Application Of Jointless Bridges In The Cold Area Of North China

Jointless bridges are also named Integral abutment bridges.Most conventional bridges possess expansion joints and bearings, which are expensive in terms of material and installation costs. For many years, expansion joints have caused considerable maintenance problems for transportation agencies, as water, salt, and deicing chemicals penetrate through them and cause extensive deterioration to the bridge components. And the phenomena of vehicle bumping at bridge-head are always occurred in highway bridges. This severely affects the operate level of highway, and damages the structure of bridge and pavement, and increases the maintenance cost. Bridge pursuers in the world wanted to find best expansion joints. At last, they draw a conclusion that the best expansion joint is jointless. Elimination of expansion joints in bridges may reduce the construction cost, overcome many of the maintenance problems and avoid vehicle bumping at bridge-head.Integral bridges have numerous favorable attributes and few limitations. Integral abutment bridges provide bridge engineers with an economical and attractive design alternative to traditional bridges with thermal expansion joints. In the United States of America, jointless bridges with integral abutments have been widely used. An investigative report of 2004 carried by AASHTO indicates that there are 13,000 Integral Abutment Bridges that are useing in America, and 4,000 of which are Semi-Integral Abutment Bridges. It is also widely used in other countries such as Japan, Australia, New Zealand, France, Switzerland, Canada.The special structure of the Integral bridges determines the complex behavior of mechanical. The real behavior of integral abutment bridges is extremely complex because of the development of secondary forces due to the movements induced by temperature, creep, and shrinkage. We attempted to construct the integral bridges at the end of last century in China. There are only several Integral bridges in China, and all of which lie in the warm area of south China. No Integral bridge has been built in the cold area of north China. The annual range of temperature of the cold area is much bigger than that of south China. Therefore the temperature stress of bridge is much bigger. But the temperature stress can be relieved because of the creep. If we want to build Integral bridges in the cold area, we must be conscious of how much the creep affects the temperature stress and shrinkage stress.Shrinkage mechanism of Concrete can be summarized as follows: Spontaneous shrinkage, drying shrinkage and carbonization shrinkage. Creep mechanism of Concrete can be summarized as follows: viscoelastic theory, seep-off theory, viscous flow theory, yield flow theory, internal force balance theory and microcrack theory and so on. Most influence factors of the shrinkage and creep are same, such as variety of the cement, aggregate, W/C, curing Conditions, Work environment and admixture etc. Their biggest difference is the creep is closely related to the stress.Shrinkage has something to do with the time but it is not dependent on the stress. For the statically indeterminate structure, the creep of concrete can affect the stress and the deformation of bridge greatly. Creep refers to a phenomenon that the strain continues to develop under the continuous stress. It is closely related to the process of the load. Temperature load include sunshine load, cooling loads and load of annual temperature changes. When we study the coupling stress of the creep, shrinkage and the temperature stress, we need only consider the annual temperature load in that the creep is a long-term effect. Shrinkage, creep and temperature effects of the concrete are unavoidable in the actual work. And their effect to the structure is complex. For example, the creep can cause prestress loss in prestressed girder, and increase the deflection of large-span beam. So the creep is harmful to the structure. But it appears as a relaxation effect in the statically indeterminate structure, which is useful to the bridge now.To a rigid fixing concrete beam, when the initial stressσ0 load on it, and after a term"t", the new stressσ(t ) is letter thanσ0. K (t ) =σ0/σ(t).To get a differential function, we always select the specific creep whose expression is usual an exponential function. After we have got K (t ) , we can get the total stress based on principle of linear superposition.The existing standard of China is that suggested by CEB-FIP, which is unfit to get an exponential function. So we introduce numerical method. And we get the numbers of elastic strain of 100 years, then we can deduce K (t ) of every day in the 36000 days.Volatility of K (t ) is big at first, which is related to the rapidly developing of the early age concrete. Because the term is shorter than 2 month, and the temperature stress is little, then we can neglect the volatility. By all numerical analysis, we can take 0.5 as the suggested reduced rate. This reduced rate interpret that the stress is reduced much. If we take good measures to improve the method of pile foundation and the abutment backfill, the integral bridges can be built in the cold area of North China.In the future, the integral abutment bridges will have a good foreground in China and it would be beneficial to our country.