Axial compression of three types of bamboo scrimber columns with different cross-sections

Abstract

Bamboo scrimber is a versatile material made by rolling and defibering bamboo into loose reticulate bundles (unbroken horizontally, loose longitudinally, and interlaced) that are subjected to drying, gluing, assembling, and hot pressing. In this study, to better understand the application value of bamboo scrimber in construction engineering, the axial compression properties of bamboo scrimber columns with solid, hollow, and I-shaped cross-sections were investigated. For each column type, three lengths of 1 m, 1.5 m, and 2 m (three specimens of each length) were selected and subjected to axial compression testing. The results demonstrated that the primary failure mode of solid bamboo scrimber columns was instability failure, whereas that of hollow and I-shaped columns was mainly debonding failure. Experimental data were further analyzed to better understand and model the failure mechanisms of bamboo scrimber columns. This study led to the establishment of a design formula for bamboo scrimber solid columns, the calculations of which matched well with the experimental results.

Full Article

Axial Compression of Three Types of Bamboo Scrimber Columns with Different Cross-sections

Yan Liu,^a Shukai Tang,^a Yanfei Guo,^a Zhongping Xiao,^b,* and Xiangyu Su ^a

Bamboo scrimber is a versatile material made by rolling and defibering bamboo into loose reticulate bundles (unbroken horizontally, loose longitudinally, and interlaced) that are subjected to drying, gluing, assembling, and hot pressing. In this study, to better understand the application value of bamboo scrimber in construction engineering, the axial compression properties of bamboo scrimber columns with solid, hollow, and I-shaped cross-sections were investigated. For each column type, three lengths of 1 m, 1.5 m, and 2 m (three specimens of each length) were selected and subjected to axial compression testing. The results demonstrated that the primary failure mode of solid bamboo scrimber columns was instability failure, whereas that of hollow and I-shaped columns was mainly debonding failure. Experimental data were further analyzed to better understand and model the failure mechanisms of bamboo scrimber columns. This study led to the establishment of a design formula for bamboo scrimber solid columns, the calculations of which matched well with the experimental results.

Keywords: Bamboo scrimber column; Axial compression; Cross-section structure; Buckling load

Contact information: a: College of Civil Science and Engineering, Yangzhou University, Yangzhou, Jiangsu 225127, China; b: School of Architectural Engineering Yangzhou Polytechnic Institute, Yangzhou, Jiangsu 225127, China; *Corresponding author: fafuxzp@163.com

GRAPHICAL ABSTRACT

INTRODUCTION

Bamboo is abundant in China, where it ranks second after trees as a forest resource. The quantity and quality of bamboo resources in China rank first in the world. Bamboo scrimber is a type of square-edged timber or board with high specification, large size, and natural bamboo texture. It is made by taking bamboo fiber bundles dried at low temperatures (until their moisture content is below 12%) and subjecting them to the stepwise processes of parallel lay-up, gluing, and hot pressing (or cold pressing), among others. Hence, these loose reticular bamboo fiber bundles are long and cross-linked, yet they retain the original arrangement of fibers. This confers several advantages to bamboo scrimber, which include a high bamboo utilization rate, excellent physical and mechanical properties, a beautiful appearance, and a low cost with good economic benefits. Currently, domestic bamboo scrimber is used mainly in indoor and outdoor flooring, enclosure structures, and veneers and furniture making, but it is rarely used in building structures in China. How to apply bamboo to architectural structure is a research topic that has just begun. Based on a search of the literature, it was found that there has been a lack of research on the performance of bamboo scrimber columns in building structures.

Zhang et al. (2015) conducted axial compression tests on four bamboo scrimber columns with a square cross-section and found ultimate load-carrying capacities that were close to the theoretical calculated values. Su et al. (2015) concluded that the primary failure mode of the axial compression of bamboo scrimber columns is ductile failure, and the failure forms consisted of the strength failure of end-crushing and instability failure of the tensile side of the middle span accompanied by fiber breaking; this led to a model formula for calculating the ultimate capacity of bamboo scrimber columns under axial compression. In other work, axial compression test results for Glulam columns with different slenderness ratios were compared with domestic and foreign codes (Xiao et al. 2015). In addition, a study of 50 axially loaded wood scrimber columns with different slenderness ratios provided the theoretical and experimental basis for the engineering application of wood scrimber (Li et al. 2015). Work by Li et al. (2015) made important contributions to the understanding of the mechanical properties of bamboo scrimber under axial loading. For example, their research examined the axial compression properties of short columns with different slenderness ratios made from different parts of bamboo (Li et al. 2013, 2015a,b). Through using lateral-pressure-glued laminated bamboo columns to perform eccentric compression tests, Li et al. (2015) found that the joint and slub parts were the weakest part of the tensile zone, and the mean strain value of the mid-span section was consistent with the value calculated from the plane-section. Based on this work, they derived a formula to calculate the eccentric cross-section bearing capacity for use in a stress-strain model under compression of glued laminated bamboo columns (Li et al. 2016a,b). A method of calculating the eccentricity bearing capacity of bamboo scrimber columns was recently proposed by Wei et al. (2016). Previously, Luna et al. (2013) drew a critical slenderness ratio diagram by studying the mechanical properties of bamboo scrimber columns with solid and hollow sections under different slenderness ratios.

In summary, previous domestic and foreign research typically used only solid sections when testing the axial compression of bamboo scrimber columns. Because the volume-weight of bamboo scrimber is large, hollow sections, I-shaped sections, and different column heights were also investigated in this study. Therefore, the axial compressive performance of each section under varied slenderness ratios was compared to expand the application potential of bamboo scrimber columns for building structures. The research work of this paper entails a certain degree of pioneering.

EXPERIMENTAL

Design of Specimen

A total of 27 bamboo scrimber column specimens were provided by the Jiangxi Chunhong Bamboo Technology Co., Ltd. (Jiangxi, China). These specimens were made from 5-year-old bamboo via cold pressing and splicing. A phenolic resin was used as the adhesive used for cold pressing, and the splice glue was a single component, moisture-cure polyurethane adhesive. Figure 1 shows the cross-sectional sizes. Three specimen heights were also considered in the tests: 1 m, 1.5 m, and 2 m, each of which was replicated with three specimens per specification (solid, hollow, and I-shape) for a total of 27 specimens. As described in Table 1, these experimental specimens were labeled as SXZ1-1~SXZ3-3, KXZ1-1~KXZ3-3, and GZZ1-1~GZZ3-3 according to their treatment combinations. The section size and test piece of bamboo scrimber columns are shown in Figs. 1 and 2, respectively, as described earlier by Su (2017).

Before axial compression testing, the physical and mechanical properties of the bamboo scrimber specimens were determined. The bamboo scrimber had an air-dried density of 1.204 g/cm³, an average moisture content of 6.6%, a compressive strength along the grain of 57.4 MPa, a standard compressive strength along the grain of 55.5 MPa, an elastic modulus along the grain of 1180 MPa, and a Poisson ratio of 0.384.

Table 1. Specimens of Bamboo Scrimber Columns for Axial Compression Testing

Fig. 1. Sectional dimensions of bamboo scrimber specimens: (a) Solid section; (b) Hollow section; (c) I-shaped section (This information appeared earlier in (Su 2017)

Fig. 2. The specimens used for axial compression of bamboo scrimber columns

Experimental Measurements and Loading

The experiment was conducted in the structural laboratory of the College of Civil Engineering of Yangzhou University in Yangzhou, Jiangsu, China. The loading system and test process followed GB/T 50329 (2013), GB 50005 (2017), and ASTM D198-15 (2015). A YAJ-5000 microcomputer (Changchun Kexin Test Instrument Co., Ltd., Changchun, China) controlled the electro-hydraulic servo test machine to perform axial loading on the study specimens. The experiment consisted of four main steps:

(1) Marking the four surfaces of a specimen clockwise as A, B, C, and D.

(2) Pasting two strain gauges to each surface of the specimens along the grain (parallel to column length) and cross-grain (perpendicular to column length).

(3) Adjusting the positions of two end-knife hinge supports and the specimens to ensure their geometric alignment and then affixing dial indicators midway onto the top of surface-A and the top, middle, and bottom parts on surface-B of the column.

(4) Using a force control method to complete the step loading applied to each specimen. The loading time was controlled within 15 min, and the loading rate was 2000 N/s. When the loading value of each stage was reached, the resistance strain gauge and percentile meter readings were recorded, and the experimental phenomena were observed (with photographs and records as required).

Figure 3, as published by Su (2017), shows the layout of the test device and measurement points. To ensure the safety of all tests, a safety net was attached around the column-testing machine, as shown in Fig. 4.

Fig. 3. The layout of the measuring points on the bamboo scrimber columns (This diagram appeared earlier in (Su 2017).

Fig. 4. The test device used for axial compression testing

RESULTS AND DISCUSSION

Failure Mode of the Bamboo Scrimber Columns

The experimental and failure phenomena of the specimens with solid, hollow, and I-shaped sections clearly differed, but the failure phenomena of specimens with the same cross-sections and different lengths were similar.

Specimens with solid sections did not appear to undergo strength failure, but they underwent overall instability failure. At the initial stage of loading, no obvious changes were observed. When the specimen was loaded to 0.7 P_u to 0.8 P_u, the dial data slowly increased, and a slight jump in the strain gauge reading occurred, which showed that deformation had increased despite the no-load increase and strength failure in the specimen. With continued loading, the specimen surface bulged slightly, and the finger joint cracked slightly. When loaded to 0.9 P_u, the specimens underwent slight lateral bending. When the loading approached the P_u of the specimens, the flexure deformation of the specimens became intensified, and buckling suddenly occurred. The typical failure phenomena of the solid column specimens are shown in Fig. 5a.

The failure phenomena of the hollow columns were mainly degumming and bursting apart. When loaded to 0.9 P_u, the specimen bent slightly. As the load continued to increase, the degree of deflection suddenly intensified. Shortly after, the specimen disintegrated from the column center into four pieces of bamboo scrimber plates, which was accompanied by a loud sound. Cracks spread rapidly along the adhesive bonding surface to the end of the column until the specimen disintegrated completely. The typical failure phenomena of the hollow column specimens are shown in Fig. 5b.

The failure phenomena of the I-shaped column were mainly degumming and cracking. When loaded in the range 0.7 P_u to 0.8 P_u, the dial data slowly increased, and the strain gauge reading jumped slightly. With further loading, the specimen’s exterior bulged slightly, and the finger joint cracked slightly. With even further loading, the flexural degree of the specimens slowly increased. When the loading was near the P_u of the specimens, the web and flange plates suddenly degummed, which made a dull “peng” sound. The flange plates were degummed and cracked in the direction perpendicular to the previous flexural direction. After flange plate degumming, the web underwent a large deflection suddenly, and then destabilization occurred. The typical failure phenomena of the I-shaped specimens are shown in Fig. 5c.