Notional Load Method For Frames Braced By Shear Bracing

Notional load method becomes a new option for design of steel structures in recent years. The aim of the notional load method was to design frames without having to determine the column effective length factor, but directly use the geometric length of columns to check stability. But the researches published in open literature had not involved braced frames, so researches on the notional load method for braced frames were necessary. Applying the notional load on the upper end of a column and taking second-order elastic analysis, one can take the effective column length factor by 1.0.A series of braced frames under different geometric and load conditions were studied by geometric and material nonlinear analysis considering member initial bow, initial sway imperfection and residual stresses. The threshold bracing area which determines the buckling modes of each frame was acquired and converted into the notional load. The notional load factor is defined as the notional load divided by column total loads. A simple notional load formula was proposed. The second-order analysis of the notional load method was also introduced. FEM result of each frame model obtained by AN SYS was compared with the results calculated from the proposed formula. The proposed formula was also compared with two formulas converted by the threshold bracing area from the theoretical derivation and the Code (GB50017-2003) respectively. The results show that the proposed formula is accurate and applicable. Meanwhile examples of the braced frame with column moments and a two-story two-bay braced frame were introduced to examine the applicability of the proposed notional load. The results showed the proposed equation is conservative and practical. Besides, the fifth chapter of the paper discussed the warping transfer at beam-to-column joints with three stiffened plates.