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The range of applications of fiber-reinforced polymer structures has become increasingly diverse in recent years. However, little is known on their performance under dynamic loads. This paper investigates the nonlinear vibrations and parametric instability of pultrude fiber-reinforced polymer thin-walled columns with a channel section under harmonically varying axial loads. The channel section is discretized into three plates, which are modeled using the nonlinear first-order shear deformation plate theory. The continuous system is discretized using Ritz method and the resulting coupled nonlinear equations of motion are solved numerically by the Runge–Kutta fourth-order method. The instability regions in the forcing magnitude-forcing frequency plane are determined as a function of the material orthotropic parameters, damping ratio and cross section geometry. The bifurcation diagrams are obtained by employing continuation techniques and the brute force method, and the stability of the solutions is subsequently investigated using Floquet theory. The bifurcation analysis allows the identification of the bifurcations associated with the parametric instability boundaries in the force control space as well as the existence of coexisting solutions. Then, the evolution of the basins of attraction of the coexisting solutions as a function of the forcing magnitude is investigated, in order to evaluate the dynamic integrity of the desired trivial stable solution. The results demonstrate that the column can lose stability at load levels well below the static buckling load and, therefore the designer has to be careful while dealing with these structures subjected to time-varying axial loads.

Recently, fiber-reinforced concrete (FRC) creep behavior has become a much addressed topic, and the main gap found in the literature is whether cracked FRC is stable in the serviceability l...

International audience; In this work, an experimental and numerical study of the behavior of a tensegrity structure constructed with natural materials is presented. Firstly, it is shown the concept of tensegrity structures, highlighting its emergence, current context, and applications within Civil Engineering. The design of a bio-based tensegrity structural module is investigated as object of study. Initially, a prototype of a single module of the structure is built in 1:25 scale. Subsequently, a prototype on a 1:5.65 scale is built using Phyllostachys aurea bamboo culms for the struts and sisal (Agave sisalana) ropes for the cable nets. Both non-destructive static and dynamic testing were performed on the module to investigate the level of prestress right after assembly of the structure and also considering the relaxation of the cable networks under laboratory conditions. The results obtained from the experimental tests are compared with the numerical predictions by means of a relative error parameter regarding the static stresses and the natural frequencies. Thus, it was possible to indirectly determine the level of prestress applied in the sisal ropes of the tensegrity module. The prestress level was equal to 15% in both static and dynamic approaches, which indicates a good agreement between them and points to the robustness of the proposed methodology. The use of bamboo as a suitable material for strut-cord joints is proposed to avoid metal components and consider the end-of-life biodegradability of the structure in the design of joints. The study deploys bio-based materials for tensegrity structures and the kit of parts as a whole, contributing to the development of extremely lightweight and sustainable structural systems.

This work presents the development of explicit equations for the prediction of elastic local buckling critical stress of castellated beams subjected to pure bending, considering the interaction between flange and web. Eigenvalue analyses are carried out for Litzka-type beams using Finite Element Method (FEM) to gather information on the buckling modes and critical stresses for combinations of usual flange-to-web width and thickness ratios. The representative compression ‘tee’ is analyzed using the Generalized Beam Theory (GBT) to study the influence of stress gradient and member length in the critical stress. The influences of flange torsional stiffness and web transverse bending in the ‘tee’ behavior are studied and an energy method is adopted to derive the prediction equations. It is shown that the proposed approach leads to a good agreement with the FEM results, with an average difference of 0.955 ± 0.063, whereas current guidelines may significantly over or underestimate actual critical stresses for low flange-to-web width ratios. It is finally shown that this buckling mode may become relevant for castellated beams using high-strength steel.

Several works in the literature have investigated creep of pultruded glass-fiber reinforced polymer (pGFRP), but mainly to characterize the material in longitudinal direction. Moreover, the empirical Findley Power Law has been widely used to describe the creep response, but this approach is particular for each case and may result in little predictive capacity for different loading conditions, resin types and fiber architecture. In this work, a linear viscoelastic model combining elastic behavior of fiber and viscoelastic nature of matrix is proposed to represent the long-term behavior of pGFRP material. To validate the model, flexural creep and recovery experimental tests were carried out for vinyl ester (VE) and orthopolyester (OPE)-based composites for specimens oriented parallel and perpendicular to pultrusion direction. To reduce uncertainties with respect to the degree of curing of resin during fabrication, tests were conducted for two different conditions: as supplied and post-cured. Dynamic mechanical analyses (DMA) were performed to evaluate qualitatively the degree of cross-linking and the fiber architecture was studied by stereomicroscopy. Tests revealed greater compliances for OPE resulting from lower viscoelastic properties of this resin, as well as by lower fiber content and less effective fiber architecture. For transverse direction, matrix dominates the behavior and compliances were more pronounced. The thermal treatment adopted for post-curing resulted in damage formation or propagation, increasing the compliances. Satisfactory agreement was achieved between proposed model and experiments.

In this paper, the findings from an experimental investigation on the flexural-torsional buckling behavior of pultruded glass-fiber reinforced polymer (GFRP) angle columns are reported and discussed. The program included the study of two sizes of equal-leg angles made with different resins (polyester and vinylester). Prior to testing, a detailed material characterization was carried out and signature curves (critical load x length) were obtained using a generalized beam theory (GBT) software for predicting critical loads. Lengths were selected in order to ensure ‘pure’ flexural-torsional buckling, in a range of slenderness not studied in previous works. Twenty-two members with fixed ends and clamped walls were tested in concentric compression and had their motions measured during loading. Load-deflection curves are presented and the influences of post-buckling reserve of strength, damage and differential rotation of legs are discussed. Experimental critical loads obtained using Koiter's method are reported and shown to be in good agreement with GBT predictions. Finally, a design recommendation through the use of a Winter-type equation accounting for the plate-like behavior is made.

The use of fabric reinforced cementitious matrix for shear strengthening of concrete structures has been widely investigated and its field of application is not an uncommon practice. However, the majority of previous works has been limited to synthetic or metallic fabrics. Thus, the present study seeks to investigate the shear mechanical behavior of a cementitious composite reinforced with natural curaua fabrics and its application as a shear structural reinforcement. The experimental program includes Iosipescu tests and the use of Digital Image Correlation technique for material characterization and flexural tests for strengthened structural beams. The developed composite material under shear tests demonstrated a shear strain-hardening behavior with well-defined stages. The beam externally reinforced with the composite presented an improved ultimate strength of 28%. Analytical model predictions were compared to experimental results and a good agreement was achieved.

One of the steps to achieve a rational approach to the shear strength of reinforced concrete members is to understand the force transfer mechanisms throughout the critical crack. This subject has been investigated for decades, but its full comprehension has not been achieved, especially in steel fiber reinforced concrete (SFRC). In the present work, an experimental/theoretical investigation on the combined action of fiber bridging and aggregate interlock was carried out. The experimental program included 12 push-off specimens in which the variables were the fiber volume content (Vf) and aspect ratio (lf/df). The results showed an increase in shear strength and an improvement in the post-cracking stage as residual flexural tensile strength (fR) and Vf lf/df increased. To gather information about the failure surface topography, 3D digital image correlation analysis was adopted for specimens without fibers. A contact density-based mechanical model is applied to plain concrete results to derive aggregate interlock parameters and, then, an extended model including fibers is proposed to evaluate the results for SFRC specimens. A reasonable agreement was achieved and the contribution of each mechanism could be estimated.

This work deals with a non-destructive approach based on dynamic testing combined with model updating techniques through finite element modelling and optimisation processes to characterise the main elastic constants of a thin-walled glass fibre reinforced polymer (GFRP) C-channel beam. The experimental modal analysis identified nine vibration modes encompassing global modes predominated by coupled flexural-torsional behaviour as well as local flexural ones related to the flanges of the beam. The longitudinal elasticity and in-plane shear moduli were less sensitive to the number of modes, indicating that only the first three modes were enough to obtain a good estimate of these parameters. In contrast, the transverse modulus of elasticity was more affected over the nine modes and the optimisation process became imperative to seek the optimal value. As corroborated by destructive tests, the non-destructive procedure gives an alternative approach for the mechanical characterisation of GFRP members, which can be advantageous for low-cost in situ quality control.

This paper presents the results of an experimental program investigating flexural creep and creep buckling behavior of pultruded glass fiber reinforce polymer (pGFRP) plate specimens. In this study (a) short-term material properties were determined; (b) 1000 h flexural creep tests at three load levels were conducted from which Findley power law parameters describing creep strain behavior were obtained; (c) creep buckling tests of slender concentrically loaded specimens were conducted; and (d) results from the buckling tests were compared with the predictions of the Findley power law which is shown to predict results reasonably well. Provided the flexural creep tests have relatively small loads and sufficient duration to pass through the primary creep stage of the material, reliable creep parameters were established. The intent of this study is to demonstrate a framework for establishing creep behavior and creep buckling performance of pGFRP materials.

In this work, explicit equations to determine local buckling critical stress of thin-walled fiber reinforced polymer (FRP) profiles in pure bending or compression are proposed. Interaction between flange and web is considered and the expressions allow for different orthotropy ratios and ranges of flange-to-web widths and thicknesses. To obtain the equations, Rayleigh Quotient energy method is adopted for assumed approximate buckled shapes. As quality of selected functions affects the accuracy, different shapes are investigated and the results are compared with those obtained with Generalized Beam Theory (GBT) for typical I-section dimensions and material properties. A comparison is made to the solutions based on discrete plate with simplified support conditions as well as to a recently proposed equation for major-axis bending. Finally, a general form for the local buckling critical coefficient is also presented along with tabulated parameters for prompt assessment of critical stresses, consisting in a simple and reliable alternative for design approach.

This paper aims to present an experimental investigation on the behavior of web-flange junctions (WFJs) rotational stiffness of pultruded fiber-reinforced polymer composites (FRP). Channels and I-sections were tested using a simple set-up, which was developed in order to experimentally characterize the junctions in a direct manner. The Digital Image Correlation (DIC) technique was used, allowing overall deflections and relative rotations between web and flange to be monitored. The WFJs' imperfections were analyzed through an optical microscope and correlated with the cracks' formation. Further, damage thresholds are identified using available stress equations for curved composite members and lower bound functions are proposed to simulate the junction stiffness retention. Finally, two Equations are developed in order to analytically predict pultruded junctions' rotational stiffness per unit of width. In general, the theoretical and experimental results agreed fairly well, with a maximum difference of 24% for I-sections and 38% for channels.

The aim of this research is to investigate the instability of castellated beams and the interaction between lateral-torsional and compression tee local buckling modes. Firstly, a comprehensive study of 197 simply supported Litzka castellated beams under pure bending is carried out. The study involves eigenvalue and fully nonlinear finite element analyses, e.g. including both material nonlinearity and geometric imperfections. After computing the critical moments associated to local and global modes and the ultimate moments, an approach based on the Direct Strength Method is proposed using regression techniques to derive strength prediction equations. The developed method is then compared with current standards procedures. The results show that the proposed equation predicts better strength values in all cases, especially those in which local and/or interaction failure modes dominate the behavior of the beam. The results also show that some of the procedures suggested in standards and guides for the design of castellated beams under flexure, either underestimate or overestimate the failure moment when considering the interaction between global buckling mode and plastification.

The present paper aims to investigate the failure modes of laterally braced non-composite Litzka-type castellated beams using finite element simulations, evaluating aspects such as mutual influence of the constituent parts to the behavior, interaction of individual modes and post-buckling reserve of strength. For a comprehensive analysis, a parametric study is carried out and the behavior is depicted in a plot (‘map’) in terms of the bending moment-to-shear force ratio, the flange and web geometries and the generalized non-dimensional slenderness depending on the plastic and buckling loads. It is shown that the failure mode of a certain beam can be roughly predicted according to the region of the ‘map’ it belongs. The results confirm the occurrence of interaction between individual modes and reveal that the normalized strength decays more quickly with slenderness for shear-dominated failure, although some post-critical reserve of strength can be observed. Finally, inspired by the Direct Strength Method, a design approach is proposed to assess the load carrying capacity of beams with usual geometries.

In this work, the development of rational explicit equations for distortional buckling critical stress of lipped channel columns subject to uniform compression is presented. An energy-based approach is adopted along with two different assumed buckled shapes (models) to obtain the analytical expressions. In Model 1, stiffened flange is assumed rigid, i.e., transverse wall bending is neglected, whereas in Model 2 the flange plate flexibility is taken into account. An analytical procedure to determine warping stresses based on cross-section displacements and considering that flange-stiffener assembly rotation center does not coincide with web-flange fold-line is described. To validate proposed equations, a parametric analysis is carried out for lipped channels with flange-to-web width ratio, bf/bw, ranging from 0.2 to 1.0 and lip-to-web width ratio, bs/bw, from 0.1 to 0.4. Results are compared to those obtained with a generalized beam theory (GBT) software for ‘pure’ distortional mode and it is shown that, within the range of typical geometries used in industry, average differences are approximately 1.062 ± 0.073 and 0.998 ± 0.001 for Models 1 and 2, respectively. The linear (natural) coupling of pure distortion and local/global deformation modes is also briefly described and its influence on the critical loads is discussed. Finally, comparison is made to benchmark methodologies currently used in design standard and specifications. The obtained results indicate that proposed equations are reliable and valuable for practical design applications.

In the present work, the flexural behavior of carbon textile reinforced concrete (TRC) I-beams is investigated. Four-point bending tests were performed in I-beams, considering SBR-laminated textile and the following conditions: (a) plain cementitious matrix; (b) plain matrix and sand-coated textile; and (c) strain hardening cement-based composite (SHCC) matrix. The main goal of the research was to correlate the improvements on interface and matrix properties with the crack pattern, failure mode and ductility. For a comprehensive discussion, full mechanical characterization was performed and a design model is proposed for a better evaluation of the experimental results. Cracking behavior was greatly improved for conditions (b) and (c) with respect to (a), due to enhanced bonding between reinforcement and matrix in the former and due to fiber bridging mechanism in the latter. Beams with plain matrix exhibited brittle failure, whereas the beam having SHCC matrix showed premature local failure near load application point, resulting in an apparent ductility due to concrete softening mechanism. The reduced cracking load and the premature flange detachment along web-flange junction for condition (a) are also discussed. The proposed model is also validated through comparison with experimental results available in the literature.

In the present study, an experimental program was carried out to evaluate the influence of the addition of 10 kg/m3 of polypropylene (PP) fibers on the tensile and flexural behavior of concrete members internally reinforced with GFRP bars. Mechanical characterization and pullout tests were performed to determine the properties of the constituent materials and to characterize their interface. In a structural level, direct tensile and four-point bending tests were performed on prismatic specimens to investigate their mechanical response. Throughout the work, digital image correlation (DIC) was used to monitor deformations, crack openings and spacings. A considerable reduction in cracking width and an increased multiple cracking behavior was observed for specimens with fiber addition. The results show also a significant influence of tension-stiffening for specimens subject to tension and bending, whereas a slight increase in structural ductility was observed for the specimens tested in bending. The tests indicate possible influence of the shear lag effect to the bar stiffness.

This work aims to investigate the influence of the exposure to moderate/high temperatures on the residual flexural properties of commercial pultruded profiles of glass fiber-reinforced polymer (GFRP) composite having different formulations. The experimental program included flexural and interlaminar shear tests for four different GFRP formulations at room temperature and after exposure to temperatures up to 320 °C. To investigate the materials’ thermal stability, thermogravimetric analyses (TGA) were performed and dynamic mechanical analyses (DMA) were carried out to evaluate the changes in the glass transition temperature. Scanning electron microscopy (SEM) and x-ray microtomography techniques were also used to analyze the microstructure and the damages after exposure to temperature. The results from bending tests showed that all but polyester-based composites showed a slight increase of modulus for intermediate temperatures, followed by a decrease as it approached the degradation temperature. All specimens failed in interlaminar shear due to porous interface between layers and a large deviation was obtained for the strength. For interlaminar shear properties also seemed to be influenced by the competing effects of post-curing and matrix degradation. For this latter test, vinyl ester matrix composite had the best performance whereas the polyester matrix composite had the lowest.

Although many researchers have already studied the shear behavior of reinforced concrete (RC) beams, the contribution of each resisting mechanism to the shear capacity is not fully understood. The problem becomes more complicated when steel fibers are added to the concrete, which may affect those mechanisms differently. The results of an investigation on the influence of the addition of hooked-end steel fibers on the dowel action by the main reinforcement of RC beams are reported here. The experimental program included 16 specimens without stirrups in which the main variables were the fiber volume content and aspect ratio and the diameter of the longitudinal steel reinforcement. Four of those specimens were analyzed using digital image correlation to gather detailed information about the displacement field and the splitting crack opening development with loading. A mechanical model for the prediction of the relationship between dowel load and crack opening is also proposed. The obtained results show the effect of the splitting crack opening along the specimen on the dowel strength and that the addition of steel fibers to the concrete enhances the dowel action load-carrying capacity and varies according to the effectiveness of fiber action along the splitting crack.

This work aims to investigate experimentally the improvements on the physical and flexural properties of Moso bamboo species (Phyllostachys pubescens) associated to the hornification phenomenon, widely used as a treatment in wood pulps or paper to reduce porosity, absorption capacity and water retention of the fibers, as well as to increase their stiffness. To accomplish this task, bamboo samples subjected up to ten wetting and drying cycles had their flexural characteristics such as stiffness, strength, neutral axis, deflection at failure and through-thickness shear modulus compared with untreated samples. The moisture content was monitored during cycles and image analysis was performed to determine the distribution of vascular bundles throughout the wall thickness of the culm’s cross-section. The experimental program showed a reduction in water retention capacity of bamboo and in the deflection at failure, as well as an increment in the modulus of elasticity up to 70% after treatment. The tests also reveal a major influence of shear deformation on the overall deflection and appropriate strain measurement techniques must be used for a more accurate moduli determination. Finally, a simplified micromechanical approach was used along with fiber content distribution to estimate the influence of hornification on the moduli under compression and tension of the constituents.

Closed-form equations to determine the local buckling critical stress of typical pultruded fiber reinforced polymer (FRP) sections – angles, I-shaped, channels and rectangular tubes – comprised of orthotropic thin walls subject to concentric compression are developed. Approximate deflected-shape functions addressing boundary conditions and compatibility of rotation between plate elements are chosen for each section having uniform thickness and material properties. The Rayleigh energy method is used to obtain equations for the local buckling critical stress. Results are compared with numerical analyses using the finite strip method (FSM) for isotropic and orthotropic sections with typical ranges of properties. Comparison is also made with the method recommended by current available standards and guidelines.

In this paper, the development of a rational and comprehensive equation to determine the strength of glass fiber reinforced polymer (GFRP) pultruded square tube columns under concentric compression is presented. Material characterization and compression tests on stubs and columns were carried out in order to establish in situ properties. Square tube columns having different lengths and sections, resulting in a range of combinations of global and sectional slenderness, were tested. Material properties, critical loads, compressive strengths and failure modes are reported and comments are made on observed post-buckling behavior and interaction between crushing, local and global buckling. Column and wall imperfections are estimated and used as inputs to the proposed equation. The strength curve is plotted along with the test results, showing good agreement. Finally, a step-by-step example is presented.

This paper aims to investigate the performance of pultruded glass fiber reinforced polymer (pGFRP) I-section columns subject to short-term concentric compression, bringing up a discussion on the relevant parameters affecting their local buckling behavior and the interpretation of tests results. An experimental program was carried out, including a detailed material characterization and twenty-nine compression tests on short I-columns made of either polyester or vinyl ester matrices, with variable flange width-to-section depth ratios ( bf/d ), column lengths and local end conditions. The theoretical critical loads predicted using generalized beam theory (GBT) and finite element method (FEM) were compared to experimental results obtained by Southwell and Koiter techniques, with better agreements obtained for the latter. It is shown that, besides the length and local end conditions for loaded edges, post-buckling with associated non-linear elastic strains distribution throughout the cross-section and damage prior to buckling have relevant influence on the measured critical loads. On the other hand, the influence of the rotational stiffness of web-flange junctions were considered small for the material and cross-sections studied. Finally, results have shown that the usual boundary condition adopted in literature approaches a clamped condition instead of simply-supported one.

In this work, the results of an experimental program intending to investigate the flexural characteristics of reinforced steel-fiber concrete beams (S/SFRC) are reported and discussed. Hooked end steel fibers with aspect ratios of 45 or 80 were used to produce matrices with fiber content ranging from 0 to 2%, in volume, and high compressive strength . A full mechanical characterization was carried, including the post-peak properties for concrete matrix in tension. Beams with reinforcing ratios of 0.28, 0.44 and 0.70% were analyzed using digital image correlation (DIC) to monitor displacement field and cracking distribution throughout the constant moment region during loading. Data reported included load-deflection and moment-curvature curves, evolution of compressive strains in concrete and neutral axis depth , spacing and openings of cracks and failure modes. Tests revealed gains of capacity ranging from 21 to 109% of R/SFRC with respect to plain RC beams and significant reductions in crack openings for a given rebar stress. R/SFRC beams exhibited sufficient plastic rotation capacity, but with a reduction in plastic hinge length as a consequence of the phenomenon of crack localization . Comparisons with current code provisions show that these are adequate for the design but residual stresses obtained from direct tensile tests are more appropriate.

In this paper, a simple accurate equation to determine the local buckling critical stress of pultruded GFRP I-sections is developed. To assess the proposed expression, an experimental program comprising cross-sectional geometry measurement, material characterization, and compression tests on stub columns were carried out. Stubs having different flange-width-to-section-depth ratios extracted from 76×76×6.4 and 102×102×6.4-mm sections made with vinyl ester and polyester matrices were tested. The proposed expression is compared with experimental results, as well as results from numerical analyses using the finite-strip method (FSM). Finally, examples are presented and critical stresses are compared to those obtained based on other comparable research and by current design guidelines, demonstrating that the latter lead to conservative results.

Physiological and structural changes in cells of Synechocystis aquatilis f. aquatilis acclimated to grow in the presence of high zinc levels (2.20-3.30mg.L - 1 ) were investigated. Growth of these cells showed a decreased specific growth rate and final yield of about 60% and 50%, respectively, of the values found for cells grown in the presence of 0.21 mg zinc.L - 1 (control culture). The higher the zinc concentration in the culture medium, the more pronounced the reduction in the chl a content. Regardless of zinc concentration, S. aquatilis possessed three distinct carotenoids. A decrease in carotenoid content accompanied the decrease of chl a, and the proportions of the pigments to each other were not affected by zinc. The photosynthetic performance of cells cultured in the presence of high zinc levels showed a decline in both the apparent photosynthetic efficiency and the photosynthetic maximal rate. In these cells the PSII reaction centers became partially closed, and the electron transport activity around PSII and PSI was reduced to 61% and 38% of the control values, respectively, which may indicate an altered PSII/PSI stoichiometry. In addition, electron micrographs revealed a reduced amount of thylakoid membranes, indicating that acclimation to high zinc levels led to a decrease in the overall number of photosynthetic units. On the other hand, light microscopic observation of negative-stained cells revealed the presence of a thick mucilaginous layer surrounding the high zinc-acclimated cells. This extracellular material could retain high amounts of metal ions from the medium, thus providing the Synechocystis cells a mechanism to circumvent toxic levels of zinc.