When the top or bottom chords of a truss are subjected to large forces, or when load application or hanger requirements necessitate a wider chord member, designers sometimes use stacked chord members, where two chord members are placed one above the other. [For images, See PDF or View in Full Issue.]
One of the most common design mistakes is assuming that two stacked members automatically act as one larger member. Simply placing two pieces together does not make them act as a single unit. True built-up behavior is achieved only when the connection is designed to make the members act together as a single composite member.
Wood trusses are typically analyzed under the assumption that forces travel through the centerline of each member. Metal connector plates are designed to transfer these forces from one member to another. If the connector plates are not positioned correctly, the load path may not develop as intended.
When load is applied to the face of a stacked chord, a common plating concern must be addressed. The simplest way to satisfy ANSI/TPI 1 (National Design Standard for Metal Plate Connected Wood Trusses) requirements is to extend the connector plates below the centerline of the lower chord to help alleviate tension perpendicular to grain. If these requirements are met, the stacked chords will act together as a single composite member and properly share the applied load.
There is no single universal connector plate layout that is appropriate for all stacked chords in girder trusses. The proper plate location depends on multiple variables such as the truss design profile, magnitude of the forces, and the load path through the joint.
An exception is when the stacked chord is not part of a typical girder truss application. For example, a stacked bottom chord used in an attic-room truss is often provided primarily for strength, stiffness, or serviceability requirements rather than for transferring girder loads. In these cases, connector plates may not need to pass through the exact centerline of each member.
In some cases, a combination of plates that do and do not pass through the centerline of the same stacked chord may be acceptable. However, such configurations should be evaluated on a case-by-case basis. If there is any uncertainty when designing trusses with stacked chords, contact a MiTek Engineer for assistance.
It is important to remember that engineering software is only a tool. Software cannot replace the knowledge, experience, and engineering judgment of a qualified designer. Every truss design should be carefully reviewed to ensure that it is suitable for the intended application and meets all performance requirements before being released for production.
For additional information, or if you have questions, please contact the MiTek Engineering department.