Flexural bending behaviour of built-up glulam box-selection beams at ambient and elevated temperatures

Abstract

The recent increasing trend of sustainable construction and advancement in the manufacturing of engineered wood have made products such as glued-laminated timber (glulam) and cross-laminated timber (CLT) preferred building materials. The intensifying demand for engineered-wood products in Canada also has prompted amendments to the building codes of several provinces by reducing the height restriction of timber structures from four to six stories. Unfortunately, the design of built-up timber beams has not yet been incorporated in most wood design standards worldwide. Thus, this lack of design guidelines brings forth the demand of acceptable methods to analyze, design and manufacture such built-up beam sections. The experimental research study detailed here in this thesis has been carried out to investigate the flexural bending behaviour of built-up glulam box-section beam assemblies fabricated using two engineered-control techniques at both, ambient and elevated temperatures. Seven full-size built-up glulam beam test assemblies were experimentally examined under four-point flexural bending to determine their maximum bending strengths at ambient temperature. 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 structural adhesive. The outcomes of ambient testing showed that reducing the spacing from 800 mm to 200 mm for the screws connecting the built-up beam section’s top and bottom flange panels to the web panels increased the beam flexural bending strength by about 45%. While reducing the spacing from 200 mm to 100 mm only for the screws connecting the bottom flange panel to the web panels over a distance equal to one-third beam span length from each support, where shear stresses are maximum, increased the beam flexural bending strength by an additional 10%. However, the experimental results of the glued beam assemblies showed considerable flexural bending strengths that are almost equal to the calculated strength of an equivalent hollow-section glulam beam. The influence of the bonding technique and configuration followed in fabricating the built-up beam sections, whether screwed or glued, was also investigated through observing the different failure modes that the built-up beam assemblies exhibited during testing. In addition, the experimental results of the ambient tests were used to verify the calculated bending strength capacity of the built-up glulam beams. Out of each of the glued and screwed assembly groups, only the strongest built-up beam assembly was examined under the effect of CAN/ULC-S101 standard fire while subjected to monotonic loading that was equivalent to the full-capacity design load of the weakest screwed built-up beam assembly with 200-mm screw spacings. The fire resistance tests were conducted using the large-size fire testing furnace accommodated at Lakehead University’s Fire Testing and Research Laboratory (LUFTRL). Outcomes of the fire resistance tests revealed that the glued built-up beam assemblies experienced greater mid-span deflections as well as beam end rotations in comparison to the screwed built-up beam assemblies. This inferior behaviour can be interpreted to the low fire resistance of the adhesive used in fabricating the built-up beam assemblies, which excessively limited the beam’s shear and bending strengths at elevated temperatures. On contrary, the self-tapping screws noticeably helped in keeping the built-up beam assemblies intact for longer time during fire testing even when the screws were exposed to direct fire heating.