Lateral-torsional buckling at the system level (global buckling) has been the subject of several research studies over the past 20 years. This buckling mode often controls over conventional lateral-torsional buckling (buckling between braces points) when the girder system is long and narrow. One of the primary contributors to global buckling resistance is the in-plane girder stiffness of the system. There is, currently, a published expression for the in-plane girder stiffness; however, recent attempts to employ this expression in more unique design situations, such as in lean-on bracing systems, have raised some questions about its efficiency. The current expression accounts only for the stiffness contribution of the two exterior girders. Though this is a conservative approach, it can, in some cases, be considered overly conservative. The primary focus of this thesis study was to derive a more broadly applicable in-plane girder stiffness expression. To that end, a new expression for the system buckling capacity, that accounts for any number of girders, was derived. This system buckling capacity expression was then used to develop an in-plane girder stiffness expression that also accounts for any number of girders. A computational study, using the program mBrace3D, was performed to determine the accuracy of the proposed expressions. These computational study models included four separate girder sections and system widths ranging from two to ten girders. Other parameters investigated in this study were number of brace points along the girder, girder spacing, and the stiffness of the braces themselves.