|Abstract or Summary
- Heavy timber framing relies primarily on bracing to withstand lateral loads due to earthquakes and wind events. Bracing configurations in heavy timber framed buildings vary widely and include cross bracing, knee bracing, and other geometries. Many heavy timber frames constructed during colonial American times are still standing, exceeding the expected life of many structures being built today. Limited research has been conducted on the lateral resistance of heavy timber frames and their connections and design aids and procedures are not readily available for engineers to assist in the design of these structures. This method of wood construction has been largely replaced with the development of light-framed wood buildings, which utilize sheathing (typically plywood or OSB) attached to the frame to resist lateral loads.
Today, the primary form of wood construction is light-frame. These structures rely on shearwalls to resist lateral loads. The shearwall consists of 2x4 or 2x6 studs regularly spaced with wood structural panel sheathing attached to the wall frame. This assembly is lightweight and ductile. Extensive research has been conducted on light-frame shearwalls since the 1950’s. The effects of construction variables (i.e., fastener schedule,
sheathing thickness and grade, anchorage, and openings) on shearwall performance have been cataloged through numerous studies. Studies have found the sheathing-frame connection, particularly the perimeter connection, is critical to the performance of a shearwall. This connection is typically nailed, although sometimes staples or adhesives are used.
The lateral load path in light-frame shearwalls relies on the sheathing-framing connection. If the load path can be modified then shearwall design can more fully utilize compressive and tensile properties of the wood materials and be less sensitive to the sheathing-framing connection properties. The idea of combining bracing typical of heavy timber framing with techniques used in light-frame construction has not been widely explored by research or analysis. This study investigates the use of bracing in conjunction with light-frame construction (a hybrid framing) to relieve the sheathing nails as the critical load path and enhance the shearwall performance under lateral loading.
A 4 by 8-ft. shearwall was designed consisting of an internal cross brace without intermediate framing studs and a lapped connection at the cross intersection. A 4x4 top-plate was used to improve vertical capacity of the braced shearwall because no intermediate stud was included. Four different types of shearwalls were tested under cyclic loading following the CUREE protocol; a conventional light-framed shearwall, a cross-braced shearwall with no mechanical connection at the corners of the walls, a cross-braced shearwall with plywood gusset plates at the corners of the walls, and a cross-braced shearwall with metal truss plates at the corners of the walls.
The conventional shearwall and the braced shearwall without mechanical connections at the corner of the wall performed similarly - the sheathing-frame connections controlled their performance. Withdrawal of the sheathing nails was the dominate failure mode. The braced shearwalls with the plywood gusset plate and the metal truss plates at the corners exhibited greater ultimate loads, greater initial stiffness and dissipated more energy
compared to the conventional shearwall. The modes of failure for these walls were shear failures in the plywood gusset plates and buckling in the metal truss plates. Some failure was observed in the sheathing nails, however, to a lesser degree than observed in the conventional shearwall.
The load path of vertical forces must be addressed in areas where intermediate studs are excluded due to the bracing configuration. Four additional walls were tested under vertical loading; two conventional shearwalls and two cross-braced shearwalls with metal truss plates at the corners. The braced shearwalls proved to adequately resist service level vertical loads similar to those resisted by the conventional shearwall.
Overall, using a hybridized shearwall as a part of light-frame construction appears to be viable option to enhance the lateral performance.