1, injury principle
Lightning strike can be divided into direct lightning and induced lightning: direct lightning is a rapid discharge phenomenon between charged clouds and a point on the earth, which will cause harm to bridges and people due to electricity, heat and machinery; Induction lightning brings different charges to a certain area of the bridge through electrostatic induction. The damage range of this lightning stroke is very large, and it can be expanded through power lines and voltage lines. To sum up, the harm of lightning can be divided into two aspects: direct lightning strike and lightning electromagnetic pulse.
2. Determination of lightning protection category
In general, according to the lightning protection classification standard of buildings in the national lightning protection standard "Code for Lightning Protection Design of Buildings" (GB50057-94), long-span bridges are identified as Class II lightning protection buildings. The estimated number of lightning strikes n in a year can be calculated from this. Where: k represents the correction coefficient, where it is1.5; Ng represents the annual average density of lightning strikes in the area where the bridge is located [times/(km2a)]; L, w and h respectively represent the length, width and height (m) of the bridge.
3. Research on lightning protection measures
3. 1 lightning receptor
Lightning arrester is one of the most commonly used lightning protection measures. When installing lightning arresters, the principles of safety, science and economy should be implemented. Lightning rods are designed and installed at the top of the main tower for direct lightning protection of the main tower and various electronic equipment at the top. The lightning rod fixed embedded bottom plate and the equipment at the top of the main tower are connected with the main grounding steel bar of the main tower. The light pole of the bridge deck lighting low pole lamp is connected with the lightning protection grounding main reinforcement of the metal box girder or the reinforced concrete box girder; All other deck facilities. The equipment shall be directly or indirectly connected with the main grounding reinforcement. For stay cables or suspension cables, it is impossible to install any lightning arrester for protection because of their large space and high height. Therefore, both ends of stay cables or suspension cables should be connected with the thematic grounding system of the bridge, so as to quickly release lightning current and play the role of lightning arrester to protect the bridge deck, pedestrians and vehicles from direct lightning.
Theoretically, if lightning rod is installed on the side, the cable tower can play a three-dimensional protection role for direct lightning. But in fact, the advantages of side lightning strike are that the striking distance is much smaller than the radius of the ball, the lightning current is smaller, and the steel bar structure on the bridge is dense and thick, which has certain lightning resistance. Therefore, for the protection of the side of the tower, it can be protected by concrete reinforcement, and there is no need to install lightning arresters on the side.
3.2 grounding
Grounding system is the key point of lightning protection design for long-span bridges. In the grounding system, the structural main reinforcement in foundation piles can be used as grounding body, and a certain number of foundation piles are selected as lightning protection grounding piles for each main tower, and the structural main reinforcement is selected as lightning protection grounding main reinforcement among lightning protection grounding piles. If the resistance is too large, the quantity should be increased appropriately. The lightning protection and grounding main reinforcement in the foundation pile is connected with the lateral lightning protection and grounding main reinforcement in the lower layer of the pile cap, and the lightning protection and grounding main reinforcement selected in the tower column under the cable tower is connected with the lateral lightning protection and grounding main reinforcement in the upper layer of the pile cap; The main reinforcement of lightning protection and grounding of the lower tower column is electrically connected all the way. So as to ensure that all lightning currents flow into a large number of parallel discharge channels after multiple re-shunting. Connect the embedded parts of lightning rod and other equipment at the top of the tower with the main reinforcement for lightning protection and grounding; Grounding embedded steel plate for connecting the lower cross beam of steel box girder. The lightning protection and grounding main reinforcement in the box girder of the approach bridge should be connected from top to bottom, and directly connected with the grounding embedded steel plate at the lower part of the box girder. The reserved steel plate at each pier is connected with the main grounding rod in the box girder to realize the grounding of the whole system.
In terms of grounding arrangement, considering that the foundation grounding of long-span bridges often doubles as other information grounding at the same time, the requirements for resistance can reach below1Ω. In general, the number of cable tower foundation piles is more than 20 meters, which can meet the requirements in areas with low soil resistivity. For special areas with high soil resistivity, we can consider extending the auxiliary artificial grounding body, that is, the cable tower cap must reserve flat steel grounding to connect with the auxiliary grounding body.
3.3 Balanced equipotential
For long-span bridges, the main tower is relatively high and the down conductor is relatively long. In order to reduce the occurrence of side lightning strike, induced voltage and counterattack voltage, generally, an equalizing ring should be set every 10 meter on the lightning protection and grounding main reinforcement of the main tower above the bridge deck. The steel mesh, plate steel bar or steel plate in the deck pavement shall be reliably welded with the steel bar in the box girder or pier as lightning protection downlead and other metal facilities on the deck, such as railings, guardrails and lamp posts. It can be regarded as equipotential bonding network and lightning protection main reinforcement. The steel mesh laid on the bridge deck shall be connected at equal points at the transverse ends of the bridge deck every 12m in the longitudinal direction. Metal railings and guardrails near expansion joints should be treated as equipotential jumpers. At the same time, in order to meet the needs of equipotential connection of other types of equipment in the future, equipotential grounding reserved parts should be embedded in their corresponding positions. Metal objects in the arch rib are connected with structural steel bars in the arch rib, embedded parts fixed at both ends of stay cables are connected with nearby lightning protection devices, and both ends of metal objects such as metal sleeves should be connected with lightning protection steel bars.
3.4 shielding
Long-span bridges are generally at a certain height from the bottom, especially steel bridge structures, which inevitably have the problem of field interference with the ground, and strong and weak lines such as power pipelines are laid on the whole bridge, which also have serious interference problems with each other. In addition, when lightning strikes, powerful electromagnetic pulses will interfere with power communication facilities and central control room. Common shielding measures are: (1) strong and weak lines should be shielded with metal pipes, which has certain safety; (2) Strong and weak shafts should be set and metal bridge should be set; (3) For important areas, such as computer rooms, a certain size of metal mesh can be used as the wall.
Lightning protection of long-span bridges is a systematic project. According to the particularity of the project, a comprehensive lightning protection system must be designed, and various measures such as lightning protection connection, shunt, voltage equalization and equipotential, shielding and grounding should be used reasonably. At the same time, in the process of construction, we must do a good job of inspection and testing, especially related concealed works, to ensure the safety of the main body and information system of long-span bridges.
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