Load path tracking

Load path as direction of travel

A load path is the route every pound of force travels through a structure from its point of application to the ground. Laurent opened the webinar with a river analogy: just as water finds its way to the ocean, every pound of load has to find its way to the foundation. "Every single pound of weight has to go down to the ground. Otherwise, your building's not gonna stand," he said.

The key distinction he drew is between tributary area and load path. Tributary area captures how much load arrives at a given point. Load path captures the direction of travel - the actual chain of elements each pound passes through. A structural model can correctly calculate tributary areas and still miss the load path if the chain is not traced explicitly from roof to footing.

Linking a gravity chain from floor to footing

The live demonstration followed a two-bedroom floor system through to its spread footing. Bedroom and bathroom joists were linked to a wood beam (engineered lumber) as line loads, reflecting the distributed nature of joist reactions along the beam span. That wood beam, along with a second parallel beam, was then linked to a steel beam below as point loads at each bearing location.

The steel beam reaction was linked to a steel column, which in turn was linked to a spread footing. Each step in the chain used the output from the upstream calculation directly as the input to the next, so every member saw exactly the load delivered to it rather than a manually entered approximation.

Automatic propagation when a load assumption changes

Partway through the demo, a design change was introduced: the floor finish specification changed from a standard finish (10 PSF dead load) to slate (25 PSF). Updating the dead load assumption immediately cascaded through the joists, beams, column, and footing. "Every single pound is accounted for," Laurent said.

The benefit extends beyond saving time on a single revision. It eliminates the risk of updating one element and missing another downstream. With a linked load path, the entire chain reflects the current design assumption rather than whatever was hand-entered when each element was last opened individually.

Eccentricity, lateral bracing, and modeling judgment

Two specific modeling questions came up during Q&A.

On eccentricity: the software does not apply eccentricity by default for steel columns, reflecting the common assumption in steel design that bearing is concentric unless the connection detail dictates otherwise. Wood columns more commonly default to d/6 or b/6 eccentricity values. Laurent's guidance: "Every situation is different. So you gotta use your judgment to some extent."

On lateral-torsional bracing: when joists frame into the side of a beam, each connection resists lateral movement at that point and can reduce the unbraced length used in the buckling check. The software includes a toggle to account for this automatically, so engineers modeling a floor framing situation do not need to manually adjust the unbraced length when the framing provides restraint.