Experimental study on the torsional mechanics of laminated structural glass beams

This thesis presents an experimental study on the torsional-mechanical behaviours of laminated structural glass beams. Glass structures are in increasing demand due to many favourable characteristics such as high compressive strength, flexibility, aesthetics, sustainability, and their positive effects on human mood and performance. However, monolithic glass is brittle in nature and fails instantaneously so laminated glass building elements are preferred, which have complex composite behaviours. An additional challenge is the lack of a finalised glass design code. This PhD study takes aim specifically at laminated glass beams and their torsional mechanics, which are crucial for supporting floor and roof plates, glass walls and other applications that enable fully transparent structures to be realised. A concise concept for quantifying the torsional stiffness of laminated glass beam elements is introduced – the Equivalent-Sectional Shear Modulus (ESSM), which is directly measured from the torque and sectional-rotation correlation in non-destructive torsion experiments. This method is advantageous as it allows for the measurement of overall rotation to torque response of laminated glass beams compositely rather than their individual components, resultantly decreasing the uncertainties of commonly adopted analytical approximations. A tailor-made, non-contact displacement measurement system based on the principles of binocular stereo-vision was developed, tested, verified, and employed to the torsion test procedure to increase the accuracy of photogrammetric measurements to be acquired. This incorporates the use of dense displacement sample targets on the glass beam which are measured and extracted using basic machine vision techniques, providing flexible, accurate, and non-intrusive measurements. Experimental torsion studies were performed on multiple samples of monolithic, two-layer, and three-layer polyvinyl butyral (PVB) and SentryGlas Plus (SGP) beams. The experimental setup, equipment, and procedures were continually improved and refined in a step-wise process throughout this work. The monolithic beams experimental ESSM results were validated against theoretical calculations from their elastic moduli relationship. Furthermore, the experimental ESSM results for two-layer and three-layer laminated beams were also compared with existing analytical solutions based on sandwich theory. Further experimental studies were also performed to evaluate the effects of ‘eccentric-torque’, where the effect of an eccentricated torsional load path on the overall torsional rigidity of the beam is studied, and on the effect of lengthened load-durations for SGP laminates. A variety of interesting and remarkable results were obtained regarding the composite torsional-mechanical behaviours of laminated structural glass beams and the optimisation of a first-of-its-kind glass beam torsion test approach. The results of this PhD thesis may help to support the further development of structural glass design codes and practices

Applicability of the Torsion Test and Photogrammetric Approach on Structural Timber Beams

The torsion test is recommended as a standard approach to determine the shear modulus of structural-size timber and glulam beams. However, there is a difficulty in measuring the rotational deformations of timber joists. A stereo camera system and photogrammetric approach is therefore proposed in this study. This study evaluates the applicability of the proposed photogrammetric approach to the torsion testing method conducted on glulamtimber beams. Tests were performed comparing rotational values of specific points determined by the photogrammetric approach with those measured by traditional devices.Remarkable findings were obtained which may help to develop an outline for future research programmes in the field of evaluating material properties of glulam. The results also showed that the optical system not only allowed assessing performance and reliability of traditional sensors, but also allowed monitoring the deformation of samples at various locations by providing more information which would be unobtainable using traditional techniques

Verification of the Photogrammetric Approach on the Torsion Test Method for Timber Beams

The photogrammetric approach was proposed in this study to evaluate torsional behaviour of glulam timber beams during the torsion test. The main objectives of the experimental work were to investigate the applicability of the photogrammetric approach on the torsion test and to monitor more details of angles of twist of glulam timber beams subjected to torsional loading. Experimental tests were conducted on glulam timber beams subjected to torque, and the photogrammetric approaches as well as the inclinometers were employed to measure, respectively, the twists on the long and short sides of the cross section of the glulam specimens. The accuracy of the photogrammetric approach was validated by comparing the average rotations of the targets with those measured by the conventional inclinometer sensors. After validating the proposed approach, its measurements were compared with the inclinometer readings. The test results showed that the optical system allowed monitoring the rotations of the samples at different locations which would not be possible to obtain using the traditional techniques. The experimental results showed that there is a non-linearity from the measurement of the shear displacements along the long side of the cross-section. However, the average rotation shows clear agreements with the inclinometer readings

Photogrammetric Evaluation of Glulam Timber Shear Modulus Using Torsion Test Method and Dual Stereo Vision System

The shear modulus of timber and timber-based composite materials is a fundamental me-chanical property which is used in the design of timber and engineered wood products. The problem of experimentally determining appropriate values of shear modulus for timber-based composite is not as simple and straightforward as in isotropic materials. Although the torsion test is a recommended standard approach to determine the shear modulus of structural-size timber and glulam beams, it is a difficult to measure the rotational deformations of the timber beams. Therefore, in this paper, a stereo camera system combined with a photogrammetric approach is proposed to evaluate the values and variations of the shear modulus of glulam beams under the torsion test. The photogrammetric approach is a non-contact method which provides an efficient and alternative approach for measuring the deformations of the torsion specimens in three dimensions. A series of experiments were conducted on glulam timber beams under the torsion test to investigate the applicability of the optical approach to evaluate the values and variations of shear modulus as well as to investigate the effect of applying torques in a clockwise or anticlockwise direction on the shear modulus of the beams. This optical system not only allows the performance and reliability of the traditional sensors to be assessed, but also allows the rotational deformation of the torsion samples to be monitored at various locations. This enables the values of shear modulus at different cross-sections of the torsion specimens to be evaluated without the need to use more devices. The test results showed that applying torques to the glulam timber specimens during loading and unloading in either a clockwise or anticlockwise direction does not influence or cause a significant change in the shear modulus of the beams. By comparing shear modulus values of glulam beams measured based on different shear spans, it was found that the larger the shear span the smaller the shear modulus value. This might indicate that the variations of shear modulus values at these different gauge lengths needs to be considered

Performance study on mounting system for displacement transducer in mechanical tests of timber samples using photogrammetry method

This paper presents a unique study to reveal the effect of the mounting system of the linear variable differential transformer (LVDT) on the mechanical property tests of timber materials. This effect has been overlooked in the past but proven to be significant enough that will distort the measurements completely if the displacement is relatively small. The reason for causing this error is because the LVDT is not directly measuring targeting points on a sample, instead, the LVDT is measuring the distance between one point on the stop-end (where the tip rested on) and the holder. The wrong common sense hopes that this point on the stop-end and holder reliably following the movement of the sample points. But this study reveals that it is not always the case with the aid from the photogrammetry method. The messages of this paper are simple but alertly useful and, important: (1) the mounting system and the method of installation of the LVDTs have a significant impact on the actual displacement measurement in a mechanical properties test of timber materials. It should be carefully designed and validated before the actual test; (2) the displacement distribution is not uniform across the timber sample. This should be taken into account when selecting the mounting points of the LVDTs. Testing standards should consider this effect when recommending displacement transducers to be used in a timber-related test

Determining equivalent-sectional shear modulus in torsion tests for laminated glass beams using photogrammetry method

This paper proposes a concise concept for quantifying the shear/torsional stiffness of the laminated glass beams experimentally by introducing the Equivalent-Sectional Shear Modulus (ESSM), that is directly measured from the torque and sectional-rotation correlation with the torsion test and tailor-made photogrammetry technique. The advantage of this method is originated from the concept of measuring the overall rotation to torque response of a laminated glass beam altogether rather than the component individually. This eliminates the uncertainties of analytical approximations that are commonly adopted by most existing methods in which the composite shear/torsion stiffness is derived from its component mechanical properties. The photogrammetry technique increased the accuracy of the sectional rotation measurement by acquiring dense displacement sample points on the glass beam simultaneously. The accuracy of the photogrammetry setup and efficacy of the test design were proven by a micrometre and a monolithic glass beam test. One sample each for the polyvinyl butyral (PVB) and SentryGlas Plus (SGP) laminated glass beams were tested multiple times non-destructively to determine the ESSM. The result of the SGP laminated glass beam showed a closer agreement with the previous studies, however the result of the PVB laminated glass beam exhibited a larger difference from the previous studies. It also suggested that mechanical properties of the interlayer played an important role in the composite behaviour of the laminated glass beam. The experimental outcomes have demonstrated the proposed method is an accurate and effective technique for measuring the ESSM of laminated glass beams

Performance study on mounting system for displacement transducer in mechanical tests of timber samples using photogrammetry method

This paper presents a unique study to reveal the effect of the mounting system of the linear variable differential transformer (LVDT) on the mechanical property tests of timber materials. This effect has been overlooked in the past but proven to be significant enough that will distort the measurements completely if the displacement is relatively small. The reason for causing this error is because the LVDT is not directly measuring targeting points on a sample, instead, the LVDT is measuring the distance between one point on the stop-end (where the tip rested on) and the holder. The wrong common sense hopes that this point on the stop-end and holder reliably following the movement of the sample points. But this study reveals that it is not always the case with the aid from the photogrammetry method. The messages of this paper are simple but guardedly/reservedly alertly useful and, important: 1) the mounting system and the method of installation of the LVDTs have a significant impact on the actual displacement measurement in a mechanical properties test of timber materials. It should be carefully designed and validated before the actual test; 2) the displacement distribution is not uniform across the timber sample. This should be taken into account when selecting the mounting points of the LVDTs. Testing standards should consider this effect when recommending displacement transducers to be used in a timber-related test