
• Nailing must match the engineer's specification (nail size, spacing at edges and field)
• Hold-downs must be properly installed and tightened
• The wall must have a continuous load path to the foundation (sill plates anchored with anchor bolts)
• Openings (doors, windows) reduce shear wall capacity and may require special detailing (portal frames, force transfer around openings)

A moment frame resists lateral loads through bending of its beams and columns, connected by moment connections (discussed in Lesson 4.4). Moment frames are the most flexible (least stiff) of the three LFRS types, but they provide the most architectural freedom because the frame bays are open — no diagonal braces or walls obstruct the space.

Types of moment frames (in order of ductility and seismic performance):

• Special Moment Frame (SMF): The most ductile, designed with special detailing requirements for high seismic zones. Columns must be stronger than beams ("strong column, weak beam" philosophy) to prevent story-level collapse mechanisms. Connections must be pre-qualified per AISC 358.
• Intermediate Moment Frame (IMF): Moderate ductility, suitable for moderate seismic zones.
• Ordinary Moment Frame (OMF): Limited ductility, suitable for low seismic zones or wind-only design.

The "strong column, weak beam" principle is critical for seismic design: during an earthquake, plastic hinges (yielding zones) should form in the beams, not the columns. If columns yield, an entire story can collapse (pancake failure). If beams yield, the building sustains damage but remains standing — the columns maintain their vertical load-carrying capacity.

The horizontal elements that connect lateral loads to the vertical LFRS:

Diaphragms (introduced in Lesson 4.3) are the horizontal floor and roof decks that distribute lateral loads to the vertical LFRS elements. The diaphragm acts as a horizontal beam:

• The web of this beam is the floor or roof deck
• The flanges are the chord members along the diaphragm edges (typically the perimeter beams or boundary reinforcement)
• The reactions are at the vertical LFRS elements

Collectors (drag struts) are structural members within the diaphragm plane that collect lateral forces from the diaphragm and deliver them to a vertical LFRS element. Collectors are needed when the LFRS element is shorter than the diaphragm it serves. For example, if a shear wall is 30 feet long but the building is 100 feet long, collectors extend from the ends of the shear wall to the edges of the diaphragm, collecting force along their length.

If the lateral loads are not balanced by the resisting elements, the building will twist (rotate in plan). This is called torsion and is caused by:

• Eccentricity: The center of mass (where forces are applied) does not coincide with the center of rigidity (where resistance is concentrated)
• Accidental torsion: Building codes require a minimum eccentricity (typically 5% of the building dimension) even if the building is nominally symmetric

Torsion is controlled by placing LFRS elements symmetrically and at the building perimeter (maximizing the distance from the center). Corner shear walls and perimeter moment frames are effective at resisting torsion.