Lateral Force Resisting Systems: Shear Walls, Braced Frames, Moment Frames, and Diaphragms

Evaluation and selection of lateral force-resisting systems (LFRS) including shear walls, braced frames, moment frames, and diaphragms, with emphasis on how each system affects building design, configuration, cost, and seismic/wind performance.

ARE · Project Planning & Design

Overview

Every building needs a strategy for dealing with forces that push sideways. Earthquakes, wind, and even soil pressure generate lateral loads that can rack, twist, or topple a structure if there's nothing designed to resist them. That strategy is the lateral force-resisting system, or LFRS.

The LFRS has two complementary halves. Vertical elements (shear walls, braced frames, or moment frames) carry lateral forces down to the foundation. Horizontal elements (floor and roof diaphragms) collect those forces across each level and deliver them to the vertical elements. Both halves must work together through a continuous load path from roof to soil.

For the ARE, the architect's role isn't to engineer the LFRS. It's to evaluate which system fits the project's design goals, budget, configuration, and code requirements, then select the appropriate layout. That evaluation requires understanding how each system behaves, what it costs, where it constrains architectural planning, and how code parameters like the response modification coefficient (R) and seismic design category (SDC) shape the options available.

This topic covers the four primary LFRS types, the diaphragm systems that tie them together, the code parameters that govern selection by seismic design category, and the architectural trade-offs that drive system selection on real projects.

Essential

The lateral force-resisting system is the structural backbone that keeps a building standing when forces push sideways. Getting this right on the ARE means understanding how each system works, what it costs, where it fits in a floor plan, and which code parameters control the selection. ## The Two Halves of an LFRS Every LFRS consists of vertical elements and horizontal elements working in tandem. Vertical elements (shear walls, braced frames, or moment frames) resist lateral forces through wall action, diagonal bracing, or rigid frame bending, carrying those forces down to the foundation. Horizontal elements (diaphragms) act like deep beams turned on their sides, collecting lateral forces from the floor area and distributing them to the vertical elements. Break one link in this chain and the system fails. A beautifully designed shear wall does nothing if the diaphragm can't deliver forces to it.

Professional

Consider a 12-story mixed-use building in Seattle with ground-floor retail, seven floors of office, and four floors of residential above. The site sits in Seismic Design Category D. The owner wants maximum flexibility on the office floors and an efficient structural cost. The structural engineer proposes three options: concrete shear walls throughout, a steel braced frame core with moment frame perimeter, or a dual system with concrete shear walls and a steel moment frame backup. Each one reshapes the architecture differently. Option 1 (concrete shear walls) provides excellent stiffness and controls drift, but it locks the architect into specific wall locations on every floor. The office tenants want open floor plans, and every shear wall segment eats into leasable flexibility. On the residential floors, the walls align naturally with unit separations. The mismatch between office openness and residential partition walls creates a stacking coordination problem.

TLDR

Shear walls: stiffest, most efficient, but require solid wall planes. Openings reduce capacity. Standard aspect ratio limit: 2:1 (up to 3.5:1 with reduced values).
Braced frames: diagonal members form efficient triangulated structures. Stiffer than moment frames, less stiff than shear walls. Most common retrofit strategy.
Moment frames: rigid beam-column connections resist lateral forces through bending. Maximum architectural flexibility, maximum cost, maximum drift. P-Delta effects matter here.
Diaphragms: horizontal elements (floors, roofs) that collect and distribute lateral forces. Flexible diaphragms distribute by tributary area; rigid diaphragms distribute by stiffness.
R-value matters: wood shear walls (WSP) R = 6.5; special reinforced concrete/masonry R = 5.0; ordinary plain concrete/masonry R = 1.5. Higher R = lower design forces but stricter detailing.
Soft story vs weak story: soft = stiffness deficiency, weak = strength deficiency. Both concentrate damage at one level.
Dual systems combine two lateral systems and require the moment frame to independently resist at least 25% of seismic forces.
Load path must be continuous from roof diaphragm through vertical LFRS elements to foundation to soil. Break one connection and the system fails.
Load combinations for LFRS: ASD: 0.6D + (0.6W or 0.7E); LRFD: 0.9D + (1.0W or 1.0E).
Seismic design category (A through F) controls which systems are allowed and what detailing is required.

ELI5

Imagine you're standing on a bus and it suddenly turns a corner. You need to brace yourself or you'll fall over. Buildings have the same problem, except the "bus turn" is an earthquake or a windstorm pushing the building sideways. A building's lateral force resisting system (LFRS) is how it braces itself. There are three main ways to do it, and each one works like a different strategy for staying upright on that bus. Shear walls are like leaning your back against a solid wall of the bus. Super stable, very strong, but you can only lean where there's wall. If someone cuts a big window in that wall, you lose your brace. That's why shear walls need to be solid; every opening weakens them. Braced frames are like grabbing diagonal handrails that go from the floor to the ceiling at an angle.

AREProject Planning & Design

Lateral Force Resisting Systems: Shear Walls, Braced Frames, Moment Frames, and Diaphragms

2 min read202 words

What Lateral Force Resisting Systems Do and Why They Matter

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