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Recent research studies have confirmed that curved I-girders are capable of developing substantial shear postbuckling resistance due to tension field action and have demonstrated that the AASHTO LRFD equations for the tension field resistance in straight I-girders may be applied to curved I-girders within specific limits. However, the corresponding demands on intermediate transverse stiffeners in curved I-girders are still largely unknown. Furthermore, a number of prior research studies have demonstrated that transverse stiffeners in straight I-girders are loaded predominantly by bending induced by their restraint of web lateral deflections at the shear strength limit state, not by in-plane tension field forces. This is at odds with present Specification approaches for the design of transverse stiffeners, which are based on (1) providing sufficient stiffener bending rigidity only to develop the shear buckling strength of the web and (2) providing sufficient stiffener area to resist the in-plane tension field forces. In this research, the behavior of one- and two-sided intermediate transverse stiffeners in straight and horizontally curved steel I-girders is investigated by refined full nonlinear finite element analysis. Variations in stiffener rigidity, panel aspect ratio, panel slenderness, and stiffener type are considered. New recommendations for design of transverse stiffeners in straight and curved I-girder bridges are developed by combining the solutions from the above FEA studies with the results from prior research.

Stiffeners are used as a means of providing additional support to columns at beam connection locations. They are added when the strength of the column is exceeded but full moment strength of the beam section is desired. In determining the design of column stiffeners, there are no specifications for determining the distribution of load between the column web and stiffeners. The AISC Load and Resistance Factor Design Specifications provides guidelines for determining the stiffener area but no specifications are given. The actual loads taken by the stiffener and web are therefore not truly known. In this study, experiments were done to determine the load supported by the stiffeners and web when tensile forces are applied to the specimen. The initial stiffener design for the test specimens was based on LRFD guidelines. The actual load distribution between the column web and stiffeners is determined from strain data obtained during testing. This distribution is compared with the assumed loads obtained from the initial LRFD calculations. Finite element analysis is also utilized to confirm the consistency of the results obtained from the experiments. Using this information, a new method is developed which better predicts the distribution of load between the column web and the stiffener. / Master of Science

A new method of connecting cross-frames to bridge girders had been proposed to alleviate concerns with current design practices. This new, half-pipe detail needs to be examined for fatigue issues that may exist which would make it infeasible as a replacement candidate for the current bent-plate design. A program of laboratory testing was carried out to determine the comparative performance between the half-pipe and the bent-plate designs. These tests were then translated into a finite element model which was examined to determine behavior over a wide range of designs scenarios. Finite element results, along with the laboratory testing data, were used to determine the appropriate use of the half-pipe stiffener. / text

Ultra-lightweight thermoformed stiffened structures are emerging as a viable option for spacecraft applications due to their advantage over inflatable structures. Although pressurization may be used for deployment, constant pressure is not required to maintain stiffness. However, thermoformed stiffening features are often locally nonlinear in their behavior under loading. This thesis has three aspects: 1) to understand stiffness properties of a thermoformed stiffened ultra-lightweight panel, 2) to develop finite element models using a phased-verification approach and 3) to verify panel response to dynamic loading. This thesis demonstrates that conventional static and dynamic testing principles can be applied to test ultra-lightweight thermoformed stiffened structures. Another contribution of this thesis is by evaluating the stiffness properties of different stiffener configurations. Finally, the procedure used in this thesis could be adapted in the study of similar ultra-lightweight thermoformed stiffened spacecraft structures.

The research addresses the effect of diagonal and horizontal stiffeners on shear transfer in rigid frame square knees. Rigid frame square knees are an integral component of pre-manufactured metal building systems. This paper examined a more efficient design of the rigid frame square knee. Five full-scale laboratory tests on square knee joints were performed to verify the effects of a diagonal stiffener on shear transfer. Two frames were fabricated without diagonal stiffeners, and three frames were fabricated with 1/8- inch- thick diagonal stiffeners, not extending the full diagonal length of the knee web. Experimental results, coupled with a finite element analysis, are compared to AISC provisions in Section F4: shear yielding and shear buckling, and in Section G3: tension field action. This research shows that if diagonal stiffeners are needed, then thin, shortened diagonal stiffeners are sufficient to restrain shear buckling of the knee web until shear yielding occurs.

This dissertation discusses the thermomechanical analyses performed on threaded fasteners and curvilinearly stiffened composite panels with internal cutouts. The former problem was analyzed using a global/local approach using the commercial finite element software ANSYS while a fully functional code using isogeometric analysis was developed from scratch for the latter. For the threaded fasteners, a global simplified 3D model is built to evaluate the deformation of the structure. A second local model reproducing accurately the threads of the fasteners is used for the accurate assessment of the stresses in the vicinity of the fasteners. The isogeometric analysis code, capable of performing static, buckling and vibration analysis on stiffened composite plates with cutouts using single patch, multiple patches and level set methods is then discussed. A novel way to achieve displacement compatibility between the panel and stiffeners interfaces is introduced. An easy way of modeling plates with complicated cutouts by using edge curves and generating a ruled NURBS surface between them is described. Influence on the critical thermal buckling load and the fundamental mode of vibration due to the presence of circular, elliptical and complicated cutouts is also investigated. Results of parametric studies are presented which show the influence of ply orientation, size and orientation of the cutout, and the position and profile of the curvilinear stiffener. The numerical examples show high reliability and efficiency when compared with other published solutions and those obtained using ABAQUS, a commercial software. / PHD / Aircraft in flight are subjected to different loads due to maneuvers and gust; there external forces cause internal loads and depend on the location of the panel in the aircraft. The internal loads, may result in the buckling of the panel. Hence, there is a need for studying structural efficiency and develop strong and stiff lightweight structures. Stiffened composite panels is a technology capable of addressing these needs. However, when used in space vehicles moving at hypersonic speeds, such structures experience significant temperature rise in a very short time resulting from the aerodynamic heating due to friction between the vehicle surface and the atmosphere. Such phenomena is more prominent during reentry and launch processes. Hence, it is really important to consider thermal effects while designing and analyzing such structures. Composite stiffened panels have many advantages like small manufacturing cost, high stability, great energy absorption, superior damage tolerance etc. One of the main failure modes for stiffened composite panels is thermal buckling. An extensive literature review on thermal buckling of stiffened composite panels was conducted in this dissertation. Thermal buckling and vibration analysis as well as a parametric study of a stiffened composite panel with internal cutouts was conducted, and verified using ABAQUS, a Finite Element Software.

Cross frames and diaphragms are essential to the stability of straight steel girder bridge systems as they help to resist lateral torsional buckling during construction and horizontal loading conditions. In skewed bridge systems, cross frames are often oriented parallel to the supports and hence, at an angle to the girder. To facilitate construction fit-up, plates, bent to match the skew angle, form the cross frame to stiffener connection. While the bent plate connection is a simple solution, it could introduce undesirable flexibility into the system, potentially compromising the effective brace stiffness. A proposed detail utilizing half pipe stiffeners may provide enhanced structural performance, while possibly reducing overall fabrication costs. Field and laboratory tests to determine the stiffness of both connection types are presented in the thesis. / text

Lateral bracing systems consisting of cross frames and their connections play a significant role in the elastic buckling strength of steel girder bridges. By providing lateral and torsional stability, they prevent lateral torsional buckling of the girder during bridge construction prior to the concrete bridge deck curing. To perform this function, the bracing system must possess adequate strength and stiffness. And since each component of the bracing system acts in series, the overall stiffness of the system is less than the least stiff component. In skewed bridges, cross frames at the ends of the girders are installed parallel to the bridge skew angle, and their connection to the girder requires that the cross frames be at an angle that prohibits welding a stiffener from the cross frame directly to the girder web. To make this connection, many states use a bent plate to span the angle between the web stiffener and cross frame. While this bent plate connection is now being widely used, it has never been rationally designed to account for its strength or stiffness in the bracing system. Results from field studies show that the bent plate connection may be limiting the cross frame stiffness thereby hampering its ability to provide stability to the girder during construction. The result is significant girder end rotations. The purpose of this research is to classify the impact of the bent plate connection on the end cross frame stiffness in skewed straight steel girder bridges and propose methods to improve the end cross frame’s structural efficiency. This research uses laboratory testing, finite element modeling, and parametric studies to recommend design guidance and construction practices related to the end cross frames of skewed steel girder bridges. In addition to recommending methods to stiffen the existing bent plate connection, an alternative pipe stiffener connection is evaluated. The pipe stiffener not only offers the possibility of a stiffer connection, but can also provide warping restraint to the end of the girder which may significantly increase the girder elastic buckling capacity. / text

The aim of this master’s thesis was to create a component model of a door handle stiffener used by the Volkswagen concern, which can be used for crash computations. Also to tune its parameters the way, its behavior corresponds the most to the real part’s. In the theoretical part the current regulations of the Euro NCAP are presented, concerning the testing and evaluation of the passive safety of new vehicles. Attention is focused on the evaluation of the side impact barrier tests, where the effect of the door handle stiffener’s damage is reflected the most. Shown are the reasons for the effort to simulate the real behavior of the stiffener, the factors, which initialized the born of the studied problem. The practical part starts with the creation of the FEM mesh of the part based on its 3D CAD model, also describes the requirements for the mesh quality, as well as the used tools and methods. Further on investigated are the characters of real damages of the door handle area during side impacts, based on which the component tests are proposed for the validation of the simulation model. Experimental research consists of the stiffener’s testing for simple bend and twist loads, three specimens each. After the execution of the tests the results get compared with the corresponding simulations. Modifications are made on the model according to the acquired results: refinement of the FEM mesh, new material model usage with failure for shell elements and definition of real material characteristics for the used thermoplastics. The latest obtained simulation dependencies are compared with the measured values again, the results are evaluated at last.