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- Pettersson, Ove, et al.
(författare)
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Fire Design of Wooden Structures
- 1983
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- The paper describes and discusses the state of the art concerning the possibilities of an analytical prediction of the real fire behaviour of wooden structures and structural members. The principles of a probability based design are outlined with reference to a physical model according to Fig. 2, connected to a natxral compartment fire exposure. The design procedure is exemplified by a Swedish method, based on a practical design format calculation, specifying characteristic values and partial safety factors for the action effects and response capacities. The model and stochastic uncertainties, to be included in the derivation of the partial safety factors, are listed. In the method, the consequences of a structural failure and the effects of the probability of a severe compartment fire, the fire brigade actions and an installed fire extinguishment system, i f any, are approximately accounted for by dividing the structures orstructural members into categories with a related differentiation of the design fire load density and the length of the fire process to be considered in the design. The state of knowledge on the thermal and mechanical response of fire exposed wooden structures is reviewed with respect to the rate of charring at varying thermal exposure and the related temperature and moisture gradients in the un-charred part of the cross section, the mechanical properties of wood at a simultaneous increase of temperature and moisture content, and the fire behaviour and ultimate load bearing capacity of the structure. The available design aid is exemplified , focusing on laminated timber structures. Design diagrams aregiven for a determination of the ultimate load bearing capacity of beams with respect to bending and lateral buckling and of the buckling load for columns. The lack of validated, practically adaptable, analytical models for the transient temperature and moisture states within the un-charred part of the cross section and for the mechanical behaviour of the material at simultaneous transient states of stress , temperature and moisture i s emphasized. The conditions prevent a reliable calculation of the deflections of fire exposed wooden structures to be performed at present and limitastructural fire design primarily to a determination of the ultimate load bearing capacity.
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