大跨度井字楼盖预应力加固方法

介绍了用低松弛高强度无粘结预应力钢绞线作体外预应力筋对大跨度井字楼盖进行加固的设计方法、节点构造、防火处理措施和施工要点,实践证明效果良好,可为类似工程实践所借鉴。     

体外预应力混凝土简支梁的应力分析

本文通过一组体外预应力简支梁的试验 ,研究了梁整体变形和体外预应力筋变形的关系 ,分析了因体外预应力偏心距引起的二次影响和转向块及预应力筋的数量及布置方式对体外预应力简支梁性能影响 ,推导出了正截面极限承载力计算公式 ,得出了一些有益的结论 ,为进一步深入研究体外预应力混凝土结构提供了实践与理论分析的依据。     

体外预应力混凝土简支梁的应力分析

本文通过一组体外预应力简支梁的试验,研究了梁整体变形和体外预应力筋变形的关系,分析了因体外预应力偏心距引起的二次影响和转向块及预应力筋的数量及布置方式对体外预应力简支梁性能影响,推导出了正截面极限承载力计算公式,得出了一些有益的结论,为进一步深入研究体外预应力混凝土结构提供了实践与理论分析的依据.     

Experimental study of the dynamic properties of cement- and lime-modified clay soils subjected to freeze-thaw cycles

Modified soils have been used in the recent construction of high-speed railways in northern China due to the lack of good fill material. But the behavior of these modified soils under increased dynamic loading of high-speed train traffic and their long-term performance based on the effects of freeze-thaw cycles have rarely been studied. In this paper, dynamic triaxial tests were conducted on cement- and lime-modified soils with different blend ratios. These soils were subjected to freeze-thaw cycles, and the threshold deviator stress and resilient modulus were studied. The results show that after repeated freeze-thaw cycles, the modified soils exhibit better performance than before modification, the cement-modified clay is superior to the lime-modified clay, and all of the soils' mechanical properties are visibly improved. Moreover, an optimal blend ratio was determined. Also, a critical deviator stress attenuation coefficient, η_f, is introduced to determine the optimal modification method and the appropriate mixture proportions to be used when soils are subjected to freeze-thaw conditions.     

Flexural behavior of steel I-beam prestressed with externally unbonded tendons

The external post-tensioning technique has been commonly used in the construction field because it facilitates the analysis of structures and is widely applicable for many types of structures. In this paper, the flexural behavior and strengthening effect of a bridge were studied using a steel I-beam that had been externally prestressed with unbonded tendons. Eleven steel beams were fabricated and tested in terms of the tendon type, the amount of tendon or prestressing force, the installation of a deviator, and the embedment of a draped tendon. The results show that the externally prestressing method creates a stiffer steel beam when the proper amount of prestressing force is applied and when detail techniques are appropriately combined.The experimental results were compared to a theoretical solution in order to verify whether the external post-tensioning method is useful for strengthening a steel bridge and whether it is applicable in the construction field.     

Experimental investigations on suffusion characteristics and its mechanical consequences on saturated cohesionless soil

The characteristics of suffusion and its mechanical consequences on saturated cohesionless soil with different initial fines contents at various stress states are presented in this paper. A series of seepage tests is performed by constant-flow-rate control mode with the measurement of the induced pore water pressure difference between the top and bottom of the tested specimen under the isotropic confining pressure. Back pressure is maintained constant in the tested soil specimen to ensure fully saturated soil condition. Cumulative eroded soil mass is continuously recorded by a consecutive monitoring system. Suffusion induced axial strain and radial strain of the 70mm-in-diameter and 150mm-in-height specimen is recorded during the seepage tests. The gap-graded cohesionless soil, which are assessed as internally unstable by existing evaluation methods, are tested. The mechanism of suffusion is demonstrated by the variation of hydraulic gradient, hydraulic conductivity, percentage of cumulative fines loss and volumetric strain during suffusion. The parametric study on the influence of two variables, effective stress level and initial fines content, on the mechanism of suffusion is elaborated. The mechanical consequences of suffusion are evaluated by conducting monotonic drained compression tests on the eroded specimens. Companion specimens without suffusion are tested for comparison purpose. The test results reveal that with the progress of suffusion, hydraulic gradient would drop and hydraulic conductivity would increase. Large amounts of fines are eroded away and correspondingly, contractive volumetric strain occurs. The larger effective confining pressure would lead to the less extent of suffusion. With larger initial fines content, more fines would be eroded away. The monotonic compression tests indicate that suffusion would cause the reduction of the soil strength at the major stage of drained shearing.     

Investigation of the fracture process with different loading path triaxial tests on saturated sandstone

 This paper discusses a detailed investigation to determine the microcrack closure and opening of a homogeneous isotropic sandstone during four different triaxial tests: undrained deviator triaxial test; undrained triaxial test with constantly increasing stress; drained triaxial test connected with decreasing confining pressure and drained triaxial test with constantly increasing stress. An analysis of the results is presented. Received: 18 May 1999 · Accepted: 4 April 2000     

Mechanoluminescent determination of the mode I stress intensity factor in SrAl2O4:Eu2+,Dy3+

The use of mechanoluminescence (ML) has enabled a new technique for the determination of the mode I stress intensity factor (SIF) in SrAl₂O₄:Eu²⁺,Dy³⁺ (SAO). The stress rate fields and its cumulative isostress contour patterns in terms of ML in the vicinity of a crack tip during compact tension were evaluated based on the two different stress criteria of the hydrostatic and deviator in triggering the trap-releasing processes of the ML mechanism. The magnitude and shape of the crack tip stress field predicted by the deviator stress criterion rather than by the hydrostatic stress criterion showed good agreement with the experimental ML evidence in a determination of the SIF from the ML intensity. The analysis derived from the deviator stress criteria not only allowed the possibility of characterizing the fracture toughness in a range of modes under static and dynamic crack propagation, but also provided a better understanding of the ML mechanism of the trap-releasing process in SAO.