Mansard-type trusses are a type of roof truss that extend outward and are supported only on top and bottom of the tall end, allowing it to cantilever out without additional support. [For all images, See PDF or View in Full Issue.]
They are commonly designed with a pin and vertical roller bearing condition. This configuration is critical because it accommodates the high horizontal push and pull forces generated by the cantilevered behavior of the truss. The top chord tends to pull away from the supporting wall, while the bottom chord pushes against it, requiring carefully engineered connections to resist these forces.
Unlike conventional trusses, mansard-type trusses generate significant horizontal reactions at their bearing points. Connections between the truss and the supporting wall must be designed to resist these horizontal thrust forces. Without proper design, the truss can push walls outward, potentially causing cracking, bowing, or long-term movement. As a result, the supporting wall often requires additional reinforcement.
When a mansard-type truss is attached to a girder truss, additional considerations are required. Trusses are inherently weak when loads are applied perpendicular to their plane. The horizontal thrust from the mansard truss acts on the weak axis of the girder truss chord, which is not designed to resist such forces. There is no practical way to fully resolve these forces within the truss or girder design alone, as trusses are designed to resist loads primarily within their own plane. To address this condition, the load path within the truss system must be designed to transfer this out-of-plane load. Both temporary and permanent bracing along the chords of the girder truss are required to prevent lateral displacement and potential instability.
When selecting hangers for mansard-type trusses, ensure the chosen hanger is capable of resisting both vertical loads and the associated horizontal thrust forces. Manufacturer catalog values often assume short-term wind loading, which can result in higher listed allowable capacities. However, mansard-type trusses are frequently governed by gravity load cases. In these situations, hanger capacities must be adjusted to reflect the applicable load duration and governing load combination. Also, when a connector is subjected to simultaneous loading in multiple directions, the allowable load shall be determined using the interaction equation provided in the applicable manufacturer’s catalog.
For additional information, or if you have questions, please contact the MiTek Engineering department.