This paper presents an analytical study on static moment redistribution and resultant reduction of lateral resistance caused by sudden beam fractures in steel moment frames. A numerical example is presented for a simple multispan frame, and the step-by-step processes of beam fractures, moment redistribution, and loss in resistance under statically increasing lateral deflection are examined. The mechanism of moment redistribution is interpreted using the three-moment equations, and the possibility of sequential fractures in the course of static moment redistribution is investigated. A sudden fracture of a beam changes moment distribution rather locally, particularly when plastic hinges are already formed in beams by the time of the fracture. Sequential fractures are less likely to occur during static moment redistribution when rotations corresponding to fracture are large (to simulate fractures after significant plastification) and vary from plastic hinge to plastic hinge (to allow for the random nature of fractures). It should be carefully noted that the above observations are applicable for static moment redistribution, and do not necessarily represent the effects of beam fractures on dynamic responses.
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