Coupled experimental and numerical investigation of structural glass panels with small slenderness subjected to locally introduced axial compression

Primary load-bearing glass constructions are often subjected to relatively important in-plane loads, transferred through so-called point-fixed connections. The according in-plane load introduction, structural resistance and failure mechanisms have been studied abundantly for axial tensile loading cases, but are relatively unknown for axial compression, in particular when buckling of the compressed component cannot occur. Consequently, stress distributions, resistance and failure mechanisms of small glass specimens subjected to locally introduced axial compression are investigated and presented in this contribution using a coupled experimental and numerical approach. The stress distributions and observed fracture patterns demonstrated that the major failure mechanism was splitting tension: the glass fractured due to high tensile stresses following the compressive stresses. However, the maximal principal tensile stresses at the crack origin were significantly lower compared to the axial tensile loading case. In addition, and in contradiction to the tensile loading case, significant maximal principal compressive stresses were found at the crack origin, leading to the conclusion that the axially compressed glass panels failed due to a complex stress state and not simply to tensile stresses, as is generally assumed in glass design. © 2009 Elsevier Ltd. All rights reserved.

Considerations on the partial safety factor format for reinforced concrete structures accounting for multiple failure modes

Miguel Fernández Ruiz, Aurelio Muttoni, Qianhui Yu

The increasing usage of nonlinear analyses for the design of reinforced concrete structures and the necessity of codes of practice to provide a consistent safety format for them is one of the challenges that new generations of codes of practice are facing. Suitable safety formats shall thus account for the peculiarities of nonlinear analysis, such as the possibility of having multiple potential failure modes. In this paper, the applicability of the classical Partial Safety Factor Format (PSFF) for the resistance of reinforced concrete structures (composed of two safety factors: gamma C for concrete compressive strength and gamma S for reinforcement yield strength) is investigated accounting for the possibility of multiple failure modes in nonlinear analysis. In addition, the similarities between nonlinear analysis and typical simple cases in the design of structural concrete are shown. Reliability analysis is performed for the design resistance of concrete structures according to PSFF under different design situations (crosssectional resistance or load-bearing capacity of structural elements and of simple structural systems). The results show that the PSFF applied to material strength variables leads to a satisfactory level of reliability, independently of the development of different failure modes induced by material uncertainties in nonlinear analysis. In addition, it is also observed that the simplification of integrating geometrical and model uncertainties into the partial safety factors for material strength variables can potentially underestimate their influence on the structural reliability in some cases. The case studies shows that occurrence of multiple failure modes can result into significantly different distribution characteristics between the tail and most probable region of the resistance of concrete structures. Attention should also be paid to a proper tail approximation of the probability distribution of the resistance when calibrating safety formats for concrete structures.

ELSEVIER SCI LTD2022

Adhesively bonded lap joints of pultruded GFRP shapes

Till Vallée

The importance of Fibre Reinforced Polymers (FRPs) as a material used for civil engineering purposes has grown in the last decade. Especially the introduction of pultrusion at an industrial level as a way to produce big batches of FRP made it possible to offer the advantages – like the high strength-to-weight ratio or the good corrosion resistance – at a reasonable cost. One issue when designing with pultruded FRPs are the connections. Up to now, connections between pultruded FRPs have been designed in the same way as structural steel connections, mainly through bolts. Because of the fibrous and layered character and the anisotropy of pultruded FRPs, bolting is not a material-adapted way to connect. Adhesive bonding is by far better suited, but has not yet been investigated for the special case of pultruded FRPs. This research is intended to fill the gap by offering designing engineers a method allowing them to dimension safe and economic adhesively bonded joints of pultruded FRPs under static loads. The present Thesis is aimed to show the steps leading to this method. After a short introduction, where the objectives and methods used are listed, the actual state of the art is presented. The review of actual literature shows that not much has been done on the special field of adhesively bonded connections of pultruded FRPs, neither experimentally nor theoretically. Some publications treat the global aspect of bonded connections for special cases like the single and double lap joints, but all on idealized mechanical systems with isotropic adherents. Also, there are no detailed reports of a mechanical failure criterion for both describing and quantifying the failure of pultruded FRPs. To overcome this, experimental investigations were carried out at different levels: the basic FRP material has been investigated in both senses of revealing the fibre architecture – with the help of burn-off tests – and the material strength – using a device the Author especially developed for this purpose: the CCLab Shear-Tensile device. This device allows the determination of the material strength subjected to combinations of out-of-plane and shear stresses. The device was also used to determine an important basic material property necessary to numerically formulate the anisotropy: the out-of-plane E-Modulus. Besides the investigations on the basic material, experiments on bonded single and double lap joints were carried out where the influence of parameters like the length of the bonded overlap, the thickness of the adherents and stress reduction methods (like chamfers of fillets) on the ultimate load was investigated. All of these experimental investigations were carried out on relatively big specimens to avoid the influence of any size effects. The experimental results showed that the adhesive layer thickness and stress reduction measures like chamfers are by far less influential that former publications expected them. For selected geometric configurations, the axial strain development along the bonded splice was experimentally gathered using strain gauges. Comparisons with FEA showed that by using the right mechanical input parameters in regard to the anisotropy, it is possible to model, with sufficient accuracy the stresses inside adhesively bonded joints of pultruded FRPs. Some single and double lap joints were filmed using a high-speed camera (up to 2000 fps) to investigate the failure process. This failure process is closely linked to the fibre architecture in the sense that it has been shown that the failure is triggered inside the laminate. The entire group of experimentally investigated specimens were then modeled with the Finite Element Method using orthotropic elements. In combination with the experimentally gathered material failure criterion, it was possible to formulate a method based on the comparison of stresses in the joint and the material resistance to predict the ultimate load of single and double lap joints, which was validated for a wide range of geometrical configurations. A simplified version of this method, based on existing analytical formulae, was then developed to make the strength prediction of adhesively bonded joints of pultruded FRP shapes available for civil engineering purposes.

EPFL2004

Stabilität von Tragelementen aus Glas

Andreas Luible

Stability of load carrying elements in glass The increasing demand in modern architecture for more slender and lighter structures requires the use of new construction materials. Glass, a material that has been used for a long time in windows as a filling material, has much to offer in this regard due to its very high compressive strength and transparency. For this reason, there is a growing trend to extend the use of glass sheets to load carrying elements such as columns, beams and panels. Due to their high slenderness and high compressive strength, such elements tend to fail because of instability (i.e. column buckling, lateral torsional buckling or plate buckling). At the moment little knowledge exists about the load carrying behaviour of glass structural elements, and existing design methods for other materials (i.e. steel) have been found to be unsuitable for direct transfer to the design of glass panels. With this in mind, the main objectives of the current thesis are: The study of the load carrying behaviour of glass elements which may fail due to lack of stability by means of laboratory tests and analytical and numerical models, as well as the study of the main influencing parameters. Discussion of possible design methods for glass elements which may fail due to lack of stability for the three main stability problems (column buckling, lateral torsional buckling and plate buckling) and proposition of possible aids for design such as buckling curves. The main influencing parameters (dispersion of the glass thickness, initial deformation) on the load carrying behaviour of glass elements which may fail due to lack of stability have been measured and are evaluated herein using statistical methods. The breakage stress, the thermal prestress and the effective tensile strength are defined and explained. Existing models to determine the tensile strength of glass are discussed. The column buckling behaviour of single layer and laminated safety glass is studied by means of column buckling tests, which are compared to analytical and numerical models. The models are used to study the influence of the main parameters, particularly the shear connection due to the interlayer (PVB) in laminated safety glass, on the load carrying behaviour and buckling strength of glass elements. On the basis of this study different possible design methods for column buckling of glass elements in compression are proposed and discussed. It is shown that a second order stress analysis is the most appropriate method for glass. As a further simplification, the cross section of a laminated safety glass structural element can be modelled as a monolithic cross section with an effective thickness. Analytical and numerical models for the lateral torsional buckling of glass beams are also verified by a comparison to test results. Along with a study of the main parameters, different methods to determine the lateral torsional buckling strength are discussed, and it is shown that buckling curves for lateral torsional buckling should be developed for glass beams using a slenderness ratio based on effective tensile strength. As a result of numerical simulations, recommendations for the future development of lateral torsional buckling curves of glass beams are given. The column buckling behaviour of single layer and laminated safety glass is also studied by means of column buckling tests, analytical and numerical models. It is shown that glass panels have a large post critical load carrying capacity but the way the loads are introduced into the panels, as well as the buckling shape, have an important influence on the plate buckling capacity. A design method with buckling curves using a slenderness ratio based on effective tensile strength seems applicable for the design of glass panels. As a result of numerical simulations, recommendations for the future development of plate buckling curves for plate glass elements under compression are given.

EPFL2004

Considerations on the partial safety factor format for reinforced concrete structures accounting for multiple failure modes

Miguel Fernández Ruiz, Aurelio Muttoni, Qianhui Yu

ELSEVIER SCI LTD2022

Stabilität von Tragelementen aus Glas

Andreas Luible

EPFL2004

Adhesively bonded lap joints of pultruded GFRP shapes

Till Vallée

EPFL2004