Hani Salim

Flexural-torsional buckling of pultruded fiber reinforced plastic composite I-beams: experimental and analytical evaluations

Abstract In this paper a comprehensive experimental and analytical approach is presented to study flexural-torsional buckling behavior of full-size pultruded fiber-reinforced plastic (FRP) I-beams. Two full-size FRP I-beams with distinct material architectures are tested under midspan-concentrated loads to evaluate their flexural-torsional buckling responses. To monitor rotations of the cross-section and the onset of critical buckling loads, transverse bars are attached to the beam crosssection and are subsequently connected to LVDTs; strain gages bonded at the edges of the top flange are also used. The analysis is based on energy principles, and the total potential energy equations for the instability of FRP I-beams are derived using nonlinear elastic theory. The equilibrium equation in terms of the total potential energy is solved by the Rayleigh-Ritz method, and simplified engineering equations for predicting the critical flexural-torsional buckling loads are formulated. A good agreement is obtained between the experimental results, proposed analytical solutions and finite-element analyses. Through the combined experimental and analytical evaluations reported in this study, it is shown that the testing setup used can be efficiently implemented in the characterization of flexural-torsional buckling of FRP shapes and the proposed analytical design equations can be adopted to predict flexural-torsional buckling loads.

Analysis and design of fiber reinforced plastic composite deck-and-stringer bridges

A comprehensive study on analysis and design of fiber reinforced plastic (FRP) composite deck-and-stringer bridges is presented. The FRP decks considered consist of contiguous thin-walled box sections and are fabricated by bonding side-by-side pultruded thin-walled box beams, which are placed transversely over FRP composite stringers. In this study, we review the modeling and experimental verification of FRP structural beams, including micro/macro-mechanics predictions of ply and laminate properties, beam bending response, shear-lag effect, and local and global buckling behaviors. A simplified design analysis procedure for cellular FRP bridge decks is developed based on a first-order shear deformation macro-flexibility (SDMF) orthotropic plate solution. The present approach can allow the designers to analyze, design and optimize material architectures and shapes of FRP beams, as well as various bridge deck configurations, before their implementation in the field. Experimental studies of cellular FRP bridge decks are conducted to obtain stiffness coefficients, and an example of a cellular FRP deck on optimized winged-box FRP stringers under actual track-loading is presented to illustrate the analytical method. The experimental-analytical approach presented in this study is used to propose simplified engineering design equations for new and replacement highway FRP deck-and-stringer bridges.

Torsion of Open and Closed Thin-Walled Laminated Composite Sections

Vlasov’s theory is expanded to perform the linear analysis of open and closed sections made of general laminated composites. Following the guidelines of Mechanics of Laminated Beams, the transverse shear deformations are included in the formulation. To overcome the overestimation of stiffness by the often assumption of negligible tangential stress, the laminate resultant force and moment are instead set to zero in this paper. The present model accounts for all possible elastic couplings in composite sections, such as extension- and bending-torsion. Applications of the formulation to flanged and box sections are presented and guidelines are provided on how to evaluate the torsional stiffness coefficients of the beam. A general solution for a fixed composite beam under tip torsional loads is derived, and the steps can be used to derive solutions for any other boundary conditions. The effect of warping-torsion on the torsional stiffness of the beam is investigated. Composite beams were tested in torsion and the angle of twist and shear strains were measured for verification of the analytical model predictions, which agree closely with the experimental results and finite element results.

Shear Moduli of Structural Composites from Torsion Tests

A combined experimental/analytical approach for effective evaluation of in-plane and out-of-plane shear moduli of structural composite laminates from torsion is presented. The test samples are produced by pultrusion and consist of E-glass fiber systems and vinylester resin. Three types of rectangular samples are used: unidirectional samples cut from plates; angle-ply and angle/cross ply samples cut from wide-flange beams. The shear moduli are obtained from the experimental torsional stiffnesses (T/) and data reduction techniques based on torsion solutions by Lekhnitskii and Whitney. The classical approach of usingpaired samples of different widths but same material orientation can grossly underestimate out-of-plane shear moduli. Thus, to overcome this problem, samples with material orientations normal to each other are used, and to obtain samples of larger dimensions than the available thicknesses of the material, the pultruded laminates are bonded flat-wise, from which transverse strips of different widths are cut. Consistent results are obtained by the two data reduction techniques, and the experimental values are compared to predictions by micro/macro-mechanics models.

Shear Lag of Open and Closed Thin-walled Laminated Composite Beams

Harmonic technique for shear lag analysis is extended to thin-walled open and closed composite beams. The cross section laminates are considered in isolation and are loaded by the shear flow and the axial stress resultants. In addition, the equilibrium and compatibility equations are established for each laminate and the boundary conditions at the laminate edges are satisfied. The shear lag model is incorporated into a beam model, from which the beam stiffness coefficients, shear flow, and stress resultants are obtained. Explicit shear lag formulas for various sections are developed. Composite beams are tested in bending for verification of the analytical model predictions, which agree closely with the experimental results.

Modeling blast wave propagation using artificial neural network methods

The paper reports on work concerned with the development of artificial neural network approaches to modeling the propagation of bomb blast waves in a built-up environment. A review of current methods of modeling blast wave propagation identifies a need for a modeling system that is both fast and versatile in its scope of application. This is followed by a description of a preliminary study that used artificial neural networks to estimate peak pressures on buildings protected by simple blast barriers, using data generated from, first, an existing empirical model and, second, miniature bomb-barrier-building experiments. The first of these studies demonstrates the viability of the approach in terms of producing accurate results very rapidly. However, the study using data from live miniature bomb-barrier-building experiments was inconclusive due to a poor distribution of the sample data. The paper then describes on-going research refining this artificial neural network approach to allow the modeling of the time-wise progress of the blast wave over the surfaces of critical structures, facilitating a three-dimensional visualization of the problem. Finally, the paper outlines a proposed novel method of modeling blast wave propagation that uses a coarse-grain simulation approach combined with artificial neural networks, which has the goal of extending modeling to complicated geometries while maintaining rapid processing.

ENGINEERING PROPERTIES OF RECYCLED PLASTIC PINS FOR SLOPE STABILIZATION

An ongoing demonstration project has shown the feasibility of using slender (90 mm × 90 mm × 2.4 m) recycled plastic pins (RPPs) for in situ reinforcement of slopes and embankments. The technique uses RPPs driven into the face of the slope in a grid pattern to intercept the sliding surface and “pin” the slope. The engineering properties of the RPPs, including the compressive, tensile, and flexural strength along with creep behavior, dictate the design and construction practice. Constituent materials and manufacturing processes are highly variable among the more than 30 U.S. manufacturers. A specification for acceptance of the members is needed; however, before an effective specification can be developed, the appropriate engineering properties and design requirements for the RPPs must be determined. The engineering properties and driving performance of four different types of members were evaluated and are reported on. Additional evaluations are under way.

Efficient 3D modeling of damage in composite materials

Due to the complexity of composite material, numerical methods are generally utilized in their analysis and design. Commercial finite element (FE) codes, such as ANSYS and ABAQUS, allow the implementation of user subroutines in the program, which provides the advantage of using high meshing and solving technologies besides the improvement of materials and/or elements models. Nonlinearities arise for many engineering problems, for example, the progressive damage of a composite element that contains sources of stress concentration or damage localization such as holes, bolts, and/or flaws causes nonlinear material behavior. In order to simulate this nonlinear behavior, especially in 3D, an accurate material constitutive model is required. Therefore, the objective of this paper was to simulate the 3D progressive damage model of composite materials by using simple numerical models. In this paper, ANSYS user subroutine (USERMAT) was used to simulate the progressive damage behavior of a composite plate containing holes using simple models. Three different material models were used: ply discount model (PDM), simple progressive damage model (SPDM) by adding an empirical progressive damage criteria to the PDM, and continuum damage mechanics model (CDMM). Good agreements were observed between SPDM, CDMM, and published experimental results. Furthermore, CDMM showed the least dependence on mesh size. Three different damage evolution laws, linear, quadratic, and degradation laws, were adjusted and tested. It was found that there was no significant difference in the predicted failure load between these selected laws.

Blast Performance of Single-Span Precast Concrete Sandwich Wall Panels

AbstractA research program was conducted to assess the capability of conventional non-load-bearing insulated precast concrete exterior wall panels to withstand blast loadings. Typical construction details from the tilt-up and prestressed concrete industries were examined. The sensitivity of insulation type, reinforcement, foam thickness, and shear tie type on the flexural resistance was assessed. Forty-two single-span static experiments were conducted on 14 different panel designs. From the results of these experiments, resistance functions and deformation limits for insulated concrete sandwich panels were determined. The resistance functions were used to develop predictive dynamic models for panels subjected to blast demands. The models were found to be accurate in comparison to measurements from four full-scale blast detonations. The findings of the research indicate that both prestressed and non-prestressed insulated concrete wall panels meet current rotational limits defined by the U.S. Army Corps of ...

Parametric study for post-tensioned composite beams with external tendons

Strengthening of bridge superstructures composite beams with external post-tensioned tendons is a good technique for strengthening the existing structures. In this study, a numerical model is illustrated to study the nonlinear simulation of composite beams stiffened with externally post-tensioned tendons. The accuracy of the developed numerical model is validated using comparisons between the numerical and existing test data. The influence of various strengthening parameters is investigated, which include draped versus straight tendons, tendon length, the effect of post-tensioning on reinstating the flexural behavior of an overloaded beam, tendon eccentricity, and the degree of shear connection. A good agreement between the proposed model and the test data is obtained. The results demonstrate that at the same tendon eccentricity, the trapezoidal profile shows better behavior for the strengthened beams. However, more ductility is obtained when using the straight tendon profile. Applying post-tensioning through the beam of full length helps to reduce the creation of fatigue cracks, which always start at stress raisers, and subsequently increases the fatigue life of the composite beam. Also, the external post-tensioning effectively maintains the flexural behavior of the overloaded strengthened beam after unloading in comparison to the un-strengthened beam. It is observed that 80% degree of shear connection or higher is recommended to obtain the desired performance of the external post-tensioning force for strengthening composite beams.