What Is a Load Path?
A load path is the continuous route by which forces travel from their point of application through a building's structural elements to the foundation and, ultimately, into the ground. A complete, continuous, and properly connected load path is one of the most fundamental requirements of structural design and a recurring theme in PPD and PDD exam questions.
Gravity Load Path
Gravity loads - dead loads (permanent, self-weight) and live loads (occupancy, furniture, snow) - flow downward through the structure in a predictable sequence:
- Roof/floor deck or slab collects loads from above.
- Beams and joists carry loads to girders or directly to columns/bearing walls.
- Columns or bearing walls transfer loads vertically downward through each level.
- Foundations (footings, mats, piles) distribute the accumulated load into the supporting soil or rock.
Lateral Load Path
Lateral loads - from wind or seismic activity - act horizontally on a building and require a distinct path to resist overturning and sliding:
- Application: Wind pressure on exterior walls or seismic inertial forces generated by the building's own mass.
- Diaphragms: Roof and floor systems act as horizontal diaphragms, collecting lateral loads and distributing them to vertical lateral-force-resisting elements. Diaphragms can be rigid (e.g., concrete slabs, which distribute load in proportion to relative stiffness) or flexible (e.g., plywood, which distribute load by tributary area).
- Vertical lateral-force-resisting elements: Shear walls, braced frames, or moment frames carry the lateral load down through the building.
- Foundation: Footings and foundation walls transfer lateral loads into the soil, often relying on passive soil pressure and friction, with hold-downs resisting overturning (uplift).
Shear Walls and Diaphragms in Practice
A shear wall resists in-plane lateral forces through a combination of shear strength, chord elements at its edges (resisting bending), and hold-downs (resisting uplift/overturning at the base). Diaphragm-to-shear-wall connections - collectors and drag struts - are critical: they gather diaphragm shear along a wall line and deliver it into the shear wall, and their absence or inadequate detailing is a classic load-path discontinuity that can cause structural failure even when individual members are adequately sized.
Common Load Path Failures
- Discontinuous shear walls that do not stack vertically between floors, requiring transfer diaphragms or beams to redirect the load.
- Missing collectors/drag struts at diaphragm-to-wall connections.
- Soft or weak stories - a level with significantly less lateral stiffness or strength than the stories above, a major seismic vulnerability.
- Inadequate connections between diaphragm and vertical elements, or between foundation and superstructure.
A clear, continuous load path from roof to foundation, for both gravity and lateral forces, should be verifiable in every set of construction documents you review on the exam.
For a broader look at framing systems, see Structural Systems, and reinforce your recall with the Structural Systems Cheat Sheet.
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