Design Proposal for Luyang Sea Crossing in Phase I Project of Yang Shan Deepwater Port Chapter VIIIChapter VIII Bridge Design for Collision Resistance8.1 Design Criteria for Collision Resistance(1) According to the probability of impact, water structures upstream and downstream of the navigated channel is more prone to collision when compared to the impact to the bridges inner and outer parameters. Also, the speed at collision is usually greater for upstream and downstream water structures. Therefore, the impact resistance design for upstream and downstream water structures should be stronger than that for the bridges inner and outer parameters.(2) As the upper part of the ship is significantly above water surface and is particularly outstanding, therefore if on impact, the impact length of the upper part of the ship will be longer and the damage to the structure will be deeper. Hence, a safe distance must be kept between the crash barrier and the main structure. (3) Greater tolerance in shape changes must be allowed in the collision resistance design, such that more energy can be absorbed and this can reduce the impact on the collision and the damage that it may caused to the existing structure. (4) Also, the proposed design should be the one with the minimum maintenance costs and the proposed crash barrier should be easily replaced.8.2 Collision at Bridge Piers Alternative StudiesThere are two major types of collision resistance facilities. The first one is indirect impact resistance and the main advantage is that the collision resistance facility can be designed around the outer parameter of the piers such that no direct forces will be imposed on the piers when in contact. Typical design example include pile group design, gravity loading design, cofferdam design, artificial island design and anchor design etc. These designs are used at shallow waters and at better geological conditioned areas. The second type is direct impact resistance and it is used to reduce and spread the impact to the piers by using designs such as cable-stayed, prestressed, buffer and spring type etc. These designs are usually used at narrow navigated channels, deep channels and in projects where there is little geotechnical works involved and the construction costs are at a minimum.According to our preliminary investigation results on local and international design experience and the existing structures, our proposed collision resistance designs should include:(1) Gravity loading design this design relies on the area of impact and the force on impact from which the displacement and fluid displacement mechanism help to absorb large amounts of energy. The major disadvantage of this system is that a large area is required, the associated construction costs is high and it is hard to carry out maintenance works.(2) Thin-shelled structure is one of the most successful collision resistance system. Its costs are comparatively lower, it is also easy to construct and requires less maintenance works. This system can be purposely made to cater for specific resistant loading required and hence can absorb large amounts of energy. Thin-shelled structures are usually circular and it is made by welding steel plates to form a circular shaped shell and is in-filled by concrete. This kind of system can effectively prevent severe damage to the ship on impact. It is noticed that the relatively poor geological ground conditions of the bridge under studied which may have significant impact to the construction of cofferdam and may add more costs to the structural and collision resistance design.(3) Artificial island design can provide good impact resistance. They are usually constructed together with the piers and the surface of the island is usually below water surface level. The surface of the island has a very gradually slope so that the ships will not crash into the island. These islands are easy to construct at a relatively low costs and it requires little maintenance. It is also easy to reinstate if damaged. The disadvantage is that it requires naturally slope and hence occupied more space within the navigated channel. It will also narrow the channel and increase its flow speed and it will increase scouring effect. It is not recommended to be used in areas with bad geological ground conditions. (4) Pile groups are often composed of raking pile or vertical piles. To cater for more shape changes, pile groups are used. The diameter of the piles can be designed to resist the required impact forces and the whole pile group can be aligned vertically or horizontally. The layout of the pile group is very convenient and the number of damaged piles under collision is relatively less which can be easily replaced or maintained.(5) Anchor system can be floating buoy system, steel ropes etc. When in impact, the