Keywords

1 Introduction

The main bridge of the Bayi Bridge project in Yangxin County adopts a 1–130 m through concrete-filled steel tubular tied arch bridge [1,2,3,4], with a calculated span of 127.04 m, a rise span ratio of 1/5 of the arch ribs, and a rise height of 25.40 m, as shown in Fig. 1. The main bridge is located on a 2.3% symmetrical longitudinal slope and a convex vertical curve with a radius of 3000 m. The transversal arrangement of the bridge deck is 2.5 m (sidewalk) + 2.0 m (arch rib area) + 0.5 m (collision barrier) + 11.5 m (three lanes) + 1.0 m (facility strip) + 11.5 m (three lanes) + 0.5 m (collision barrier) + 2.0 m (arch rib area) + 2.5 m (sidewalk) = 34 m.

The main bridge adopts a full-width design with two arch ribs, each of which adopts a dumbbell-shaped steel tube concrete structure. The diameter of the upper and lower chords of the bridge is 1264 mm, with a wall thickness of 28 mm, a web height of 0.684 m, a wall thickness of 28 mm, and a total height of 3 m for the arch ribs. It is made of Q390D steel. The chord of the arch rib and the interior of the abdominal cavity are filled with C60 micro expansive concrete, and the concrete is poured using the pumping-up method. Each steel pipe arch rib is manufactured in 7 sections, with the middle section being the closure section. The steel pipe arch rib is constructed using the support method [5,6,7].

A total of 5 wind braces are set up for the entire bridge, including an “I” type wind brace and four K-type wind braces. The “I” type wind bracing uses two steel pipes with a diameter of 900 mm to form a space frame, and the web pipe uses a steel pipe with a diameter of 600 mm. K-type wind bracing is composed of one horizontal steel pipe with a diameter of 1200 mm and two slant support steel pipes with a diameter of 900 mm. All wind braces have a wall thickness of 20 mm. The wind support is manufactured in the factory and constructed by on-site lifting and welding.

Fig. 1.
figure 1

Layout of Main Bridge Type 1/2 of Bayi Bridge Project (cm)

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The tie beam adopts a prestressed concrete structure with a height of 3.0 m and a width of 2 m. The thickness of the top and bottom plates is 0.65 m, and the thickness of the web plate is 0.5 m. A chamfer of size 0.2 × 0.2 m is set inside the box. A total of 28 prestressed steel strands of 19–15.2 and 1860 MPa are set up for a single tie beam, and the tie beam is constructed using the full support method. The middle and end crossbeams are set between the tie beams, both of which are prestressed concrete structures, and a concrete bridge deck is set above the crossbeam.

In the design guidance plan of the bridge, the arch rib assembly bracket is removed after the completion of the steel pipe concrete construction of the arch rib. Considering that the arch rib assembly support bears a high load and poses a high safety risk during the construction process of steel tube concrete pressure injection jacking, the dismantling sequence of the arch rib assembly support is optimized, and a comparative analysis is conducted on the pre and post optimized schemes from the perspective of structural stress.

2 Dismantling Plan for Steel Pipe Arch Rib Assembly Support

2.1 Original Plan

This article mainly studies the removal sequence of steel pipe arch rib assembly brackets, so only attention needs to be paid to the removal of steel pipe arch rib brackets and previous construction steps. According to the design drawings, the construction process of the original plan is shown in Fig. 2. The specific steps are ① Sequentially construct the pile foundation, bearing platform, pier column, and cover beam, set up brackets at the corresponding positions of the tie beam, pour concrete for the tie beam and arch foot on the brackets, and embed the steel pipe arch rib embedded section. ② Symmetrically tensioned tie beams with longitudinal prestressed steel tendons (N1, N8). ③ Symmetrically lift the arch ribs of the first, second, and the arch top sections in sequence, place the connections on the arch rib assembly support platform, complete the closure of the arch ribs, and symmetrically install the wind braces. ④ Pour concrete into the upper chord. ⑤ Pour concrete into the lower chord (after the concrete in the upper chord forms strength). ⑥ Pour concrete into the abdominal cavity of the chord (after the concrete in the lower chord forms strength). ⑦ The second batch of prestressed steel tendons (N2 and N7) for symmetric tensioning tie beams. ⑧ Cast in place the end crossbeam, remove arch rib assembly support.

Fig. 2.
figure 2

Schematic diagram of the construction process of the original plan

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2.2 Optimization Plan

In order to reduce the stress on the arch rib assembly bracket, reduce construction safety risks, optimize the construction process, and facilitate on-site construction organization, an optimization plan for dismantling the steel pipe arch rib assembly bracket is proposed. Compare with the guidance plan and advance the two steps of “symmetric tensioning of the second batch of prestressed steel beams (N2 and N7) and dismantling of the arch rib assembly support” until the arch rib closure. The specific steps are ① sequentially construct the pile foundation, bearing platform, pier column, and cover beam, set up brackets at the corresponding positions of the tie beam, pour concrete for the tie beam and arch foot on the brackets, and embed the steel pipe arch rib embedded section. ② Symmetrically tensioned longitudinal prestressed steel tendons on tie beams (N1, N8). ③ Symmetrically lift the first, second, and arch ribs in sequence, connect and dispose of them on the arch rib assembly support platform, complete the closure of the arch ribs, and symmetrically install the wind braces. ④ Symmetrically tensioned the second batch of longitudinal prestressed steel tendons on tie beams (N2 and N7). ⑤ Cast-in-place end crossbeam; remove arch rib assembly support. ⑥ Pour concrete into the upper chord. ⑦ Pour concrete into the lower chord (after the concrete in the upper chord forms strength). ⑧ Pour concrete into the arch rib abdominal cavity (after the concrete in the lower chord has strength).

3 Structural Calculations

To verify the rationality of the optimization plan and compare the stress conditions of the original plan and the optimized plan, the finite element analysis software Midas Civil was used to establishing an analysis model and calculate the two plans.

3.1 Model and Construction Conditions

The prestressed tensioning of the tie beam, the assembly of arch ribs, and the pouring of concrete in the pipe are all constructed symmetrically upstream and downstream; The construction of intermediate tie beams and bridge decks does not need to be considered in the proposed construction steps; The wind braces and end tie beams are only subjected to self-weight and have relatively small loads. From the above factors, it can be seen that the construction simulation of wind braces and end tie beams can be ignored. Therefore, only one side of the arch rib calculation can reflect its regularity.

Fig. 3.
figure 3

Finite element model diagram

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The construction process of steel pipe arch ribs was simulated using Midas Civil to establish a finite element model, and the stress differences between the original plan and the optimized plan were compared and analyzed. The steel pipe arch rib and concrete inside the pipe are simulated using beam elements, and the construction process of concrete inside the pipe is simulated using the dual element method [8,9,10]; The arch rib assembly support is simulated using only compression node elastic support, and the elastic stiffness of each support from the side span to the middle span is 1097568 KN/m, 487808 KN/m, and 365856 KN/m; The tie beam adopts beam element simulation, and the full support of the tie beam adopts general support simulation to simulate the prestressed tensioning construction process of the tie beam; Elastic connection is used between the arch rib and the tie beam. The finite element model is shown in Fig. 3.

According to the construction steps, ten construction stages are divided. The main difference between the original plan and the optimized plan is that the two construction stages of “Tensioning prestressed N2 and N7; Dismantling the arch rib assembly support” in the optimized plan have been advanced to the lifting and closure of the arch rib. The specific construction steps are shown in Table 1.

Table 1. Recommended and Optimized Plan Construction Stage Working Conditions Table
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3.2 Results

The calculation results are shown in Table 2. Compared with the recommended design scheme, the maximum steel pipe stress in the optimized scheme increased by 11.7 MPa, while the compressive stress of the upper and lower chord concrete decreased by 1.8 MPa and 2.4 MPa, respectively. This is mainly due to the transfer of some concrete weight to the steel arch ribs, resulting in an increase in the stress on the arch ribs and a decrease in the stress on the concrete inside the pipes. According to the design calculation sheet, the maximum compressive stress of the arch rib steel pipe under frequent combined actions is 116.7 MPa. Considering an increase of 11.7 MPa, the maximum steel pipe stress in the optimized plan is 128.4 MPa. Since the steel pipe uses Q390 material, the maximum stress is less than the design strength of Q390 material, which is 335 MPa [11].

The arch crown deform 3.6 mm upwards in the design scheme and 4.6 mm downwards in the optimization scheme, with significant differences between the two. The main reason is that after the prestressed tension of the tie beam, the tie beam is shortened, which can cause the arch ribs to deform upwards. In the design plan, the bracket was removed late, the arch rib stiffness was high, and the downward deformation was small, so the final deformation is still upward; In the optimization plan, the support was removed early, and the concrete of inside the pipe was constructed in stages. The arch rib stiffness was relatively small, and the downward deformation was large, which was still more significant than the deformation caused by the prestressed tension of the tie beam. Therefore, the final deformation was downward. Although there are significant differences in the deformation of the arch ribs, the difference in deformation has little impact on the selection of the two schemes. It can be solved by setting the pre-camber.

In the optimization plan, the compressive stress of the beam concrete during the tensioning stage of N2 and N7 prestressed steel tendons is −11.5 MPa, the maximum stress of the steel pipe is 20.9 MPa, and the vertical deformation of the arch crown is 12.4 mm, all of which are not significant. This indicates that the pre-tensioning of N2 and N7 prestressed steel tendons in the proposed optimization plan has little impact on the structural stress.

Table 2. Recommended and Optimized Construction Stage Working Conditions Table
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4 Conclusion

In the design guidance plan for the main bridge of the Bayi Bridge project in Yangxin County, the arch rib assembly support is removed after the completion of the arch rib steel pipe concrete construction. Considering that during the construction process of steel pipe concrete top pressure injection, the arch rib assembly support bears a large load and has high safety risks, an optimization plan is proposed to remove the arch rib assembly support first after the steel pipe arch rib assembly is completed, and then top pressure injection of steel pipe concrete. By using Midas Civil to establish an analysis model for the stress analysis of the two schemes, the results showed that the maximum stress of the steel pipe arch rib in the optimized scheme increased by 11.7 MPa to 128.4 MPa, which is less than the material design strength; The compressive stress of the upper and lower chord concrete decreases by 1.8 MPa and 2.4 MPa respectively; The difference in arch rib deformation has little impact on the selection of the two schemes, and can be solved by setting pre camber; The tensioning of N2 and N7 prestressed steel tendons has little effect on the structural stress. In summary, the optimization scheme meets the stress requirements.