Dynamic fail-safe behaviour of steel structures

University dissertation from Luleå : Luleå tekniska universitet

Abstract: The fundamental behaviour and the capacity of steel structures subjected to loss of interior load-bearing elements are studied. The ultimate load-bearing and deformation capacity of certain beam-to-column connections and the dynamic fail-safe behaviour of some ordinary steel structures are investigated in detail. A number of different geometrical models of steel structures in the area of primary damage are analysed. Both bending and catenary action of the models are treated and the strength properties of both members and connections are considered. Two types of connections are investigated, viz. the "bolted heel connection" and the bolted end-plate connection (with a degree of moment rigidity of 25%). No stability problems are treated. A rigid-body method of analysis is applied. Design methods for the bolted heel and bolted end-plate connections under catenary action are proposed, which safely predict the over-all behaviour and the ultimate load-bearing and deformation capacity of each connection. For structures having these two types of connections, the static damage endurance capacity under catenary action is approximately equal. The applicability and accuracy of the riqid-body method is evaluated. The accuracy is determined by comparison with an elasto-plastic vibration theory and an equivalent mass-spring method. The rigid-body model proves to be a suitable model in order to determine the failsafe behaviour of most steel structures. The ultimate dynamic load-bearing capacity under bending action regarding member characteristics and under catenary action regarding partly member characteristics and partly joint characteristics is evaluated in great detail. It is found that the dynamic capacity under bending action approximately equals the static one provided that the greater deformations obtained under dynamic conditions can be absorbed. No regard to the time of removal of the load-bearing element is thus necessary if only the deformation capacity of the structure is verified. Also, strain-hardening effects and geometrical non-linearity need not be considered. The dynamic capacity under catenary action regarding member characteristics is only half of the static one, and regarding joint characteristics only one third of the static one for structures with bolted heel connections, and half of the static one for structures with bolted end-plate connections. The high efficiency of catenary action compared to bending action found under static conditions, is reduced considerably under dynamic conditions. For many practical cases only full end constraints, momentary loss of load-bearing elements and geometrical linearity need to be considered. The maximum column reactions at ultimate dynamic load always fall below the maximum reactions at ultimate static load. Deflections and reactions determined theoretically from the rigid-body models employed agree well with those measured.

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