Crack Control Technology of SPMT Transportation Structure of Long-span and Special-shaped Concrete Box Girder

By analyzing the engineering characteristics of SPMT transportation of long-span and special-shaped box girder, this paper took asynchronous displacement as the master control parameter of large concrete box girder transportation and stress as the auxiliary control parameter, and put forward the crack control method of spatial four-pivots driven concrete box girder transportation structure. Through numerical simulation, this paper set up the control values of girder posture and stress in box girder transportation, constructed the information monitoring system of girder posture and stress to realize remote real-time monitoring and real-time display, and shaped the stress and posture control technology of SPMT transportation structure for long-span specialshaped box girder, which helped the box girder of Cross Bay Link, Tseung Kwan O project to be successfully barged. The research results of this paper can be used for reference for similar projects in the future.


Introduction
From a global perspective, large-scale sea-crossing transportation infrastructure projects are developing in the direction of being "large-scale, standardized, industrialized and assembled", that is, large prefabricated components are adopted, standardized and industrialized production is realized, and then large mechanical equipment, large floating cranes and offshore working platforms are adopted to install components, so as to minimize offshore operations, speed up project progress and reduce construction risks [1] .
Concrete box girder has a fragile structure, requires large storage space, and has poor overall tensile and torsional resistances. During the transportation of concrete box girder, the concrete structure is at risk of being damaged by tensile crack [2][3] . In the construction process, cracks of concrete box girder must be well controlled to ensure the smooth transportation of concrete box girder. At present, the transportation control of concrete structures is generally based on simple stress and deformation index control. Although stress and strain can reflect the force and deformation of key sections of box girders, they cannot directly instruct how to make adjustment after early warnings, and cannot define the main reasons for the deformation condition of the structure in time. However, the transfer and barge process of long-span concrete box girder is continuous, and especially the floating barge is often uninterrupted, In order to support the girder crack control and on-site rapid process adjustment during box girder transfer and barge of Cross Bay Link, Tseung Kwan O project, this paper proposed to take the girder posture as the master control parameter of SPMT transfer and barge of long-span box girder, and the girder stress as the auxiliary control parameter to form the girder stress and posture control technology of SPMT transportation of long-span special-shaped concrete box girder, which finally helps the box girder of Cross Bay Link, Tseung Kwan O project to be barged smoothly.

Project Background
Cross Bay Link, Tseung Kwan O project and its related projects builds another large-scale seacrossing landmark bridge in Hong Kong after Tsing Ma Bridge and Stonecutters Bridge. The project involves the prefabrication of 18 large-span prestressed curved special-shaped box girders, and the transfer and rolling to barge of SPMT transportation. Barges are transported to the site of Cross Bay Link, Tseung Kwan O for installation. The maximum length of a single box girder is 75m and the maximum weight is 3344t. It is the first time in China that SPMT sets are used for transferring and barging of large-tonnage and long-span prestressed concrete box girders, and there is little experience to learn from.

SPMT Transportation Control Parameters for Long-span Special-shaped Box Girder
According to the occurrence probability of each working condition and its influence on the structure upon occurrence, the transportation control parameters of long-span box girder can be divided into master control parameters and auxiliary control parameters. The master control parameters are subdivided into single parameter control and multiple parameter control. The auxiliary control parameters are the collection of other remaining control parameters.
The SPMT sets adopt 4-pivots synchronous jack-up, which is converted into 3-pivots support during transportation [4][5][6] . The asynchronous SPMT jack-up, asynchronization caused by uneven ground 3 during transportation, the hull deformation and posture change during the floating barge of the box girder, and the height difference between the hull and the wharf will all cause the change of the box girder postures, causing the box girder to twist, thus adversely affecting the stress of the box girder structure. On the contrary, the posture of box girder can also reflect the postures of SPMT sets and ships to a certain extent, so controlling the posture of box girder to avoid torsion can not only control the stress on girder, but also provide decision support for the posture adjustment of SPMT sets and ships.
This paper proposes to take the girder posture as the master control parameter of SPMT transfer and barge of long-span box girders, and the girder stress as the auxiliary control parameter. The force bearing form of long-span box girder SPMT transportation is simplified as a four-pivots support system, and the girder posture is controlled by controlling the four-pivots asynchronous displacement. The calculation formula of asynchronous displacement is: △= (△1-△3) -(△2-△4), and 1, 2, 3, 4 are the four corners circling around the girder. The elevation variation is respectively △1, △2, △3, △4.

Structure Stress and Posture Control Value for Long-span Special-shaped Box Girder
The finite element calculation model of box girder NE4-5 is constructed, as shown in Figure 3. The elastic modulus and bulk density of C60 concrete in the model are 3.65×10 7 kN/ m 2 and 25 kN/ m 3 respectively. The prestressed 19Φ15.7 steel strand is used for the prestressed steel base plate, 27Φ15.7 prestressed steel strand is used for the web plate, 19Φ15. 7 prestressed steel strand is used for the top plate, and 1*7 steel strand is used. The nominal area of the section is 150 mm2, the stress of tension control is 1395 MPa, and the SPMT jack-up force is applied to the supporting beam in the form of uniformly distributed load, as shown in Figure 3.

Figure 3 NE4-5 Simulation Model
This paper carries out simulation analysis of the whole process of jack-up, transfer and barge of box girder NE4-5, and the sensitivity analysis of asynchronous displacement of the girder. Based on the crack control requirements of box girder, the girder posture and stress control values are put forward. The standard value of C60 concrete tensile strength is 2.85MPa and the standard value of compressive strength is 38.5 MPa. As the transportation of long-span box girder is a temporary working condition during construction, 70% of the standard value is taken as the stress control value, which means the tensile and compressive stresses are controlled within 2MPa and 27MPa respectively. After calculation and analysis, the asynchronous displacement and strain control values of box girder NE4-5 are shown in Table 1 and Table 2 respectively. The asynchronous displacement of the box girder is the main control parameter and three-level early warnings are set here. When the monitored data is close to the value of first-level early warning, a warning is given without having to make adjustment. When the monitored data is close to the value of second-level early warning, the box girder should be slowed down and the box girder moves while making adjustment. When the monitored data is close to the value of third-level early warning, the box girder stops running and cannot continue until the posture is adjusted. 1#, 2#, 3# and 4# in Table 2 are support beam numbers.

Information Monitoring of Structure Stress and Posture
During transportation, the box girder is in a multi-support structural state, with the maximum positive bending moment in the midspan and the negative bending moment at the support beam. Therefore, a test section is selected at the support beam to control the tensile stress of the box girder's top plate and a test section is selected in the midspan to control the compressive stress of the base plate, as shown in Figure 4. Static water levels are arranged at the four corners of the box girder, as shown in Figure 5 (both sides are arranged in the same way). One of the measuring points is taken as the base point to test the elevation difference of the other three measuring points relative to the base point. An automatic monitoring system is adopted to realize real-time monitoring of the uneven displacement of the four corners of the girder during the transfer and barge of the box girder. This paper proposes for the first time to use static water level to test the posture of box girder. The static water level applies the principle of communicating vessels to keep the liquid surfaces of multiple liquid reserve tanks connected by communicating vessels always at the same level. The real-time test of the elevation difference of static water levels can be realized by using information monitoring system, as shown in Figure 6. With the help of information technology, an information monitoring system for structural stress and posture of long-span box girder SPMT is constructed. The system consisting of a perception layer, a transmission layer and an application layer, realizes real-time remote collection, transmission and display of monitoring data.

Results of Structure Stress and Posture Control
The posture monitoring results of box girder NE4-5 are shown in Figure 7. As can be seen from Figure 7, the torsion of the girder is basically controlled within 10 mm, which does not reach the early warning value, during the process of box girder barging and transferring.
While ensuring that the girder posture meets the control requirements, the girder stress is monitored in real time. The stress monitoring results of the box girder NE4-5 are shown in Figure 8.