Q: How do structural engineers estimate a bridge's lifespan? Anand Sikka, Toronto, Canada
Jerome Lynch: Dear Anand,
Bridge life expectancy is primarily considered during the design stages. Certain design choices can lead to more robust and durable bridges (hence, ones with longer lifespans). However, once a bridge is opened and in service, visual inspection remains the bridge engineer's primary tool for evaluating the condition, or health, of a bridge. Based on an assessment of the bridge's current health, a rough estimate of the remaining operation lifespan can be made.
Q: On a scale of one to 10, if 10 means safe and walking down the sidewalk is a nine, how safe is crossing an average bridge in the U.S.? Greg M., Springfield, Missouri
Lynch: Dear Greg,
I would estimate the safety of U.S. bridges to be between 9.5 and 10. While tragedies like the I-35 bridge collapse understandably shake public confidence, bridge structures are extremely safe infrastructure elements. But while American bridges have an impressive safety record, there is always room for improvement. This is where monitoring systems come into the picture.
The primary goal of a bridge monitoring system is to add more objectivity to the management process. If engineers are provided with more quantitative information on the behavior of the structure, they can be more cost-effective in managing the health of the structure. For example, instead of inspecting and renovating bridges on fixed schedules, bridge engineers can pursue condition-based maintenance strategies.
Q: As someone who travels the same bridge every day, I'm curious: Which brand new sensing systems are likely to be implemented soonest? How "foolproof" are they? Vikram Reddy, New York, New York
Lynch: Dear Vikram,
Some of the emerging sensor systems likely to be implemented soonest on bridges include fiber optic strain sensors, wireless sensors, and sensors that use acoustic and ultrasonic waves to sense cracks. Notably, Mike Todd of the University of California at San Diego was featured in the episode for his work in acoustic and ultrasonic wave sensors.
In terms of how "foolproof" these sensors are, no one sensor will ever provide an identification of damage with 100 percent accuracy. There is always the question of interpreting the data collected by the sensor. In fact, the development of automated algorithms that can interrogate sensor measurements for indications of structural damage is still an open research area in the academic community. Significant work is still needed in the area of automated damage detection algorithms.
Q: What does it mean when a bridge is designated structurally deficient or functionally obsolete? If these labels are as bad as they sound, why have transportation authorities allowed tens of thousands of American bridges to fall into disrepair? Jane S., Germantown, Tennessee
Lynch: Dear Jane,
All bridges in the United States have to undergo visual inspection every two years by federal law. Federally mandated inspections have been standardized by the Federal Highway Administration (FHWA) based on a sophisticated rating system termed the National Bridge Inventory (NBI) rating scale. The terms "structurally deficient" and "functionally obsolete" are technical terms that are part of the NBI scale. A structurally deficient bridge is one in which the bridge deck, superstructure, or substructure is rated as in "poor" condition. Also, if a cresting river can submerge the bridge deck during flooding, the bridge would also be rated as structurally deficient. A functionally obsolete bridge rating is given if the bridge load capacity is below current design standards. This classification generally applies to older bridges designed more than 50 years ago when trucks simply carried less weight than today's trucks.
Keep in mind, bridges rated as deficient or obsolete do not necessarily pose a danger to the community-such structures can still be safely used by motorists. Rather, these designations are intended to assist local departments of transportation in prioritizing the repair, retrofitting, and replacement of these bridges ahead of other tasks associated with the upkeep of the remainder of the bridge inventory.
Viewers would have the wrong impression if they felt that bridge engineers have in some way neglected these deficient or obsolete bridges and let them fall into disrepair. These engineers have the difficult task of managing hundreds and thousands of bridges with fixed budgets while every year more vehicles and heavier trucks continue to wear on these structures.
Q: If the US has 600,000 bridges and structural monitoring technology costs $50,000 per bridge, where do bridge owners find $30,000,000,000? Jeff, Santa Fe, New Mexico
Lynch: Dear Jeff,
You have hit the nail on the head! While many technologies exist for monitoring bridges, the challenge is how to pay for such monitoring systems. Many bridge owners have to contend with limited budgets that often do not permit the selection of structural monitoring systems as part of their management strategies. In response to this, the research community is working hard towards developing new sensor technologies with improved performance that cost an order of magnitude less than current sensors.
For example, one sensor technology that has generated considerable interest in recent years is the wireless sensing system. This is essentially a wireless local area network in the structure set up in much the same way as your wireless internet network at home, but with sensors using the wireless network to send data to one another. A major benefit of this is that installation of the monitoring system is cheaper compared to the tethered monitoring systems in which extensive lengths of wiring are needed to communicate data between sensors. If monitoring systems continue to reduce in cost (as I believe they will), it is likely we will see greater adoption of these systems for a larger fraction of the U.S. bridge inventory.
Q: Great segment. Systems like the carbon-fiber skin sound like they'd be great at sensing problems, but they also sound expensive. What's the most cost-effective type of structural health monitoring? Anonymous
Lynch: Some of the ingredients that go into the manufacturing of the sensing skin are very costly. Of particular note are the carbon nanotubes, which are the most expensive ingredient. The good news is that as more manufacturers enter the marketplace with high-quality nanotubes, the cost of this impressive nanostructure will continue to decline. Today, the sensing skin is estimated to cost about $1 per square inch, which can add up considering the lengths and heights of many bridges. In general, cost is one of the most important determinants in the decision process to adopt or not to adopt a sensor technology for an actual bridge.
Q: How will these new nano-electrical sensing technologies stand up to extreme weather conditions? Anonymous
Lynch: This is a great question, because for any sensor to be considered for adoption by the bridge engineering community, it must offer reliable service over long time periods on the order of decades. Keep in mind the idea of installing sensors in bridges is to reduce the complexities inherent to managing bridges over years of service. Obviously, if the sensors fail or degrade, then we have only added another system component that must be maintained by the engineer.
At this point, the nanoengineered sensing skin is just making its way out of the laboratory to be tested in field conditions (for example, exposure to wet weather and direct sunlight). The field tests currently being carried out using actual bridges will allow us to quantify the reliability and durability of the sensor technology over many years of service. If our sensor is durable over a decade of use, then our design will be considered successful. On the other hand, should the sensor degrade overtime, we will learn from our study and will seek to improve the sensor performance.
Q: I understand that the Brooklyn Bridge will by "refurbished" in 2010. Should we be concerned? What are the problems with the bridge? Michael Robertson, Oceanside, California
Lynch: Dear Michael,
There is nothing to be alarmed about-bridge renovation and repair is a common practice in the bridge engineering community. It is one way in which engineers can ensure such structures remain safe structures for public use. Given the harsh conditions many bridges are exposed to (in the case of the Brooklyn Bridge, high levels of urban traffic, harsh winters, exposure to a moist environment), deterioration will gradually occur; agencies such as the New York City Department of Transportation, which oversees the Brooklyn Bridge, are extremely proactive in identifying when repair and renovation are prudent and economically convenient to carry out, which is well before it is actually necessary.
Rehabilitation of an aging bridge is not a management strategy unique to the bridge engineering community. For example, airlines commonly take their aircraft out of service after a certain number of miles flown; the aircraft is literally rebuilt to ensure the aircraft is in a safe operational condition. Other examples can be found in many other industries dealing with expensive and complex engineered systems.
Q: Where are the world's most advanced structural sensing systems being implemented, and what are these systems like? D.J. Williams, Jackson, Mississippi
Lynch: Dear D.J.,
Advance structural monitoring systems are being implemented throughout the world. Many bridges in the United States, Japan, China, Hong Kong, Korea, and Europe have sophisticated monitoring systems that record their response to loading. Many of these monitoring systems are one of a kind, uniquely designed for the specific bridge being monitoring. For example, in the United States many landmark bridges such as the Vincent Thomas Bridge (Los Angles) and Golden Gate Bridge (San Francisco) are implemented with monitoring systems designed to record the bridge response to earthquake loads.
This data leads to better understanding of bridge behavior during seismic loading and improved design practices. Many of the long-span bridges in Hong Kong have extensive monitoring systems installed with hundreds of sensors. These systems are intended to monitor the behavior of the bridge during typhoon events, which commonly occur in Asia during the summer months. Excessive bridge motion due to a typhoon is important in deciding when to close the bridges to traffic.
Q: I've heard that if a lot of people walk with their footsteps in sync across a bridge, they can cause the bridge to collapse. Is this true? Could a marching band or soldiers really exert that much damage upon a structure? Ryan Simmons, Chicago, Illinois
Lynch: Dear Ryan,
This common public belief is actually rooted in a historic bridge failure that occurred in France in the mid-1800s. Reportedly, the Angers suspension bridge failed when hundreds of troops were marching in step across the bridge. Today it is impossible to know if the Angers Bridge had a design or construction flaw leaving it vulnerable to failure, but it does appears that the troops contributed to the disaster. When troops march in cadence, they are essentially impacting the bridge at a fixed rate, or frequency. Every structure has a "natural" frequency; if the loading frequency (whether from troops or other sources) matches the natural frequency of the structure, a phenomenon known as resonance can occur. The Tacoma Narrows Bridge is widely cited as a modern day example of resonance-induced collapse. In this case, the frequency of the wind matched the natural frequency of the bridge deck resulting in higher than normal response amplitudes and subsequent failure.
In more recent times, the Millennium Bridge spanning the River Thames in London suffered from pedestrian induced resonance. On opening day, thousands of pedestrians crossed the Millennium Bridge, resulting in a noticeable sideways oscillation. The instinctual reaction of the pedestrians was to compensate for the humanly perceptible motion by walking in step. Pedestrians walking in step further amplified the sideway oscillation, a classic example of pedestrian-induced resonance. The bridge was closed after three days of service to be retrofitted with a tuned mass damper, a device that limits structural motion in the face of resonance.
In modern day bridge design, resonance is accounted for in the design process. Therefore, if troops were to cross a modern U.S. bridge in step at a frequency equal to the natural frequency of the structure, higher bridge amplitudes would be encountered consistent with physics. However, such responses would likely remain within the safe operational envelope of the bridge, resulting in no damage to the structure or cause for concern.