Biaxial Wrinkling of Thin-Walled GFRP Webs in Cell-Core Sandwiches

Fiber-reinforced polymer (FRP) sandwich structures offer several advantages compared to structures made of traditional materials, such as high specific strength, good corrosion resistance, low thermal conductivity and rapid component installation. In this context, glass fiber-reinforced polymer (GFRP) cell-core sandwiches composed of outer GFRP face sheets, a foam core and a grid of GFRP webs integrated into the core to reinforce shear load capacity are well suited for load-bearing applications in civil engineering i.e. in bridge deck and roof construction. Despite the great potential of these structural concepts, the use of heterogeneous materials in FRP sandwiches results in more complex failure mechanisms compared to conventional structural components and lack of knowledge regarding the prediction of failure modes makes the design of structural components difficult. This is one of the major disadvantages limiting the acceptance of cell-core sandwiches in civil engineering applications. One of the critical failure modes of cell-core sandwich structures is wrinkling in the webs. A great deal of information exists concerning the phenomenon of skin wrinkling failure of sandwich laminates loaded in compression but comparatively little on wrinkling in the webs of sandwich structures where the pure compression loading is complicated by supplementary transverse tension. The purpose of this research is to develop an appropriate model for the prediction of wrinkling in the webs of cell-core sandwich structures. Two new approaches were developed to predict the wrinkling loads of webs. The first approach examines the wrinkling behavior in webs as an in-plane biaxial compression-tension buckling problem according to the rotated stress field theory. In this regard, extensive experimental, numerical and analytical studies were performed to investigate the interaction between the compression and tension stress tensors during the buckling/wrinkling instability phase of GFRP plates and sandwich panels subjected to biaxial compression-tension loading. The investigations demonstrated that the transverse tension in the biaxial compression-tension set-up induced two simultaneous counteracting effects: a stabilizing and a lateral contraction effect. The stabilizing effect tends to push the plate back to the median plane and thereby delays the onset of buckling/wrinkling instability. In contrast, lateral contraction accelerates the bending of the plate, which leads to a significant decrease in buckling/wrinkling loads. In composite plates, the first effect predominates and increases the buckling loads while in sandwich panels the second effect is dominant and decreases the wrinkling loads. Using the second approach, the wrinkling behavior of foam-filled web-core panels was modeled by applying an improved mixed-mode interaction formula in which two approximate models are developed based on the energy method in order to determine the critical loads when the pure shear and bending stresses act independently on the web. The application of both approaches to a real case study, the GFRP cell-core sandwich roof of the Novartis Campus Main Gate Building proved that they are sufficiently accurate to be used as valid tools assisting the optimum design of sandwich structures whereas existing models result in too conservative predictions

Direct Strength Method and Response of Cold-Formed Steel Storage Rack Uprights in Global Biaxial Bending

This study seeks to investigate the global (lateral-torsional) buckling capacity of cold-formed steel (CFS) storage rack uprights under biaxial bending. A previously validated biaxial bending numerical model for local and distortional buckling of CFS rack uprights is used for global buckling. Biaxial bending response of nine unperforated upright cross-sections, each with nine different biaxial bending configurations, were considered. The findings demonstrate that the biaxial bending of the investigated uprights is governed by a nonlinear interaction behavior. DSM predictions including the classical method and the use of inelastic reserve capacity are compared to numerical capacities. The use of the DSM with inelastic reserve capacity as in the AISI-S100 and AS/NZS 4600, results in an overall 3% improvement of the predictions when compared with the classical DSM. A new extended range of the inelastic reserve capacity for global buckling is proposed. When compared with the classical DSM approach, the new extended range results in 14% improvement of the DSM predictions

Prospect for new guidance in the design of FRP

Over the last twenty years, many innovative solutions have confirmed the usefulness of composite structures realized with FRPs (Fibre Reinforced Polymer or Plastic). The need of European standards for use of fibre-reinforced polymer composites in civil engineering was justified in 2007 in the JRC Report EUR 22864 EN. The new European technical rules will be developed using the existing organization of CEN/TC250. The present report has been worked out in the frame of CEN/TC250/WG4 activities. The report encompasses: • Part I, which introduces the policy framework and the CEN/TC250 initiative • Part II, which gives a prospect for CEN guidance for the design and verification of composite structures realized with FRPs The report presents scientific and technical background intended to stimulate debate and serves as a basis for further work to achieve a harmonized European view on the design and verification of such structures. This has been the main impulse to include the work item of the Fibre Reinforced Polymer Structures in the Mandate M/515 with high priority.JRC.G.4-European laboratory for structural assessmen

Prospect for new guidance in the design of FRP : support to the implementation and further development of the Eurocode

Over the last twenty years, many innovative solutions have confirmed the usefulness of composite structures realized with FRPs (Fibre Reinforced Polymer or Plastic). The need of European standards for use of fibre-reinforced polymer composites in civil engineering was justified in 2007 in the JRC Report EUR 22864 EN. The new European technical rules will be developed using the existing organization of CEN/TC250. The present report has been worked out in the frame of CEN/TC250/WG4 activities. The report encompasses: • Part I, which introduces the policy framework and the CEN/TC250 initiative • Part II, which gives a prospect for CEN guidance for the design and verification of composite structures realized with FRPs The report presents scientific and technical background intended to stimulate debate and serves as a basis for further work to achieve a harmonized European view on the design and verification of such structures. This has been the main impulse to include the work item of the Fibre Reinforced Polymer Structures in the Mandate M/515 with high priority

Local structural effects in fiber-reinforced polymer web-core sandwich structures

Glass fiber-reinforced polymer (GFRP) pultruded decks and sandwich panels currently represent two of the most extensive applications of FRP materials for load-bearing structural components in the bridge and building domains. Based on the state of the art, the global structural behavior of both systems has been fairly well investigated. Nonetheless, local effects governing in most cases the global behavior have been barely addressed. Selected local structural effects relevant to the global structural performance of pultruded GFRP bridge decks and GFRP-foam web-core sandwich structures are therefore investigated in this research. The effect of the core geometry of pultruded GFRP decks on the systemâs behavior in its transverse-to-pultrusion direction was experimentally investigated. The experimental work conducted on two deck designs with trapezoidal- and triangular-cell cross sections showed that the transverse structural performance depends on the cell geometry. Furthermore, the systemsâ transverse bending and in-plane shear stiffness were evaluated and the results indicated that a triangular core causes a more pronounced bi-directional behavior of the deck when it is subjected to concentrated loads. The local behavior of the web-flange junctions (WFJs) of the pultruded deck with trapezoidal cells was experimentally investigated regarding energy dissipation capacity and recovery subsequent to unloading. The experimental responses reported for two junction types with similar geometry and fiber architecture but different initial imperfections demonstrated that dissimilar imperfections could significantly affect WFJ behavior and change it from brittle to ductile. The time-dependent recovery and energy dissipation mechanisms of the WFJs exhibiting a ductile response were evaluated; the viscoelastic effects were found to be small in both cases. The rotational behavior of all WFJ types present in the trapezoidal-core deck was characterized. An experimental procedure based on three-point bending and cantilever experiments conducted on the web elements was developed and used for this purpose. The rotational stiffness, strength and failure modes of the WFJs differed depending on the web type, location of the WFJ within the deck profile, existing initial imperfections and direction of the applied bending moment. Numerical simulations of the full-scale deck were performed to demonstrate the validity of the experimental moment-rotation (M-Ï) relationships and simplified M-Ï curves provided. The effects of creep on the load-bearing behavior of GFRP-foam web-core sandwich structures were investigated. A study of the creep behavior of polyurethane (PUR) foams was conducted and showed that in order to assess the long-term structural performance of the sandwich system, the foam anisotropy, density and loading type should be considered. The creep behavior of web-core sandwich panels, and specifically the structural aspects affected by the web-core interaction, were analyzed using the GFRP-PUR sandwich roof of the Novartis Campus Main Gate Building as case study and currently available design guidelines. The resulting sandwich designs depended on the applied design recommendations. Finally, provisions for the cross-sectional design of the hybrid web-core were proposed

A new mixed model based on the enhanced-Refined Zigzag Theory for the analysis of thick multilayered composite plates

The Refined Zigzag Theory (RZT) has been widely used in the numerical analysis of multilayered and sandwich plates in the last decay. It has been demonstrated its high accuracy in predicting global quantities, such as maximum displacement, frequencies and buckling loads, and local quantities such as through-the-thickness distribution of displacements and in-plane stresses [1,2]. Moreover, the C0 continuity conditions make this theory appealing to finite element formulations [3]. The standard RZT, due to the derivation of the zigzag functions, cannot be used to investigate the structural behaviour of angle-ply laminated plates. This drawback has been recently solved by introducing a new set of generalized zigzag functions that allow the coupling effect between the local contribution of the zigzag displacements [4]. The newly developed theory has been named enhanced Refined Zigzag Theory (en- RZT) and has been demonstrated to be very accurate in the prediction of displacements, frequencies, buckling loads and stresses. The predictive capabilities of standard RZT for transverse shear stress distributions can be improved using the Reissner’s Mixed Variational Theorem (RMVT). In the mixed RZT, named RZT(m) [5], the assumed transverse shear stresses are derived from the integration of local three-dimensional equilibrium equations. Following the variational statement described by Auricchio and Sacco [6], the purpose of this work is to implement a mixed variational formulation for the en-RZT, in order to improve the accuracy of the predicted transverse stress distributions. The assumed kinematic field is cubic for the in-plane displacements and parabolic for the transverse one. Using an appropriate procedure enforcing the transverse shear stresses null on both the top and bottom surface, a new set of enhanced piecewise cubic zigzag functions are obtained. The transverse normal stress is assumed as a smeared cubic function along the laminate thickness. The assumed transverse shear stresses profile is derived from the integration of local three-dimensional equilibrium equations. The variational functional is the sum of three contributions: (1) one related to the membrane-bending deformation with a full displacement formulation, (2) the Hellinger-Reissner functional for the transverse normal and shear terms and (3) a penalty functional adopted to enforce the compatibility between the strains coming from the displacement field and new “strain” independent variables. The entire formulation is developed and the governing equations are derived for cases with existing analytical solutions. Finally, to assess the proposed model’s predictive capabilities, results are compared with an exact three-dimensional solution, when available, or high-fidelity finite elements 3D models. References: [1] Tessler A, Di Sciuva M, Gherlone M. Refined Zigzag Theory for Laminated Composite and Sandwich Plates. NASA/TP- 2009-215561 2009:1–53. [2] Iurlaro L, Gherlone M, Di Sciuva M, Tessler A. Assessment of the Refined Zigzag Theory for bending, vibration, and buckling of sandwich plates: a comparative study of different theories. Composite Structures 2013;106:777–92. https://doi.org/10.1016/j.compstruct.2013.07.019. [3] Di Sciuva M, Gherlone M, Iurlaro L, Tessler A. A class of higher-order C0 composite and sandwich beam elements based on the Refined Zigzag Theory. Composite Structures 2015;132:784–803. https://doi.org/10.1016/j.compstruct.2015.06.071. [4] Sorrenti M, Di Sciuva M. An enhancement of the warping shear functions of Refined Zigzag Theory. Journal of Applied Mechanics 2021;88:7. https://doi.org/10.1115/1.4050908. [5] Iurlaro L, Gherlone M, Di Sciuva M, Tessler A. A Multi-scale Refined Zigzag Theory for Multilayered Composite and Sandwich Plates with Improved Transverse Shear Stresses, Ibiza, Spain: 2013. [6] Auricchio F, Sacco E. Refined First-Order Shear Deformation Theory Models for Composite Laminates. J Appl Mech 2003;70:381–90. https://doi.org/10.1115/1.1572901

Composites for hydraulic structures: a review

Composites for hydraulic structures: a review Composites have evolved over the years and are making major in-roads into the marine, aviation and other industries where corrosions and self-weight are the major impediments to advancing the state-of-the-art. Civil Works engineers have been reluctant to make use of these composite advantages, partially because of the absence of well documented success stories, accepted design and construction practices or specifications, and limited understanding of composites, higher initial costs and others. A few navigational structures using FRP composites have been designed, manufactured and installed in the United States of America and Netherlands, recently. US Army Corps of Engineers is embarking on higher volume applications of composites for navigational structures. This report is aimed at summarizing the state of the art of fiber reinforced polymer (FRP) composites for hydraulic structures including design, construction, evaluation and repair. After a brief review of history and introduction of fundamentals of composites, their manufacturing techniques, properties, and recent field applications are presented, including FRP rebar for bridge decks, other highway and railway structures, gratings, underground storage tank, pavement, sheet and pipe piling, FRP wraps, moveable bridges, utility poles, etc. Focus is placed on applications of composites in waterfront, marine, navigational structures including lock doors, gates, and protection systems. Design of hydraulic composite structures is presented for the cases available, such as design of FRP recess panel, Wicket Gates, Miter Gates, FRP slides and repair of corroded steel piles. This report also reviews engineering science issues such as fracture and fatigue, durability, creep and relaxation, UV degradation, impact resistance, and fire performance. The report concludes with summary remarks and recommendations after a discussion on operation and maintenance guidance including nondestructive evaluation inspection techniques. Intention is to provide up to date information on composite design, manufacturing and evaluation methodologies that are applicable for fabrication and maintenance of navigational structures. This report is a living document with advances taking place with time as waterborne transport infrastructure community makes progress with FRP systems. This report is expected to be useful for those decision-makers in government, consultants, designers, contractors, maintenance and rehab engineers whose focus is to minimize traffic interruptions while maximizing cost effectiveness