Where the force actually goes

Load paths, and how to trace one through a structure by eye.

A force applied anywhere on a structure has to travel to the ground or to whatever is holding that structure up, and tracing the actual route it takes on the way there, its load path, means following the specific members that carry that force, since the structure almost never shares it evenly.

Force takes the stiffest, most direct route

Any structure that is standing still is passing every force applied to it through some sequence of members until it reaches a support that can absorb it into the ground or into whatever the structure is fixed to. That route is rarely spread evenly. A force follows the stiffest, most direct connection available, the way water finds the lowest route down a hillside instead of spreading across the whole slope. A member sitting right next to a loaded point but outside the direct route to a support can carry almost none of the load, while a member forming part of that route can carry nearly all of it. Two structures that look almost identical from outside, differing only in which joints connect to which, can therefore handle the same load in completely different ways, one spreading it through several members and the other funnelling nearly all of it through one.

Hands around a loaded bedsheet

A group of people holding the edges of a taut bedsheet with something heavy set in the middle shows how uneven the sharing is. The pull concentrates in the hands holding the fabric nearest the load, since that fabric is the most direct route for the weight to reach anyone's grip, while hands at a far corner, connected only through a long, indirect stretch of cloth, feel comparatively little. Move the weight towards one edge and the hands nearest it suddenly take far more pull, though nobody's grip and nothing about the fabric has changed. Someone at a far corner who feels their grip go slack as the weight moves away has felt a load path shift off a member entirely.

Tracing the path before cutting weight

Knowing the load path is what lets a structure be made lighter without becoming weaker. A member carrying almost none of the load can often be thinned or removed, while one sitting on the main path needs to stay as substantial as it is or grow. Guessing instead of tracing is a common way to add weight in the wrong place, reinforcing a member that was never working hard while a loaded one nearby stays undersized because it did not look special from outside. Following the shortest, stiffest chain of connected members from load to support turns a drawing of shapes into a picture of where the forces flow, and it is far easier to do on paper than to discover after welding that a supposedly lightly loaded mounting point was carrying most of the force.

The same scrutiny applies in reverse. Removing a member that sits on a load path forces everything it carried onto a longer, less direct route through members never sized for it, so a structure only slightly lighter on paper can behave very differently under load. A member believed to be doing nothing deserves the same tracing before it is cut as a new one does before it is added.