Every truss requires at least two bearings, or points, to transfer its loads to. Typically, it is a 2x4 or 2x6 wood frame wall or panel in residential construction but can also be a light gauge steel wall or block wall in commercial or industrial applications. Likewise, bearings can be beams, ledgers, headers, or posts made from any of the above-mentioned materials. Regardless, the bearing must be strong enough to support the loads transferred from the trusses above. If the bearing material is not strong enough, crushing can occur when the truss is under designed loads. This results in serviceability issues like cracked sheetrock and dips in ceilings above and floors below. In extreme cases failure can occur, albeit rare.
Other trusses can also be used to support adjacent trusses connected at some type of angle, usually 90 degrees with a metal connector joist or truss hanger. In some instances with very light loads, trusses can be nailed together to act as a bearing condition. The carrying truss that supports other trusses is known as a girder truss. The bearing supporting the girder truss is most likely to suffer from the crushing effects mentioned above. Enhancements can be made to the side of the truss where it connects to the wall known as bearing enhancers or squash blocks to better transfer the loads of the girder over a greater surface area of the bearing.
Additional bearings can be designed to the interior area to help support the truss mid span, lessening the load reactions at the outer bearing and potentially reducing the amount of lumber and the size of the metal connector plates required to build the truss. These bearings need to be supported by an adequate support structure downward though the wall to any floor systems below and into the foundation system of the structure.
Bearing width knowledge is critical to truss design to ensure the truss meets expectations in the construction process. An example of this is the seat cut of a scissors truss. If the bearing size varies from the truss design, the ceiling area can be impacted where it meets the wall and modifications will be necessary when installing drywall or other finish products.
Typically, trusses are connected to the bearing temporarily with nails and permanently with a metal connector designed for uplift and lateral load considerations.