Abstract: According to the current design codes, such as the 2005 AISC Seismic Provisions for Structural Steel Buildings, the vertical boundary elements (VBE’s) of steel plate shear walls (SPSW’s) are sized as beam-column members and the codes require a minimum moment of inertia for VBE’s to avoid “pull-in” behavior. This paper reviews the derivation of the flexibility factor, ω_t, and how the factor was incorporated into current code design requirements for SPSW’s. Then, analytical models to prevent VBE shear yielding and to estimate the elastic out-of-plane buckling strength of VBE’s are developed by theoretical analysis and finite element calculation. It is shown that the existing limit by codes on ω_t does not correlate with satisfactory out-of-plane VBE performance. The significant inward VBE inelastic deformation and out-of-plane buckling were due to excessive VBE flexibility or other causes, such as shear yielding at the ends of the VBE’s. Therefore, the VBE’s of SPSW’s should be designed according the proposed approach to avoid the VBE shear yielding and ensure out-of-plane stability.