There are no more than three cable-stayed bridges, and Jiashao bridge is the first six tower cable-stayed bridge. Moreover, without the use of cooling water pipes, Jiashao bridge starts with materials to reduce the hydration heat of concrete and solve the problem of bridge service life caused by concrete cracking. In addition, Jiashao bridge has 7 closure openings, and engineers have explored a new closure scheme - Geometric closure method.

Author Yu Qianqiu, doctor of civil engineering, Tsinghua University

When it comes to the sea crossing project in Hangzhou Bay, many people first think of Hangzhou Bay sea crossing bridge. In fact, there is also a bridge across Hangzhou Bay upstream of Hangzhou Bay Bridge, which is Jiashao bridge.

It starts from Haining City, Jiaxing City in the north and connects Shangyu District, Shaoxing City in the south. It is an integral part of Changtai Expressway (Changshu, Jiangsu - Taizhou, Zhejiang). Although it is only one third of the length of Hangzhou Bay Bridge, Jiashao bridge is a long and wide multi tower cable-stayed bridge in the world. The number of cable towers and the length and scale of main bridges rank first in the world.

The cost per kilometer is 600 million yuan, and Jiashao bridge can be called a hundred yuan banknote_ one

(Jiashao bridge - multi tower cable stayed bridge)

（1） This is an unprecedented six tower cable-stayed bridge

The construction of a bridge must divide the river into navigable reach and non navigable reach. Among them, the non Navigable Bridge section has small span and low cost; The Navigable Bridge section has large span and high cost. The general bridge will only set aside one navigable reach, but why should Jiashao bridge build a six tower cable-stayed bridge and set aside six navigable reach?

It turns out that Jiashao bridge is located in the upstream of Hangzhou Bay sea crossing bridge. The water surface width here is much narrower than that of the downstream Hangzhou Bay sea crossing bridge. In addition, the water is shallow, so the water flow is more turbulent. It also happens to be at the intersection of river and sea. The tide is strong, the current is rapid and the sediment content is large. The riverbed of the river section is silty sandy soil, which is very easy to scour and the scouring and silting changes violently. This makes the main channel here change frequently, with a range of 1-3.3 kilometers.

Qiantang River is a navigable reach, and cargo ships travel frequently every day without delay. Therefore, in order to prevent the impact of the swing of the main channel on navigation, several main channels must be reserved in advance to adapt to the swing of the main channel of the riverbed. Even if one channel is blocked, ships can pass through other channels.

In order to deal with the turbulent river water, the world's large diameter bored piles with a diameter of up to 3.8m are also widely used in the approach bridge in the water area, which not only solves the problem of gravity, but also greatly reduces the water blocking area.

The cost per kilometer is 600 million yuan, and Jiashao bridge can be called a hundred yuan banknote_ two

(there are two bridges on Hangzhou Bay, Jiashao bridge on the left and Hangzhou Bay sea crossing bridge on the right)

At present, in addition to Jiashao bridge, most of the multi tower cable-stayed bridges built at home and abroad are 3 towers, while Jiashao bridge adopts the 6-tower cable-stayed bridge scheme. This makes the length of the main bridge reach 2680 meters, divided into five main channels, and the number of cable towers and the length and scale of the main bridge rank first in the world. With the cable distribution areas on both sides, the total width of the main bridge is 55.6m.

However, the stress of multi tower cable-stayed bridge is different from that of conventional cable-stayed bridge. Because there are no auxiliary piers and transition piers on both sides of the middle tower, it can not effectively help the stiffness of the main beam and cable tower, increase the amplitude and range of the live load influence line of each response of the structure, and increase the live load effect strain of each component. Therefore, measures must be taken to improve the vertical stiffness of the main beam.

Therefore, according to the structural characteristics of Jiashao bridge, technical experts proposed that the cable tower adopt X-shaped bracket, and set longitudinal double row supports for the main beam to restrict the relative rotational freedom between the main beam and the cable tower to improve the stress of multi tower cable-stayed bridge. In this way, the load proportion transmitted from the main beam to the upper tower column is reduced. Therefore, the stress of the upper tower column can be relieved, and the stiffness of the main beam has been significantly improved.

The cost per kilometer is 600 million yuan, and Jiashao bridge can be called a hundred yuan banknote_ three

(longitudinal double row support of main beam)

The span of Jiashao bridge is too long, so the steel box girder of the main bridge is long, and the temperature deformation of the main beam has a great impact on the stress of the cable tower and foundation. The traditional structure can not adapt to this long main beam structural system and the special construction environment of Jiashao Bridge.

In order to solve the problem of temperature deformation of long main beam scientifically and reasonably, the innovative structural system of setting rigid hinge device in the middle of the whole bridge span is adopted in the project. This structure is different all over the world. The so-called rigid hinge is to disconnect the steel box girder at the mid span position. Its basic structure is to place a small box girder inside the steel box girder on one side, the small box girder is fixed on the steel box girder on the other side, and the other end is free.

The rigid hinge releases the longitudinal relative linear displacement at both ends of the main beam, restricts the rotation angle and shear displacement of the main beam, and can not only meet the stress requirements, but also ensure the driving comfort.

The cost per kilometer is 600 million yuan, and Jiashao bridge can be called a hundred yuan banknote_ four

(structural drawing of rigid hinge)

（2） How to cool the cushion cap without cooling water?

Jiashao bridge is located in the marine environment, and the bearing platform is deeply buried, which requires high concrete durability. However, the concrete volume of a single bearing platform of the main bridge is up to 8000 cubic meters, which belongs to typical mass concrete.

The cost per kilometer is 600 million yuan, and Jiashao bridge can be called a hundred yuan banknote_ five

(night view of Jiashao bridge)

The hydration heat of mass concrete is not easy to lose, and it is easy to crack when subjected to external constraints. In the marine environment, concrete cracks mean rapid corrosion, and the consequences are very serious.

Generally, the temperature control of mass concrete mainly depends on a large number of cooling water pipes arranged inside the concrete. The heat generated by concrete hydration is carried out by circulating water through the cooling water pipes. This temperature control method has indeed played a certain degree of cooling effect in a sense, but there are many disadvantages.

The control of cooling water temperature and flow in cooling water pipe is complex. The water temperature is too high, the flow is too slow, and there is no cooling effect; If the water temperature is too low and the flow rate is too fast, it is very easy to lead to excessive temperature difference of concrete around the pipe wall, resulting in a large number of shrinkage cracks, which will have an adverse impact on the concrete.

In addition, in the later stage, no cavity shall be left in the cooling water pipe, and grouting shall also be carried out. However, the later grouting can not be compacted, which has a weak link affecting the durability of concrete. Especially for marine high-performance concrete, harmful ions are easy to enter the concrete through the grouting pipe, causing reinforcement corrosion.

The cost per kilometer is 600 million yuan, and Jiashao bridge can be called a hundred yuan banknote_ six

(cooling water pipe in concrete reinforcement cage)

Therefore, the cushion cap of Jiashao bridge does not adopt the traditional water pipe cooling method, but starts from the material to reduce the hydration heat.

On the one hand, cement with low calorific value is selected to reduce the total hydration heat, so as to reduce the temperature difference inside and outside the concrete. On the other hand, select the aggregate with good gradation and small porosity, which can improve the volume of aggregate in concrete, improve the compactness of concrete, save cement and reduce the total hydration heat and water consumption of concrete. Moreover, the use of continuous aggregate can also increase the total strength of concrete and improve the crack resistance of concrete itself.

After, Jiashao bridge also used a large number of high-quality fly ash and mineral powder as admixtures. Fly ash can improve the workability of concrete, greatly improve the working performance and durability of concrete, replace cement and reduce the heat of hydration. However, when the content of fly ash is large, it has a great impact on the early strength, which can be compensated by mineral powder. It can also reduce the hydration heat and improve the early strength compared with fly ash.

In the end, 77% of various mineral admixtures and 23% of cement are used in the concrete of the bearing platform. By skillfully adjusting the material ratio and pouring scheme, the long-term performance and durability of the concrete have reached an excellent level, and the crack resistance, impermeability, sulfate corrosion resistance and frost resistance have reached the standard.

Combined with sand and gravel aggregate, watering by pump truck for cooling, mixing water with ice and other measures, the temperature rise of bearing platform is very low and there is no temperature shrinkage crack.

In order to achieve information construction, truly reflect and test the temperature control effect of each layer of concrete, many temperature sensors are also arranged inside and outside the bearing platform during concrete pouring. All monitoring items shall be carried out immediately after concrete pouring and continuously until the temperature change is basically stable.

The temperature test results show that the tensile strength of each layer of concrete is greater than the tensile stress produced by cooling in the same age period, and has a high anti crack safety factor.

（3） How did Jiashao bridge close?

The construction of cable-stayed bridge needs to be carried out gradually from the middle to both sides, and finally involves the closure with the bridge decks on both sides.

In civil engineering, closure means that small errors in construction will be amplified, so error control is very difficult. The main channel bridge of Jiashao bridge is the first cable-stayed bridge with six towers, two spans and four cable planes in the world. There are 7 closure openings in the whole bridge, and the closure scheme is very complex.

People have explored a new closure scheme - Geometric closure method.

The cost per kilometer is 600 million yuan, and Jiashao bridge can be called a hundred yuan banknote_ seven

(closure of cable-stayed bridge)

The so-called geometric closure method is to make the closure opening width reach the design closure section length by applying jacking force. No matter what temperature, it only needs to change the jacking force according to the different closure width, and there is basically no requirement for environmental conditions. When geometric control method is used for construction control, it is only necessary to accurately control the stress-free state of components.

After the closure of the bridge by this method, the main beam alignment is smooth and very consistent with the design theoretical alignment. The measured alignment basically coincides with the theoretical alignment, and the errors are within 5cm, indicating that the closure index is excellent and the quality is fully up to standard.

In addition, the geometric closure method can also ensure that the design length of the closure section does not change and the established closure time, which also provides a reference for the closure control of cable-stayed bridges in the future.

The cost per kilometer is 600 million yuan, and Jiashao bridge can be called a hundred yuan banknote_ eight

(the error in closure is very small, and the degree of coincidence between theory and practice is high)

epilogue

The total cost of Jiashao river crossing channel is about 13.9 billion yuan, and the investment of the bridge is 6.35 billion yuan. On average, the cost of the bridge is 600 million yuan per kilometer, and the whole bridge is equivalent to being paved with 100 yuan bills.

But the investment is well worth it. After the completion of the bridge, Shanghai Hangzhou Expressway, Zha Jiasu expressway, Hangpu expressway, Hangyong Expressway and shangsan expressway will be connected. The driving distance from Shaoxing to Shanghai will be shortened by at least half from the original three hours, further promoting the integrated economic and social development of the Yangtze River Delta.

As a highly recognized label in the world, made in China is seeking strategic upgrading. " The "made in China" column strongly invites industry authorities and senior players to present the path of Chinese innovation in their eyes.