In order to prevent collisions, guard rail in highway not only needs to have sufficient height to stop vehicles from crossing over, but also needs to prevent vehicles from tilting or sliding underneath the guardrails. In the past, it was believed that the height of the guardrail was the height of the top of the guardrail. However, in guardrail systems with guard rail columns and combination guardrail systems, the resistance R of the highway guardrail is usually not located at the top of the guardrail, but slightly lower. The effective height of the highway guardrail is defined as the height between the guardrail resistance R and the road surface. Therefore, when considering the impact of highway guardrail height on vehicle rollover in highway guardrail specifications, the effective height of the highway guardrail is more important than the total height of the guardrail. In the United States, it was found that many guard rail in highway were breached not because of insufficient strength, but because the effective height of the highway guardrail was not high enough.
There are many types of cars produced around the world, ranging from heavy commercial vehicles to lightweight micro-cars, and there is a huge difference in their weights and shapes. However, each country has certain restrictions on the exterior dimensions of specific types of vehicles, such as cars or trucks. With the opening up of various markets and the international standardization, vehicle specifications in different countries are becoming more similar. Therefore, we can refer to the regulations of different countries on the effective height of guard rail in highway for collision prevention.
In the United States, the effective height of guard rail in highway is derived from collision tests and field investigations. In the United Kingdom, the minimum height of guard rail in highway for collision prevention is 1.0m, and the range of main longitudinal effective components (effective height) is 53.5cm~68.5cm. The maximum height of secondary longitudinal effective components is 38.5cm. The Japanese highway guardrail specifications specify that the center height range of the main longitudinal beam of the highway guardrail is 60~80cm, and the height of the secondary beam under the main beam is 25~55cm. Japan is a country where trucks are the main vehicle type (40% of the vehicles are cars and 60% are trucks). In Europe and the United States, cars are the majority of vehicles. However, the regulations on the effective height of guard rail in highway in different countries are similar when considering the collision conditions of both small cars and trucks.
According to our long-term design experience of highway guardrail boards and columns, when designing guardrails on bridges, we consider that when the bridge deck is more than 3m above the ground or water level, the top of the guardrail should be higher than the human center of gravity, which is equivalent to 2/3 to 3/5 of the height of a person, assuming an average height of 1.7m, the center of gravity is 1.0~1.1m high. Therefore, the height of the guard rail in highway on the bridge deck should generally be no less than 1.0m. Drivers sitting in the cab also have a fear of heights.
When the guard rail in highway is used as a pedestrian handrail and the total height of the guard rail in highway needs to be increased, there are two methods:
Use a three-beam collision-resistant guardrail system
Increase the non-effective component in the longitudinal direction on the top of the guard rail in highway
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