Abstract: The elastic-plastic analysis was performed for seven chevron-braced frames with weak beams, with the brace slenderness varying from 43 to 132. The braces are of two types: carrying and free of gravity load. The effect of gravitational axial force in braces on the lateral resistance of the braced frames was studied. The results showed that the gravity load caused the braces buckle (or yield) earlier; it weakened the peak strength of the chevron braces and the overall braced frames; and those effects were greater for slender braces used in regions of smaller seismic fortification intensity. Based on the overall, the brace’s and the frame’s lateral force-drift curves, the paper explained why the beam should be designed as if no bracing point at midspan, why the United States and Japan treated the braces of moderate slenderness as the structure having the worst seismic behaviour. It was also found that the braces reached its ultimate state at a much smaller drift than the frame, implying that the current design method of linear combination is not fully justified, and an improved design method is proposed. A plastic mechanism was proposed to evaluate the relations between the midspan-deflection of the beams, the out-of-plane displacement of the braces and the inter-storey drift.