Structural analysis of the main bridge of Li Shi viaduct?

Lishi High-tech Development Zone Longfeng Street planning width of 80m, the design route and its 30 ° angle of intersection. Considering the layout of urban pipeline network on both sides of the road, the net span of the bridge needs to be more than 120m, after a comparison of multiple programs, the final use of 85m +135m +85m three-span double-tower, single cable-stayed pre-stressed concrete partially cable-stayed bridge, the structural form of the tower and beam cementation, pier and beam separation, with the bottom of the beam with bearings. The lower part adopts reinforced concrete slab pier with bored pile foundation. The construction method is cantilever casting for the main girder. The general arrangement is shown in Figure 1.

2 Liushi viaduct main bridge technical standards

(1) highway grade: highway;

(2) bridge width: 2 × net -11m + 2 × 015m collision guardrail + 3m central median;

(3) design load: automobile - super 20, trailer -120;

(4) seismic basic intensity: 6 degree.

(3) Design load: car- super 20, trailer-120;

(4) Earthquake basic intensity: 6 degrees.

3 main bridge structural design

3.1 overall design

Leaving the stone viaduct single cable-stayed layout in the highway central isolation zone, beautiful shape and low cost. Structural form due to the use of tower and girder cementation, pier and girder separation, the bottom of the girder with bearings, so that part of the cable-stayed bridge is closer to the beam system, the force is clear, the structure is simple. And this structural form extends the basic self-oscillation cycle of the structure, reduces the seismic effect, and is conducive to improving the seismic and overall stability of the structure. The bearing adopts basin type rubber bearing. The piers use reinforced concrete slab piers to adapt to changes in temperature, concrete shrinkage and creep and other loads.

3.2 Main girder design Main girder adopts single box and three chambers with large cantilever section, outer web diagonal placement, box girder top plate width is 26m, web slope rate is unchanged, box girder bottom plate width from 15.6m gradient to 16.864m. 4.2m girder height at the top of the main pier, x in the middle, the side span direction of the girder height within the 45m range of the change of the second parabola, and the rest of the girder section of the same height, the girder height is 2.4m. The thickness of bottom plate of box girder closing section is 25cm, and the thickness of bottom plate at the end of block 0 is 46.2cm, in the section of girder height change, the change of bottom plate thickness adopts quadratic parabola. The thickness of the top plate is unchanged, the thickness of the side chamber is 28cm, the thickness of the middle chamber is 45cm, the thickness of the side web is 50cm, the thickness of the middle web is 35cm, the anchorage area of the diagonal cable is equipped with cross partition, the thickness of the side chamber cross partition is 30cm, the thickness of the middle chamber cross partition is 40cm. the cross section of the box girder is shown in Fig.2.

The main girder adopts a three-way prestressing structure, with stranded wire and high-strength fine-rolled threaded rebar in the longitudinal direction, stranded wire in the transverse direction, and high-strength fine-rolled threaded rebar in the vertical direction.

The main girder is constructed by hanging basket cantilever casting method, the section length of block 0 is 10m, the section length of girder 1 and 2 is 3m, the section length of girder 3 is 3.5m, the length of combined section is 2m, and the length of the rest of the girder sections is 4m. the maximum cantilever casting weight is 223.5t.

3.3 Design of the main tower

The main tower is calculated to have a tower height of 18m, and it is made of solid rectangle cross-section. The main tower is arranged in the center divider, and a saddle is provided on the tower to allow the cable to pass through. The diagonal cable is arranged in 2 rows across the bridge, and the saddles are also arranged in 2 rows.

The saddle adopts the form of split wire tube, and each split wire tube wears 1 stranded wire to facilitate the future change of a single cable. Anti-slip anchor plate is set at the exit of both sides of the diagonal cable to prevent the strand from sliding.

The cable is a single cable, considering the tensioning equipment, construction capacity and construction convenience, the single cable is arranged in 2 rows in the direction of the transverse bridge, and each cable consists of 31 epoxy sprayed strands. The saddle is also set up in 2 rows, adopting the form of split-wire tube. Anti-slip anchor plates are set up at the exit of the diagonal cables on both sides to prevent the strands from sliding. The cable adopts multiple anti-corrosion measures, single stranded wire for epoxy spraying, outsourcing single-layer PE, stranded wire cable outsourcing HDPE casing.

3.4 Substructure design

According to the drilling reveals, the stratum of the bridge site consists of Q4 alluvial deposits and Permian sandstone interbedded. the thickness of Q4 alluvial deposits is 7.8~8.8m, and the Permian sandstone consists of fully weathered, strongly weathered, weakly weathered and slightly weathered sandstone interbedded. The groundwater level is about 2.3~3.7 m. The foundation adopts drilled piles and is designed as embedded rock piles.

4 Static analysis of the main bridge structure

The abutment, main girder and main tower all use three-dimensional finite element beam unit, the cable-stayed cable adopts the tensile-only cable unit, according to the construction stage of the girder section and prestressing beam arrangement, the main girder is divided into 92 units, and the variable cross-section is handled as a variable cross-section unit within the range of the main tower is a rectangular cross-section, and each main tower is divided into 12 units; each pier is divided into 5 units; the entire bridge is divided into 5 units, and each pier is divided into 5 units, and each pier is divided into 5 units. The main tower is rectangular section, each main tower is divided into 12 units; each abutment is divided into 5 units; the whole bridge 44 cable ties are divided into 44 cable units, the whole bridge *** counts 170 units.

According to the construction design, in the construction stage, it is assumed to be cemented at the top of the right and left piers; in the operation stage of the bridge, the left pier is a fixed bearing, and the right pier and the two ends of the main girder are set up as a rolling bearing along the direction of the bridge, and the calculation schema is shown in Figure 3.

The following load combinations are adopted for the whole bridge:

(1) constant load + preload + concrete shrinkage, creep + car-super 20;

(2) constant load + preload + concrete shrinkage, creep + trailer-120;

(3) constant load + preload + concrete shrinkage, creep + car-super 20 + bearing sinking 1cm + deck plate Warming of 10℃;

(4) Constant load + Pre-loading + Concrete shrinkage, creep + Automobile-super 20 + Bearing sinking 1cm + Cooling of bridge deck plate by 10℃;

(5) Constant load + Pre-loading + Concrete shrinkage, creep + Automobile-super 20 + Bearing sinking 1cm + Warming of the whole by 20℃;

(6) Constant load + Pre-loading + Concrete shrinkage, creep + auto-super 20 + support sinking 1cm + overall cooling 20℃.

Calculation results show that in the use stage of the main beam is in full cross-section compression state, the maximum compressive stress appears in the root of the tower, to meet the design requirements. According to the results of local stress analysis, the stress distribution at the anchorage end of the prestressing beam and the support connection is complicated, and the local reinforcement should be strengthened.

5 Self-vibration characteristics of the main bridge structure

According to the above calculation model, I calculated the self-vibration characteristics of the structure. According to the need of seismic analysis, the first 100 steps of the structure were calculated, and the structural modes are shown in Fig. 4 to Fig. 7, and the first 15 steps of the structure's self-oscillation characteristics are listed in Table 1.

Table 1: First 15 orders of self-oscillation

The study shows that the cumulative longitudinal, transverse, and vertical contributions to the first 20 orders of vibration are very close to 100%, which is more than the 90% required by the UBC code. The first 20 steps of the vibration mode have covered the main dynamic characteristics of the structure. From the vibration mode diagram, the first three steps are mainly vertical vibration modes, and the transverse vibration modes appear in the fourth step, which indicates that the structure has good transverse stiffness; the vibration modes reflected in the three-dimensional solid unit are comprehensive, which can grasp the dynamic characteristics of the structure well and provide reliable theoretical analyses for the dynamic measurement of the bridge. The structure has good transverse stiffness.

6 Conclusions and Recommendations

The design practice of Lishi Viaduct provides another valuable experience for the successful application of partially prestressed concrete cable-stayed bridges in China. The partial cable-stayed concrete cable-stayed bridge with double towers and single cable face is clear in stress, simple in structure, beautiful in shape and low in cost. Partial cable-stayed bridges can reasonably choose the dimensions of each part according to the actual situation, so that the design freedom is greater. Through the structural static calculation and dynamic characteristic analysis, it shows that this type of bridge can fully meet the requirements of the use function, with less construction difficulty and shorter construction period, which provides a broader space for future bridge construction.

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