Analytical determination of the ultimate strength of sandwich beams

An analytical determination of the ultimate strength of a typical GRP/PVC sandwich beam has been performed. These beams represent common building practise in marine applications. Equations describing the behaviour of a sandwich panel under beam loading and various failure modes have been developed. The method has been applied to predict the ultimate load for a simple supported sandwich beam. The critical loads have been compared with those from the experimental investigation of a typical bulkhead-to-hull GRP/PVC sandwich T-joint under pull out forces.     

木质包装箱结构优化设计

目前我国市场使用大量的包装箱仍以木材为主,经调查木质包装箱用料过剩的现象比较严重.以某厂一款产品的包装箱为例,对其进行了结构分析,发现在设计方法上造成木材大量的浪费.为此提出了木质包装箱优化设计的方案,阐述了木质包装箱优化设计的方法和步骤.采用新的木质包装箱优化设计的方法,使该款产品包装箱在满足使用条件的情况下,其木材用量节约30%左右.     

Flexure of pultruded GRP beams with semi-rigid end connections

Details of a series of three point flexure tests on 101.6 × 101.6 × 6.4 mm Wide Flange (WF)-section, pultruded Glass Reinforced Plastic (GRP) beams are presented. The beams were tested at four spans (L/d=21–45) under four sets of end conditions: clamped, bolted web and flange cleats, bolted web cleats and simply supported. Mid-span deflections and beam end rotations were recorded in the tests. The observed deflections and end rotations are compared with semi-rigid shear deformable beam theory predictions. Reasons are given for the differences (not large in the majority of cases) between the observed deflections and end rotations and the theoretical predictions. It is shown that deflection reduction and load enhancement performance indices provide a better basis for comparison of the experimental results and the theoretical predictions. The comparisons demonstrate and quantify the improvements in structural performance, arising from the inherent rotational stiffness of practical end connections, which could be utilised at no extra cost by adopting a semi-rigid design philosophy.     

Design shear strength formula for high strength concrete beams

After a brief review on the concrete shear strength mechanisms, two very reliable expressions for predicting the shear strength of beams without transverse reinforcement are reported: the one proposed by Ba?ant and Kim [7], which is valid for Normal strength Concrete (NSC) beams, and the other recently proposed by the authors, which is valid for High Strength Concrete (HSC) beams. Hence a summary of a shear strength model for beams with stirrups is provided, which was derived [27] on the basis of the Ba?ant and Kim expression and therefore is adequate only for NSC beams.On the basis of the expression obtained for HSC without stirrups and of the model already proposed for NSC with stirrups, a shear strength expression for HSC beams with stirrups is derived. The obtained expression is applied to an experimental program of 116 HSC beams with stirrups, and is found to predict the test results better than ACI Code [1], Eurocode [12] and CEB/FIP Model Code [9].A design formula is hence proposed, which is adequately conservative and accurate.A design example of a HSC beam with stirrups is carried out, and the various design expressions previously considered are compared.     

Fir and chestnut timber beams reinforced with GFRP pultruded elements

High-performance fibers are being widely researched for repair and rehabilitation in civil engineering structures. The potential benefits, liabilities, and architectural considerations regarding the use of high-performance fibers for reinforcing wood beams are discussed. An experimental program based on a four- and three-point bending test configuration is proposed to characterize the stiffness and strength response of wood beams reinforced with pultruded GFRP (glass fibers reinforced polymers) elements. Improving wood mechanical characteristics through the use of fiber reinforced polymers often involves the use of adhesives, generally epoxy resins. For this reason mechanical, calorimetric and thermo-gravimetric analyses were performed on the resin utilized and bonding effectiveness was studied. Mechanical tests carried out on full-scale wood beams showed that the reinforcement with GFRP beams may produce strong increases in flexural stiffness and capacity. In addition, an analytical investigation based on a simple linear analysis was conducted to predict ultimate load. At the end of this paper results of the experimental program are presented and used for comparison with the analytical procedure.     

Pre-buckling imperfection sensitivity of pultruded FRP profiles

This paper presents a geometrically non-linear one-dimensional model suitable for analyzing thin-walled fiber-reinforced polymer profiles, which accounts for the effect of manufacturing imperfections. The kinematic model is developed under the hypotheses of small strains and moderately large rotations of the cross-sections, and is able to take into consideration the contribution of shear strains and the effects related to warping displacements.The aim of the study is to develop a proper tool to analyze the pre-buckling behavior of such beams, since current approaches based on two-dimensional finite element method analysis demand significant computational efforts to be applied to real structures. The numerical results underline the effectiveness of the proposed mechanical model in analyzing case studies of technical interest in Civil Engineering, and the relevant influence of geometrical imperfections on the structural performance of FRP components with regard to serviceability design requirements.     

Analysis of pultruded glass reinforced plastic beams with semi-rigid end connections

The influence coefficient method of analysis has been used to derive closed-form expressions for the mid-span deflection and end rotations of shear deformable uniform section beams with semi-rigid end connections. The formulae have been recast into performance indices which define the reduction in mid-span deflection, the increase in load carrying capacity and the increase in span relative to an otherwise identical simply supported beam for two practical load distributions: (1) a point load at mid-span and (2) a uniform load over the entire span. Expressions are also presented for the required rotation capacity of the semi-rigid end connections. Initial rotational stiffness data, derived from full-scale tests on web and web and flange cleat connections between two sizes of pultruded glass reinforced plastic (GRP) WF-section are used in the formulae to evaluate the performance indices for the practical range of span-to-depth ratios for load case (1). The values obtained quantify the benefits to be derived from exploiting semi-rigid end connection stiffness in the design of pultruded GRP beams with the current, very limited, range of section sizes.     

Optimum laminate design for strength and stiffness

A method is presented for minimum weight optimum design of symmetric fibre-composite laminates subject to multiple in-plane loading conditions which takes into account membrane stiffness requirements and strength limitations. The problem is cast as a non-linear mathematical programming problem in which the thicknesses of material placed at preassigned orientation angles are treated as the only design variables. The non-linear programming formulation is transformed into a sequence of linear programs employing an adaptation of the method of inscribed hyperspheres in which only critical and near critical constraints are considered at each stage in the procedure. Example applications illustrate that the method presented offers an efficient and practical optimum design procedure for the fundamental and recurring problem treated.     

Prediction of the ultimate strength of reinforced concrete beams FRP-strengthened in shear using neural networks

In the last years, a great number of experimental tests have been performed to determine the ultimate strength of reinforced concrete beams retrofitted in shear by means of externally bonded fibre-reinforced polymers (FRP). Most of design proposals for shear strengthening are based on a regression analysis from experimental data corresponding to specific configurations which makes very difficult to capture the real interrelation among the involved parameters. To avoid this, an artificial neural network has been developed to predict the shear strength of concrete beams reinforced with this method from previous tests. Furthermore, a parametric study has been carried out to determine the influence of some beam and external reinforcement parameters on the shear strength with the purpose of reaching more reliable designs. Finally, some modifications of the design expressions are proposed and checked with experimental results.     

高比强高孔隙率泡沫铝合金三明治梁

研究了高比强泡沫铝合金和泡沫纯铝的单向压缩和剪切性能,对以高比强泡沫铝合金为夹芯的三明治梁失效模式的尺寸范围和承载能力进行了理论计算,结果与实验结果符合得很好.给出了泡沫铝合金三明治梁的设计方法,表明在较小的设计载荷下,三明治梁是以刚度作为设计的控制条件;在较大的设计载荷下,以相对强度为设计的控制条件.泡沫铝合金三明治梁与泡沫纯铝三明治梁的对比表明,在刚度设计控制条件下,前者的刚度是后者的1.00～1.185倍.在强度设计控制条件下,剪切破坏和压凹破坏失效模式下的泡沫铝合金三明治梁比泡沫纯铝三明治梁的极限强度分别提高57%～180%和90%～220%.     

Pre-buckling and post-buckling failure at web-flange junction of pultruded GFRP beams

Separation at the web-flange junction is a common failure mode of pultruded glass fiber-reinforced (GFRP) beams subjected to bending. The causes of this separation appear to depend on the presence of lateral supports to prevent lateral buckling. To clarify the driving mechanisms, four-point bending experiments were carried out on pultruded GFRP girders. Lateral buckling was prevented by using lateral supports. Web-flange separation failure due to exceeding the shear strength was observed before any buckling was seen. Furthermore, nonlinear FEA was performed to identify the critical stress states of GFRP beams from the research literature, without any lateral supports in the post-buckling phase. In this case, based on numerical calculation, the critical stress states and their locations depended significantly on the shape of initial imperfections. The ultimate loads, with or without lateral supports, were predicted by a modified von Mises criterion applied to the stress states at the web-flange junction.     

Suspended carbon nanotube nanocomposite beams with a high mechanical strength via layer-by-layer nano-self-assembly

The fabrication and characterization of single-walled carbon nanotube (SWCNT) composite thin film micropatterns and suspended beams prepared by lithography-compatible layer-by-layer (LbL) nano-self-assembly are demonstrated. Negatively charged SWCNTs are assembled with a positively charged polydiallyldimethylammonium chloride, and the composite thin film is patterned by oxygen plasma etching with a masking layer of photoresist, resulting in a feature size of 2 μm. Furthermore, the SWCNT nanocomposite stripe pattern with a metal clamp on both ends is released by etching a sacrificial layer of silicon dioxide in the hydrofluoric acid vapor. I-V measurement reveals that the resistance of SWCNT nanocomposite film decreases by 23% upon release, presumably due to the effect of reorientation of CNTs caused by the deflection of about 50 nm. A high Young's modulus is found in a range of 500-800 GPa based on the characterization of a fixed-fixed beam using nanoindentation. This value is much higher than those of the other CNT-polymer composites reported due to organization of structures by self-assembly and higher loading of CNTs. The stiff CNT-polymer composite thin film micropattern and suspended beam have potential applications to novel physical sensors, nanoelectromechanical switches, other M/NEMS devices, etc.     

Selective reinforcement of LLDPE components produced by rotational molding with thermoplastic matrix pultruded profiles

This work is aimed to study the use of pultruded profiles for the selective reinforcement of linear low density polyethylene (LLDPE) parts produced by rotational molding. A preliminary screening on different types of pultruded profiles was performed, highlighting the relevance of adhesion to LLDPE in order to prevent debonding of the reinforcing pultruded profiles. As expected, high density polyethylene (HDPE) matrix pultruded tapes are characterized by a very high adhesion to rotomolded LLDPE. Therefore, HDPE matrix pultruded tapes, fastened on the inner surface of the mold, are incorporated into LLDPE during rotomolding. Plate bending tests performed on reinforced rotomolded plates and pressurization tests performed on the box shaped prototypes showed a significant increase of the stiffness with a negligible amount of reinforcement and increase of the weight of the component.     

Design and evaluation of various section profiles for pultruded deck panels

Local high stresses and buckling may be critical to the composite deck panels with moderately thick walls. Design and evaluation of section profiles for pultruded deck panels with the reduced local stress and improved buckling strength are discussed. The deck panels with several different cross-sectional profiles are assessed based on finite element analysis. The assessment covers the global and local stiffness, the maximum stresses and buckling strength. Compared to the original design, one of the panels with the modified profiles possesses significant improvement in all aspects evaluated. Most profiles provide lower compressive stress and higher local stiffness and significantly higher buckling strength.     

Optimum thickness of joints made of GFPR pultruded adherends and polyurethane adhesive

This paper presents results of experimental and numerical investigations on double-lap joints composed of pultruded GFRP profiles and polyurethane adhesive subjected to quasi-static axial tensile loading. The objective was to investigate the effect of the joint geometry on the structural response of adhesively-bonded joints and, in particular, to seek for experimental evidence of an optimum adhesive layer thickness. The influence of the adhesive thickness (0.3–10.0 mm) and the overlap length (50–200 mm) on the joint behavior was investigated. It was found that there is an optimum adhesive thickness of approximately 1.0 mm and joint strength consistently increases with the overlap length.     

Flexural analysis of reinforced concrete beams strengthened with a cement based high strength composite material

The structural behaviour of reinforced concrete beams strengthened with a system made by fibre nets embedded into an inorganic stabilized cementitious matrix named Fibre Reinforced Cementitious Mortars (FRCM), was investigated in this paper. The main issues focussed in the paper are: (i) the strengthening effect of the FRCM system on the flexural behaviour of reinforced concrete beams in terms of both ultimate capacity, deflections and ductility and (ii) the influence of the fibre reinforcement ratio on the occurrence of premature failure modes.The analysis refers to a FRCM system made by ultra-high strength fibre meshes such as the Polypara-phenylene-benzo-bisthiazole (PBO) fibres; PBO fibres have, in fact, great impact tolerance, energy absorption capacity superior than the other kind of fibres and chemical compatibility with the cementitious mortar.A total of 12 reinforced concrete beams strengthened in flexure with the PBO-FRCM system have been tested. The influence of some mechanical and geometrical parameters on the structural behaviour of strengthened beams, is analysed both at serviceability and the ultimate conditions. Results of a comparison between experimental results and theoretical predictions, obtained by models usually adopted for the analysis of FRP strengthened concrete structures, are, also, presented and discussed.     

Flexural analysis of reinforced concrete beams strengthened with a cement based high strength composite material

The structural behaviour of reinforced concrete beams strengthened with a system made by fibre nets embedded into an inorganic stabilized cementitious matrix named Fibre Reinforced Cementitious Mortars (FRCM), was investigated in this paper. The main issues focussed in the paper are: (i) the strengthening effect of the FRCM system on the flexural behaviour of reinforced concrete beams in terms of both ultimate capacity, deflections and ductility and (ii) the influence of the fibre reinforcement ratio on the occurrence of premature failure modes.The analysis refers to a FRCM system made by ultra-high strength fibre meshes such as the Polypara-phenylene-benzo-bisthiazole (PBO) fibres; PBO fibres have, in fact, great impact tolerance, energy absorption capacity superior than the other kind of fibres and chemical compatibility with the cementitious mortar.A total of 12 reinforced concrete beams strengthened in flexure with the PBO-FRCM system have been tested. The influence of some mechanical and geometrical parameters on the structural behaviour of strengthened beams, is analysed both at serviceability and the ultimate conditions. Results of a comparison between experimental results and theoretical predictions, obtained by models usually adopted for the analysis of FRP strengthened concrete structures, are, also, presented and discussed.