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Structural reliability analysis for a three-hinge timber portal frame subjected to fire was undertaken. Eight modes of failure were identified for the frame and limit state function was formulated for each failure mode. The limit state functions were based on the Eurocode 5 design criteria. Uncertainties in the timber material properties were generated from laboratory test results performed on five commonly used timber species in Nigeria, namely Alstonia boonei (Ahun), Triplochiton Scleroxylon (Obeche), Terminalia Ivorensis (Idigbo), Terminalia superba (Afara) and Lophira Alata (Ekki). Uncertainties in loading and geometrical properties were obtained from international references. The limit state functions were evaluated using nonlinear constrained optimization technique. The optimization was executed using Genetic Algorithms(GA) based First Order Reliability Method (FORM) algorithm, through a developed computer programme in MATLAB. The results indicated that, the predominant mode of failure for a three-hinged timber portal frame in fire is the failure of the rafter-column connection that resulted to least safety levels for all the considered fire exposure times. Also, it was observed that at the critical mode of failure the portal frame can sustain fire for up to 50 minutes before failure, however, the target safety index of 3.8 recommended in the Eurocode can only be achieved at fire exposure time less than or equal to 25 minutes.