Every Building Needs a Lateral System - the Architect's Program and Planning Decisions Constrain the Options
Wind and seismic forces impose lateral loads on buildings that must be resisted by a defined structural system. Unlike gravity systems (which collect loads from columns and beams to foundations in a fairly straightforward vertical hierarchy), lateral systems require horizontal diaphragms to collect lateral forces and vertical elements to carry them to the foundation. The structural engineer designs the lateral system, but the architect's design decisions directly affect which systems are feasible: a plan with no interior walls eliminates shear wall options; a facade of all glass eliminates braced frames on the exterior. Understanding the major lateral system types and their implications for building plan organization, facade design, and structural depth is a core ARE PPD skill.
Shear Walls
Shear walls are rigid planar elements - concrete walls, concrete masonry unit (CMU) walls, wood structural panels, or light-gauge steel sheathing - that resist lateral forces by acting as cantilever beams fixed at the foundation. Shear walls are the simplest, stiffest, and most economical lateral system for most low-to-mid-rise buildings. Requirements: shear walls must be located so that they can transfer lateral loads from the diaphragm through the building plan to the foundation; they must be distributed in plan to minimize torsional eccentricity (centers of mass and rigidity should be close); they must be designed with adequate length-to-height ratio for overturning resistance. Plan implications: shear walls occupy significant wall area (a 6-inch concrete shear wall in an office building takes up wall space that could otherwise be glazed or open); they are most practical in buildings where program naturally generates walls in the right locations (apartments, hotels, institutional buildings with regular floor plans).
Braced Frames
Braced frames use diagonal steel members (braces) in a vertical bay to resist lateral forces through axial action (tension and compression) in the braces. Types: concentric braced frames (CBF) with braces meeting at a single point; eccentric braced frames (EBF) with braces offset from beam-column joints to introduce a ductile "link" for seismic energy dissipation. Braced frames are more efficient than shear walls for tall buildings because they use less material and allow more architectural flexibility - the braced bay can be as few as one or two bays in a floor plan. Plan implications: braced frames require specific bays dedicated to the lateral system; diagonal brace members may conflict with window openings or door locations and must be coordinated with the architectural plan.
Moment Frames
Moment-resisting frames resist lateral forces through bending at beam-column joints, which are made rigid through welded or bolted connections designed to transfer moment. Moment frames provide lateral resistance in the plane of the frame through the rigid-joint action without diagonals or walls, allowing architectural openness in the elevation. Types: ordinary, intermediate, and special moment frames (OMF, IMF, SMF) with increasing ductility requirements and more demanding connection design and testing for higher seismic zones. Plan implications: moment frames allow open facades (no braces interrupting the wall elevation) but require significant structural depth in beams to achieve lateral stiffness; they are more expensive than braced frames and are most common in seismic zones where ductility requirements favor moment frame behavior.
Key Exam Points
- Shear walls: stiff; economical; consume wall area; best for regular plans in low-to-mid-rise buildings.
- Braced frames: axial behavior; steel efficiency; diagonal members must coordinate with fenestration.
- Moment frames: open facades; ductile; expensive; beam depth requirements; preferred in high seismic zones.
- Lateral system selection is constrained by architectural plan and facade design - the architect and engineer must coordinate early.
- Diaphragm action: floors and roofs collect lateral forces and distribute to vertical elements (shear walls, frames).
AREprep's PPD structural systems content covers lateral force-resisting systems in depth - shear walls, braced frames, moment frames, their design principles, and the architectural planning implications of each system - giving ARE PPD candidates the structural systems literacy the exam tests when it asks about building structural system selection and coordination.
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