Load Path Is the Most Fundamental Concept in Structural Engineering
Every force that acts on a building must find a continuous path from where it originates to where it is ultimately resisted - the ground. This path is called the load path. If the load path is interrupted at any point - a missing connection, an undersized member, or a discontinuity in a shear wall - the structure can fail. Understanding load path is perhaps the single most important structural concept for architects, because it explains why structures behave the way they do and why certain configurations are strong while others are vulnerable. The ARE PPD tests load path knowledge consistently and at a conceptual level that architects must master.
Gravity Load Path
Gravity loads (dead and live) follow a hierarchical path from the most local element to the most global:
- Roof/floor deck or slab: Distributes load to underlying framing members (joists, decking, secondary beams).
- Secondary beams (joists): Carry deck loads to primary beams or girders.
- Primary beams and girders: Span between columns or bearing walls.
- Columns or bearing walls: Transfer vertical loads down to the foundation.
- Foundation (footing, mat, or pile cap): Distributes loads to the soil or transfers to deep bearing strata.
- Soil: Provides the ultimate resistance to all gravity loads.
Each level of this hierarchy must be sized to carry the cumulative load from all elements above it. A ground-floor column in a 10-story building must carry 10 times the load of a roof-level column (approximately), which is why lower-story columns are always larger than upper-story columns.
Lateral Load Path
Lateral loads (wind, seismic) follow a different path than gravity loads, though they share some elements:
- Cladding and exterior surfaces: Wind pressure acts on the building skin and is transferred to structural framing at panel or girt connections.
- Floor and roof diaphragms: The horizontal structural elements (concrete slabs or steel decks with concrete topping) distribute lateral forces in-plane to the lateral force resisting system.
- Lateral force resisting system (LFRS): Shear walls, moment frames, or braced frames carry the lateral load from each floor level to the foundation.
- Foundation and grade beams: Transfer lateral forces to the soil through friction and bearing.
- Soil: Provides passive resistance and friction to resist overturning and sliding.
Load Path Discontinuities and Soft Stories
Load path discontinuities are among the most serious structural deficiencies. A common example is a "soft story" - a building story with significantly less lateral stiffness than the stories above (often where an open ground floor with parking or retail is topped by residential floors with shear walls). During an earthquake, the soft story concentrates all of the building's lateral deformation at one level, leading to collapse of that story. The 1994 Northridge and 1989 Loma Prieta earthquakes produced many soft-story failures that inform current code requirements.
Key Exam Points
- Gravity load path: deck → joists → beams → columns/walls → foundation → soil.
- Lateral load path: cladding → diaphragm → LFRS (shear walls/frames) → foundation → soil.
- Every connection in the load path must be adequately sized - the weakest link governs.
- Diaphragms are critical horizontal elements that distribute lateral loads to the LFRS.
- Soft stories concentrate lateral deformation - a common seismic failure mode.
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