Laterally-loaded connections in wood structures are designed based on a series of "yield limit equations" when fasteners such as bolts, lag screws, wood screws, or nails are used to transmit loads in shear. Each yield limit equation describes a unique connection yield mode based on analysis of the fastener as a beam in continuous contact with idealized elastic-plastic materials (i.e.- the wood, steel, concrete or masonry members in the connection). This "yield model" of connection behavior describes connection yield as an interaction between plastic hinge formation in the fastener and deformation/crushing of wood fibers in bearing against the fastener. It should be noted that the yield limit equations do not describe ultimate connection failure. Rather, they estimate the load at which inelastic connection behavior begins to occur (i.e.- the "yield point"). Published connection capacities (Z) are determined by calculating a connection yield point, then applying safety adjustment factors.
In general, connections may exhibit any of six possible yield modes when a sufficiently large load is transmitted through the connection. (Note that some types of connections have a smaller number of possible yield modes.) Connection capacity (Z) will vary based on parameters such as fastener diameter (D), fastener bending yield strength (Fyb), main member thickness (tm), main member bearing strength (Fem), side member thickness (ts), and side member bearing strength (Fes). For connections with large diameter fasteners (i.e.- bolts or lag screws), wood bearing strength (Fe) is dependent upon the angle of load to grain (i.e.- the angle between the longitudinal axis of the member and the direction of the applied load). The yield mode with the smallest calculated capacity (Z) governs connection behavior. The various connection yield modes can be described as follows:
|Mode Im ||connection yield is governed by crushing of wood fibers in the main member.|
|Mode Is ||connection yield is governed by crushing of wood fibers in the side member(s).|
|Mode II ||connection yield is governed by rigid rotation of the fastener at the shear plane (only applicable for single-shear bolted connections).|
|Mode IIIm ||connection yield is governed by formation of a single plastic hinge in the fastener at each shear plane, combined with wood crushing in the main member.|
|Mode IIIs ||connection yield is governed by formation of a single plastic hinge in the fastener at each shear plane, combined with wood crushing in the side member(s).|
|Mode IV ||connection yield is governed by formation of two plastic hinges in the fastener at each shear plane.|
(Note that localized crushing of wood fibers also occurs at the face of wood members in yield modes II, III and IV.)
Yield modes III and IV provide enhanced connection ductility due to the formation of plastic hinges in the metal fasteners. Thus yield modes III and IV may be more desirable than yield modes I or II for connections in wood structures in regions of high seismic activity.
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