Bridge Comparison of Erection Solutions for Steel Box Girders Spanning from Anchorage in the Sea on Shenzhen-Zhongshan Bridge

Abstract

The non-navigable hole bridge in the east and west flooding area of the Shenzhen-Zhongshan Bridge 110 m continuous steel box girder system, each with 2 holes of steel box girders spanning the Lingdingyang Bridge sea anchor, which is affected by the anchor and the subsidiary structure, and its installation is difficult. In view of the characteristics of the project, three installation solutions were proposed: large section lifting, small section incremental launching and large section incremental launching. Combined with the comparative analysis of construction efficiency, temporary structure usage, equipment input and construction risk, the large section incremental launching solution not only has less temporary structure input and controllable construction period, but also avoids the input of new equipment and has the best comprehensive economy, which is recommended as the preferred solution for this project. The finite element analysis of the incremental launching process shows that the structural forces meet the requirements after local reinforcement, and the scheme is safe and feasible.

1 Introduction

The Shenzhen-Zhongshan Bridge is about 30 km from Humen Bridge in the north and 38 km from Hong Kong-Zhuhai-Macao Bridge in the south. The project connects the Airport-Load Highway in the east, crosses the Pearl River Estuary, and reaches Ma’an Island in Zhongshan in the west. It connects with the planned Zhongkai and eastern outer ring highway to land in Shenzhen, Zhongshan and Nansha District of Guangzhou City. The total length of the project is about 24.03 km, of which the cross-sea section is 22.39 km. The design speed of 100 km/h and the technical standard of two-way eight-lane highway are adopted. It is a system cluster project integrating bridge, island, tunnel and underground interchange.

The Lingdingyang Bridge in the middle of the Shenzhen-Zhongshan Bridge is a twin pylons three-span full suspended suspension bridge with the spans arranged as (500 + 1666 + 500) m. The two ends of the bridge are connected to the non-navigable span bridge in the east and west flood discharge areas. The non-navigable span bridge in flood discharge area adopts continuous steel box girder system, 24 × 110 m girder in east area and 22 × 110 m girder in west area. The area above the anchorage of Lingdingyang Bridge is a six-span continuous beam bridge with a span arrangement of 6 × 110 m = 660 m. The main girder is a framing equal section and ship-shaped steel box girder with a single width of 20 m and a height of 4 m. The ratio of beam height to span is 1/27.5. Segmented length is 133.5 m + 4 × 110 m + 86.5 m, weight is 1513t + 1258.1t + 2 × 1252.6t + 1295.3t + 1025t. Steel box girder members are high strength low alloy structural steel Q345qD or Q420qD [1] (Fig. 1).

Fig. 1

Layout of steel box girder in east anchorage area

The east anchorage of Lingdingyang Bridge is a gravity anchorage structure in the sea. The foundation adopts an 8-shaped diaphragm wall foundation. The diaphragm wall is 2 × 65 m in diameter and 1.5 m in thickness. It is embedded in a moderately weathered granite layer of 5 m and the lining thickness is 1.5/2.5/3 m. Anchorage foundation top elevation is + 3.0 m, bottom elevation is −39.0 m. The bottom elevation of the diaphragm wall is −44.6 to −63.0 m. The east anchorage adopts the combination scheme of locking steel pipe pile and I-shaped sheet pile to construct the cofferdam, and the diameter of the cofferdam is 150 m round. Construction area (diameter 200 m) after inserting plastic drainage board construction lock steel pipe pile cofferdam, cofferdam internal backfill sand. Outside the cofferdam throws sandbag for slope protection and use block stone for erosion protection. The upstream and downstream sides of the cofferdam are equipped with the steel platform of the mixing station and the steel platform of the living area [2] (Fig. 2).

Fig. 2

Photo of structure layout in east anchorage area

2 Engineering Situations

The 110 m-span steel box girders in the east and west flood discharge areas of this project are all constructed by the floating crane large section installation technology. However, there are island cofferdams in the east and west anchorage areas, which have large influence areas and high requirements for the lifting amplitude of the crane ship. At the same time, due to close to the Lingdingyang Bridge, the maximum pier height is more than 63 m, which has exceeded the maximum lifting height of the ‘Tianyi’ special lifting ship for 110 m steel box girder installation. According to the structure and environmental characteristics of this project, three construction schemes of steel box girder above anchorage are proposed, including consolidation of long segment girder, small segment jack-in method and large segment jack-in method.

2.1 Consolidation of Long Segment Girder

Consolidation of long segment girder with precise positioning after hoisting the whole steel box girder to the bridge position by large floating crane is the main method for installation of prefabricated steel box girder of cross-sea bridge at present. Due to its high construction efficiency, small amount of temporary measures and mature technology, it has become the preferred installation technology for large section steel box girder of sea-crossing bridges [3].

This project hoisting affected area is 27#–29# pier interval two holes steel beam, weight is 1295.3t + 1025t. When the large floating crane is installed, the mixing station platform outside the cofferdam needs to be removed, and the temporary wharf and the steel platform in the aquatic living area have the lifting conditions. Two hoisting schemes are considered: The first scheme is to remove only the outer steel platform of the cofferdam without removing the outer protective structure for hoisting the 27#–28# section steel box girder. At this time, the maximum lifting amplitude is not less than 91 m, the maximum lifting height is not less than 77 m, and the maximum lifting weight is 1470t. The maximum arm length is determined according to the inclination angle of the floating crane. In the second scheme, in addition to the removal of the outer steel platform, the outer protective structure and some locking steel pipe piles are also removed to excavate the island building area of the cofferdam to meet the requirements of floating crane entry. At this time, the maximum lifting amplitude is not less than 61 m, the maximum lifting height is not less than 77 m, and the maximum lifting weight is 1470t. According to the parameter selection floating crane, using scheme 1, floating cranes rated lifting capacity is not less than 4500t, jib length is not less than 120 m. Scheme 2, floating crane rated lifting capacity is not less than 3500t, jib length is not less than 90 m. The plan layout is shown in Fig. 3.

Fig. 3

Layout of large section hoisting scheme

Consolidation of long segment girder has mature technology and high construction efficiency. But for this project, there are the following problems: (1) The affected steel beam above a single anchorage has only two holes and four a single anchorage, which need to be installed by large floating crane alone, and the cost is high [4]. (2) When the scheme 1 is adopted, 4500t large floating crane is needed, and the market resources are scarce, and the equipment schedule is difficult to meet the requirements of the project duration. In the scheme 2, the cofferdam protection structure and part of the locking steel pipe pile need to be demolished, the demolition and subsequent recovery work is heavy, the cost is high, and the safety risk is high. (3) Both schemes need to dismantle the steel platform of the water mixing station and the platform of the living area, and recover after hoisting, which has a great influence on the subsequent construction of the project. In summary, for this project, the cost of consolidation of long segment girder is high. Although the hoisting workload is small, there are a large number of temporary structure demolition and restoration work, which has a great impact on the subsequent construction.

2.2 Small Segment Jack-In Method

Jack-in method is a method to design a bridge into several segments, which is completed in the factory, and use vehicles or ships to arrive on-site assembly. Then, the multi-point jacking equipment is used to assemble the assembled beam structure step by step forward or dragged bridge. At present, it is widely used in bridge construction under the conditions of water depth, pier height and span, namely cable.

The installation process of steel box girder above anchorage by small segment jack-in method is as follows: first it uses consolidation of long segment girder to install 23#–27# pier section steel box girder. Portal beam lifting station, assembled bracket and 1# temporary pier are set up in pier 27#–28#, and 2# temporary pier is set up in pier 28#–29#.One leg of the lifting station supports the steel box girder bridge deck at the top of 27# pier, The other end of the leg landing support. It is equipped with four 200t lifting jacks and longitudinal jacks, which can be suspended and lifted vertically along the bridge. The top of the assembly bracket and temporary pier is equipped with 1200t walking push equipment. The span of the assembly bracket and temporary pier is 51 + 80 m, and the assembly area is 24 m. Rails are laid on the surface of the steel box girder of pier 26#–27#, and the beam trolley moves along the track of the steel beam carried on the bridge deck.

It makes steel box girder growth about 20 m beam segment. The 600t floating crane is used to lift the beam section to the trolley of the bridge deck of 26#–27# pier and transport it to the beam pick-up position at the beam lifting station, and the beam section is lifted to the assembly platform at the beam lifting station. It lifts the guide beam to temporary pier and assembly platform, and complete the assembly of guide beam and steel box beam segment. The assembled beams were pushed forward section by section by using multi-point walking pushing equipment, each assembly section is pushed up once, so as to circulate until the whole two-hole steel beam is pushed up to the design mileage, the construction of the whole bridge is completed by adjusting the falling beam accurately, removing the guide beam and temporary pier. The scheme is shown in Fig. 4.

Fig. 4

Layout of jacking scheme for small sections

Two-hole beam needs 20 times pushing in place (double), pushing weight is 4641t, temporary measures input about 2600t. Small segment jack-in method uses the erected steel beam as the hoisting and transportation site to avoid the influence of anchorage cofferdam area. After dividing into small sections, no large floating crane is needed, and the beam can be directly dropped after pushing in place. It is lower risk. But there are also the following problems: (1) Due to the high height of the bridge, it is necessary to enter a single 600t floating crane, and the lifting station is far away from the beam position to the assembled bracket. The maximum lifting weight reaches 280 t, and the overall temporary measure reaches 2600t. (2) The beam section is pushed several times on the assembly platform, with many components and large lifting weight. Under the action of self-weight, the pushing steel beam will have an impact on the docking posture of the assembled steel beam, and the overall assembly line control is difficult [5]. (3) There are many pushing times in small sections, and a single cycle includes multiple processes such as floating crane hoisting, transportation, beam lifting station lowering and pushing. The process is complex and the work efficiency is low. Overall, the cost is high, the construction period pressure.

2.3 Large Segment Jack-In Method

Different from the small segment jack-in method, the large segment jack-in method requires the jacking construction of the steel box girder across the anchorage before the installation of the steel box girder in the 25#–26# pier section. The process is as follows: In order to meet the requirements of the maximum lifting height of the ‘Tianyi’ crane ship at 61 m, the hanging beam area is set up in the 25#−26# pier section. The jacking area is equipped with 1#−4# temporary pier, of which 1#, 2# pier is water pier, 3#, 4# pier with cofferdam lock steel pipe pile as the foundation, for temporary pier on land. It installs 1200t jacking equipment at the top of temporary pier and 26#−28# permanent pier as jacking fulcrum, span arrangement is 45 + 45 + 65 + 50.5 + 59.5 + 79 m. The steel box girder is equipped with front guide beam and rear guide beam, and the length is 53 m and 43 m respectively.

The large section steel box girder is manufactured in the factory. The front guide beam is welded and hoisted to the top of 25# pier and 1# temporary pier in the beam feeding area of 25#−26# pier by ‘Tianyi’ crane ship after being transported to the site [6]. The top of the steel box girder is pushed forward to the end of the beam body to the top of 25# pier by using the jacking equipment of the pier top. After installation, the guide beam continues to push. When the support of pier 26# and temporary pier 2# is reached, the rear guide beam is removed, and the next steel box beam is installed. The steel box beam is docked with the jacking steel beam, and the jacking moves forward. In this cycle, the steel box beam of pier 26#−29# three holes is pushed to the design mileage, and the jacking equipment is removed after the bridge axis is accurately adjusted. The steel beam is dropped to the permanent pier by using the falling beam jack, and the steel box beam is docked with the rear installed steel box beam to complete the whole bridge construction. The scheme is shown in Fig. 5.

Fig. 5

Layout of large section jacking scheme

The large section jacking scheme is divided into 6 times of jacking construction of 3 hole steel beam (double), the weight of jacking is 7146t, and the amount of temporary measures is about 2200t. The process uses the existing floating crane for steel beam hoisting, and uses the combination of permanent pier and temporary pier as the jacking fulcrum, which greatly reduces the amount of temporary measures and has better economy. The hoisting and docking times of large section jacking construction are less, the construction efficiency is higher, the linear control is simple, and the bridge quality is easy to guarantee. However, due to the use of permanent piers as push fulcrum, steel beam push in place after the need to be converted into beam jack for beam falling. The falling beam height is about 1.5 m, the process is mature but there is a certain risk.

According to the description of the above three erection schemes, the comparison of the advantages and disadvantages of each scheme is shown in Table 1.

Table 1 Comparative analysis of steel box girder erection schemes above anchorage

Considering the work efficiency, construction quality, equipment investment and construction risk, the large segment thrusting method is recommended as the optimal scheme for the installation of the steel box girder across the anchorage in this project. Detailed design and calculation analysis are carried out to determine the structural safety.

3 Process and Simulation Analysis of Incremental Launching Construction

3.1 Construction Process

Large segment jacking construction structure mainly includes temporary pier, front and rear guide beam, equipment frame, falling beam cushion beam and 16 1200t jacking equipment. The general construction steps of large segment pushing steel box girder are as follows:

• (1) The first section 86.5 m steel box girder is hoisted to the top of 25 # pier and 1 # temporary pier by floating crane (see Fig. 6a).

  Fig. 6

  

  Installation steps of large section jacking of steel box girder

• (2) The steel box girder is pushed to 17.5 m, and the guide beam is installed at the top of 25# pier (see Fig. 6b).

• (3) The steel box girder is pushed forward 95 m, the rear guide beam is removed, and the second section 110 m steel box girder is hoisted to the graphic position by floating crane (see Fig. 6c).

• (4) The steel box girder of the second section is welded into a whole by pushing forward about 8.6 m and accurately docking with the steel box girder of the first section. The pushing forward is continued for 15 m and the guide beam is installed (see Fig. 6d).

• (5) It installs the third section steel box girder according to the steps 2–4, push the steel box girder over 2 m to the graphic position, and remove the guide beam at 29# pier (see Fig. 6e).

• (6) It removes 25# pier jacking equipment and 1# temporary pier, hoist 25–26 # pier interval large section steel box girder, the steel box girder back to the design mileage, accurate adjustment axis deviation. The jack and cushion beam of the falling beam are installed, and the jacking equipment on the 26#−28# permanent pier is removed, and the bridge support is installed. The jacking equipment on the jack and the temporary pier is gradually dropped to the support. The jacking beam is accurately docked with the steel box girder of the 25–26# pier and welded into a whole to complete the jacking construction (see Fig. 6f).

3.2 Load Combination

The fulcrum position of steel box girder is changing in the construction process and the stress state is also changing during jacking construction. The loads at different jacking stages vary greatly, so it is necessary to check and analyze the strength of steel beam, the force of temporary structure and the local stability of steel beam. The most unfavorable loading conditions at each stage of construction are as follows:

Maximum tail section cantilever state. Condition 1: The three-hole main beam is pushed to large cantilever state of tail. In 1# temporary pier backward cantilever 65 m, 28# pier forward cantilever 72.1 m.

Front end maximum cantilever state. Condition 2: The three-hole girder is pushed to the tail of the large cantilever state, in the 1# temporary pier backward cantilever 58.1 m, 28# pier forward cantilever 79 m.

Beam lowering state. Condition 3: The main beam is pushed in place, the beam is dropped, and the synchronization error of the fall beam fulcrum reaches 20 mm.

3.3 The Computation

ANSYS finite element software was used to simulate the pushing process of steel box girder. Shell181 element was used to establish the shell element model. Material properties are ideal elastic–plastic constitutive model. In the modeling process, the structure is simplified accordingly. The attached structures such as guardrail are loaded on each node and unit with distributed mass, and the connection and jacking support of each component are simplified as common nodes and contact simulation. The load is mainly loaded by node force and acceleration.

The finite element analysis shows that in the process of steel box girder jacking, the maximum stress extremum appears in the front 79 m cantilever state, and the maximum single point fulcrum reaction force is 6657 kN. The local buckling of the web was rechecked. Compression zone σ/σcr1 + (σ_c/σ_c,cr1)2 + (τ/τcr1)2 = 1.48 > 1 [7], local stability does not meet the requirements. Local reinforcement is needed. The reinforcement method is to increase 16 mm thick transverse stiffeners along the bottom of the middle web according to 400 mm spacing, increase the effective distribution length of the pushing fulcrum and reduce local compressive stress. Compression zone after reinforcement σ/σcr1 + (σ_c/σ_c,cr1)2 + (τ/τcr1)2 = 0.47 < 1,local stability meets the requirements. The reinforced model is analyzed and the results are shown in Fig. 7.

Fig. 7

Comprehensive stress nephogram of structure under the most unfavourable stress condition

In the process of pushing, due to the possible uneven settlement of support, steel plate over-cushion height difference and pushing system error and other factors. There is a relative height difference between the two sides of the steel beam. The sensitivity analysis of the jacking process is carried out. Considering the influence of 30 mm height difference at the maximum reaction fulcrum under working condition 2, the maximum stress of the steel box beam during the jacking process f_max = 261.3 MPa < f = 295 MPa, for local stress concentration, pushing the maximum stress of temporary pier f_max = 193.2 MPa < f = 295 MPa, The strength and stiffness of permanent structure and temporary structure meet the requirements, and the structure is safe and reliable.

Considering the convenience of subsequent demolition of the pile foundation, the temporary pier adopts the form of pile lifting. The maximum vertical load of a single steel pipe pile is 1140 kN, the soil depth is 20.7 m, and the maximum bearing capacity of the pile foundation is 1222 kN. The bearing capacity meets the requirements.

4 Conclusions

The 110 m continuous steel box girder system is adopted for the non-navigable bridge in the east and west flood discharge areas of Shenzhen-Zhongshan Bridge, which both cross the sea anchorage of Lingdingyang Bridge. Affected by the anchorage itself and cofferdam, the installation of steel box girder is difficult. Through the scheme research, the large section jacking scheme saves the construction period and reduces the temporary structure investment, avoids the new hoisting equipment investment, and has better comprehensive cost and quality control. Through further detailed design and finite element analysis, the structural stress meets the requirements after local reinforcement of steel beams, and the scheme is safe and feasible, which can provide reference for similar projects (Fig. 8).

Fig. 8

Large section jacking of steel box girder