| CHARACTERISTICS AND COMMON VULNERABILITIES INFRASTRUCTURE CATEGORY: RAILROAD BRIDGES | SENSITIVE HOMELAND SECURITY INFORMATION – LAW ENFORCEMENT SENSITIVE | September 22, 2003 | Download |
| POTENTIAL INDICATORS OF TERRORIST ACTIVITY INFRASTRUCTURE CATEGORY: RAILROAD BRIDGES | SENSITIVE HOMELAND SECURITY INFORMATION – LAW ENFORCEMENT SENSITIVE | September 22, 2003 | Download |
Common Characteristics
Thousands of railroad bridges exist in the United States (U.S.). Some date back to the
19th Century and were designed to withstand the weight of heavy locomotives. Today, computer
technology is applied to the design and construction of bridges. Although the bridges vary in
type, they all have a common goal: to safely cross over an obstacle, whether it is a body of water
or a crack in the earth’s crust.
The national economy is based on timely rail deliveries, especially in light of industry’s current
practice of just-in-time stocking arrangements. Railroad bridges can be critical chokepoints for
high-volume rail lines moving freight from geographic areas of supply to other areas of demand.
Furthermore, critical rail bridges are vital assets of the Strategic Rail Corridor Network
(STRACNET), a 38,800-mile interconnected network of rail corridors. The STRACNET
supports the deployment of military forces across the U.S. to strategically located ports of
embarkation.
Railroad bridges must be strong enough to support their own weight (dead load) and the weight
of a crossing train (live load). The dynamic characteristics of live load and the load caused by
wind blowing across the structural members (parts) of the bridge require that bridges be stiff
enough so as to not vibrate uncontrollably.
The distance that a bridge extends between supports is called the “span.” There are two types:
the simple span and continuous span. The middle supports for a span are known as piers, while
the end supports of a bridge are called abutments. The forces that act on a bridge result in
stresses to its members. There are several types of stresses: tension stress – stretching or pulling
apart; compression stress – squeezing or pushing together; bending stress; shear stress; and
torsional stress. Every member in a bridge must be designed to handle the different types of
stresses to which it is subjected. The Fracture Critical Member is the key point of a railroad
bridge that, if destroyed, would severely reduce the design load capacity of the bridge or cause
the entire railroad bridge to collapse.
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CONSEQUENCE OF EVENT
Failure of a railroad bridge that supports the shipment of rail cars containing hazardous materials
could result in a severe chemical release. A hazardous materials release in or around a railroad
bridge will cause environmental damage, particularly for releases into the underlying water body.
It could also cause injury or death to nearby residents or railroad employees. Some victims may
experience persistent or long-term health effects or illnesses. The water supply may be
contaminated, depending on the amount of chemical release into the waterway and/or the
proximity of the receiving water treatment system. The physical circumstances of a railroad
bridge failure would hamper response, rescue, and cleanup efforts. Specialized equipment would
be necessary to effect any response (e.g., fire boats, dredging equipment). Failure of a railroad
bridge that supports passenger transportation could result in injury or death to passengers and
railroad employees. Failure of a railroad bridge due to a large explosion could impact
neighboring residents and businesses due to the resulting explosive forces (e.g., fire and
pressure).
The worst-case scenario, as defined by the railroads, would be the interruption of service for any
extended time. High-traffic corridors are especially critical. Railroad logistics are directly
affected by the loss or damage of bridge assets for indefinite amounts of time. It is clear that
many minor incidents created on a railroad system could interrupt traffic. The railroads haul
sensitive military shipments on the STRACNET, 20% of chemicals, 40% of grain harvest,
64% of coal used for electric power, and 42% of intercity ton-miles essential to the viability of
the nation’s day-to-day operations. Despite the availability of alternate transportation through
added barge and truck travel, many industries would be forced to close if they could not receive
the required materials for operation. Additionally, they would not be able to ship their products
to market.
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Truss Bridge. The truss bridge is very widely used for today’s transportation needs, because it is strong and relatively inexpensive. It can be made of various materials, but steel and pre-stressed concrete are those most commonly used for its construction (like the beam bridge described below). The guiding principle behind the truss design is the use of triangles, which are very strong if used correctly. The greater amount of triangles a in a truss bridge, the greater the load the bridge can support. The structure of a truss, in fact, is usually combined with other structures to increase its strength.
Beam Bridge. The beam bridge has a very basic structure; it uses a beam resting on two or more piers. The weight of the load pushes down on the beam and is transferred to the piers, which determines its loading capacity. When a load pushes down on the beam, the beam’s top edge is pushed together (compression), while the bottom edge is stretched (tension). Reinforced concrete is the ideal material for beam bridge construction, because the concrete efficiently withstands compressive forces, and the steel rods, imbedded within, resist the forces of tension.
The length of a beam bridge is limited, and for long spans, such bridges may require multiple piers. Therefore, the capacity of a beam bridge decreases with increasing span unless other reinforcing measures are included in the design. This does not mean beam bridges are not used to cross great distances; it only means that they must be daisy-chained together, creating a “continuous span.” Most beam bridges, however, are extremely simple and span short distances.
Cantilever Bridge. In a cantilever bridge, the beams are supported at only one end and carry a load at the other end, or distribute the load toward the center of the bridge. Long cantilevers are used in structures where clear space is required below, such as over a shipping channel or harbor.
Arch Bridge. Arch bridges are one of the oldest types of bridges and have great natural strength. They rely on the concept that the arch displaces the weight from going straight down on the supports to having a portion of the force going straight down, the other portion being displaced diagonally to either side.
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