Oakland+Bay+Bridge+Eyebar+Failure

=San Francisco- Oakland Bay Bridge (Sept.-Dec. 2009)= toc Rebecca Dick, BAE/MAE, Penn State, 2012

Keywords:
Eyebar; Oakland Bay Bridge; Tie Rods; Fatigue failure

=Abstract:= The Oakland Bay Bridge was opened on November 12, 1936 to connect San Francisco with Yerba Buena Island and Oakland, which is situated directly across the bay from San Francisco. Over the past few years, the Oakland Bay Bridge has been undergoing a seismic retrofit in addition to the construction of a new eastern span. In September 2009, a crack in an Eyebar was found during an inspection on the eastern span of the Oakland Bay Bridge. The cracked Eyebar was quickly repaired and additional structural steel was added to help prevent the repair from experiencing any further failure. Unfortunately, on October 27, 2009 a Crossbar and two Tie Rods used in the repair failed and fell onto the upper deck of the Oakland Bay Bridge. This case study will explain why the Eyebar failed initially, and why the subsequent repair also failed. The final repair performed will be discussed. Additionally, this case study will take a look at what can be learned from these structural failures and how this can be applied to future bridge construction.

=History of the Oakland Bay Bridge:= Established as a trade city during the Gold Rush, San Francisco had a prime location. Unfortunately, the bay surrounding San Francisco is wide and runs for miles to the north and south of the city, which made it difficult to get to cities across the bay by any means other than by boat. Eventually, bridges were constructed to connect the city to it's neighbors across the bay. Engineered by Ralph Modjeski ("San Francisco- Oakland Bay Bridge", 2011), the San Francisco- Oakland Bay Bridge was constructed in the the early 1930's to connect San Francisco with Alameda County across the San Francisco Bay. See Figure 1 above for a view of the east span of the Oakland Bay Bridge, taken at night.

= = =Structural System of the Original East Span:= Since the bay is so wide, the traverse was placed so Yerba Buena Island would intersect it at about the halfway point. This allowed for easier construction, and hence is the reason for the different types of construction. On the west span, closest to San Francisco, the Bay Bridge consists of two double-decker, two-span suspensions connected by a central concrete pier. The east span, closest to Oakland, consists of a 5-span truss bridge connected to a causeway. The connection between the two types of bridges occurs at the tunnel dug through Yerba Buena Island. = = When the bridge was constructed, it was built by "walking-out" from the supports (Alfrey, 2010). This means that each additional structural element helps to stabilize the preceding structural element. This method, unfortunately, makes it very difficult to replace or repair damaged members of the structural system. Additionally, in the 1930's, eyebars were made using dyes and during the dye-press process, sharp edges were created at the area where the slender bar met the hooked head of the eyebar (Upton, 2010). This created an area of stress concentration when loads were applied (Upton, 2010).

== =Retrofit and New East Span:= In recent years, it has been discovered that the Bay Bridge is not seismically adequate to continue to provide transportation services to Bay Area residents. After the partial collapse of the upper deck onto the lower deck in the Loma Prieta Earthquake in 1989, the bridge was deemed not seismically sound ("Eastern Span Replacement...", 2011). Begun in 2004, the Bay Bridge is undergoing a full retrofit (Caltrans, 2011). On the west span, retrofit efforts include replacement of rivets with high-strength steel bolts, addition of new steel plates, new steel bracing under the deck, and encasing the piers in concrete (Caltrans, 2011). Dampers and isolation materials were also added to absorb energy and distribute dynamic loads created during an earthquake (Caltrans, 2011).

The east span is being completely reconstructed. The current east span continues to provide transportation between Yerba Buena and Oakland, but it's replacement is being constructed only meters away to the north. Figure 2 to the left shows the original east span and the new span under construction. The new east span will consist of a self-anchored suspension bridge connected to a causeway. The self-anchored suspension portion is designed to withstand an 8.5 magnitude earthquake-- larger than the 6.9 Loma Prieta Earthquake of 1989-- ("Eastern Span Replacement...", 2011), using several load absorbing structural techniques, including shear link beams (Caltrans, 2011).

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=Labor Day Weekend Failure:= Over the Labor Day holiday weekend in 2009, a 300 foot long detour span (Public Information Office, 2009) was being placed to connect the Yerba Buena tunnel transition with the current east span so construction of the new east span could continue without interrupting traffic (Carlsen, 2009). During a routine maintenance check of the structure, it was found that there was a large crack, 1.5 inches wide (Upton, 2010), in a 65 foot long eyebar in one of the the truss spans (Reid, 2010). An eyebar is a structural element designed to carry tensile loads in the truss configuration. They were used in pairs on the Oakland Bay Bridge, each half consisting of three lengths of eyebar. The failure occurred at the top end of the middle eyebar noted in the picture to the side (Alfrey, 2010). The location of the failed eyebar can be seen in Figure 3 to the right.

According to Caltrans, the crack would have originated at the outer edge of the eyebar head and worked inward (Upton, 2010), which is consistent with a fatigue failure due to stress concentrations at the transitions between bar and head. Federal law mandates that bridges constructed using eyebars must be inspected every two years (Reid, 2010). Since the crack was visibly rusty when discovered, it can be deduced that it was there for some time (Public Information Office, 2009). It was not discovered in the inspection performed two years prior, so the initiating crack occurred some time between 2007 and 2009 (Public Information Office, 2009).

Each member in the truss system carries about 500 kips of dead load, and this load was redistributed to other members upon failure (Reid, 2010). Surrounding members were not designed to carry this extra load, so repairing this member became vital to the safety of the structure. But how? With the construction being what it was in the 1930's, the eyebar could not just be removed and replaced since it would destabilize the truss system. The bridge was immediately closed and efforts to find quick but effective repair were begun.

Caltrans knew they needed to transfer the load from the eyebar to surrounding members to complete the repair and allow it to carry the 500 kip tensile load. To do this, they contracted C.C. Myers to design the repair (Wilson, 2009). This repair consisted of a brace-like system. A saddle was placed over the pin at the ends of the eyebar and a crossbar was then welded to the saddle to provide stability and a connecting surface (Alfrey, 2010). Four 1-3/4" diameter tie rods were bolted to the saddle and crossbar configuration to connect each end and carry the tensile load evenly (Reid, 2010). The repair was completed in about 70 hours (Carlsen, 2009), and when it was completed and was loaded it carried the tensile load and relieved the damaged eyebar. A view of the upper saddle, crossbar, and tie rod configuration in it's finished state can be seen in Figure 4 to the right. = = =October Failure:= On October 27, 2009, during evening rush hour traffic, the repair from the Labor Day weekend eyebar crack failed in a dramatic style. 5000 pounds of structural steel used in the repair fell onto the upper deck including two tie-rods and the waler (Wilson, 2009). Although three vehicles were struck by the falling debris, thankfully no one was injured from this incident (San Francisco- Oakland Bay Bridge, 2011).

During investigation, it was found that the weld between the crossbar and the saddle at the top connection failed (Alfrey, 2010). Additionally, one of the tie rods was found to be cracked at the location where it would connect into the crossbar (Alfrey, 2010). So the question became "Which failed first, the weld or the tie-rod?" Metallurgists were used to determine if the failure was due to a design failure, an installation failure, or a material failure ("San Francisco Artery...", 2009).

Warning signs of this failure were noted by Caltrans officials. It was noted that the repair had a high degree of movement during high winds, but no precautions were taken to reduce this movement (Wilson, 2009). It is believed that this is the culprit for the October failure since wind speeds of 50+ mph were recorded on that day (Alfrey, 2010). October 27 was an extremely windy day, producing a lot of vibratory action in the eyebar repair. Since the repair used a connection of steel-on-steel at a rotation point, this produced very high stresses in addition to the existing stresses due to traffic and temperature effects. The rotation initiated by the wind forces caused grinding between the two members to occur, which resulted in a fatigue failure of the member (Reid, 2010).

California based MCM Construction, Inc. was called in to perform the second repair ("San Francisco Artery...", 2009). The second repair was a variation on the first repair with the understanding that the tie rods experience a great deal of stress. The holes where the tie rods are threaded through the crossbar were radiused to allow for vibrations without stress concentrations. Plates and a coupling member were also used to stiffen the crossbars. Additionally, the nuts used at the connection between the tie rods and the crossbar and saddle arrangement were radiused and butted against a spherical mating surface to complete the connection with minimal stresses. The weld strength between the crossbars and the saddles was also increased (Alfrey, 2010). Protective sleeves were used on the tie rods to prevent any contact between them and the damaged eyebar (Toll Bridge Oversight Committee, 2009). Cables and tethering straps were also used on the tie rods, saddle, and crossbar to restrain the elements from falling onto the upper deck in case another failure occurs (Toll Bridge Oversight Committee, 2009). Together, the alterations made in the second repair resulted in a better, higher strength connection. Many precautions were taken to design and construct a stronger repair and to prevent another failure of this magnitude.

=Final Reconstruction:= In December of 2009, American Bridge Company and Fluor Corporation were called in to perform the final repair (Reid, 2010). The final repair for the failed eyebar consisted of a six-step process. With the second repair still in place, the first step was to cut and remove the cracked eyebar head and part of the shaft. Then a structural hairpin and lower jacking bracket was installed at the cut plane of the eyebar. Next, a new section of eyebar was installed with splicing plates at each end. Upper jacking brackets were installed and the rod was then pre-stressed. The new eyebar was then bolted at the splices and the jacking system was removed (Reid, 2010). A diagram detailing the final repair is shown in Figure 5 to the right. The new components are colored in red, yellow, and green, while the original material is gray and maroon. The final repair, which is still in place today, is monitored with stress-strain gauges, and is inspected daily (Alfrey, 2010). Caltrans is confident that the repair will last well beyond the decommissioning of the current east span in 2013 (Wilson, 2009). = =

=What Can We Learn?= The main lesson that can be learned from this case study is that wind forces and resulting vibrations and stresses should be of significant consideration in design of bridge members. Material properties play a huge role in the behavior of structures. It was from material fatigue that the the repair failed, and likely the reason for the initial failure. Although neither of the failures discussed here resulted in casualties, it is not to say that such failures would not result in such a serious manner.

More maintenance checks should also be performed on older bridges. After over 70 years of operation, members will have seen considerable stresses and loads, including traffic loads, stressed from temperature, environmental stresses, and wind load. The Oakland Bay Bridge is located in an area with high winds, corrosive salt water, and a large amount of vehicular traffice. This bridge has seen it's fair share of stresses over the years. Thankfully Caltrans inspects the bridge daily, significantly decreasing the possibility of a major failure from occurring again. It is not surprising that a failure of this sort occurred, and should be kept in mind during the continued construction until the bridge is decommissioned in 2013.

=Bibliography:=

 Alfrey, Tony. "BrokenBridge." // Sci-Experiments //. Jan. 2010. Web. 01 Oct. 2011. . //This web page analyzes the failure and discusses the causes of the original failure and the repair.//

"Bay Bridge Info." // Latest Bridge Stories | Bay Bridge Info //. Bay Bridge Public Information Office, 2011. Web. 30 Oct. 2011. .  Caltrans, Bay Area Toll Authority, California Transportation Commission. // San Francisco-Oakland Bay Bridge East Span Eyebar Repair Project //. Powerpoint Presentation.//San Francisco-Oakland Bay Bridge East Span// //This web page provides pictures and videos of the Oakland Bay Bridge under construction and before construction.//

Caltrans. "Self-Anchored Suspension Span (SAS) | Bay Bridge Info." //Latest Bridge Stories | Bay Bridge Info//. California Department of Transportation, 2011. Web. 20 Oct. 2011. . //This website describes what is involved in the East span seismic project. //

Caltrans. "The Retrofit of the West Span | Bay Bridge Info." //Latest Bridge Stories | Bay Bridge Info//. California Department of Transportation, 2011. Web. 20 Oct. 2011. . //This website describes the retrofit program for the West span.//

Carlsen, Robert. "Unexpected Bay Bridge Crack Slightly Delays Reopening." //ENR// 9 Sept. 2009. Web. Oct. 2011. //This source describes what was discovered during the// //detour installation over Labor Day weekend, 2009, and who was called in to fix the failure.//

 "Eastern Span Replacement of the San Francisco- Oakland Bay Bridge." // Wikipedia //. Web. 1 Oct. 2011. . //This sources describes what the Eastern Span Replacement Project encompasses and describes the Eyebar failure and possible reasons for the failure. // Eyebar Repair Project. The San Francisco.Oakland Bay Bridge Seismic Safety Project. Web. 2 Oct. 2011. //This Powerpoint uses diagrams to show the area of the eyebar failure, the repairs made to the failed members, and the overall construction methods used in the Eastern Span Replacement Project. //

 Public Information Office. // Emergency Repair and Detour Connection Completed on Bay Bridge //. // The San Francisco.Oakland Bay Bridge Seismic Safety Projects //. 8 Sept. 2009. Web. 2 Oct. 2011. . //This press release explains what was found during the Labor Day inspection and what the impact was on the structure and the construction process. //

 Reid, Robert L. "Damaged Eyebar Section Replaced on San Francisco- Oakland Bay Bridge." // Civil Engineering // (2010): 18-23. // American Society of Civil Engineers //. Web. 2 Oct. 2011. //<span style="color: black; font-family: Arial,Helvetica,sans-serif;">Robert L. Reid describes the necessity of the Eyebar in the overall structure of the bridge, where and why it failed, and what the repairs entailed. //

"San Francisco Artery Reopens After Second Emergency Fix." // Engineering News Record // 9 Nov. 2009: 52. Print. //<span style="color: black; font-family: Arial,Helvetica,sans-serif;">This article discusses the companies involved in the second repair and the details of the structural components of the second repair. //

<span style="font-family: Arial,Helvetica,sans-serif;"> "San Francisco- Oakland Bay Bridge." // Wikipedia //. Web. 1 Oct. 2011. <http://en.wikipedia.org/wiki/San_Francisco_%E2%80%93_Oakland_Bay_Bridge>. //<span style="font-family: Arial,Helvetica,sans-serif;">This source describes the Oakland Bay Bridge and its history. //

<span style="font-family: Arial,Helvetica,sans-serif;"> Toll Bridge Program Oversight Committee. // 2009 Third Quarter Project Progress and Financial Update //. Publication. // CA.GOV //. California Department of Transportation, Nov. 2009. Web. 2 Oct. 2011. <http://www.dot.ca.gov/baybridge/>. //<span style="color: black; font-family: Arial,Helvetica,sans-serif;">The California Department of Transportation describes in detail the scope of work of the San Francisco- Oakland Bay Bridge Seismic Retrofit Project. Additionally, factsheets, diagrams, and detailed information are provided on the Eyebar failure and repair on the east span. //

<span style="font-family: Arial,Helvetica,sans-serif;"> Upton, John. "Bay Bridge Eyebar Woes Date to 1930s." // San Francisco Examiner //. 1 Apr. 2010. Web. 2 Oct. 2011. <http://www.john-upton.com/baybridge.htm#top>. //<span style="font-family: Arial,Helvetica,sans-serif;">Upton describes the reasons why such an eyebar would fail and the history behind the use of Eyebars in bridge construction. //

<span style="font-family: Arial,Helvetica,sans-serif;"> Wilson, Bill. "The Gloomy Gate." // Roads & Bridges // 47.12 (2009): 16-20. // Academic Search Complete //. <span style="font-family: Arial,Helvetica,sans-serif;">Web. 2 Oct. 2011. //<span style="color: black; font-family: Calibri,sans-serif; font-size: 11pt;">Bill Wilson details the causes of the failure of the Eyebar and the specifics of the repair process. //

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