This study aims to guide a method for assessment of brittle fracture performance of a multi-planar welded pipe structure that will be adopted as substructure of an extremely large-scale infrared ray telescope (Thirty Meter Telescope, TMT). In this structure, the brittle fracture from narrow part of welds between pipes that include a crack like welding defect can most likely occur under large-scale seismic load at a minimum service temperature (around -15°C). The tensile fracture testing for a "narrow-weld joint specimen" that has a crack in coarse-grained heat affected zone (CGHAZ) of final welding pass is conducted, and the brittle fracture occurred from the CGHAZ at a temperature 40°C lower than the service temperature. This test temperature is employed assuming a fracture toughness deterioration due to the large scale seismic loading. The fracture load is predicted based on the Weibull stress concept, where the Critical Weibull stress distribution of the CGHAZ is identified from testing and FE-analysis conducted for standard fracture toughness specimens at -30°C. Predicted results taking ductile crack growth into account show a good agreement with the experimental results. This result implies that the Weibull stress concept can be applicable for brittle fracture assessment of the "narrow-weld joint specimen" where a notch is located in the CGHAZ irrespective of temperature as long as brittle fracture occurs. Namely, it is expected that the Weibull stress concept is applied for fracture assessment of a multi-layer welded pipe structure subjected to large-scale seismic loading.