A regular alternation of lamellas and voids filled by insulating material within each layer of CLT can lead to cellular panels with improved acoustical, thermal and fire performance. In order to support the development of these innovative and lighter engineered wood products, their mechanical behavior is investigated in this paper by means...
Board width-to-thickness ratios in non-edge-glued cross laminated timber (CLT) panels influence the in-plane shear stiffness of the panel. The objective is to show the impact of board width-to-thickness ratios for 3- and 5-layer CLT panels. Shear stiffnesses were calculated using finite element analysis and are shown as reduction factors relative to the shear stiffnesses of edge-glued CLT panels. Board width-to-thickness ratios were independently varied for outer and inner layers. Results show that the reduction factor lies in the interval of 0.6 to 0.9 for most width-to-thickness ratios. Results show also that using boards with low width-to-thickness ratios give low reduction factors. The calculated result differed by 2.9% compared to existing experimental data.
In the last decade, cross laminated timber (CLT) has been receiving increasing attention as a promising construction material for multi-storey structures in areas of high seismicity. In Japan, application of CLT in building construction is still relatively new; however, there is increasing interest in CLT from researchers as well as construction companies. Furthermore, the Japanese government is providing construction cost subsidies for new CLT structures as it is a carbon neutral and sustainable material. The high shear and compressive strength of CLT makes it a good candidate for use as shear walls in mid-rise buildings. One important aspect of CLT walls, and one that is presently poorly understood, is the influence of openings on the shear carrying capacity. Openings are often necessary in CLT panels either in form of windows, doors, lift shaft openings or installation of building services. Concerning this aspect, the code regulations in Japan are relatively strict, such that if openings exceeded certain prescribed limits, the entire CLT panel is considered as a non-structural element, and its contribution to lateral strength is totally ignored. Furthermore, as the maximum opening size is usually governed by edge distance constraints, the size of openings that designers can use is inevitably limited by the standard sizes supplied by the manufacturers. As a result, designers are obligated to adopt very small opening size. This is thought to be a very conservative approach. The main purpose of this paper is to experimentally evaluate the influence of openings on seismic capacity; strength and stiffness reduction, as well as failure mode with changing opening size and opening aspect ratio. In addition, check the validity of the Japanese code regulations with regards to openings in CLT panels.
In this study, six 5-layer CLT panels containing different openings were tested. The parameters considered include the size and layout of the opening. The panels were specifically designed with openings that would render them ineffective in resisting lateral loads according to the Japanese standard. However, in addition to the six panels, one panel without openings and one panel with openings that meet the Japanese standard was designed. All the CLT panels were tested in uniaxial diagonal compression in order to simulate pure shear loading. The CLT panels and the loading setup were designed such that the resulting failure mode will be governed by a shear mechanism. The main focus of the experiment was to relate the deterioration of the lateral strength and stiffness of the panels to the size and layout of the opening.
The results showed that the panels with openings with the same area have relatively different failure direction and reduction factors for panel shear strength and stiffness, and that is due to the shear weak and strong direction that CLT panels have. Also, the effect of openings on the reduction of stiffness for CLT panels was found to be greater than their effect on the reduction of shear strength. The prescribed equation in the Japanese CLT Guidebook underpredicts stiffness reduction, and has discrepancies with regard to strength as the difference of panel strengths in weak and strong directions are not considered.
This master thesis is on post-tensioning cross-laminated timber stability cores for multiple story buildings. When designing a CLT core, significantly larger core sections will be needed than when designing a stabilizing core in concrete. This is for one part due to the limited stiffness of the CLT compared to concrete. For another part it is due to the limited stiffness of connectors in CLT. Sliding and uplift can occur in connections in CLT loaded in tension and shear respectively. The CLT panels behave like rigid bodies, with most of the displacement occurring at the connections. In addition, cooperation between flange and web may be limited, depending on the stiffness of the corner connection and the occurrence of shear lag. Post-tensioning is suggested as a solution to diminish uplift and sliding in the connectors. In this way, with the same core section, a taller building may be realized compared to the non-post-tensioned case. In the thesis also the long-term effects on the prestress level is assessed, as estimating these effects is important for the safety of the system.This thesis adds to the body of knowledge on post-tensioned CLT structures. Firstly, previous studies on post-tensioned CLT focus on individual shear walls and on seismic design situations. This thesis explores how beneficial post-tensioning is from the perspective of serviceability limit state governed design. Furthermore, though post-tensioning as a prestressing method has been applied often in concrete structures, prestressing of CLT is a novel research subject. Especially the estimation of long-term force loss is a topic that still requires research. This thesis provides the designer with a straightforward calculation method (using python) for estimation of prestress force loss in the long-term.The research was carried out with a literature study and a case-study. The literature research comprised of studies on structural design with CLT loaded in-plane; the effective flange of a CLT core; stiffness of connections in CLT; prestressing of CLT; a design approach for post-tensioning; time dependent losses in post-tensioned CLT. The case study was based on a fictitious floorplan including a “minimal core”, and at expressing the benefit of post-tensioning in terms of height gain.The degree to which the flange and the web cooperate showed highly dependent on the connection between flange and web and the core height. In the case study, the effective flange width showed to depend highly on the height of the core and the stiffness of the connection between flange and web.In the case-study, without post-tensioning, approximately half of the displacements could be attributed to the connections. With post-tensioning, the uplift and sliding displacements in the horizontal joints was eliminated. Consequently, the attainable height was significantly increased: from 5 storeys in the un-post-tensioned case, to 8 storeys in the post-tensioned case. Long-term effects on the prestress loss were considerable. In the case-study, approximately 40% loss of post-tension force in the lifetime of the building was predicted and included in the design. Largest causeof force loss was due to changes of moisture content during construction. The remaining lateral displacements after post-tensioning were due to bending and shear.Post-tensioning of CLT cores is a powerful method for reducing lateral displacements in cases where uplift and sliding are dominant contributors to the lateral displacements. This is especially the case in light-weight buildings. Uplift and sliding displacements can be eliminated altogether with post-tensioning. The designer should realize that post-tensioning does not increase the bending and shear stiffness of the core. The thesis also concludes that with the post-tensioning of CLT walls, the compressive strength of the CLT in the so-called “compression-toe” might be exceeded. It is an important check in design. Furthermore, depending on the decision to re-tighten the rods at some point or not, the post-tension force loss should be calculated and included in finding the right prestress level. For this estimation of the moisture level of the CLT proved to be an important but difficult step. It is likely that the 40% force loss in the case-study is on the conservative side, since a large change in moisture content has been assumed. In practice, the moisture content can be measured on site. This can help verify the assumptions on the moisture content used in force loss calculations. This can help in assuring the structure is safe in the long-term.