Every Building Needs a Lateral Force Resisting System
Wind and seismic forces push buildings horizontally. Gravity-only structural systems - simple beams and columns with pin connections - cannot resist these lateral forces. Buildings must have a lateral force resisting system (LFRS) to carry horizontal loads from the floor and roof diaphragms to the foundation. The two most common LFRS types are moment frames and shear walls. A third type - braced frames - is also commonly used. The ARE PPD exam tests the behavior, advantages, limitations, and appropriate applications of each system.
Moment Frames
A moment frame resists lateral loads through rigid connections between beams and columns. When a lateral load is applied, the frame deforms in a "racking" pattern, and the rigid beam-column connections develop bending moments that resist the deformation. Moment frames are called "rigid frames" in older literature because the connections are rigid (moment-resisting), not pinned.
Advantages: Moment frames do not require solid walls or diagonal bracing, which makes them ideal where open, flexible floor plans are desired. Glazed facades, open lobbies, and retail spaces often use moment frames because the perimeter can remain open. Steel moment frames are common in mid-rise to high-rise construction.
Disadvantages: Moment frames are less stiff than shear walls. They deflect more under lateral load, which can cause glass, cladding, and partition damage. They are more expensive because all beam-column connections must be detailed for moment transfer, which requires more steel and more complex welded or bolted connections.
Shear Walls
A shear wall is a structural wall that resists lateral loads in its own plane through in-plane shear. Wood-framed shear walls (plywood or OSB sheathing with nail patterns), concrete shear walls, and masonry shear walls are all common. The wall acts like a wide, short vertical cantilever - it transfers horizontal forces from the diaphragm above to the foundation below through shear and overturning moment.
Advantages: Shear walls are very stiff, which limits building drift and protects nonstructural components. They are efficient in material and are inherent in most wood-frame residential construction (exterior and interior sheathed walls). In concrete or masonry construction, the core walls of elevator and stair shafts frequently serve as shear walls.
Disadvantages: Shear walls limit plan flexibility because solid wall segments must be maintained in the structural grid. They impose overturning forces on the foundation that must be resisted, which can require hold-down hardware or enlarged footings.
Braced Frames
Braced frames use diagonal members that resist lateral loads primarily in axial tension or compression. Concentrically braced frames (CBFs) have diagonals arranged concentrically; eccentrically braced frames (EBFs) incorporate a link segment that dissipates energy through shear yielding. Braced frames are stiffer than moment frames and simpler to detail than moment frames, but the diagonals block openings.
Comparison Summary
| System | Stiffness | Plan Flexibility | Cost | Best Use |
|---|---|---|---|---|
| Moment frame | Low-medium | High | Higher | Open plans, glazed facades |
| Shear wall | High | Medium | Lower | Residential, concrete core |
| Braced frame | High | Low-medium | Medium | Industrial, where plan allows |
Key Exam Points
- Moment frames: rigid connections, more flexible, best for open plans.
- Shear walls: stiffer, require solid wall segments, common in residential and concrete core buildings.
- Braced frames: efficient, diagonal members, blocks openings.
- Dual systems combine moment frames with shear walls for redundancy.
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