Hybrid Cross-laminated Timber Research Articles

Prediction of withdrawal resistance for a screw in hybrid cross-laminated timber

The aim of this study was to predict the withdrawal resistance of a screw in hybrid cross-laminated timber (CLT) composed of two types of lamina layers. A theoretical model to predict the withdrawal resistance was developed from the shear mechanism between a screw and the layers in hybrid CLT. The parameters for the developed model were the withdrawal stiffness and strength that occurs when a screw is withdrawn, and the penetration depth of a screw in layers of a wood material. The prediction model was validated with an experimental test. Screws with two different diameters and lengths (Ø6.5 × 65 mm and Ø8.0 × 100 mm) were inserted in a panel composed of solid wood and plywood layers, and the withdrawal resistances of the screws were evaluated. At least 30 specimens for each group were tested to derive the lower 5th percentile values. As a result, the developed model predictions were 86–88% of the lower 5th percentile values of hybrid CLT from the properties of the lamina layer. This shows that the withdrawal resistance of hybrid CLT can be designed from the properties of its layer.

ASSESSMENT OF SHEAR ANALOGY AND TIMOSHENKO METHOD FOR ANALYZING HYBRID CLT UNDER OUT-OF-PLANE LOADING

Timber is a natural material which offers superior mechanical properties in parallel to fiber direction when compared against those in perpendicular to the fibers. Cross-laminated timber (CLT) is made up of layers of structurally graded timber, orthogonally oriented in layers whereby it can sustain loading in both directions. CLT is often used as floor panels, and hence, its performance under out-of-plane loading is of significant interest. Low rolling shear modulus resulting in higher shear flexibility of the cross-layers tend to decrease the effective bending stiffness of CLT sections. Developing hybrid CLT using timbers with higher rolling shear modulus as cross-layers in CLT is considered a viable option to improve its performance under out-of-plane loading. The present study investigates the performance of shear analogy and Timoshenko methods in predicting the deflection of hybrid CLT panels while considering different span-to-depth ratios and various combinations of rolling shear modulus. Numerical models were developed to conduct a parametric study and obtained deflection results were compared against those calculated from the shear analogy method and Timoshenko method. It was observed that for CLT with a small span-to-depth ratio and cross-layers made from material with higher rolling shear modulus, the deflection calculated from the analytical methods deviates from the values obtained from the numerical model.

Evaluating Timoshenko Method for Analyzing CLT under Out-of-Plane Loading

Cross-laminated timber (CLT) is an engineered wood product made up of layers of structurally graded timber, where subsequent layers are oriented orthogonally to each other. In CLT, the layers oriented in transverse direction, generally termed as cross-layer, are subjected to shear in radial–tangential plane, which is commonly known as rolling shear. As the shear modulus of cross-layers is significantly lower than that in other planes, CLT exhibits higher shear deformation under out-of-plane loading in contrast to other engineered wood products such as laminated veneer lumber (LVL) and glue laminated timber (GLT). Several analytical methods such as Timoshenko, modified gamma and shear analogy methods were proposed to account for this excessive shear deformation in CLT. This paper focuses on the effectiveness of Timoshenko method in hybrid CLT, in which hardwood cross-layers are used due to their higher rolling shear modulus. A comprehensive numerical study was conducted and obtained results were carefully analyzed for a range of hybrid combinations. It was observed that Timoshenko method could not accurately predict the shear response of CLTs with hardwood cross layers. Comprehensive parametric analysis was conducted to generate reliable numerical results, which were subsequently used to propose modified design equations for hybrid CLTs.

Shear properties of hybrid CLT fabricated with lumber and OSB

Recently, to enlarge the source of raw materials and improve the mechanical properties of cross-laminated timber (CLT), some local wood species, hardwood and Engineering wood products were used to fabricate CLT. This project studied mainly the feasibility of using construction oriented strand board (COSB) to fabricate hybrid CLT (HCLT). The shear properties of the HCLT were reported in this paper. The mechanical properties of COSB in major and minor strength directions were firstly obtained by various span bending tests and compression tests. The shear resistance and stress in major and minor strength direction of five groups of 3- and 5-layer HCLT, having different lay-up arrangements, were evaluated by experiments, shear analogy theory as well as FE methods. Results showed COSB had higher mechanical properties in minor strength direction compared with SPF dimension lumber. Different HCLT groups had different failure modes observed for major and minor direction. Except to the rolling shear failure of SPF transverse layer and tension failure of gaps between the lumber of the bottom layer, the longitudinal and transverse shear delamination failures and tension failure of COSB were observed during four-point short-span bending tests of HCLT. All the HCLT specimens had higher shear resistance than that of regular CLT with the same number of layer. The difference of the shear resistance between the two strength directions of HCLT decreased as the number of COSB layer increased. In addition, the shear resistance and stress of HCLT can be predicted according the shear analogy theory, and the developed FE models can predict the shear stress of HCLT well. The results presented in this paper can provide fundamental basis for supporting the potential engineering application of CLT panel products fabricated with COSB.

Thermal and Energy-Efficiency Assessment of Hybrid CLT–glass Façade Elements

Façade elements are a building component that satisfies multiple performance parameters. Among other things, “advanced façades” take advantage of hybrid solutions, such as assembling laminated materials. In addition to the enhanced mechanical properties that are typical of optimally composed hybrid structural components, these systems are energy-efficient, durable, and offer lighting comfort and optimal thermal performance, an example of which is the structural solution developed in collaboration with the University of Zagreb and the University of Ljubljana within the Croatian Science Foundation VETROLIGNUM project. The design concept involves the mechanical interaction of timber and glass load-bearing members without sealing or bonding the glass-to-timber surfaces. Following earlier research efforts devoted to the structural analysis and optimization of thus-assembled hybrid Cross-Laminated Timber (CLT)-glass façade elements, in this paper, special focus is given to a thermal and energy performance investigation under ordinary operational conditions. A simplified numerical model representative of a full-size building is first presented by taking advantage of continuous ambient records from a Live-Lab mock-up facility in Zagreb. Afterwards, a more detailed Finite Element (FE) numerical analysis is carried out at the component level to further explore the potential of CLT–glass façade elements. The collected numerical results show that CLT–glass composite panels can offer stable and promising thermal performance for façades similar to national and European standard requirements.

Study on the Mechanical Properties of Tropical Hybrid Cross Laminated Timber Using Bamboo Laminated Board as Core Layer

This study was performed to analyze the mechanical properties of tropical hybrid cross-laminated timber (CLT) with bamboo laminated board as the core layer in order to evaluate the possibility of its use as a CLT material. Bamboo board was used as the core layer and the tropical species Acacia mangium willd., from Indonesia, was used as the lamination in the outer layer. The modulus of elasticity (MOE), modulus of rupture (MOR), and shear strength of the hybrid CLT were measured according to APA PRG 320-2018 Standard for Performance-Rated Cross-Laminated Timber. The results show that the bending MOE of the hybrid CLT was found to be 2.76 times higher than SPF (Spruce Pine Fir) CLT. The reason why the high MOE value was shown in bamboo board and hybrid CLT applied bamboo board is because of high elasticity of bamboo fiber. However, the shear strength of the hybrid CLT was 0.8 times lower than shear strength of SPF CLT.

Influence of Aspect Ratio on Rolling Shear Properties of Fast-Grown Small Diameter Eucalyptus Lumber

Eucalyptus is a major fast-grown species in South China, which has the potential for producing structural wood products such as cross-laminated timber (CLT). Aspect ratio (board width vs. board thickness) of eucalyptus lumbers is small due to the small diameter of fast-grown eucalyptus wood. To evaluate its rolling shear modulus and strength for potential CLT applications, three-layer hybrid CLT shear block specimens with different aspect ratios (2,4,6), were tested by planar shear test method. Digital image correlation (DIC) was employed to measure the rolling shear strain distribution and development during the planar shear tests. The mean values of rolling shear modulus and strength of eucalyptus lamination were 260.3% and 88.2% higher than those of SPF(Spruce-pine-fir) lamination with the same aspect ratio of 4, respectively. The rolling shear properties of eucalyptus laminations increased as the aspect ratio increased. Aspect ratio had a significant influence on rolling shear modulus compared to rolling shear strength. The high shear strain regions were primarily found around the gaps between segments of cross layer. The quantity of high shear strain regions increased as the aspect ratio of lamination decreased. Other high shear strain regions also occurred around the pith and along the glue line. The sudden failure of specimen occurred in the high strain region. In conclusion, the rolling shear strength and modulus of fast-grown eucalyptus laminations exceed the respective characteristic values for softwoods in the current standard by roughly factors of 3 and 8, indicating great potential for fast-grown eucalyptus wood cross-layers in CLT.

Moment and shear capacity of Ply-lam composed with plywood and structural timber under out-of-plane bending

The aim of this study was to investigate the moment and shear capacity of Ply-lam which is composed with plywood and structural timber. The moment and shear capacity of Ply-lam were predicted by using prediction methodologies for typical cross-laminated timber (CLT). The predicted values by the methodologies were verified by comparison with measured capacities. Fifty-eight specimens of Ply-lam (five layers) were manufactured and half of the specimens were tested for bending (span-to-depth ratio (R_{text{sd}} ) was 25:1). The other specimens were tested for rolling shear (R_{text{sd}} was 12:1) in accordance with EN 16351. The results show that typical prediction methods for structural properties of CLT can also be used to predict those of Ply-lam, since the measured values were higher than the predicted values. In rolling shear test, 52% specimens failed by bending at the bottom layer. This indicates that the rolling shear test method in EN 16351 was not suitable for Ply-lam because the typical CLT would be reinforced by replacing the cross layer with plywood. Thus, the span-to-depth ratio needs to be reduced for hybrid CLT like Ply-lam.

Environmental benefits of using hybrid CLT structure in midrise non-residential construction: An LCA based comparative case study in the U.S. Pacific Northwest

In this study, the cradle-to-gate environmental impact of a hybrid, mid-rise, cross-laminated timber (CLT) commercial building is evaluated and compared to that of a reinforced concrete building with similar functional characteristics. This study evaluates the embodied emissions and energy associated with building materials, manufacturing, and construction. Two alternative designs are considered for fire protection in the hybrid CLT building: 1) a ‘fireproofing design’, where gypsum wallboard is applied to the structural wood; and 2) a ‘charring design’, where two extra layers of CLT are added to the panel. The life cycle environmental impacts are assessed using TRACI 2.1 and the total primary energy is evaluated using the Cumulative Energy Demand impact method.Results show that an average of 26.5% reduction in the global warming potential is achieved in the hybrid CLT building compared to the concrete building, excluding biogenic carbon emissions. Except ozone depletion, where the difference in impact between scenarios is <1%, replacing fireproofing with charring is beneficial for all impact categories. The embodied energy assessment of the building types reveals that, on average, the total primary energy in the hybrid CLT buildings and concrete building are similar. However, the non-renewable energy (fossil-based) use in the hybrid CLT building is 8% lower compared to that of the concrete building. As compared to the concrete building, additional 1,556 tCO2e and 2,567 tCO2e are stored in the wood components of the building (long-term storage of biogenic carbon) in the scenario with fireproofing and with charring, respectively.

Hysteretic Response of Metal Connections on Hybrid Cross-Laminated Timber Panels

AbstractThe development of cross-laminated timber (CLT) has created new opportunities for the use of lumber from low-grade and low-density species in structural applications. In order to ensure the...

Lateral and Withdrawal Capacity of Fasteners on Hybrid Cross-Laminated Timber Panels

AbstractCross-laminated timber (CLT) is an orthogonally laminated timber panel product made using dimensional lumber. It is gaining traction in the construction industry, as is evident by the incre...

Experimental and numerical performance of shear connections in CLT–concrete composite floor

The hybrid cross-laminated timber (CLT) and concrete system has drawn attention as an attractive application for residential and commercial buildings. However, it is crucial to fully understand the mechanical characteristics of the CLT–concrete composite connections before such a hybrid structure becomes widely applicable. This paper assesses the structural performance (force–slip response, slip modulus, and failure mode) of a CLT–concrete composite by conducting fifteen pushout test specimens, which were designed by using the γ-method (Eurocode 5). In these composite specimens, the CLT floor panels were connected to concrete slabs using common mechanical connectors, and the effect of different connection types, connection angles, the penetration depth of the connector, and concrete topping thickness were investigated. The results show that all of the specimens had good trend in the failure mechanisms with a little crush occurred in the CLT and concrete. The finding revealed that the inclination connectors were appeared as an attractive connector layout for CLT–concrete composite components. Non-linear 3D finite element analysis was also developed to simulate the load-slip behavior of the CLT-concrete specimens under shear load. In general, a reliable non-linear finite element model using ANSYS is a great tool for capturing shear capacity and the overall behavior of the hybrid push-out specimens.

Full-scale static and dynamic experiments of hybrid CLT–concrete composite floor

The Cross-Laminated Timber (CLT) concrete composite floor can successfully address the issues of conventional wood product and become an attractive alternative material for conventional reinforced concrete floors in mid to high-rise building. The purpose of this research was to fully evaluate the capacity of this novel hybrid composite material under both dynamic and static conditions, before such this hybrid system becomes widely employed. This was achieved by performing a full-scale testing of five floor specimens. One specimen was designed according to Eurocode 5 to satisfy the design load for common office buildings, and four specimens were prepared with a concrete topping with different connector types, connector angles, and connector spacing. The test results illustrated that the CLT–concrete floor specimens had excellent structural performance in terms of strength capacity, 3 –5 times higher than the conventional CLT floor. The vibration performances of this composite floors were also enhanced, resulting in a remarkably effective use of this innovative materials.

Thermal conductivity values for laminated strand lumber and spruce for use in hybrid cross-laminated timber panels

This study examined the thermal conductivity as a function of specific gravity and moisture content for laminated strand lumber (LSL) and red spruce. As part of a larger study of heat and mass transfer in cross-laminated timber panels using laminate comprised of both LSL and spruce, the authors measured the thermal conductivity at four moisture content levels. The results showed that the LSL had a higher thermal conductivity value across the entire moisture content range tested. The average difference was just over 8% and the range for both LSL and spruce was from 0.081 W /m-K to 0.126 W / m-K. Comparisons with published solid wood thermal conductivity values across the range were good. There were no reported values of LSL thermal conductivity at various moisture content levels.

Planar shear and bending properties of hybrid CLT fabricated with lumber and LVL

Planar (rolling) shear properties in cross laminated timber (CLT) is an important factor that should be considered for CLT structural components with short span or openings. The planar shear properties of SPF (Spruce-pine-fir) dimension lumber and laminated veneer lumber (LVL) were measured using a modified approach in this study. The failure modes in planar shear tests were investigated by visual inspection and optical microscope. Hybrid CLT (HCLT) was fabricated using lumber and/or LVL, and its bending properties (in major strength direction) were evaluated. It was found that SPF had higher planar shear strength and modulus than LVL. The failure mode of HCLT with LVL as cross layer in planar shear test was shear failure along the glue line direction, however, the typical failure modes of HCLT with SPF as cross layer in planar shear test included rolling shear failure along the growth ring direction, shear failure along the wood ray direction and shear failure along glue line direction. The bending properties of the generic CLT had been improved by using LVL as parallel layers and been reduced by using LVL as cross layer. The planar shear failure of cross layer in the zone between loading point and support point was the typical failure mode of CLT and HCLT in bending test.

Structural performance of hybrid SPFs-LSL cross-laminated timber panels

The bending and shear performance of hybrid cross-laminated timber (CLT) panels made from Spruce-Pine-Fir (South) (SPFs) and laminated strand lumber (LSL) are examined. Four configurations of three-layer CLT were fabricated: all-SPFs control specimens, all-LSL specimens, hybrid specimens with SPFs faces and an LSL core, and hybrid specimens with LSL faces and an SPFs core. Bending tests were conducted to assess flexural strength and stiffness. Additionally, three-point bending tests were performed to assess shear performance. The incorporation of LSL in the core of CLT panels increased mean panel bending stress at failure by 23% through mitigation of rolling shear failure.

Hybrid cross-laminated timber plates with beech wood cross-layers

A hybrid, three-layered, softwood-hardwood cross-laminated timber build-up with outer layers of European spruce (Picea abies) and a center cross-layer of European beech (Fagus sylvatica) has been investigated with regard to out-of-plane bending. The determination of the rolling shear properties of the beech cross-layer performed by different test and measurement methods comprising bending and compression shear tests was of primary interest. The shear capacity of the composite is significantly influenced by the spruce longitudinal shear strength at the beech-spruce interface. The characteristic values of rolling shear modulus and strength of the beech cross-layer from the bending tests were Gr,mean=350N/mm2 and fv,r,05=2.6N/mm2, respectively. Direct strain gauge measurements and compression shear tests resulted in 10–20% higher values. The high rolling shear properties render the shear lag implications of the softwood CLTs to a negligible quantity. The hybrid build-up can be designed as a rigid composite with small error versus a more exact analysis. The novel investigations reveal the great potential of mixed softwood-hardwood CLT build-ups for structural elements in the building sector.

Mechanical properties of laminated strand lumber and hybrid cross-laminated timber

Hybrid cross laminated timber (HCLT) was fabricated using lumber and/or laminated strand lumber (LSL), the mechanical performances of which were evaluated. To reach this goal, the mechanical properties of LSL and the bending properties of CLT and HCLT were measured in this study. The properties of LSL measured included the tension strength (only in the major direction), shear strength, shear modulus, and modulus of elasticity (MOE) and modulus of rupture (MOR). The failure mode of each kind of specimens was visually examined and recorded. Four types of CLT panels, one generic CLT (used as control) and three types HCLT were fabricated. The properties measured included the bending properties (in the major direction) and planar shear properties (in both major and minor directions). It was found that the HCLT had better bending and planar shear properties than that of generic CLT. The MOE and MOR of HCLT having LSL as the outer layers were 19% and 36% higher than those of generic one, respectively. The MOE and MOR of HCLT having LSL as core layer (replacing the cross lumber layer) were 13% and 24% higher than that of generic CLT, respectively. The failure modes of four types of CLT observed included the planar shear failure of cross lumber layer, tension failure of bottom LSL, and tension failure of bottom lumber, especially tension failure of lumber originated at a knot(s).

합판을 코어로 사용한 교호 집성재의 물리·기계적 성질

본 연구는 기존에 제조되었던 교호집성재의 휨강도의 단점을 보완하고 새로운 특성을 가진 교호 집성재 즉, 합판을 코어로 이용한 집성재가 가진 기계적강도의 효과를 알아보기 위해 수행되었다. 집성재와 합판의 구성 방법, 적층 방향에 따라 그 값을 비교하였으며, 그에 따른 휨강도와 탄성계수를 측정하여 분석한 결과, 중심부를 집성판과 합판을 혼합하여 구성한 합판 코어 집성재의 휨강도(MOR) 값이 59.6% 강도가 향상되어 교호집성재구조 대조군보다는 우수하고, 집성재구조 대조군과는 유사한 강도를 나타냈다. 휨탄성계수(MOE)는 합판 코어 집성재의 구조 및 적층 방향성에 상관없이 모두 집성재구조 대조군과 유사한 MOE 값을 나타냈다. 치수 안정성 실험에서는 합판을 코어에 사용한 합판코어 집성재가 합판 사용으로 인하여, 수축 팽창률 모두 집성재와 교호집성재구조 대조군에 비해서 더 안정적인 것으로 나타났다. This study was performed to study physical and mechanical properties of hybrid cross laminated timber (HCLT) with plywood as core layer in order to improve its mechanical properties for wooden housing. MOE, MOR, and dimensional stability of the HCLT were determined, depending on plywood composition and lamination direction. MOR value of the HCLT was improved as much as that of the glued laminated timber, which was 59.6% stronger than that of the cross laminated timber (CLT) control group. All MOE values of the HCLT were similar to glued laminated timber structure control group regardless of plywood composition and lamination directions. The dimensional stability of the HCLT was better than those of the glued laminated timber and CLT control group, owing to the use of plywood in the core.