In all these seemingly unrelated disasters, ships or barges deviated from shipping channels, striking the bridge and causing the collapse. In the last 25 years, straying vessels caused five high-profile U.S. bridge collapses, causing 105 deaths. So what can engineers do?
A recent study of collapsed bridges found that engineers designed piers adjacent to shipping channels that were capable of withstanding a strong collision, sometimes a collision much more forceful than they were ever likely to experience. However, other piers were often weaker in design and unable to survive. The operating assumption apparently is that ships wont leave their regulated path. This is a mistake. Fortunately, there are a number of ways public agencies can mitigate potential bridge/ship collisions, including:
Designing bridges so any piers vulnerable to impact can survive.
Reinforcing and/or better protecting vulnerable and weak piers.
Installing collapse detection and warning systems for motorists.
The I-40 Bridge is a good example why these improvements are needed. The bridge had two types of piers constructed of reinforced concrete with spread footings bearing on hard shale. There were no piles and no footing keys. The main channel piers consisted of a solid concrete stem and a continuous spread footing. The remaining piers consisted of a concrete cap supported by two concrete columns with individual footings. The two columns were connected by a concrete web wall. Pier protection consisted of two cells constructed on the upstream side of the main span.
 |
| Dead End. I-40 sight lines were too short. |
An errant barge struck a downstream pier adjacent to the navigation channel pier with an impact force estimated at 1,144 kips. The stricken pier could only sustain an impact load of approximately 630 kips. The failure load for the main channel pier was approximately 12,370 kips, or 20 times stronger than the stricken pier and more than 10 times the impact.
How can we have increased pier strength? One way would be by extending the concrete web wall to the base of the foundation and extending the concrete web wall full height to the top of the pier. That done, the pier probably would have been damaged but may not have collapsed. Pier stiffening may be an alternative but it is not always the preferred option. Retrofitting bridges to improve web walls could be very expensive.
Installing pier protection systems might be more cost effective. Options include building timber pile clusters, called dolphins, protective barriers and artificial islands around vulnerable piers to deflect vessels or absorb their impact. Another option is moored cable arrays to snag a ship and bring it to a halt.
 |
| ASSIS |
A final but critical improvement, especially for long, causeway-type structures, is a driver warning system. Experts investigating the I-40 accident examined the available sight distances and determined that the stopping distances were much greater. Drivers could not stop.
Collapse warning systems are very simple. Most have an electric or fiber-optic cable running along the underside of the bridge, usually at the edge. If a section collapses, the system is triggered turning on warning lights and activating safety gates.
Engineers are proficient at designing bridges to withstand a wide range of adverse conditions. But we must work with public agencies to better protect their bridges from collisions.
.
