Experimental Behavior of L-Shaped and T-Shaped Cross-Laminated Timber to Evaluate Shear Walls with Openings

Using cross-laminated timber (CLT), a mass timber product, a number of mid- and high-rise buildings have been constructed (van de Lindt et al. 2019; Izzi et al. 2018; Sustersic et al. 2016). The Japanese “Act on Promotion of Use of Wood in Public Buildings,” instituted in 2010, promoted use of wood in public buildings (Isoda et al. 2021). The Japanese Agricultural Standard (JAS) product specification for CLT was established in 2013 (Ministry of Agriculture, Forestry and Fishery 2013). Subsequently, the “Design and Construction Manual for CLT buildings” (JHWTC 2016), henceforth referred to as CLT manual, was published.

Two construction methods are prevalent in the Japanese CLT manual: narrow- and wide-panel CLT walls. Narrow-panel walls, with 1 to 2 m width panels and without openings, have higher ductility (Sato et al. 2019). The narrow-panel walls integrate window and door openings through lintel and parapet beams (e.g., Casagrande et al. 2021a; Zhang et al. 2021; Yasumura et al. 2016). Wide-panel walls can exceed a width of 10 m and integrate cut-outs for window and door openings (Zhang et al. 2021; Sato et al. 2019). Under lateral loads, however, the wide-panel CLT walls are prone to cracks at the corners of the opening (Fig. 1). According to the Japanese CLT manual, the strength limit state of the wide-panel CLT walls is defined as the initiation of cracks at the opening’s corners (Yasumura et al. 2016).

Shahnewaz et al. (2017) and Dujic et al. (2008) carried out a parametric study on the aspect ratios of the opening and wall and developed analytical models to estimate the stiffness and strength. Mestar et al. (2021) experimentally quantified the effect of CLT shearwalls with openings on the base shear and hold-down tensile load. Pai et al. (2017) carried out a numerical study on the force transmission around the openings. Araki et al. (2018) analyzed the strength of CLT walls with openings and proposed a method that accounts for the dimensions of the opening. Casagrande et al. (2021b) investigated the distribution of internal forces in single-story symmetric CLT shearwalls with openings. Casagrande et al. (2021a) analytically and numerically investigated behavior CLT walls with openings. Khajehpour et al. (2022) extended this study to numerically investigate the effect of lintels and parapets on the mechanical performance of small CLT walls. Popovski and Gavric (2016) conducted a static load test on a 2-story large CLT panel building with opening. From the test results, brittle local failure occurred in the corners of the wall panel with a large opening.

Aljuhmani et al. (2022) carried out in-plane shear strength and stiffness experimental tests on 24 CLT panels with different sizes and orientations of openings. The loads were applied at 45-degree angles for shear-only principal loads. The experimental tests showed the location of the initiation and propagation of the cracks. With increasing size of the openings, as expected, the stiffness and shear strength capacity of the CLT panels were reduced. To investigate the elastic design principle, Yasumura et al. (2016) conducted a full-scale test and finite element model (FEM) analysis on a two-story CLT structure. Two structures, with narrow (three $\mathrm{1,000}\times \mathrm{2,700}\mathrm{mm}$ CLT wall panels connected by lintels) and wide (6,000-mm-long and 2,700-mm-high panels with two openings in the first and second stories) CLT panels were considered. This FEM was useful in predicting the mechanical properties of the CLT building, but the elastic calculation overestimated the stress.

In previous studies, the authors of this paper tested CLT shear walls with and without openings to evaluate the Japanese design and construction practices. With wide CLT panels incorporating openings, the main problem that was identified is crack initiation and failure at the opening’s corners (Fig. 1). For a CLT of wide panels with openings, the objectives of this paper are to: investigate the initiation and propagation of cracks; quantify shear stiffness and bearing capacity; determine the dominant failure mechanism of the panel zone (i.e., bending or shear); and develop simplified analytical models to understand the capacity of the CLT panels under these loads. A series of tests are carried out by varying the number of layers (3 or 5), shape (L-shape or T-shape, Fig. 2), and web thickness ${\mathit{d}}_{\mathit{c}}$ (240, 360, or 480 mm). Additional three-point bending and shear tests were carried out to obtain mechanical properties of the CLT panels. The next two sections present the specimen design and experimental setup. This is followed by a results and discussion section of the bending and shear tests. This is followed by results on the details of each member, the strain around the corner, and stiffness. Finally, conclusions and future directions are given.

In this study, 3- and 5-layer CLT panels with ${\mathrm{M}}_{\mathrm{x}}60\mathrm{B}$ properties (JAS), composed of M60B and M30B Japanese Cedar laminae (Fig. 3), were selected. The laminae were visually selected to minimize the effect of knots and finger joints. An aqueous polymer isocyanate-based adhesive was used for the adhesion of the laminated surface and finger joints. Edge glue was not used in this study. The average, lower, and upper bending Young’s modulus values of the laminae are summarized in Table 1. For the 3- and 5-layer specimens, average elastic modulus, obtained through experimental testing, were 7.02 and $6.68\mathrm{kN}/{\mathrm{mm}}^2$, respectively.

For all specimens, depth of beam (${\mathit{d}}_{\mathit{b}}$) and width of column (${\mathit{d}}_{\mathit{c}}$) have been selected by equating effective section moduli of the beam and column [Eqs. (1) and (2)]. For L-shape elementswhere t is the thickness of the single layer and n is the number of layers within the CLT panel. For T-shape elements having two beams

From the preliminary test, bending failure of the L-shaped column was observed. As the intention of this study is to investigate initiation of cracks at the corners and shear failure in the panel zone, the sides of the 3-layer columns were reinforced with plywood [28 mm thickness, 200 mm width, and 900 mm height, Fig. 4(a)]. The 5-layer specimens were reinforced both on the side and back layers of the column [Fig. 4(b)].

The shear stress In the panel zone ($\tau$), in terms of bending stress (${\mathit{\sigma }}_{\mathit{b}}$) and section modulus (${\mathit{Z}}_{\mathit{e}}$), is defined as

Considering total cross section (Nihon System Sekkei Architects & Engineers 2013), the shear strength ranges of the 3- and 5-layer specimens are $2.83–2.97\mathrm{N}/{\mathrm{mm}}^2$ and $3.16–3.38\mathrm{N}/{\mathrm{mm}}^2$, respectively. If the shear strength of the cross section and panel zone is considered to be equal, bending failure occurs prior to shear failure. The mean bending and shear strength values were determined to be $37.6\mathrm{N}/{\mathrm{mm}}^2$ and $6.26\mathrm{N}/{\mathrm{mm}}^2$, respectively. A rolling shear strength of $1.64\mathrm{N}/{\mathrm{mm}}^2$ is assuming, based on work by Okabe et al. (2014). The rolling shear has two planes, but considering the stress concentration in torsion, it is reduced by half.

In this study, from CLT walls with cut-out openings, L- and T-shaped specimens were extracted and their strength and failure properties were analyzed. The findings are summarized as follows:

The experimental results describe the force transfer around the cut-out opening. Furthermore, a quantitative evaluation of the strength of CLT shear walls with a cut-out opening will be evaluated in the future through analytical studies. Moreover, strength and stiffness reduction factors that can be used for design will be proposed. Analysis of the glued surface of the panel zone using the Cohesive Zone Model will be conducted to investigate the stress conditions in the panel zone.

Some or all data, models, or code generated or used during the study are available from the first author by request. Experimental data is available from the first author by request.

This study was based on the building standards development promotion project promoted by the ministry of land, infrastructure, transport and tourism “S7: Study on Design Method of Wooden Structures Using CLT (2013)” and the housing market development promotion project “Sophistication of Wooden Building Standards Using CLT (2014).” We would also like to thank all people and institutes involved, including Mami Wada who was a former student of RISH and the Hideyuki Nasu’s Laboratory of Nippon Institute of Technology, which cooperated in the experiment.