Using timber in construction has a lot of advantages from its lightness, its environmental benefits over concrete production, and the prefabrication opportunities. But engineers are also familiar with timber’s downsides, including stiffness, low strength, and load limitations. The industry has used sandwiched wood and steel plates since the nineteenth century and continues with limited success to explore hybridizing timber using steel and reinforced concrete. Timber beams combined with bonded steel tendons have shown promise, but considerations related to timber strength and degradation need to be addressed during the design phase.
In a study in the Journal of Structural Engineering, researchers Reyhaneh Hosseini and Hamid R. Valipour evaluated the performance of hybrid timber–steel encased beams. The laboratory testing and analytical modeling outlined in their paper, “Timber-Encased-Steel Beams: Laboratory Experimentation and Analytical Modeling,” used radiata pine and Douglas fir timber lamellae bonded to encased steel bars. Their testing analyzed 18 beams of various sizes and the size and location of the encased steel bars. The authors also studied the effect of the adhesive type and the groove size. Using a four-point bending test, they tested the beams to failure (failure being the load dropping below 75% of the peak load or the midspan deflection exceeding 120 mm). Learn more about this study and how timber-encased beams could be used in new designs at https://doi.org/10.1061/JSENDH.STENG-12898. The abstract is below.
Abstract
The flexural behavior of hybrid timber–steel encased beams comprising coniferous radiata pine (Pinus radiata) (MGP10) and Douglas fir (Pseudotsuga menziesii) (F8) timber lamellae with bonded-in steel bars is studied. The effect of cross-section depth, steel bar size, timber species/grade, and steel bar arrangements (only bottom and top-and-bottom) on the hybrid beams’ stiffness, failure mode, ductility, and load-carrying capacity were investigated. The flexural capacity and stiffness of the doubly (top-and-bottom) reinforced beams are increased by 127% and 71%, respectively. However, in the singly (bottom) reinforced beams, the flexural capacity and stiffness are increased only by 41% and 25%, respectively, highlighting the important role of the compressive bars. The failure of all beams was associated with tensile flexural failure of timber, but the steel bars improved the ductility of the beams. The maximum coefficient of variation of the peak load in hybrid beams (CoV=14.3%) is lower than that of the bare timber beams (CoV=21.7%). Two analytical models were developed based on a linear and a bilinear stress–strain relationship for timber. The analytically predicted peak load and stiffness agree well (less than 13% and 12% difference) with the experimental results.
Learn more about these test results and their potential in your projects in the ASCE Library: https://doi.org/10.1061/JSENDH.STENG-12898.
