Importance of the Weld Ductility Factor in Bracing Connections
When designing bracing connections, have you been using a weld ductility factor on welds subjected to shear and transverse loads to allow for adequate redistribution of stresses? Although there have been multiple tests and papers showing the importance of this factor, some engineers are still unaware of its necessity which can result in underperforming bracing connections.
In the February 2018 SEU presentation Introduction to the Design and Cost of Steel Seismic Connections – Braced Frames, Patrick Fortney, PhD, PE, SE, P.Eng., from the University of Cincinnati reviewed using the uniform force method in braced frame connections and covered the various strength and detailing requirements for braced frames and the associated costs. In his presentation, Pat walked through the design of seismic steel connections, which also included many important thoughts on braced connection design, in general.
Pat specifically highlighted the importance of using a weld ductility factor of 1.25 to increase the load for which welds in all bracing connections subjected to shear and transverse loading. This weld ductility factor accounts for issues with proximity and distortion within the braced frame connection and is necessary to allow adequate force redistribution within the weld.. Pat referenced the 2004 paper by Hewitt and Thornton, Rationale Behind and Proper Application Of the Ductility Factor for Bracing Connections Subjected to Shear and Transverse Loading, as a good resource to further understand the need for this ductility factor. Although the Code does not specifically require this weld ductility factor, Part 13 of the AISC Steel Manual does recommend using it.
It is important to note that welds that are designed to develop the strength of the gusset plate need not be increased by the weld ductility factor, since the ductility of the plate allows the redistribution of load. Also, the weld ductility factor does not apply to welds in shear only. It is the combination of shear and tension on the weld that triggers the need for force redistribution within the bracing connection but only if proximity issues or distortion issues are present.
Many times these bracing connections may be delegated the steel fabricator, however, it is still important for EORs to be aware of the necessity of the weld ductility factor and its relevance in connection design. Although not specific to seismic systems, Pat’s explanation of the weld ductility factor and its relevance is a welcomed reminder to engineers that may not be familiar with its use, or don’t typically design their own steel connections.