Comparison to Post-Fire Beam Experiments

The bending stiffness of the specimens was estimated from the second-order lateral deformations. The values at the end of fire exposure (WCl/2, obtained from Chapter 7) and after cooling (post-fire, P-WCl/2) are summarized and compared to the reference value (P-REF) in Table 8.6 [14]. A decrease to 42% and 36% of initial stiffness resulted at the end of fire exposure (for the stiU-hot specimens), while after cooling the post-fire values significantly increased to 76% and 70% of the initial values (values close to the compressive post-fire stiffness given in Table 8.4). 

These results can be compared to the same set of results obtained from the study on beam specimens (SLCOl and SLC02) in Section 8.2, which were subjected to four-point bending during 60 and 120 min fire exposure from the underside, see Table 8.6. In the former study, six-cell specimens were used in contrast to the four-cell specimens used here. The bending stiffnesses obtained from Section 8.2 were therefore corrected by a factor of 4/6 in order to make them comparable. At the end of fire exposure, the bending stiffness of the still-hot specimens (SLCOl/02) dropped to 46% and 43% of the initial value, which almost matched the values obtained for the column specimens. The post-fire stiffnesses (64% and 60%), however, were slightly lower than those of the column specimens (76% and 70%). 

In this chapter, the post-fire behavior of FRP composites was evaluated and modeled on the stmctural level. Results from the models compared well with results from fuU-scale post-fire experiments on cellular GFRP beam and column specimens that had been subjected to mechanical and thermal loading up to 120 min with inclusion of different thermal boundary conditions. On the basis of the previously proposed thermal and mechanical response models, existing approaches for post-fire evaluation can be applied. Predicted temperature profiles and the conversion degrees of decomposition can be used to estimate the post-fire stiHhess from existing two- and three-layer models. The borders between different layers can be determined either by a temperature criterion or a RRC criterion. 

The recently developed models to predict time and temperature-dependent material properties and post-fire properties showed good agreement with the experimental results. On the basis of the proposed models, the post-fire stiffness of FRP composite materials can be predicted before fire exposure. As a result, the post-fire behavior can be predesigned based on the functionality and importance of the stmcture. 

Pering, G.A., Farrell, P.V., and Springer, G.S. (1980) Degradation of tensile and shear properties of composites exposed to fire or high temperatures. J. Compos. Mater., 14, 54-68. 

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