I had a chance not long ago to grab coffee with an old college roommate. We’re both engineers by trade, but our careers took different paths — he went into structural consulting, while I went into component design, working with wood trusses. As often happens, the conversation quickly turned to work. He mentioned a recurring challenge in his projects: delegating truss design. In the middle of that discussion, he brought up a term he had only recently encountered — drag truss. He described trying to explain to a builder that a particular truss needed to be designed not just for gravity, but to transfer lateral loads into a shear wall below. That load condition had been missed in the initial truss design.
That conversation stuck with me. Because the more I thought about it, the more I realized something important. For most designers, trusses are intuitive when it comes to gravity. They support vertical loads and transfer them down through the structure. But lateral load behavior is different. And that raises a fundamental question: How are trusses used within a building’s lateral system?
Understanding the Load Path
In contrast to gravity loads, which move vertically through a structure, lateral loads are first distributed horizontally through the diaphragm — typically the roof or floor system — before being transferred into vertical resisting elements such as shear walls. [For all images, See PDF or View in Full Issue.]
Drag trusses serve as collector elements within this system. Their role is to transfer forces from the diaphragm to the shear walls below, helping complete the lateral load path.
Unless the Engineer of Record (EOR) explicitly identifies drag truss locations and forces, it can be surprisingly difficult to determine where they are required, what forces they are intended to carry, and how they integrate into the overall lateral system.
What Changes When a Truss Becomes a Drag Truss
When a truss takes on this role, its behavior changes significantly. Instead of carrying primarily gravity loads, the truss must now resist axial forces introduced by the lateral system. These forces can:
- Add significant tension or compression to chords and webs
- Introduce load reversal under seismic conditions
- Increase the demand on truss plate connections.
Equally important are the end and collector connections, which govern how forces enter and leave the truss. Without a clearly defined load path through these connections, the truss cannot effectively perform its role as a collector.
Where Drag Trusses Typically Occur
Based on their function, drag trusses can appear in several common conditions:
- At or above shear walls to deliver diaphragm forces directly into the lateral system
- At offset shear walls where forces must be transferred across the diaphragm
- Over large openings (such as corridors or storefronts) to redirect forces to adjacent shear walls
- At diaphragm discontinuities caused by complex geometry or framing changes.
In many cases, the truss is not originally intended to serve as a collector — but becomes one because it is the most direct available load path.
The Real Design Challenge: It’s Not the Truss — It’s the System
One of the biggest challenges in designing drag trusses is that the problem is rarely isolated to the truss itself.
If the load path is not clearly defined in the construction documents, critical information is often missing, including:
- The magnitude of the drag (collector) forces
- How the diaphragm connects to the truss
- Where and how those forces are ultimately resisted.
Without this information, even a well-designed truss may not function as intended within the system.
Why Boundary Conditions Matter
A crucial — and often overlooked — aspect of drag truss design is understanding the boundary conditions, or how the truss is connected within the structure. Truss design software requires assumptions about how loads are applied and resisted. But in the case of drag trusses, those assumptions must align with the actual load path.
Key questions include:
- Are forces distributed along the entire bottom chord, or only a portion?
- Is the load transferred through the top chord using straps or collectors?
- Where are the forces delivered into the shear wall system?
These decisions directly impact how forces flow through the truss and whether the design accurately reflects real-world behavior.
Closing Thought: From Component to System
Understanding how trusses function — not just as gravity elements, but as part of the lateral load path — is essential for today’s wood construction. Drag trusses highlight an important shift in perspective. Trusses are not just individual components. They are part of a larger structural system. And when that system isn’t clearly defined, even experienced designers can find themselves filling in the gaps.