1 Introduction

The special-shaped inclined tower cable-stayed arch bridge is a composite bridge with beam-arch system. The structure of the bridge is highly statically indeterminate, and the stress is extremely complex [1]. The most significant characteristic is that the internal force distribution of the structure can be changed by adjusting the hanger cable force, so as to optimize the overall structure stress. In the process of construction, the system transformation of the bridge is complicated due to the phased removal of the arch rib and the beam support, and the change of the linear shape and internal force is uncertain, which brings hidden trouble to the construction [2]. At present, there are few researches on construction monitoring of special-shaped cable-stayed Bridges [3]. Taking the pedestrian landscape bridge in Anyi County, Nanchang City, Jiangxi Province as an example, this paper introduces the construction monitoring of such Bridges.

2 Project Summary

The length of the bridge is 194.449 m, the full width of the deck is 6.0 m, the clear width is 5.4 m, and the span is arranged according to 15 + 2 × 75 + 15. The main arch ring is pentagonal concrete filled steel tube structure, the main span calculated span 65.0 m, lateral rotation front vector height 24.377, vector span ratio 1/2.666, the arch axis is circular curve. The transverse bridge of the arch rib itself tilts outward 22.088°, and the stability of the arch rib itself is guaranteed by the tension of the inclined hanger rod. Overlooking the main beam is s-shaped, steel box girder is used in design, steel box girder material is Q345qC, steel box girder is 1.5 m high, 6.0 m wide, 1.5 m wide cantilever is set on both sides. The thickness of the roof and bottom plate is 20 mm, the thickness of the web is 16 mm, and the diaphragm is set every 2 m or so. A 1 m wide beam is arranged at the cable tension position of the steel box girder. There are 16 suspender rods in the whole bridge. The upper end of the suspender is projected to be 7 m along the bridge design axis, and the lower end is projected to be 6 m along the bridge design axis. The steel box girder of the bridge deck is rigidly connected by diagonal braces and arch feet. The beam and arch combined stress system of two arch rings and main beam configuration special-shaped double inclined tower cable-stayed arch bridge is shown in the Fig. 1.

Fig. 1.
figure 1

Layout of bridge structure

3 Finite Element Analysis Model

The large-scale finite element analysis software MADIS/Civil2019 was used to establish the analysis model of the construction stage of the bridge according to the actual construction process [4], so as to simulate and analyze the whole construction process. In the model, arch ribs and steel box beams are simulated by beam element. The suspender is simulated by tension truss element, and the arch rib support and main beam support are simulated by compression only elastic connection. The Finite element analysis model is shown in the Fig. 2.

Fig. 2.
figure 2

Finite element analysis model

4 Construction Monitoring Content

The purpose of construction monitoring is to monitor and control the main beam installation, arch rib installation, hanger tension, bracket removal and other stages in the construction process, so as to ensure that the construction process and its structure are in absolute safety control. According to the actual state of the structure, the linear and internal force control data of each construction stage is given, which can be used to guide and control the construction, prevent the accumulation of errors in the construction, and ensure that the linear and internal force after the bridge meets the design requirements [5].

4.1 The Linear Monitoring

The alignment monitoring of the main beam adopts level elevation monitoring. The alignment monitoring point of the main beam selects the truncation point of the beam and the contact point of the boom and the main beam. The Layout of main beam alignment monitoring point is shown in the Fig. 3 and Fig. 4.

Fig. 3.
figure 3

Layout of main beam alignment monitoring point I

Fig. 4.
figure 4

Layout of main beam alignment monitoring point II

The arch rib alignment monitoring adopts total station displacement monitoring, and the arch rib alignment monitoring point is selected as the cutting point of the arch rib and the contact point between the boom and the arch rib. The Layout of arch rib linear monitoring point is shown in the Fig. 5 and Fig. 6.

Fig. 5.
figure 5

Layout of arch rib linear monitoring point I

Fig. 6.
figure 6

Layout of arch rib linear monitoring point II

4.2 Stress Monitoring

For the stress monitoring of the superstructure, the pre-embedded and pre-attached strain gauge method is adopted. The stress and strain monitoring points are arranged for the key parts and the stress concentration places, which are the adjacent points of tension on the arch and the adjacent points of tension on the beam. The stress monitoring points are shown in the Fig. 7 and Fig. 8.

Fig. 7.
figure 7

Layout of stress monitoring points of arch ribs

Fig. 8.
figure 8

Layout of stress monitoring points of main beams

4.3 Cable Force Monitoring

The vibration signal of the cable under the vibration excitation is picked up by the precision vibration collector, and then the natural vibration frequency of the cable is determined according to the spectrum diagram after filtering and amplification and spectrum analysis, and then the cable force is determined according to the relationship between the natural vibration frequency and the cable force.

5 Construction Monitoring Results

5.1 Linear Monitoring Results

Before the removal of the steel box girder support, the measured lateral displacement value is slightly less than the theoretical displacement value. The transverse displacement and vertical displacement of the arch rib are slightly larger than the theoretical value. After the bracket is removed, the linear state of the bridge is relatively consistent with the theoretical value, and the difference is within a reasonable range. The displacement of main girder is shown in the Fig. 9 and Fig. 10. And the displacement of arch rib is shown in the Fig. 11 and Fig. 12.

Fig. 9.
figure 9

Vertical displacement of main girder

Fig. 10.
figure 10

Transverse displacement of main girder

Fig. 11.
figure 11

Vertical displacement of arch rib

Fig. 12.
figure 12

Transverse displacement of arch rib

5.2 Stress Monitoring Results

Before the removal of steel box girder support, the measured stress value of arch rib is slightly greater than the theoretical stress value, considering that the spot welding anchorage between the bottom of the box girder and the support has the effect of transverse limit, and the theoretical value does not consider the transverse limit. After the removal of the support, the transverse resistance of the support is eliminated, and the arch rib is stretched. The stress state of the bridge is relatively consistent with the theoretical value, and the difference is within a reasonable range. The stress of bridge girder is shown in the Fig. 13 and the stress of arch rib of bridge is shown in the Fig. 14.

Fig. 13.
figure 13

Stress of bridge girder

Fig. 14.
figure 14

Stress of arch rib of bridge

5.3 Cable Force Monitoring Results

The suspender is divided into the initial and final tension. The initial tension is tensioned when the arch rib support is removed and the main beam support is not removed. The final tensioning is completed in the load construction of the main girder deck and is tensioned before the main girder is removed [6]. Finally, the main beam support was removed, and the structure formed a combination system of beam and arch. The cable force measured at this time is the final cable force of the bridge [7]. The comparison between finished cable force and design cable force is shown in the Fig. 15.

Fig. 15.
figure 15

Comparison between finished cable force and design cable force

6 Conclusion

To Anyi landscape bridge in Nanchang, Jiangxi Province as an example, this paper discusses the special cable stayed arch bridge construction monitoring, the following conclusions:

  1. (1)

    the demolition of bracket horizontal displacement value of the measured before the overall displacement value is less than the theoretical value, considering the reason is spot welding at the bottom of the box girder with transverse displacement between stent anchoring and restrictions, The theoretical value does not consider the lateral displacement, and the horizontal displacement is within the allowable deviation range [8]. The measured vertical displacement value is slightly higher than the theoretical value, which is considered to be due to the slight lift caused by the vertical component of the boom, and the deviation is within the allowable range. The transverse displacement and vertical displacement of the arch ribs are slightly larger than the theoretical value because the transverse displacement of the box girder caused by the welding between the bottom of the box girder and the bracket is insufficient [9]. The linear state of the bridge after removing the support is consistent with the theoretical value, and the difference is within a reasonable range.

  2. (2)

    In the construction process of the bridge, the pressure arch ribs and beam tension of adjacent points are close to the theoretical value as a whole, and the key points and beams on the average section of the arch ribs are close to the theoretical value [10].

  3. (3)

    The cable stress monitoring, after four times of fine adjustment, meets the design value, the error meets the monitoring requirements, the cable stress distribution is reasonable. The cable force is consistent with the design value, and the difference is controlled within ±5%