Behavior of Structural Composite Lumber T-Beam Bridge Girders after Fatigue Loading

Recent innovation in the engineered wood industry has produced structural composite lumber (SCL) that achieves excellent strength, stiffness, and efficient use of wood. Product variations of SCL, such as laminated veneer lumber (LVL) and parallel strand lumber (PSL), are currently being used in transportation to produce bridge girders and decks for rural and other low traffic volume roads. Although the elastic and shear properties of SCL are available, no attempt has been made to estimate the fatigue performance of bridge girders. This study tested 12 new and 2 old, weathered SCL T-beam bridge girders with material and preservative variations for AASHTO-specified flexural fatigue under a stress-controlled test setup simulating 60?years of service. Transverse posttension was applied to the girders simulating a real-life situation. Results from the study indicate that the girders are capable of withstanding the repetitive loads without much physical damage. A few of the LVL girders had severe delamination at the SCL-epoxy interface. The fatigued girders were loaded statically up to failure and compared with the ultimate flexural strength of fresh girders. The girders did not show any appreciable strength loss because of one million cycles of fatigue loading. There was no effect of SCL type and preservative treatment on fatigue strength.     

Finite-Difference Solution of a Both-End-Fixed Orthotropic Composite Beam under Uniformly Distributed Loading Using Displacement Potential Function Formulation

The elastic solution of an orthotropic composite beam under the influence of uniformly distributed loading is obtained numerically. Two ends of the beam are rigidly fixed, and the fibers are oriented along and perpendicular to the direction of the length of the beam. An efficient finite-difference computational scheme based on displacement potential formulation is used to analyze the present mixed-boundary-value elastic problem. The effect of several important parameters such as beam aspect ratio and fiber orientation on the elastic field are investigated. Solutions are presented mainly in the form of graphs and deformed shapes. Finally, the reliability as well as superiority of the present computational scheme is discussed by comparing the present solution with the corresponding finite-element solutions.     

Experimental Validation of a 10-m-Span Composite UHPFRC–Carbon Fibers–Timber Bridge Concept

This paper presents the experimental study of a new structure for a 10-m-span bridge deck, which takes into account the range of possibilities offered by new and high-strength materials along with the advantages of a traditional environmental friendly material. This 10-m-span element is formed by wooden beams braced at their ends on supports, a thin (7-cm-thick) upper slab made of precast ultrahigh performance fiber-reinforced concrete (UHPFRC), and fiber-reinforced polymer at the lower chord of these beams. The test program has been aimed at identifying the major critical aspects involved in producing an initial estimate of safety margins as well as validations of the design process and its underlying assumptions. Under the first loading configuration derived from live traffic loads, both the transverse and local bending of the thin UHPFRC slab were activated and confirmed by means of a three-dimensional finite-element computation. The second loading configuration corresponds to pure global longitudinal bending, with the bearing capacity being monitored up to the theoretical ultimate limit state loading and then beyond, up to experimental failure. Critical mechanisms and safety factors have also been identified. Though concept feasibility has been demonstrated, some aspects still need to be further optimized in order to obtain greater ductility and safer control over failure modes and occurrences.     

Robust Dynamic Sliding Mode Control of a Rotating Thin-Walled Composite Blade

This paper presents a robust control methodology for the dynamic response control of a rotating blade exposed to external excitations and modeling uncertainty. The blade is modeled as a tapered thin-walled beam with fiber-reinforced composite material. The dynamic response characteristics of a rotating composite beam are investigated; namely, ply angle, taper ratio, and various selected rotational speeds. To extend life and improve efficiency, a robust control methodology is implemented and structural tailoring composite properties are used. To this end, considering modeling uncertainties and external disturbance conditions, a sliding mode control is proposed and its performance and robustness are compared with the conventional linear quadratic Gaussian control.     

Performance of Glued-Laminated Timbers with FRP Shear and Flexural Reinforcement

Glued-laminated wood beams (glulams) have low allowable shear stresses relative to competitive engineering wood products such as parallel strand lumber and laminated veneer lumber. For heavily loaded applications such as garage door headers, the lower shear allowable stresses typically necessitate the use of larger glulam members relative to other engineered lumber. This paper reports on experimental research aimed at increasing the shear capacity of glulams. To increase the shear strength, a series of fiber-reinforced polymeric (FRP) pins are inserted into holes drilled transversely across the plies of the glulam. These pins are epoxy-bonded into place after the glulam has been produced. The test results show that the shear strength scatter in the pinned set of glulams is significantly reduced as compared to the unpinned specimens. Two- and three-parameter Weibull estimates of the service-level allowables increases between 40% and 100% for specimen sets reinforced by the FRP pins. The transverse reinforcing scheme also shows promise for aiding in engagement of composite flexural reinforcement plies into the laminate stack. Previous research has shown that the bonding between wood and FRP plies has potential long-term durability problems. Therefore, the pinning technology reported on in this work shows promise for providing a means for direct engagement (through bearing of the pins) of the FRP plies to the wood plies. This behavior has been demonstrated though the testing of larger-scale specimens with both transverse shear reinforcements and longitudinal FRP plies. The reinforcing strategies show potential structural and economic advantages, especially when the FRP is coupled with low-grade, low-value finger-jointed lumber.     

自由结构 柯宅

文章通过5个方面展示了柯宅从设计到建成的思想脉络。柯宅延续了春晓砖宅的"自由结构"思想,是其第二个原型。设计试图开启另一种结构的组织方式,探讨光与结构融合的可能性。整个柯宅内部弥散着均质的漫射光,只有在有限时段才会出现内部的直射光。柯宅还采用了"无"结构的手法,在内部与外部形成风、光、水的渗透,而这些被引导的自然元素经过精心组织,进一步形成了新的抽象形式,即光缝、断路、空梁等。柯宅与砖宅一样,具有不可质疑的内向化,而这些特殊的"光带"则试图与外界建立一个抽象的景观联系,从"有限的室内"转向"无限的室内"。     

经圆孔衍射的轴对称偏振光束在湍流大气中的平均光强分布和偏振特性

本文对经圆孔衍射的轴对称偏振光束在湍流大气中偏振特性进行了研究.分析表明,平均光强和偏振度与轴对称偏振光束的偏振方向无关,而与湍流大气折射率结构常数和截断参数密切相关.     

非守恒荷载作用下叠层梁的动力稳定性

利用动力学标准对次切线荷载作用下的叠层梁结构进行稳定性分析。该荷载由守恒和非守恒正切载荷组合而成,具有非守恒加载参数的特性,有助于研究荷载变化程度对于叠层梁稳定性的影响。利用动力虚功原理对叠层梁进行分析得到其切线刚度和质量矩阵。对各向同性叠层梁的守恒荷载计算结果与文献中的结论非常吻合。对于部分守恒的荷载,分析结果显示：针对各向同性叠层组合梁,在无阻尼情况下,所有小于或等于0.5的非守恒荷载参数的数值由发散不稳定性决定,其他的由则颤振不稳定性来决定。     

应用多收发孔径实现高分辨率宽测绘带的合成孔径雷达研究

测绘带宽和分辨率是合成孔径雷达(SAR)系统的两个关键指标,但SAR的脉冲工作方式限制了分辨率和测绘带宽的同时提高。为了解决这一问题,该文采用多个发射机和接收机进一步展宽测绘带和提高分辨率,同时增加了平均发射功率,提高了信噪比。文中介绍该技术的实现方法,并通过实际参数仿真与常规SAR和采用单发射机的多波束SAR进行了对比,证明了进一步展开测绘带的可行性。     

Torsional Stiffness of Prestressing Tendons in Double-T Beams

When a prestressed double-T beam is subjected to torsion, a pair of prestressing tendons resists torsional rotation because of the restoring action of the displaced prestressing tendons. A comprehensive formulation to account for the torsional restoring action of double-T beams is presented, based on Vlasov’s hypothesis of considering warping displacement in an open-section. The deformation energies of prestressing tendons and reinforcing bars are calculated based on the deformed geometry to obtain the total potential energy. A two-noded beam element with seven degrees of freedom per node approximates an axial displacement, two translations, two flexural, and one torsional rotations, and a warping displacement to derive the finite-element equilibrium equations by minimizing the potential energy function. The role of prestressing forces of the tendons on the torsional resistance and the limitations of the traditional transformed section approach are addressed when it is applied to torsional problems. As a numerical example, an existing three-span continuous double-T beam is analyzed, and the bimoment and angle of twist are compared to those calculated using conventional three-dimensional finite-element analysis and the analytical solution of governing differential equations.