The latest developments in seismic design philosophy have been geared towards developing of so called "resilient" or "low damage" innovative structural systems that can reduce damage to the structure while offering the same or higher levels of safety to occupants. One such innovative structural system is the Pres-Lam system that is a wood-hybrid system that utilizes post-tensioned (PT) mass timber components in both rigid-frame and wall-based buildings along with various types of energy disspators. To help implement the Pres-Lam system in Canada and the US, information about the system performance made with North American engineered wood products is needed. That information can later be used to develop design guidelines for the designers for wider acceptance of the system by the design community. Several components influence the performance of the Pres-Lam systems: the load-deformation properties of the engineered wood products under compression, load-deformation and energy dissipation properties of the dissipators used, placement of the dissiaptors in the system, and the level of post-tensioning force. The influence of all these components on the performance of Pres-Lam wall systems under gravity and lateral loads was investigated in this research project. The research project consisted on two main parts: material tests and system tests.
In the material tests part of the program, a total of 110 compression tests were conducted to determine the load-deformation properties of four different engineered wood products (LVL, LSL, Glulam and CLT) in various directions. The LVL, LSL and Glulam specimens tested under compression parallel to grain had similar linear elastic behaviour with limited ductility. The CLT specimens tested under compression in the major-axis direction had linear elastic behaviour with moderate plasticity. Depending on the type of engineered wood product, typical failure modes included crushing, shear, wedge split and splitting. The compressive strength of the products tested ranged from 42.1 to 53.5 MPa, the global MOE (of the entire specimen under compression) varied between 6390 and 9554 MPa, the local (near the crushing surface) MOE parallel to grain was in the range of 2211 to 5090 MPa, while the local to global MOE ratio ranged from 29.2 to 58.0%, and was higher with the increase in the oven-dry density.
The specimens of the four different engineered wood products tested under compression perpendicular to grain or in the minor-axis direction had elastic-plastic behaviour with a clearly defined plastic plateau. Crushing (densification) of the fibres perpendicular to grain was the main failure mode for all specimens, and was in some cases followed by in-plane shear failure or cracking perpendicular to grain. Compression parallel to grain in the middle layer that was followed by its delamination and buckling was a unique failure mode for CLT specimens tested under compression in the minor strength direction. The compressive strength of the engineered wood products tested were in the range of 4.8 to 27.8 MPa, while the global and local MOE perpendicular to grain were in the range of 244 to 2555 MPa, and 320 to 1726 MPa, respectively. The compressive strength and global MOE perpendicular to grain increased with an increase in the oven-dry density. The results show no well-defined trend for the local MOE perpendicular to grain. The specimens loaded in the centre perpendicular to grain had higher strength, global and local MOE than those loaded at the end.
A convenient and timesaving design for the axial energy dissipators (fuses) was developed by replacing the epoxy in the original design with two half-tubes. Compared to the original design of fuses with epoxy, the new design with two half-tubes had similar necking failure mode and a longer failure displacment, thus providing user-friendly fuses that performed similar or even better than the original design.
In the system tests part of the program, a total of 17 different PT and Pres-Lam CLT walls with six different configurations were tested under monotonic and reversed cyclic loading. The studied parameters included the level of PT force, the position of the fuses, and the number of UFPs. CLT shear walls subjected only to post-tensioning, had non-linear elastic behaviour. The behaviour of the PT walls with and without energy dissipators was relatively similar under monotonic and cyclic loading. The strength degradation observed during the cyclic tests was low in all wall configurations suggesting that very little damage was inflicted upon the structure during the first cycles at any deformation level. Four major failure modes, including yielding and buckling of fuse, crushing and splitting of wood at the end of wall, and buckling of lumber in the exterior-layer of CLT wall, were observed in the tests. The yielding in fuses occurred at the early stage of loading as designed and the other failure modes happened when the lateral drift reached or beyond 2.5%.
The initial stiffness of the single-panel PT CLT walls tested ranged from 1.80 to 2.31 kN/mm, the load at the decompression point and 2.5% drift were in the range of 4.2 to 14.9 kN and 32.7 to 45.9 kN, respectively. The initial stiffness of the single-panel Pres-Lam CLT walls tested ranged from 1.69 to 2.44 kN/mm, the load at the decompression point and 2.5% drift were in the range of 21.0 to 30.2 kN and 59.6 to 69.8 kN, respectively. All the mechanical properties increased with an increase in the PT force. The average initial stiffness and the load at 2.5% drift of the coupled-panel Pres-Lam CLT walls tested were 4.59 kN/mm and 151.3 kN, respectively, while the load at the decompression point increased from 58.4 to 69.7 kN by increasing the number of UFP. The test results show that the behaviour of the Pre-Lam CLT shear walls can be de-coupled and a “superposition rule” can be applied to obtain the stiffness and resistance of such system.
The test results gave a valuable insight into the structural behaviour of the PT and Pres-Lam CLT shear wall under in-plane lateral loads. The data from the testing will be used in the future for development of numerical computer models. They will also be used for development of design guidelines for this system. All tests conducted in this study and the analyses in the future modelling research will form the basis for developing future design guidelines for PT and Pres-Lam mass timber systems.