Experimental study on LBL beams

Six specimens were made and tested to study the mechanical properties of LBL beams. The mean ultimate loading value is 68.39 MPa with a standard deviation of 6.37 MPa, giving a characteristic strength (expected to be exceeded by 95% of specimens) of 57.91 MPa, and the mean ultimate deflection is 53.3 mm with a standard deviation of 5.5 mm, giving the characteristic elastic modulus of 44.3 mm. The mean ultimate bending moment is 20.18 kN.m with a standard deviation of 1.88 kN.m, giving the characteristic elastic modulus of 17.08 kN.m. The mean elastic modulus is 9688 MPa with a standard deviation of 1765 MPa, giving the characteristic elastic modulus of 6785 MPa, and the mean modulus of rupture is 93.3 MPa with a standard deviation of 8.6 MPa, giving the characteristic elastic modulus of 79.2 MPa. The strain across the cross-section for all LBL beams is basically linear throughout the loading process, following standard beam theory.

Some researchers have investigated the basic mechanical properties of LBL. Tensile, compressive and bending performance of layered laminate bamboo composite (LLBC) have been studied by Verma and Charier [12], and the specimens have a cross-section of 16mm x 10mm. Yeh and Lin [13] investigated how the growth height influence the bending strength of LBL, and both un-jointed and jointed specimens with the length of 1000mm and the cross-section of 30mm x 30mm have been tested. Considering the glue spread rate and moisture content influencing factors, Lee et al. [14] studied the bending properties of 24 laboratorymanufactured LBL specimens, and found the elastic modulii ranging from 7411 MPa to 9204 MPa and rupture strengths between 67.7 MPa and 107.2 MPa.
As for the structural elements, Li et al. [15][16][17][18][19][20] examined the mechanical performance for the columns in detail, and proposed a tri-linear model with an elastic portion, and elasto-plastic portion and a purely plastic portion. A fine stress-strain relationship model for LBL under axial compression was also put forward by Li et al. [16] based on the short compression tests. Considering many influencing factors, both the LBL columns under axial compression and eccentric compression have been investigated by Li et al. [17][18][19][20], and the ultimate bearing capacity calculation equations were proposed. The axial compression performance of LBL column piers along three directions were studied and compared by Su et al. [21], and the relationship models for load-axial displacement along three compression directions could be used the same tri-linear model.
As for the structural beam members, Sinha et al. [22] evaluated the potential application for the laminated bamboo lumber (LBL) and bamboo glulam beams (BGBs)'s in structures. Li et al. [23][24][25][26] also investigated the mechanical performance for LBL beams considering the influencing factors of shear span ratio and height to width ratio, and the ultimate load calculation equations were proposed. Zhang et al. [23][24][25][26] has studied how AFRP efect on parallel bamboo strand lumber beams.
As mentioned above, even though some studies about LBL beams have been done by some researchers, the work is still limit and more research on the mechanical properties of LBL beams need to be done. Thus, this study examines in detail at the behaviour of specimens constructed from laminated bamboo lumber.

Specimens
The lower growth portion of the Moso bamboo (Phyllostachys pubescens, from Fujiang province) tubes were chosen with the age of 3-5 years to produce the specimens. After removing the outer skin (epidermal) and inner cavity layer (pith peripheral) by a planer, all the culm strips were then dried and charred. With the final thicknesses of 7 mm and the widths of 21 mm, the strips were produced and made into laminated bamboo lumbers. Six beam specimens were made with the size of 50 mm × 160 mm × 1960 mm and the cross-section for the beam specimen could be seen from Fig. 1.

Test methods
The beam test arrangement could be illustrated in Fig. 2. Five Laser Displacement Sensors were arranged to measure the displacements of the specimen. The beams were strain gauged longitudinally at the middle cross section, with five strain gauges pasted on one side face at even spacing through the depth, and one strain gauge pasted on each of the bottom face and the top face, as shown in Fig. 2. A microcomputer-controlled electrohydraulic servo universal testing machine (Fig. 3) with a capacity of 300 kN was chosen for the beam tests. Fourpoint loading method was used for the tests and the clear span for the beam is 1770 mm. All beam specimens were divided into three even parts by four loading points.
Load b is the width which is 50 mm; and h is the height of the beam cross-section which is 160 mm.

Load-displacement response
The load-displacement curves for beam specimens could be seen from Fig. 5. The load-displacement response is consistency in the original elastic stage. When the loading value was bigger than 28 kN, five curves kept good consistency except one curve.
Micro-cracks within the material were audible and are also observed in small drops along the loaddisplacement curves for all test specimens, no cracks were visible before the ultimate state. The overall behaviour for all the beams is substantially the same, with an initial elastic response followed by non-linear softening, and a brittle failure.

Strain profiles
The strain profiles through the loading for the mid-span cross-section for all test beams could be seen from Fig. 6. The strain across the cross-section for all LBL beams is basically linear throughout the loading process, following standard beam theory.

Conclusions
Six specimens were made and tested to study the mechanical properties of LBL beams. According to analysis of the test data, the following conclusions can be drawn.  This work was financially supported by the National University students practical and innovation training project