Bearing: A structural support, usually a beam or wall, which is designed by the building designer to carry the truss reactions to the foundation. [For all images, See PDF or View in Full Issue.]
MiTek’s engineering software uses two common types of bearings, roller and pinned. There is a third type, fixed, which should not be used for truss design. These bearing types are used in the structural analysis to determine the forces in the truss. Please note that real world supports do not behave exactly as the idealized pin or roller, but they are the best way available to achieve accurate results from the software.
A typical truss design has a pin-type bearing at one end and a horizontal roller-type bearing at the other end. If the truss has more than two bearings, then only one bearing is a pin-type and the rest are horizontal roller-type bearings. Bearings may be located anywhere along the truss, at the ends or at midpoints of the chords, or at any other intermediate points.
Pinned Bearings
A pinned support allows rotation but does not allow movement in vertical or horizontal directions. A single pinned connection is not sufficient to make a structure stable. Another support must be provided, to prevent rotation of the structure. Except for special cases, there should be only one pinned bearing per truss.
Roller Bearings
A roller support is free to move along the surface upon which the roller rests. A roller can resist vertical or horizontal forces, but not both. An H-roller cannot resist forces in the horizontal direction. A V-roller cannot resist forces in the vertical direction. Since most trusses are subjected to lateral loads of some kind, a truss must have at least one other type of support in addition to roller supports.
A typical design with a pin-type bearing at one end and a horizontal roller-type bearing at the other end will have some amount of horizontal deflection at the roller bearing. This can sometimes cause a failure warning on scissor-type trusses due to excessive horizontal displacement. Changing the bearing conditions to pin-type bearing at both ends when a truss is failing with a pin-type bearing on one end and h-roller bearing on the other end is not a valid option, unless special measures are taken by the Building Designer to resist the horizontal thrust that develops at the pin-type supports. You must consult with MiTek Engineering in this situation.
Sometimes trusses are designed with a pin and vertical roller bearing condition (mansard-type trusses). The main concern with mansard-type trusses are the horizontal reactions. When you attach a mansard-type truss to a wall at the top and at the bottom along the same vertical, the top will try to pull away from the wall while the bottom will push on the wall. A connection is required for this horizontal pull/push (it is the horizontal reaction on the truss design drawing). Additionally, the wall itself must be capable of resisting these horizontal forces. If a truss is attached to a girder truss, this thrust, or push, is acting on the weak dimension of the chord and could push the girder sideways. Since trusses are designed to resist loads within the plane of the truss only, there is no solution in the software to check a girder for out-of-plane forces.
For additional information, or if you have questions, please contact the MiTek Engineering department.