Drag Loads
Question:
Can you explain drag loads and how to calculate a drag load pertaining to roof trusses?
Answer:
Drag loads are due to lateral (horizontal) loads generated in high-wind or seismic events. These loads are generated within the structure and transferred into load carrying elements (like drag strut trusses, shear walls or roof diaphragms) which then transfer the loads to the foundation and then safely into the ground. Therefore, the loads must be calculated by the building designer (usually required to be a structural engineer in seismic areas on the west coast) based on the expected environmental loads and the configuration of the structural elements.
The location, magnitude, and direction of the relevant drag loads should be provided by the building designer. The truss technician or the truss designer is neither qualified nor responsible for calculating drag loads in the structure. Some truss design software programs analyze lateral loads so the design of a drag truss should be straightforward once the loading information is provided. If not, check with the truss designer to make sure all information is provided before the truss is designed and manufactured.
Another point to consider: load transfer from the structure to the drag truss is only as good as the connector used. Proper connector detailing and installation is required for the drag truss to perform as expected.