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Lu, Y., Xie, W., Wang, Z., & Gao, Z. (2018). "Shear stress and interlaminar shear strength tests of cross-laminated timber beams," BioRes. 13(3), 5343-5359.

Abstract

The interlaminar shear stresses of the three-layer, five-layer, and seven-layer cross laminated timber (CLT) and those of the oriented laminated beams were calculated according to Hooke’s law and the differential relationship between the beam bending moment and shear force. The interlaminar and maximum shear stresses of the CLT beam are related to the number of CLT layers and to the elastic modulus ratio EL/ET (or EL/ER) of the parallel and perpendicular layers. The interlaminar shear strength of the Hemlock CLT was positively correlated with the elastic modulus of its parallel layer. The results showed that the CLT short-span beams had three failure modes when subjected to a three-point bending test, namely perpendicular layer rolling shear failure, CLT interlaminar shear failure, and parallel layer bending failure. The shear stress of the oriented laminated beam followed a parabolic distribution along the height of the section, while the shear stress of the orthogonally laminated beams tended to be balanced, rather than parabolically distributed along the height of section. The short beam three-point bending method was able to effectively test the interlaminar shear strength of CLT due to its stable and readable load.


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Shear Stress and Interlaminar Shear Strength Tests of Cross-laminated Timber Beams

Yao Lu, Wenbo Xie, Zheng Wang,* and Zizhen Gao

The interlaminar shear stresses of the three-layer, five-layer, and seven-layer cross laminated timber (CLT) and those of the oriented laminated beams were calculated according to Hooke’s law and the differential relationship between the beam bending moment and shear force. The interlaminar and maximum shear stresses of the CLT beam are related to the number of CLT layers and to the elastic modulus ratio EL/ET (or EL/ER) of the parallel and perpendicular layers. The interlaminar shear strength of the Hemlock CLT was positively correlated with the elastic modulus of its parallel layer. The results showed that the CLT short-span beams had three failure modes when subjected to a three-point bending test, namely perpendicular layer rolling shear failure, CLT interlaminar shear failure, and parallel layer bending failure. The shear stress of the oriented laminated beam followed a parabolic distribution along the height of the section, while the shear stress of the orthogonally laminated beams tended to be balanced, rather than parabolically distributed along the height of section. The short beam three-point bending method was able to effectively test the interlaminar shear strength of CLT due to its stable and readable load.

Keywords: Cross-laminated timber; Interlaminar shear stress; Interlaminar shear strength; Test

Contact information: College of Materials Science and Engineering, Nanjing Forestry University, Nanjing, Jiangsu 210037, China;*Corresponding author: wangzheng63258@163.com

INTRODUCTION

Cross-laminated timber (CLT) is the basic unit of production for heavy wood structures used in mid- to high-rise buildings. The advantages of CLT as a building material include factory prefabrication, simple installation, light weight, high strength, good structural integrity, high thermal insulation performance, and durability (Que et al. 2017; Wang et al. 2017).

As a new solid wood composite building material, it is particularly important to control the design, processing technology, and processing parameters of CLT (Wang et al. 2011; Gagnonet al. 2012; Cao et al. 2016; Sikora et al. 2016). The interlaminar shear strength of CLT, which is widely used to evaluate the interlaminar mechanical properties of CLT, refers to the strength limit under shear stress between the laminates.

At present, the shear strength of CLT is commonly measured using the short beam bending method. In other words, to ensure shear failure rather than bending damage, CLT short beams are subjected to load measurement during the three-point bending test.

According to the ANSI/APA PRG 320-2012 standard (ANSI/APA PRG 320-2012), the span should be 5 to 6 times the thickness of the specimen when the interlaminar shear strength of the CLT is tested. The ASTM D198 standard (ASTM D-198-2015) shows that the short beam three-point bending method is suitable for testing the interlaminar shear strength of engineered wood products with irregular sections, including those with rectangular sectional wood materials, wooden joists, and circular columns.

Moreover, the interlaminar shear strength of the rectangular section wood specimen is 1.5 times the average shear stress of the section, namely 3P/4bh, where P is the interlaminar shear failure load of specimen, b is the width, and h is the thickness. It is uncertain whether the calculation formula of interlaminar shear strength based on the ASTM D198 standard is suitable for CLT. It is necessary to find a way to estimate the accuracy of the test. One of the purposes of this work is to solve these two problems.

In the current work, the formula describing the normal stress of the CLT beam along the height of the rectangular section was obtained first according to the orthogonal anisotropy of CLT. Then, the calculation formula for the shear stress of the CLT beam was derived according to the differential relationship between the bending moment and the shear on the beam section. After that, the interlaminar and maximum shear stresses of the three-layer, five-layer, and seven-layer CLT beams were calculated. The interlaminar shear strength of the CLT hemlock was measured using a short-span three-point bending method with a span-to-depth ratio of 6 (ANSI/APA PRG 320-2012, ASTM D-198-2015).

EXPERIMENTAL

Interlaminar Shear Strength Test of Three-layer CLT

According to the ANSI APA PRG320-2012 standard, the interlaminar shear strength of CLT was tested by means of short span three-point bending method.

The designed CLT boards had dimensions of 5,500 mm × 1,200 mm × 105 mm. Figure 1 shows the unit structure of the boards.

Fig. 1. CLT unit structure

The full-scale CLT boards (Fig. 1) were made on the CLT production line of the Ningbo Sino-Canada Low-Carbon Technology Research Institute Co., Ltd. according to the CLT manual (Wang et al. 2011) and conventional Canadian CLT production technology.

There are two types of full-scale CLT boards: those consisting of three parallel layers of Hemlock Grade 1 (E=13.8 GPa), and those consisting of three parallel layers of Hemlock Grade 2 (E=10.3 GPa). These layers were numbered 1, 2, 3, 4, 6, and 8. The perpendicular layers of the CLT boards of Grades 1 and 2 were comprised of the hemlock with an average elastic modulus of 7.4 GPa. From one full-size CLT board, four specimens (735 mm × 305 mm × 105 mm) were obtained to achieve three-point bending with the span of 630 mm, i.e., a span-to-depth ratio is 6.

The test instruments consisted of one universal mechanical testing machine with a maximum load of 10T (including one set of load displacement analysis software) produced by Jinan Tianchen Testing Machine Manufacturing Co., Ltd. and one self-made vacuum-pressurized circulation system, including a vacuum pressure tank (60 m3), vacuum pump, air compressor, and tubes.

The interlaminar shear strength of the three-layer CLT was tested using the three-point bending method. The loading point was located in the middle of span, while the loading direction was perpendicular to the specimen surface (Fig. 2).

During the test, to record the load-displacement curve the loading rate was set as 4 mm/min. The failure location and failure mode of the specimen were represented by the point at which the load-displacement curve deviated from the characteristic of load-displacement curve. During the test, with the load-displacement curve was automatically recorded. The failure position, failure type, and the corresponding characteristics of load-displacement curve were observed until the specimen was destroyed. The load values were recorded when the interlaminar shear failure occurred in the specimens. It was found that interlaminar shear failure load was the maximum load in the load-displacement curve, from which the interlaminar shear strength of CLT could be calculated.