Niz | Puni Prikaz

wangkiky · 08-11-2016, 04:39 AM

key highway bridge stretching is one of the bridge construction and maintenance problems. Long bridge stretching dimension of exposure to the atmosphere, the environment was quite poor, the bridge structure is most easily damaged parts but more difficult to repair. Bridge expansion key slight defects in design, construction, or lack of, will cause its early destruction; the destruction of the bridge expansion joints, and may cause a lot of vehicle impact load, driving conditions deteriorate drastically reduce the life of the bridge. In this paper, combined with practical work experience for jointless bridge some influencing factors were analyzed, and put forward concrete support conclusions. 
 1. Construction details jointless bridges 
 concrete beams and steel beams adopt detailed structure jointless integral abutment. These figures detailed structure is just a basic skeleton, it can not reflect the important details and other design aspects. Detailed structure just to give the initial contact jointless bridges people with a basic concept and impressions, can not be described in more detail specific and describe in this article, to which we will be further addressed in future discussions. However, a preliminary understanding of the detailed structure for a comprehensive understanding of jointless bridge performance, integrity and durability will be very helpful. 
 2. Jointless bridge design factors 
 Because of jointless bridge stringers embedded in the concrete abutment, the upper part of the structure to be bound, so to consider the impact of the secondary load design. It summarizes the impact of the secondary load below. 
 (1) the size of the creep strain influence Creep depends span upper structure, age of concrete when subjected to loads, load duration, the quality of concrete, the shape of the ambient temperature and concrete elements. No concrete composite beam bridge stretching - with age varying effective elastic modulus and creep analysis of small and medium-span, steel this paper. The results show that the presence of creep can be reduced abutment and pier at the top of the cross section of the upper edge of the tensile stress, but also increases the compressive stress of the lower edge of the steel stringers (less than the stress caused by dead load of 10%, will not force structure adversely affect). Therefore, in the design of small and medium-span, steel - concrete composite beam bridge without expansion joints may not be considered when the creep in. 
 (2) shrinkage of the concrete bridge deck and steel stringers contraction will cause inconsistencies from stress within the structure. In addition, jointless bridge ends were partially or fully constrained, leading to shrinkage, creep can cause stress within the structure. Some scholars believe that the role of creep and shrinkage contrary, the two cancel each other. However, this paper shows that shrinkage and creep effects can not be fully offset, will be more than the presence of stress may lead to progressive development of cracks in concrete, and to carry out concrete cracks will be reduced so that the shrinkage stress portion. Therefore, when the expansion joints on the bridge without the analysis and design should consider the effect of the contraction of the upper structure. 
 (3) the effect of temperature on the effect of temperature jointless bridge is undoubtedly jointless bridge design in a very important factor, but in fact As previously noted, many bridges stress tests show stress due to temperature effects and determined to be smaller than expected, one can analyze the reasons is due to the expansion or contraction of the concrete temperature, it will produce creep. Creep will cause stress to the extent not foreseen in the design. So in order to make the theory better reflect the actual situation, some departments to consider the temperature of the elastic modulus of concrete is reduced elastic modulus to dynamic loads 1/3 of the design. The second is due to the relatively large size of most concrete structures , so that changes in the surrounding temperature which is less sensitive. AASHTO (American Association of State Highway and Transportation Officials) in the calculation of the temperature change which has been stipulated: "must take into account the massive concrete building or structure on the internal temperature of the atmospheric temperature is lagging behind." Maximum temperature cycle for a concrete bridge smaller than the steel bridge. Due to a larger volume of concrete to absorb heat, so its temperature does the same with the theoretical expected? In a way this can explain, why the thermal expansion coefficient [alpha] is almost the same as the two materials (concrete α = 0.00001, steel a = 0.000012), while the steel bridge changes in temperature much more sensitive than the concrete. As for the effect of temperature on jointless bridge in the quantitative analysis on the study needs further discussion. Two things can: 
 (4) Settlement Settlement of the substructure will cause the jointless bridge superstructure considerable stress, its impact depends on the structural characteristics and geometry of the bridge, as described above, the rigidity, the lower part of the structure of the settlement amount, cross number, span, and the support of high station and so on. GangaRao, who pointed out that the settlement has little effect on the single-span bridge in the 1981 study; for multi-span bridges, subsidence caused by dead load allows abutments and piers suffered a reaction force substantially equal to be resolved. Therefore, by rational design, jointless bridge can not consider the settlement of. 
 (5) Earth Pressure of thermal expansion when the upper structure, bridge abutment after the squeeze caused by passive earth pressure filling station. Calculation of earth pressure behind the station, there are many uncertain factors, such as the size of passive earth pressure and its distribution in the longitudinal and transverse directions. Although passive earth pressure causes axial forces and moments in the upper structure, but this study shows the effect of earth pressure but negligible, the authors recommend that in the analysis and design of jointless bridge can not consider earth pressure. 
 (6) taking into account the stress pile pile foundation and superstructure of the integral bridge together against the action of longitudinal movement has a direct relationship, during jointless bridge design: ① restrictions form the basis of integral bridges, the most good elongated vertical piles made of a single row. ② limit pile type. ③ to adjust the H-pile weak axis, so that it is consistent with the direction of movement. ④ provide a hinge device to control the deflection of the pile. ⑤ limit the size of the structure of the skew angle. 
 There are other problems, such as: how to balance the pressure on the initiative of integral abutment behind? Whether the whole bridge together after leaving a free space to allow for thermal expansion? If desired detail what kind of structure? and many more. Of course, to solve these problems, it is necessary in the future in the absence of expansion joints in bridge design continue to conduct research and practice. 
 By design considerations, if you set the control loop seam, the seam stress release, approaches the plate or arrange a certain sequence, and so construction can overcome some of the pitfalls jointless bridge, but the span restrictions and other aspects of the design to make some there are those who still tend to use stretch stitch bridges. 
 Although no bridge expansion joints in the United States has successfully built a long time, but so far there is not a more scientific design theory. The current design method is essentially dependent on experience and observation, is still not resolved analysis and structural design of the bridge expansion joints without fundamentally. Most states do use jointless integral abutment bridge, but the investigation also shows that most of the state's Department of Transportation in the design are also very conservative, you should be able to build more bridges. But to make the design more to be accepted there is still a need for some reasonable research and analysis. Although no bridge expansion joints in China has not yet carried out in-depth research, but in essence, this is the analysis and design of a bridge structure. 

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