Drag Trusses: What Are They? Why Are They Required?
Let’s start with the definition of a drag truss according to the Structural Building Components Association (SBCA):
Drag Truss: “A truss or trusses designed to assist in resisting the effects of seismic events by acting as a drag strut. This drag strut, drag truss or collector is a single element or component designed to transmit lateral loads to lateral load resisting systems that are parallel to the applied force.”
Wind and earthquake forces place loads on a structure that must be considered. Building Designers will call out “drag trusses” or “collector trusses” to be installed to help resist these loads. For those of you not familiar with this type of truss, this article will help you to understand how this works.
As component designers, we lovingly call these loads that are applied “drag loads” in the MiTek software.
As MiTek’s guide for applying drag loading states: “While the application of wind and earthquake loads is very different when it comes to truss designs, they both are designed as if they are horizontally applied to the structure when considering the overall resisting system.”
As you can see in the picture below, the building designers has placed drag trusses as several locations to “square off” the building and transfer loads from one wall to another. This is done by aligning trusses perpendicular to walls and applying a load that is then transferred through the truss.
Why are drag trusses necessary?
MiTek’s guide goes on to state: “Wind loads develop pressure on the wall and roof elements. The magnitude of this pressure is proportional to the square of the wind speed. Earthquake loads create ground movements and the structure within the specific earthquake zone must be able to withstand the lateral accelerations caused by these movements. Both wind and earthquakes create powerful torsional forces within the structure what can shear a building apart. Wall, floor and roof systems must be designed to resist these lateral forces in addition to supporting vertical loads. In accordance with ASCE 7, a Drag Strut is a structural element (could be a truss) installed parallel to an applied load that collects and transfers diaphragm shear forces to the vertical-force-resisting element or distributes forces within the diaphragm or shear wall. Properly designed drag strut trusses, shear walls or roof diaphragms and their connections will transfer lateral loads to the foundation and then safely into the ground.
The truss is then connected to the building structure using a special connector, like the one shown here, a Simpson DSC.
Applying drag loads in MiTek
It has been my experience that:
- Drag loads are very much misunderstood outside of a seismic region
- Drag trusses are not used in high wind zones and, if they were, could prevent some of the failures that happen when a hurricane or other wind storm makes landfall
- Truss designers are given very little, if any, training to fully understand just how critical they are to a structure’s stability
In reality, all load bearing walls are “shear walls”, whether we call them that or not. But we will save that for another article…
Why is it that only the seismic regions use drag trusses? Why aren’t drag trusses and shear panels used everywhere?