The bending behavior of T-section beams composed of a glulam web and an upper cross-laminated timber flange was studied. The influence of two fundamental factors on the bending strength and stiffness was considered: the wood species used for the webs and pretensioning with unbonded tendons. Sixteen specimens with a 9 m span were tested until failure: eight of them were nontensioned (4 Picea abies webs and 4 Quercus robur webs) and the other eight were pretensioned using threaded bars with 20 mm diameter anchored in plates fixed at the ends of the specimens (4 Picea abies webs and 4 Quercus robur webs). Pretensioning with unbonded tendons showed a clear improvement in the load capacity of the specimens with Picea abies webs, while the difference was not significant for the specimens with Quercus robur webs. Considering deflection, pretensioning gave the advantage of an initial precamber but also generated slight variations in the stiffness as a result of increasing the portion of the section that was in compression. The variation in the stiffness depended on the relation between the compressive and tensile moduli of elasticity parallel to the grain, and its influence on the deflection was analyzed using a finite element method.
The possibility of improving the bending strength of timber beams through prestress is discussed. Different prestressing and anchoring procedures are investigated: (i) post-tension, with mechanical anchorage on the ends of the wood elements; (ii) pre-tension, with the anchorages being either mechanical, as in the previous system, or glued, with the force being gradually transferred through shear stresses along a certain length of the wire/wood interface. The main conclusion is that any of the procedures may be advantageously used with either small (mass-production) or long glued-laminated timber beams, as a possible alternative to CFRP reinforced beams.