Redundancy and Stress Redistribution of Rigidly Connected Steel Frames subject to Fire

Abstract

Today I make a presentation on the redundancy and stress redistribution of rigidly connected steel frames subject to fire. Stress redistribution is a main theme of this presentation. Redundancy has the same meaning. I will take rigidly connected steel structures, since a majority of structures are designed to be rigidly connected in Japan to increase both rigidity and strength for seismic design. A rigidly connected frame indicates, of course, a frame with moment connections. In such frames, when they are overloaded, local plastification and subsequent sound stress redistribution often occurs without loss of strength. The situation is similar when they are subjected to fire. I would like to present my recent research, and my talk will be quite brief and simple. The more refined, more specific details and discussions, I’ll leave to my colleague Dr. Jun-ichi Suzuki. Now I discuss three typical examples of stress redistribution that can be often observed in heated frames. About the first and second examples you probably know well: the first example is stress redistribution that occurs in plastified beams and the second example is stress redistribution that occurs in plastified columns. The third example is probably a new topic to you. This involves the overall stress redistribution that occurs in a whole heated frame subsequent to column buckling. The phenomenon may be similar to local buckling of a plate in that, as Dr. Knobloch pointed out, post-local buckling behavior of a plate remains stable if sound stress redistribution works after the onset of buckling. Here you can see stress redistribution created in plastified beams as shown in Figure 2.3.3. This is a T-shaped frame—the left half of a symmetrical frame and we assume the lower column and the beam are equally heated. When the member temperature is elevated, the beam is plastified first since beam load is assumed to be large. The Fire Science and Technology Vol.26 No.2(2007) 87-92 87