Koror-Babeldaob+Bridge+Collapse

Koror-Babeldaob Bridge Collapse //M. Julia Haverty, BAE, Penn State 2015 // toc = Introduction = On September 27, 1996 around 5:45 PM local time, the 18 year old Koror-Babeldaob Bridge collapsed. The collapse occurred a short time after a repair, which lead many to believe that the repair was t he cause of the collapse. The bridge had served as the only vehicular connection point between Palau's two major islands: Koror and Babelthuap. Most of the country's inhabitants resided on Koror, where the capital is located, but many services, such as the airport and the freshwater supply, were on Babelthuap.
 * Country of Palau- September 27, 1996 **

The unexpected collapse resulted in 2 fatalities and 4 injuries (Ernst and Verlag 1998). Due to the remote location of the islands and the unexpected break in the supply, the country was forced to declare a state of emergency.

The bridge was a concrete box girder design, and spanned approximately 790 ft (241m), which at the time made it the longest concrete box girder cantilever bridge in the world. The sudden collapse, which o ccurred during a period of calm weather and typical loading conditions, was caused by a compression-induced delamination of the top flange of the bridge.

Keywords

 * bridge collapse
 * Palau
 * retrofit
 * delamination

= Palau =

Palau is a small and remote Oceanic island nation located in the Pacific Ocean, East of the Philippines and North of the Indonesian Island of Papua. The country is made up of over 300 islands and is home to approximately 21,000 inhabitants. Palau was a German territory until World War I, after which point the Japanese government seized control of the islands. Following the end of World War II, the islands became part of the UN Trust Territory for the Pacific, and remained that way until the country gained independence in October 1994 (CIA 2014). In 1978, the Koror-Babeldoab Bridge was constructed in order to properly link the territory's two major islands: Koror and Babelthuap.

Though most of Palau's inhabitants resided on Koror, many services, including freshwater and the airport, are found on Babelthuap, deeming it necessary for these two islands to have a connection point that could be easily traversed. The bridge, which spanned across the 30m (98 ft) Toachel Channel, was the easiest way to get from one island to the other by car or by foot (Burgoyne and Scantlebury 2006). Getting between the airport and the capital is of paramount importance for the country, which counts tourism as its primary economic source (CIA 2014).


 * = [[image:https://lh5.googleusercontent.com/QqtiI1EtEH1tEku1TwoVjWYMqtWeKCqmqp9owvpmTO_o8TBVAKtOGmdxocmY0DpK0lvrEx0HfGqYlzwnprHXpbsC9aDVwhtrTMYN27M2sOOntjJYIhwjVqw8Od1km7JtiA width="497" height="451" caption="Figure 2: Map of Oceania (Courtesy: Google Maps)"]] ||= [[image:http://upload.wikimedia.org/wikipedia/commons/7/74/Palau-CIA_WFB_Map.png width="402" height="320" caption="Figure 3: Map of Palau (Courtesy: Wikimedia Commons)"]] ||

= Pre-Collapse Events and Construction =

The $5.2 million bridge was constructed in 1978 by the Socio Construction Co. of Korea with the intention of forever linking the two major islands. The location of the bridge was chosen due to the small land masses on either side of the Toachel Channel (see Figure 4). The bridge was designed by two firms, Alfred A. Yee and Associates and Dyckerhoff and Widmann Ag (Ernst and Verlag 1998). The main span of the bridge was 241 m (790 ft) and the height from the bottom of the midspan to the water level was 20.42 m (67 ft). The bridge's width of 9.62 m (~31.5 ft) allowed for two lanes of traffic plus a pedestrian walkway.

The bridge was designed symmetrically, with two large main piers supporting cantilevered portions that met above the middle of the channel. The bridge was a true box girder design, and featured a fixed width with depths getting smaller as they approached the midspan. A cross section of the design can be seen in Figure 5. The bridge's main piers were prestressed and attached to the "approach spans" which were supported by end piers. Supporting the large main piers were vertical piles. These piles were designed to resist horizontal forces created by the channel. Conversely, the end piers were simply supported by vertical piles. Each cantilevered side had 25 cast in place segments (Bazant et al. 2010) and the two components were joined at the channel's midpoint using a central hinge. The hinge was designed only to carry a small load, and allowed longitudinal moment and relative rotation of each portion of the cantilevers (Burgoyne and Scantlebury 2006).
 * = [[image:cross section.PNG caption="Figure 5: Bridge cross section (Courtesy: Burgoyne, The Structural Engineer)"]] ||= [[image:elevation.PNG caption="Figure 6: As-built Bridge Elevation (Courtesy: Burgoyne, The Structural Engineer)"]] ||

From the time of construction until 1984, it was the longest post-tensioned concrete box girder bridge in the world. Over the course of the bridge's lifetime, the cantilevers began to deflect due to shrinkage and creep. In 1990, the bridge's midspan deflection was observed to be larger than one meter, and the central hinge connection between the two cantilevers had a change of angle, which caused overall discomfort and concern between locals and government officials alike (Burgoyne and Scantlebury 2006). This deflection can be seen in Figure 7.

After numerous complaints regarding the deflection, the Palauan government appointed two teams of structural engineers to investigate the bridge. The teams concluded that the bridge would continue to deflect an additional meter over the next century but was technically still safe for both vehicular and pedestrian traffic. Ultimately the Palauan government decided to correct the deflection rather than let the bridge continue to sag. Retrofit work began in October 2005 and started with the removal of the top layer of concrete (Bazant et al. 2010). In July 1996, after a year of work, "retrofit" changes to the structure's midspan were completed. Flat jacks were inserted at the center of the bridge, and the central hinge was removed and grounded in order to make the bridge continuous. Eight additional post-tensioned prestressed cables were also added to each side of the span. Lastly, the central span of the bridge was resurfaced (Burgoyne and Scantlebury 2006).

= Collapse =

The Koror-Babeldaob bridge collapsed on September 27, 1996 in the early evening. The collapse occurred during a period of typical loading and calm weather and caused two fatalities and four injuries. Due to the bridge's importance, the severed connection between the two major islands caused the country to go into a state of emergency.

Eyewitness accounts reported hearing popping sounds and the sound of concrete hitting metal shortly before the collapse occurred. The Babelthuap side of the bridge was the first to fail, with the assumed initial failure location occurring roughly 1/3 into the mainspan. Additionally, there was a blow out failure occurring near the bridge's midspan. These failures proved too catastrophic for the cantilevered structure to remain upright, and the bridge's main span collapsed into the channel below (Burgoyne and Scantlebury 2006).

Many parties were quick to theorize that the bridge's retrofit repair actually lead to the collapse of the structure. Following the collapse, the Palauan government brought in forensic engineering firms to determine why the structure actually failed.

A report created by SSFM concluded that the collapse most likely occurred in four related parts. The collapse started due to delamination of the top flange on the Babelthuap side of the bridge. This resulted in large moments over the main pier, causing large tensile stresses in the top slab. When the top slab and top portions of the webs failed, nearly all of their shear capacity was lost. This loss in shear capacity lead to a shear failure next to the Babelthuap main pier. Following that, the self weight of the structure was shifted to the Koror side of the bridge. The structure would not withstand the increased load, and the bridge began to rotate and temporarily lift into the air. Lastly, the compressive stresses induced on the main pier of the Koror side crushed the box girder, the top slab failed in tension, and the central span of the bridge fell into the water below. SSFM used both eyewitness accounts and remaining visible portions of the structure to help support their failures mechanism. The eyewitnesses were said to have heard popping sounds and saw concrete fall onto the metal portion of the bridge, both of which helped support SSFM's theory (Burgoyne and Scantlebury 2006).

Following SSFM's report, the US forensic firms of T.Y. Lin and Wiss, Janney, Elstner, Associates Inc (WJE) performed a series of field test including strain-relief testing and underwater investigation. Following the investigation, WJE concluded that root cause of the collapse was a delamination of the top flange of the structure. When the bridge underwent its repair, compressive stress increased at the midspan due to the removal of the central hinge. Additionally, this small change resulted in changes in temperature based stress. Though the repairs made actually kept the structure within its accepted stress limits, the top flange became vulnerable to delamination due to the low tensile strength in the concrete and a lack of vertical reinforcement. This investigation ultimately found the poor retrofit repair and the team behind it responsible for the collapse (WJE 2014).

Concrete Delamination
Delamination is a concrete failure which is caused by air and water that are trapped within the concrete. The entrapped air and water settle within the concrete during a process called bleeding. If the bleeding process has not completed before the concrete is finished, voids are created in the concrete at the location of the trapped air and water. The voids lead to weaker zones in the concrete, which could eventually detach from the slab. Delaminations may be avoided by giving the concrete time to bleed properly, using precautions when working with air entrained concrete, and warming the concrete or using set accelerator (Seegebrecht 2014). The American Concrete Institute states that "Delamination is a separation along a plane parallel to a surface (ACI Committee 301 2004)." This separation could be a horizontal split, crack, or separation within a slab, that generally appears near the upper surface of concrete. Delaminated areas can be found using non-destructive tests such as chain dragging or tapping with hammers (ACI Committee 301 2004).

= Bridge Replacement =

Due to the location and importance of the bridge, a replacement structure was deemed necessary. Using the report written by WJE, the government of Palau was able to win their lawsuit against the retrofit repair team. This lawsuit provided the country with enough funding to begin the process of designing and constructing a replacement bridge.

Construction on a new bridge near the same location as the original structure began in 1997 and finished in December 2001. The new bridge was designed similarly to the original bridge, using a prestressed concrete box girder in its main span. This time however, the bridge is not as deep in the main pier or center, and it is supported with stay cables. The country of Japan aided in the construction of the new bridge, so for that reason, the bridge was name the "Japan-Palau Friendship Bridge." The bridge has approximately the same span, but now the prestressed concrete box-girder is supported with stay cables. The bridge opened to the public in January 2002 (Burgoyne and Scantlebury).

= Similar Cases =

Another devastating bridge collapse that was inspected by WJE was the I-35W Bridge in Minneapolis, MN. The collapse parallels that of the Koror-Babeldoab Bridge since the failure occurred during a period of construction and since there was no weather event to trigger it.

The bridge had been under renovation for two months at the time of collapse.The bridge collapsed on August 1, 2007 during 6 PM rush hour traffic and resulted in 13 deaths and 145 injuries. The day the catastrophic failure occurred, four traffic lanes were closed for repair, and construction materials and equipment were stored on the south end of the bridge (Barden 2009).

Following the tragic collapse, two major failure theories came to light. One failure theory, by Thornton Tomasetti, theorized that a beam buckled due to a hot weather failure. The other popular theory was that a gusset plate had been undersized by the design team and that it was not equipped to carry the correct load. WJE completed an full failure investigation and report by November 2008. In the report, they detailed that the failure had been caused by an improperly designed gusset plate (Barden 2009).

For more information, please visit the I 35 W Minneapolis Wiki Failures page created by Benjamin Barden.

=Lessons Learned=

Since the collapse, the bridge has been used as a learning tool for future box girder structures. Zdenek Bazant, a professor of civil and environmental engineering at Northwestern University spent several years writing a report detailing the cause of collapse. At the time of his report, the results of the failure analysis were not available to the public. Bazant was angered by the idea that information that could positively benefit the structural engineering community could be withheld for such a long period of time. In 2007, he submitted a resolution to the Structural Engineers World Congress that called for the immediate release of collapse data for the sake of engineering ethics. Additionally, he believed that releasing data like that collected at the collapse would help prevent future collapses from occuring. The motion went on to pass (Khal 2009).

Following the collapse and release of information, an article was published (Bazant et al. 2012) that detailed how widely used prediction models for shrinkage and temperature were inaccurate. Using traditional analysis methods of box girder structures, such as the Koror-Babeldoab Bridge, calculated deflections have errors up to 20% from actual values. Upon this realization, other models, including those of ACI, Japan Society of Civil Engineers, and Comite Euro-International du Beton were studied. These studies showed that the estimated 18 year deflection were between 50-77% lowered than measured. The models also calculated 18 year prestress loss at around 50% lower than the measured values (Bazant et al. 2012). These errors have been brought to light in recent years as a reason to change industry standard of deflection calculation.

= Conclusion =

The findings of each structural team were not made public until January 2008. During the 12 years between the collapse and the publication of the reports, many parties theorized the reasoning behind the collapse. Ultimately, the WJE team's findings of the failure due to delamination of the top flange became the widely accepted cause of failure, and the retrofit repair team was found responsible for collapse.

The failure of the Koror-Babeldaob Bridge became the catalyst for the investigation of 66 other long span prestressed concrete box girder bridges. The bridges investigated had already begun to exhibit signs of long term deflections (Engineering Business Journal 2011). Ultimately, the collapse of this bridge helped save other bridges from possible collapse.

In the time since the collapse, the people of Palau have adapted to the new bridge and view it as a symbol of the continued future. No structural issues with the bridge have been recorded.

= Bibliography =


 * American Concrete Institute (ACI). (2004) "Troubleshooting Surface Imperfections- Delamination." //American Concrete Institute//. **
 * < http://www.concrete.org/Tools/TroubleshootingSurfaceImperfections/Delamination.aspx > (accessed December 11, 2014). **
 * This page, written by ACI, provides a concise definition of delamination, as well as ways to determine if it is present in the concrete.


 * Barben, Benjamin R. (Fall 2009) "I 35W Minneapolis." //Failures Wiki//. **
 * < http://failures.wikispaces.com/I+35+W+Minneapolis > (accessed December 1, 2014). **
 * This article includes a detailed account of the I 35W Minneapolis Bridge collapse.


 * Bazant, Zdenek, Yu, Qiang, Li, and Guang-Hua ****. June 2012. "Excessive Long-Time Deflections of Prestressed Box Girders. I: Record-Spain Bridge in Palau and Other Paradigms. //American Society of Civil Engineers, June 2012, 676-696.// **
 * This journal article outlines the extreme deflections faced by the Koror-Babeldoab Bridge. It details errors within the current prediction models for shrinkage and creep.


 * Bazant, Zdenek, Yu, Qiang, Li, Guang-Hua, Klein, Gary, and Kristek, Vladimir ****. June 2010. "Excessive Deflections of Record-Span Prestressed Box Girder." //Concrete International, 32(6), 44-52.// **
 * This journal article details lessons learned from the Bridge collapse, focusing on the bridge's excessive deflections pre-collapse. It also recalls the release of information about the collapse on the grounds of ethical engineering practices.


 * Burgoyne, Chris and Scantlebury, Richard. (June 6, 2006) "Why Did Palau Bridge Collapse." //The Structural Engineer.// Cambridge, UK. **
 * < __ [] __ > (accessed September 29, 2014). **
 * This article includes excellent graphics and theories as to why the bridge collapsed. It has a strong analysis of the bridge's repair and cantilever system.


 * Central Intelligence Agency. (June 20, 2014). "Australia-Oceania: Palau" //The CIA World Factbook.// **
 * <[]> **** (accessed September 30, 2014). **
 * A short fact sheet outlining the country of Palau's precise location, population, economy, means of transportation, and communication, among other things. This was deemed necessary to this investigation due to the unique location of the failure.


 * Engineering Business Journal. (December 28, 2011). "Structural Engineering; Data on Structural Engineering Described by Researchers at University of Pittsburgh. ****"//NewsRx.// **
 * <[]> **** (accessed September 30, 2014). **
 * A brief article detailing how the collapse of the Koror-Babeldoab Bridge prompted a search for signs of failure on similar structures. The article details how researchers at the University of Pittsburgh created a model that simulates creep, similar to the creep that aided in the collapse of the bridge.


 * Ernst, Wilhelm and Verlag, Sohn. 1998. "Koror-Babeldoab Bridge." //Structurae: International Database for Civil and Structural Engineering.// **
 * <[]> (accessed September 30, 2014). **
 * This database entry provides technical material information and dimensions of the bridge pre-collapse. It also details all parties involved with the construction of the original bridge.


 * <span style="background-color: #ffffff; font-family: Arial,Helvetica,sans-serif;">Google Maps. (2014). "Palau. " < https://www.google.com/maps/place/Palau/@7.3663306,134.4340941,7z/data=!4m2!3m1!1s0x328445b4a2af0399:0x12ed1edd39a1ebbb > (accessed December 2, 2014). **
 * <span style="background-color: #ffffff; font-family: Arial,Helvetica,sans-serif; font-size: 12px;">A world map, with focus on Oceania and Palau.


 * <span style="background-color: #ffffff; font-family: Arial,Helvetica,sans-serif;">Kahl, Nathan. (November 2009). "Learning From Failures. " //CEES Ethics Column//. **
 * <[]> **** (accessed September 30 2014). **
 * <span style="background-color: #ffffff; font-family: Arial,Helvetica,sans-serif; font-size: 12px;">A brief article detailing how engineers can learn from the collapse of the bridge. The article also describes how the engineering code of ethics should be the primary concern when constructing a new structure, particularly after a collapse.


 * OPAC Consulting Engineers. (1998). "Koror-Babeldoab Bridge." //OPAC Projects.// **
 * <[]> (accessed September 30, 2014). **
 * This brief project summary details the scope of work done by OPAC Consulting Engineers and outlines how the firm was used in the legal defense of the bridge collapse. Used as a photo resource.


 * Seegebrecht, George. (2014). "Delaminations in Concrete Slabs." //Concrete Network.// **
 * < http://www.concretenetwork.com/concrete-delamination.html > (accessed December 11, 2014). **
 * A page detailing how delaminations occur and why they are problematic. Goes into detail explaining how to avoid delaminations.


 * <span style="background-color: #ffffff; font-family: Arial,Helvetica,sans-serif;">Wikimedia Commons. (2014). "Japan-Palau Friendship Bridge." < ****http://commons.wikimedia.org/wiki/File:Japan-Palau_Friendship_Bridge_1.JPG**** > (accessed December 2, 2014). **
 * <span style="background-color: #ffffff; font-family: Arial,Helvetica,sans-serif; font-size: 12px;">A photo of the Japan-Palau Friendship Bridge, built to replace the Koror-Babeldaob Bridge.


 * <span style="background-color: #ffffff; font-family: Arial,Helvetica,sans-serif;">Wikimedia Commons. (2014). "Palau" < http://commons.wikimedia.org/wiki/Palau**** > (accessed December 2, 2014). **
 * <span style="background-color: #ffffff; font-family: Arial,Helvetica,sans-serif; font-size: 12px;">A location map of the bridge as well as a geographic map of the country of Palau, detailing the location of the two major islands.


 * <span style="background-color: #ffffff; font-family: Arial,Helvetica,sans-serif;">Wiss, Janney, Elstner Associates. (2014). "Koror-Babeldoab Bridge: Collapse Investigation." //WJE Project Profiles.// **
 * <[]> **** (accessed September 30, 2014). **
 * <span style="background-color: #ffffff; font-family: Arial,Helvetica,sans-serif;">A project summary outlining the scope of work of WJE, their findings, and the eventual cause of the bridge collapse. Includes a brief description of the bridge constructed to replace Koror-Babeldoab.

= Additional Resources =


 * <span style="background-color: #ffffff; font-family: Arial,Helvetica,sans-serif;">Chen, Wai-Fah and Duan, Lian. (2014). "Concrete Bridges." //Bridge Engineering Handbook, Second Edition: Construction and Maintenance//. CRC Press, Boca Raton, FL, 449. **
 * <span style="background-color: #ffffff; font-family: Arial,Helvetica,sans-serif;">A portion of handbook illustrating lessons learned in the bridge collapse and its original construction. Additionally, it compares this bridge collapse with the collapse of the Lowe's Motor Speedway Pedestrian Bridge.


 * Ketchum, Mark. "The Koror-Babeldoab Bridge" //Mark Ketchum's Bridge Collapse Page//. **
 * < **** [] **** > (accessed September 30, 2014). **
 * This article very briefly describes the bridge collapse and includes photos from before the failure. It's great achievement is how it links a variety of bridge failures together and details the differences between collapses.