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Publication Number:  FHWA-HRT-13-105    Date:  December 2013
Publication Number: FHWA-HRT-13-105
Date: December 2013


Evaluation of Member and Load-Path Redundancy on The US-421 Bridge Over The Ohio River

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FHWA Publication No.: FHWA-HRT-13-105
NTIS Accession No. of the report covered in this TechBrief: PB2013-110588
FHWA Contact: Justin Ocel, HRDI-40, (202) 493-3080, justin.ocel@dot.gov


This document is a technical summary of the unpublished Federal Highway Administration (FHWA) report Evaluation of Member and Load-Path Redundancy on the US-421 Bridge over the Ohio River, available through the National Technical Information Service at www.ntis.gov.


The sudden collapse of the Silver Bridge in 1967 demonstrated that failure of a single member could result in failure of the entire bridge. This failure and other fractures led to the development of fracture critical member (FCM) provisions for design, fabrication, and inspection. The design and fabrication aspects were originally published by the American Association of State Highway Transportation Officials (AASHTO) as a guide specification in 1978.(1) The guide specification was withdrawn in 1989 and largely adopted into the AASHTO Load and Resistance Factor Design (LRFD) Bridge Design Specifications, and American Welding Society (AWS) D1.5 Bridge Welding Code.(2, 3) Regulations regarding the field inspection requirements of fracture critical members are mandated by federal law.(4)

The AASHTO LRFD Bridge Design Specifications describe an FCM as “a component in tension whose failure is expected to result in the collapse of the bridge or the inability of the bridge to perform its function.”(2) Bridges containing FCMs require a “hands-on” inspection every 24 months.(4) These inspections are more rigorous than routine inspection and often require closing down portions of the bridge to traffic to gain access. As of December 2011, there were 18,770 bridges with FCMs, out of a total 91,850 steel bridges in the National Bridge Inventory (approximately 20 percent).(5)

Under the AASHTO specification, FCMs are non-redundant steel members either partially or wholly in tension. The specification defines redundancy as “the quality of a bridge that enables it to perform its design function in the damaged state,” and redundant members are those in which “failure does not cause failure of the bridge.”(2) However, determining redundancy is often difficult, as demonstrated by bridges in service where full-depth fracture of an FCM did not result in collapse of the structure as described in detail within NCHRP Synthesis 354.(6) In fact, many bridges have carried traffic for some time prior to a fracture being discovered. The performance of these structures after fracture implies that redundancy, though unaccounted for, does exist by backup mechanisms not considered in the engineering design.

Redundancy is often separated into three different types: internal redundancy, structural redundancy, and load-path redundancy, described as follows:

A technical memo published by FHWA with the subject “Clarification of Requirements for Fracture Critical Members,” dated June 20, 2012, provides guidance and recognition regarding structural redundancy for members traditionally classified as FCMs.(7) This memo for the first time recognizes system performance (i.e., structural redundancy) as a method to assess redundancy and classify FCMs. The memo also encourages the use of internal redundancy as good detailing practice but does not allow it for classifying whether a member is fracture critical.


The objective of the project was to assess the after-fracture performance of a two-line, simple-span truss bridge. The bridge was slated for explosive demolition and offered the ability to examine both internal and structural redundancy. Since this study focuses on a single bridge, the results and methodologies developed through this research are rather specific. Nevertheless, as will be discussed, they do show that the universal assumption of collapse following failure of an FCM is not always valid.


The US-421 bridge spans over the Ohio River between Madison, IN, and Milton, KY, and is referred to as the Milton-Madison Bridge. It was constructed in 1921 and consisted of 19 spans of riveted, built-up members. An overall view of the bridge is shown in figure 1.

Figure 1 does not capture the approach-span trusses leading up to the river span. The first approach-span truss on the Indiana shore is the focus of this research effort. A picture of a test truss is shown in figure 2; the lower chord is close to the ground and spans over a city park property, making access for instrumentation easy. The test truss spans 149 ft and is in a Pratt configuration as shown in figure 3. The truss had some corrosion throughout, which was quite severe in some places and left holes and partially severed members, as seen in figure 4.

This picture shows the overall view of the bridge standing from the Indiana shore. Through the middle of the picture are six concrete piers protruding from the river. It is a foggy morning, and the furthest two piers and bridge sections over them are barely visible. The morning is still and the Ohio River is smooth as glass, showing a mirror-image reflection of the bridge in the bottom half of the figure. The extreme right middle of the picture shows the deck truss approach spans that transition to the through truss spans over the river through the middle of the photo.

Figure 1. Photo. Overall view of bridge from Indiana shore.

This photo focuses on just one of the approach span trusses, which takes up most of the width of the photo. The truss is simply supported and in a Pratt configuration. There is a white van parked beneath the bridge that demonstrates that the lower truss chord is approximately 15 ft off the ground. Atop the bridge is a yellow dump truck used as a controlled load; in the picture, it appears to be over the second node away from the Indiana shore.

Figure 2. Photo. View of the test span.

This figure illustrates a schematic drawing of the truss to denote some dimensions and node numbering. The right side of the figure shows an elevation view of the truss. On the elevation view, the left side is the Indiana side, and the right side is the Kentucky side. The lower nodes of the truss are labeled sequentially, left to right, from L0 to L7. The same number scheme is used for the upper nodes, except from left to right and using the labels U0 to U7. The elevation also shows that the total length of the truss is 147 ft from centerline to centerline of bearings, made from 7 equal length panels of 21 ft. The distance from the top and bottom chord centerlines is 22 ft. There is a section called out between the second and third panels, and the section view is on the left side of the figure. The section view depicts that there is X-bracing used to prevent sway.

Figure 3. Illustration. Elevation view of truss.

This photo is a close-up shot of a lower chord member near a bearing. The chord member is primarily a riveted, built-up member. The overall member appears to be built up from two built-up C-sections. Each C-section is a plate with two angles riveted at the top and bottom of the plate. Within the field of view, only one of the C-sections can be seen, and there is a thicker plate bolted to the web plate of the C-section. Most of the member has some minor surface rusting, though it is clear that one of the lower angles is completely severed from corrosion.

Figure 4. Photo. Severe corrosion in lower chord near abutment.


A simple, three-dimensional analysis model of the bridge was created with beam elements, and shell elements for the deck. The bridge deck was replaced with a filled grid deck in 1996, and this was modeled with shell elements with different longitudinal and transverse properties. The model indicated which members should be removed in a simulated fracture event and guided placement of strain gauges.

Forty-eight weldable strain gauges were applied to select members on the truss. Remote monitoring was performed for five months under ambient traffic loading before the truss was removed from service. These data were used to conduct fatigue analysis from rain-flow stress histograms, which is covered in detail within the full report.

Additionally, static-load and slow-crawling tests were performed on the bridge with a sand truck of a known weight.


Controlled load testing was performed on the bridge to understand its behavior, determine stresses in the bridge under a known load, and calibrate the finite element analysis. The two-part test included crawl tests and park tests, which were used to examine the response of the bridge to moving loads and to obtain static data (i.e., with the loading at defined locations) that are helpful in calibrating the FE model. The gross vehicular weight of the test truck exceeded the posting on the bridge in order to ensure that sufficiently large stresses were produced. Analysis showed that the weight of the truck would not produce any overstressing in the bridge.

The results of the monitoring revealed that a substantial number of trucks that crossed the bridge produced stresses that were equal to or greater than those that the test truck produced. It was concluded that random vehicles crossing the bridge must have been at least as heavy as the test truck and that the posted weight limit for the bridge was exceeded on a daily basis.

The park tests were used to determine if the model could accurately capture the behavior of the bridge. The field measured stresses that were later compared to those predicted by the FE model for all nine park test locations. In general, the model yielded a conservative prediction of the stresses in the members.


As far as this project was concerned, the controlled demolition plan was to completely sever the L3-L4 bottom chord on the upstream truss while monitoring the strain gauges. Sand was placed on the bridge to represent about 2/3 the original design live load. It was difficult to select this loading, as there is no guidance regarding what amount of live load a fractured bridge should be able to sustain. After much consideration, the selected load was deemed reasonable, as it is much higher than any loads seen during long-term monitoring and was thought to be a reasonable upperbound load. This resulted in 145 kips of sand distributed over the deck. However, this load was focused over the three interior panels of the truss (i.e., between nodes U2 and U5) and resulted in stresses in members that were approaching the original design loading. Figure 5 is a photo of the sand atop the bridge.

The chord member was fabricated from built-up channels that were laced together. Each built-up channel was comprised of two angles and a plate that served as the web. In some locations, two web plates were utilized. Each built-up channel was severed separately to evaluate the internal redundancy of the member (i.e., each half of the member was cut separately). To facilitate this, the lacing was flame cut between the two sets of shape charges to ensure that no load was carried through the lacing. Shape charges, as shown in figure 6, were used to instantaneously sever the entire built-up channel, in order to represent a fracture event as much as possible. Figure 7 is a photograph taken after one half of the lower chord was severed.

Figure 8 shows a record from one of the strain gauges installed on the L3-L4 chord on the downstream truss during the two explosive events on the same member of the upstream truss. The plots include the effects of dead load and sand loads. Therefore, this plot shows the total stress in the member. The dead-load stresses were obtained from the computer model, while the stress from the sand load was measured. With the sand in place, the member was subjected to 9.4 ksi prior to the simulated fracture. After the first half of the upstream chord was severed, the stress increased to 9.9 ksi. After the member was completely severed, the stress further increased to 11.9 ksi. The dynamic amplification due to the instantaneous load release following the simulated fracture events was also observed. For all members that were instrumented, the dynamic amplification from the first blast ranged from 1.08 to 1.30. For the second blast event, the amplification factors ranged from 1.17 to 1.41.

Vertical displacements were also monitored during the two blast events using high-resolution pictures and surveying equipment. Four truss nodes were monitored, and the results are shown in table 1. The measurements were taken at a distance of 164 ft and have a resolution of approximately 0.16 inches. The measured displacements were close to the resolution but are still thought to be representative of the real displacement. The agreement between the simple analysis model and the real bridge in terms of vertical displacement were reasonably close after the first blast event. However, after the second blast event, the model over-predicted vertical displacement by a factor of almost two. The breakdown in the correlation between the model and the test was likely due to the inability of the model to capture the complex alternate load paths used by the structure following removal of a member and the transfer of shear forces in the truss. This is believed to be a major result of the difficulty associated with modeling the filled grid deck and the rigidity of the connections to the primary members and floor system.

This photo is taken from the deck of the approach span deck truss on the Indiana side. In the background is the Kentucky shoreline, and the view is looking right down the middle of the bridge, so the through truss spans are also in the background. In the immediate foreground of the photo is sand, distributed evenly over the entire width of the truss and for about 52 ft along the truss. In the right foreground is a man in a yellow safety vest walking towards the sand.

Figure 5. Photo. Sand placed on deck before simulated fracture event.

This photo is a close-up of a shape charge used in the explosive demolition. Running from left to right across the photo is the built-up chord member made from angles and plates riveted into a C-shape. In the center of the figure, the outstanding legs of the angles have been flame cut away for approximately 2 inches, flush to the web of the built-up C-shape. A copper clad rod (the shape charge) is clamped to the C-shape web in a vertical orientation where the outstanding legs had been removed.

Figure 6. Photo. Shape charges applied to one web plate of chord member.

This photo was taken from the ground, looking up to the chord member after the first explosive event. The chord member is running left to right across the photo, and one of the two built-up C-shape elements of the member has been distinctly severed, leaving an approximate 1/2-inch gap where the shape charge was mounted. The background of the photo generally shows the bottom of the exodermic deck.

Figure 7. Photo. Web plate fracture after first explosive event.

This is a graph showing the measured response of the chord through the two explosive events. The vertical axis shows total stress in ksi, plotting from 0 to 16. The horizontal axis plots time in seconds from 0 to 400 seconds. There are three distinct plateaus shown in the plot. The first is about 9.5 ksi from 0 to 60 seconds, the second is about 10 ksi from 60 to 230 seconds, and the third is about 12 ksi from 230 to 400 seconds.

Figure 8. Graph. Data record of L3-L4 downstream chord during both explosion events.

Table 1. Comparison of measured and predicted vertical deflections.



Measured Vertical Displacement

Predicted Vertical Displacement

First Blast Event













Second Blast Event














The following important conclusions were derived from this project:


As a result of this research, recommendations for future research have been developed. These recommendations are as follows:


  1. AASHTO (1978). Guide Specification for Fracture Critical Non-Redundant Steel Bridge Members. American Association of State Highway and Transportation Officials, Washington, DC.
  2. AASHTO (2012). LRFD Bridge Design Specifications—6th Edition. American Association of State Highway Officials, Washington, DC.
  3. AASHTO/AWS (2010). D1.5M/D1.5 Bridge Welding Code. American Welding Society, Miami, FL.
  4. Code of Federal Regulation (2013). 23 CFR 650C.
  5. FHWA. National Bridge Inventory ASCII Files, accessed July 3, 2012, at https://www.fhwa.dot.gov/bridge/nbi/ascii.cfm.
  6. Connor, R.J., Dexter, R., and Mahmoud, H. (2005). “NCHRP Synthesis 354 - Inspection and Management of Bridges with Fracture-Critical Details.” Transportation Research Board, Washington, DC.
  7. FHWA (2012). “Clarification of Requirements for Fracture Critical Member.” Memorandum from Office of Bridge Technology, dated June 20, 2012. Washington, DC.

Researchers—This work was conducted by Purdue University under the direction of Dr. Robert Connor, Lindsey Digglemann, and Ryan Sherman. The work was performed under contract DTFH61-10-D-00017-T-11004.

Distribution—This TechBrief is being distributed according to a standard distribution. Direct distribution is being made to the Divisions and Resource Center.

Availability—This TechBrief may be obtained from the FHWA Product Distribution Center by email to report.center@dot.gov, fax to (814) 239-2156, phone to (814) 239-1160, or online at https://www.fhwa.dot.gov/research.

Key Words—Fracture critical, steel bridges, truss, internal redundancy, structural redundancy.

Notice—This document is disseminated under the sponsorship of the U.S. Department of Transportation in the interest of information exchange. The U.S. Government assumes no liability for the use of the information contained in this document. The U.S. Government does not endorse products or manufacturers. Trademarks or manufacturers’ names appear in this report only because they are considered essential to the objective of the document.

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