Study On Single Expansion Joint Bridge Applicable For Large Or Medium Bridge

Expansion joints are commonly used for bridges to accommodate temperature-induced deformations. However, practice has shown that expansion joints are inclined to damage and are difficult to maintain, which arises as a disturbing problem in the practice of bridge engineering. To solve the above problems, a semi-integral abutments full jointless bridge was proposed by researchers at Hunan University. However, this kind of bridge is only suitable for short- and middle-span bridges. To extend the application scope of jointless bridges, a novel single expansion joint bridge(SEJB) is proposed in this paper. An SEJB consists of a bridge girder, one expansion joint(installed at the original temperature centre of the bridge), a semi-integral abutment, a continuous reinforcement concrete pavement(CRCP) with saw kerfs, and ground beams. In doing so, conventional expansion devices installed at the end of the bridge against the abutments are canceled. To validate the safety, feasibility, and applicability of this kind of novel bridge system, this dissertation systematically studies the structural performance of the SEJB through theoretical and parametric analyses, experimental tests, and in-situ observations. Based on the present studies, the following original observations are notable:(1) CRCP is a primary component that absorbs the temperature-induced expansion of the main girder. In a decreasing temperature situation, the CRCP is in an eccentric tension state, caused by tension forces transferred from the main girder and friction forces transferred from the underlying basement. To analyze such behavior, a calculation model was proposed for the CRCP. By taking into account the mechanical characteristics of the pavement and the contact relationship between the pavement and the basemen, the pavement was divided into a number of segments at the crack positions, and the external force in each segment was assumed to have either a complete friction or an incomplete friction. Based on the above assumptions and the bonding-sliding theory applicable for the steel reinforcement bars and the concrete, the calculation model and differential equation formulas of each segment was established for the CRCP exposed to decreasing ambient temperatures. By solving the differential equations, a group of results could be obtained, including the stress in the steel bars, the average stress in the concrete, stress distribution over the depth of the concrete, and crack width. A calculation program was assembled to facilitate the solution of the equations.(2) A simplified calculation model is proposed for the SEJB according to its mechanical characteristic. Based to the balance condition that the horizontal forces should be in equilibrium to temperature center, theoretical formulas were deduced for the calculation of internal force and deformation. Then extensive parametric analysis was performed through a compiled calculation program. The analysis revealed that the following observations are notable. Friction coefficient between the bearing and the main girder has a significant effect on the mechanical characteristic of the SEJB. Thus, it is desirable to select bearings having small surface friction coefficient for an SEJB. An appropriate reinforcement ratio should be determined for the pavement so as to reduce the stress in the steel rebar and to disperse cracks. The pavement should neither be too long nor be too short, and a suitable pavement length for SEJBs up to 300 m is 20-30 m. Materials to be used for the basement should be selected with cautions, because the friction coefficient between pavement and basement influences the behavior of the SEJB significantly. There is more flexibility in selecting backfill materials for the SEJB relative to normal jointless bridges. Span length has very limited influence on the mechanical character of the SEJB, which should be attributed to that much of the thermal deformation in the main girder can be released by the single expansion joint installed at the original temperature centre. The analysis has validated that it is safe to extend the span length of the SEJB up to 280 m. Temperature actions, especially the actions caused by decreasing temperatures, has a significant effect on the mechanical character of the SEJB. The higher the temperature difference, the larger the internal force in the girder caused by decreasing temperatures. Consequently, it is preferable to make the closure of the bridge attained at low temperatures(e.g. at night or in winter).(3) To reveal the relationship among the extension deformation, the tensile stress, and crack of the CRCP under decreasing ambient temperatures, a full-scale experimental specimen(28m long) was fabricated and tested. The maximum tension displacement of the specimen was 4.363 mm, simulating a temperature decrement of 44° in the CRCP of a 300 m SEJB. The displacement and force were measured at the loading end and the fix end, respectively. It was also measured the strains and crack widths of the specimen, at certain intermediate sections. The test showed that cracks with uneven opening widths appeared at the saw kerfs and that no cracks developed between two adjacent saw kerfs. The displacement and force at the fix end were very small, which indicated that the pavement could absorb the force and deformation effectively. The test results were compared to the theoretical results obtained from theoretical analysis. The comparison indicated that the strains and average crack width derived from the theoretical analysis were only slightly larger than the tested results. The agreement implied that the theoretical analysis can reflect the mechanical behavior of the SEJB and can capture the crack width of pavement. In addition, the comparison showed that the calculated results were on the safe side.(4) A Parametric study was undertaken on factors that influence the stress distribution over the cross section of the pavement. Some useful recommendations were given based on the analysis results, shown as follows. It is favorable to adopt different segmental reinforcement ratios for the pavement. For the segments near the abutment, relative large reinforcement ratio should be used and the steel bars should be close to the upper surface. While for the remaining segments beyond the abutment, the reinforcement ratio can be reduced and the steel bars can be arranged near the middle of the cross section. The analysis also revealed that the spacing of the saw kerfs had significant influence on the cracking law of the pavement as well as the crack width. In addition, the analysis showed that the higher the cross section, the larger the stress gap between the upper and the bottom surface of the pavement.(5) Damage of slabs caused by uneven settlement in the embankment, located at the back of the abutment, is one of the important reasons that prevent the integral and semi-integral abutment bridges from being widely used. To study the stress and deformation of the slabs caused by uneven settlement in the embankments, by analyzing the construction features of the slab and the main form of the uneven settlement, the slab was assumed to be an elastic foundation beam, with the two ends simply supported and some intermediate positions detached from the ground. Parametric analysis was performed and some useful suggestions are therefore given. Double-layer steel bars should be arranged in the slab so as to prevent the slab from cracking at the negative moment zones; one layer is near the upper surface and the other one is near the bottom surface. The steel bars in the slab and the ground beam should be connected effectively, and the length of the detached zone between the slab and embankment should be appropriately controlled by relevant measures. It is recommended to adopt well-graded gravel and pebble as materials for the groud.(6) Based on the aforementioned serial studies, the first single expansion joint bridge of China was designed; i.e. the Dayankeng Bridge, a bridge in Qingyuan of Gudong province. The Dayankeng Bridge is 244 m long, which was opened to traffic in October of 2013. To observe the behavior of the real bridge, numerous sensors were instrumented to the bridge so as to monitor the temperature, the expansion relative movement, the strain of the CRCP, the earth pressure of the abutment and ground beam. The in-situ observation was performed every 15 days and lasted for 19 months, during which time the bridge experienced at least once the annual highest and lowest temperature. The obtained data revealed that the bridge performs well so far. The expansion joint works well, and there is not any visible cracks at the structure surface. In addition, there is no sign of uneven settlement, bump, and damage at the end of the bridge.