Comparitive study on the flexural behaviour of glulam built-up beams based on ambient and standard fire tests

Abstract

The experimental study presented in this paper proposes a type of glulam beam with a hollow cross-section that can further optimise solid glulam beams. The study investigated the structural behaviour of glulam built-up box-section beams under four-point flexural bending at both ambient and elevated temperatures. A total of eleven 3100-mm long simply supported beam assemblies were experimentally examined: seven beams were tested at ambient temperature; and four beams were subjected to CAN/ULC-S101 standard fire. Five of the seven beam assemblies tested at ambient temperature were fabricated using self-tapping screws, while the other two assemblies were built using industrial polyurethane adhesive. Each built-up beam assembly was made of four glulam panels, all of 44 mm thickness except the bottom flange panel which had 86 mm thickness. Through ambient testing, it was concluded that when the spacing of the screws connecting the top and bottom flange panels of the built-up section to its web panels was decreased from 800 to 200 mm, the flexural resistance of the beam assembly was increased by about 45%. Whereas further reduction of the spacing from 200 to 100 mm for the screws connecting the bottom flange panel to the web panels over a distance equals to one-third of the span length of the beam from both supports, where shear stresses are maximum, increased the flexural resistance of the beam additionally by 10%. However, the experimental results of the glued beam assemblies show significantly high flexural strength that was almost equal to the calculated strength of an equivalent hollow-section glulam beam. In a subsequent phase, only the strongest beam assemblies (two screwed and two glued) were tested under the effect of standard fire while subjected to monotonic loading that caused a bending moment that is equivalent to the beam full resistance moment calculated for ambient condition. Fire tests' results show that both screwed and glued glulam built-up box-section beams can sustain the applied design load under standard fire exposure for more than 30 min without any fire protection.