Thursday, April 11, 2013

Suspension bridges: Concepts and various innovative techniques of structural evaluation

A typical configuration of a suspension bridge.
Small perforations in the cable wrap surface. These are potential locations for water infiltration.
The author performing a structural evaluation of Throngs Neck Bridge in New York City.
A typical configuration of a suspension bridge.
by Avinash Prasad, P.E., L.S., PhD. (C), Metropolitan Transit Authority – New York City Transit.

A greater utilization of advanced NDT methods can help spot potential problems in suspension bridges.

During the past 200 years, suspension bridges have been at the forefront in all aspects of structural engineering. Their spans have grown from 50 to 2,000 meters (164 to 6,561 feet) with designs for 3,000 meters (9,842 feet) under consideration.1

A suspension bridge is a special type of bridge in which loads from the bridge deck are carried by vertical suspenders that are supported by suspension cables suspended between towers and anchored at both ends of the bridge. The anchorage should be strong enough to take the high-tensile forces of suspension cables. The main suspension cable of modern bridge cables are made from multiple strands of wire. This contributes greater redundancy in case of structural failure.

Suspension bridges are showing signs of distress due to aging, improper repair or rehabilitation or lack of proper maintenance. Normally, visual inspections (including 100 percent hands-on inspection of fracture-critical fatigue prone details) are done for structural evaluation of critical components of a suspension bridge.

Extending the service life of aging suspension bridges is important for the transportation industry. Proper non-destructive testing (NDT) can identify most of the structural problems. Thereafter, necessary repair/rehabilitation can be executed with minimum funding. Additional research is required on this topic, which would be beneficial to the industry.

As it pertains to structural evaluation of suspension bridges carrying railroad traffic and/or vehicular traffic, it is important to include in the design of a suspension bridge that the centerline of the railroad tracks and associated structures are symmetrical with respect to the transverse centerline of the suspension bridge to avoid unnecessary and/or additional torsional forces. Additionally, the dead load of the suspension bridge should be significant compared to other types of load.2

Why suspension bridges fail

The Silver Bridge over the Ohio River at Point Pleasant collapsed in 1967 due to a brittle fracture of an eye-bar of the suspension system. Some of the suspected reasons were poor quality control during construction, inability to inspect, ignorance of fatigue behavior and lack of redundancy, among other factors.3 After this incident, the Federal Highway Administration made it mandatory for biennial inspection of all bridges.

An additional outcome was the National Bridge Inventory (NBI), which currently contains data on more than 600,000 vehicular bridges, 200,000 railroad bridges and more than 100,000 culverts. Fracture-critical details became an important part of the inventory and special provisions were designed for their inspections.4

Structural failure of a suspension bridge may be a combination of poor design, construction, maintenance, failure to inspect fracture-critical and fatigue prone details thoroughly, severe weather conditions or a combination of all these factors. Normally, suspension bridges are visually inspected along with checking of interior wires through a wedging process for selected critical locations biannually. There are limitations in current visual inspection of suspension cables, as visual inspection reports vary when compared with NDT method results.

Taking the above points into consideration, it is imperative to develop various NDT techniques to evaluate the structural condition of suspension bridges to avoid catastrophic failures.

Structural evaluation of suspension bridges

Suspension bridges are more susceptible to vibration due to flexibility compared to other rigid bridges. In design, there are variations in the type of vehicular and railroad live load. The dynamic effect by railroad traffic is more significant than vehicular traffic on a suspension bridge. Railroad live load, in a train length of approximately 600 feet, can be uniformly distributed as the load crosses over the bridge. In the case of vehicular loading, live load is a series of point loads as vehicle wheels cross over the bridge.

Railroad traffic can be allowed on a suspension bridge under specific circumstances and the following considerations are required:

  1. The dead load of suspension bridge should be higher compared to other types of load on the bridge.
  2. A provision should be made for increased stiffness of the bridge (by providing inclined cable stays, etc.).
  3. It is important to make consideration in design that the railroad centerline of track and associated structures are symmetrical with respect to the transverse centerline of suspension bridge to avoid unnecessary and/or additional torsional forces.

In New York City, Manhattan Bridge and Williamsburg Bridge are examples of suspension bridges, which carry both highway and railroad traffic.

The Williamsburg suspension bridge is an example where the railroad centerline of track and associated structures are symmetrical with respect to transverse centerline of the bridge, which avoids unnecessary and/or additional torsional forces.6 However, in the Manhattan suspension bridge, the railroad centerline of track and associated structures are unsymmetrical with respect to the transverse centerline of the bridge causing unnecessary additional torsional forces. As a result of this positioning, the Manhattan Bridge, over the years, has suffered damage and had to be retrofitted with a torsion tube to increase its resistance to torsional forces. This special arrangement is not proving effective year after year and serves as one of example of additional stress caused to a suspension bridge due to poor design, carrying railroad and vehicular traffic together.6

Advancement of NDT methods

Current NDT methods can be used to check the material properties of a structure and to verify the structural condition without damaging the infrastructure. The numerous NDT methods used on suspension bridges are based on the bridge structures inherent material properties, various types of electromagnetic radiation, etc. When electromagnetic radiation passes through suspension bridge structural elements, it shows distinct characteristics on account of material flaw. Dye penetrant testing for nonmagnetic materials and a magnetic particle test for magnetic materials can be performed to discover new cracks and/or any discontinuity in material properties.7, 8

Alternative NDT methods include electromagnetic testing, infrared and thermal testing, radiographic testing, C-scan, laser technology testing, ultrasonic testing, hardness testing, vibration analysis and Magnetic Resonance Imaging (MRI).

Even with these options, advancement of NDT methods for structural evaluation is needed. One reason is that the current practice of visual inspections and subjective observations are not sufficient enough for precise structural evaluation of suspension bridges. Additionally, even the current practice of visual inspection of suspension bridges is not done properly, sometimes due to fear factor, as well as improper access to bridge inspectors.

Apart from regular visual inspection, wedging of cables is performed to check the condition of cable strands, but this method covers only a portion of cable surface area as most of the cable area remains inaccessible. Inspecting a 20-foot length cable with wedging at eight points exposes less than 0.1 percent of the wire for a typical suspension bridge (4,000 foot main cable with 15,000 wires).4 The factor of ignorance in cable inspection is high due to the current wedging technique, as mentioned above. This process of wedging is tedious and can be preferably performed for the smaller size of cable strands.

In structural evaluation of suspension cables, visual inspection cannot detect the actual deterioration (corrosion, breakage of wires, etc.) of cable strands due to the protective cable wrap over the actual load bearing cable. Hence, we are limited to visual inspection (that includes free climbing on suspension cables) and limited wedging of cable strands. This situation forces us to think about various innovative NDT methods to check the actual condition of the suspension cable.

It is highly disappointing that not many NDT methods are commonly used for structural evaluation of suspension bridges. Catastrophic failures have occurred for various reasons on a few suspension bridges. This could have been avoided had various advanced NDT methods been put to use.

Saving aging suspension bridges is important for the transportation industry. It is highly recommended to use various advanced NDT testing for structural evaluation of aging suspension bridges. Since every NDT method is unique, it is imperative to select particular methods based on the suitability for a successful inspection.

Some of the NDT methods, such as MRI, are costly and need specialized training for effective structure evaluation. Normally, most of the high technology NDT tests are not performed on account of higher cost and effort in developing and using these methods for structural evaluation of suspension bridges. It is better to spend time and resources to use newly-developed NDT techniques to find a hidden structural defect. The transportation industry has to gear up to make use of these NDT methods. By following these norms, there will be higher chances of avoiding catastrophic failures of suspension bridges in the future.

There are not enough NDT methods that are practiced in structural evaluation of suspension bridges. It is the author's opinion that visual inspection cannot cover many areas of suspension bridges. Many inspectors are not able to pay close attention during inspection due to the height factor and poor access. The current structure evaluation is based on the visual observation and its subjective interpretation. Based on the limitations of visual structural evaluation, there is an urgent need of sophisticated NDT technology, which can extend the structure evaluation coverage. To further solidify the use of NDT technology, the author suggests that federal and state transportation agencies consider mandating the use of additional NDT methods of suspension bridges as part of the existing biennial inspection.

Future work

With the increasing pace of changes in the technology and the current economic downturn, organizations around the world are focused more on cost-effective and value-added technology to develop new NDT methods to pinpoint the critical defects of the different components of suspension bridges. A broad level of research with adequate funding could be initiated down the road to promote the use of NDT methods of suspension bridges in the transportation industry. The author is continuing his research on this topic to explore additional facts, which will be published in the future when available.


The author acknowledges the help of Ms. Indira Prasad, PMP, in peer reviewing this article. The author sincerely thanks his professors at NYU-POLY, as well as his colleagues at Metropolitan Transportation Authority-New York City Transit (MTA-NYCT) for their help and encouragement.

Disclaimer, about the author

Even though the author works for MTA-NYCT, any opinions, findings and conclusions or recommendations expressed in this material does not reflect the views or policies of MTA-NYCT nor does mention of trade names, commercial product or organizations imply endorsement by MTA-NYCT. MTA-NYCT assumes no liability for the content or the use of the materials contained in this document. The author makes no warranties and/or representation regarding the correctness, accuracy and or reliability of the content and/or other material in this paper. The contents of this file are provided on an "as is" basis and without warranties of any kind, either express or implied.

Avinash Prasad works for MTA-NYCT as a civil engineer-level III and has more than 25 years of professional experience. He is a registered professional engineer and land surveyor in multiples states and is a structural evaluator as a professional engineer of high-rise structures (suspension bridges, towers, etc.) in the United States and abroad.


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