Residual Strength of Fire-Damaged Laminated Beams

Three 50-foot, glued-laminated, Douglas-fir beams damaged by fire were tested for residual strength by, bending 10-foot lengths from each end for longitudinal-shear strength, and by bending the remaining central 30-foot lengths for extreme fiber strength. The beams, nominally 7 x 30 7/8 inches in cross section, averaged 6 3/8 x 30 1/2 inches after char was removed by scraping. A 0.0025 inch flat feeler gauge penetrated gluelines 3/8 inch, leaving a net glueline width of 5 1/2 inches. Symmetrical two-point loading was applied to the six short beams. Critical shear stresses developed, based on not glueline width, ranged from 510 to 670 psi with an average of 595 psi. Only the weakest of these failed in shear, the other five failed in bearing at a reaction. A single concentrated load at mid-span developed extreme fiber strengths of 3600 to 5600 psi, with an average of 4830 psi, in the three 30-foot long beams. Full-span (314 inches) linear deflection was measured to indicate moduli of elasticity based on the appropriate bending moment deflection formula ranging from 1.56 to 1.60 (106) psi. The six short beams performed satisfactorily, the minimum critical longitudinal shear stress developed being 3.1 times the allowable working stress of 165 psi for normal duration and dry service. The weakest long beam developed 1.5 times the allowable bending, stress of 2400 psi for normal duration and dry service. A steeper-than-permitted grain angle coupled with an irregular edge in the outer tensile lamination was considered to be responsible for the low strength of this specimen. No evidence was found to suggest that the fire had reduced the beam strengths beyond what was measurable in reduced cross section and destroyed glueline.

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