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Steel Saddle Can’t Save Bridge Schedule

Wednesday, July 10, 2013

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Repairing dozens of broken bolts on the San Francisco-Oakland Bay Bridge will push back the span's highly anticipated Labor Day opening by months, officials have conceded.

The previously scheduled Sept. 3 opening of the new east span of the San Francisco-Oakland Bay Bridge (and its multimillion-dollar party) has been postponed, the Toll Bridge Program Oversight Committee announced Monday (July 8).

Along with that announcement, the committee released its investigative report into the bolts that broke on the bridge in the spring. The 130-page analysis includes the panel's review of other bolts on the bridge, an approved fix for the problem, and a final investigative report that lays out the chronology of events and who was responsible.

Bay Bridge investigation

The investigation into problems and setbacks on the Bay Bridge project was detailed in a 130-page report released Monday.

The document is the result of a months-long scramble to keep the $6.4 billion bridge project on track after 32 massive bolts, also called anchor rods, popped loose in March, just days after crews started tightening them.

Experts previously described the galvanized steel rods as "vulnerable," and the California Department of Transportation (Caltrans) had banned their use on other bridges because of the risk.

The contractor, XKT Engineering, is using a steel saddle retrofit to make repairs and now estimates that that work will take until Dec. 10. However, opening the bridge may take longer. Bridge officials will select an opening date based on actual completion of the east pier retrofit work, traffic impacts, weather, and other factors. 

The steel saddle solution is expected to cost from $5 million to $10 million, according to the Bay Area Toll Authority, which manages the project with Caltrans and the Metropolitan Transportation Committee.

'Less than Ideal'

The Toll Bridge Program's "Report on the A354 Grade BD High-Strength Steel Rods on the New East Span of the San Francisco-Oakland Bay Bridge With Findings and Decisions" details the investigation into why the rods failed and whether the other 2,210 rods on the bridge are at risk, what course of action is needed to address all of the rods, and what the findings and recommendations mean for the now-postponed opening of the bridge.

Bay Bridge bolts
Caltrans

Massive bolts broke during construction of the bridge's eastern pier only days after crews started to tighten them. The failure has delayed the opening of the bridge until at least mid-December.

The rods are used for seismic stability and range from nine to 24 feet long and three inches in diameter. In April, Caltrans released hundreds of pages of documents that showed its inspectors had found structural integrity issues with some of the bolts years ago. The agency's engineers ordered tests on the bolts in 2008, but those tests were never done after the contractor disputed whether they were required.

As part of the investigation, the Oversight Committee gathered and analyzed project records pertaining to the design, specifications, fabrication and construction related to the rods.

A metallurgical investigative team examined the cause of the failures of the high-strength steel rods from 2008. Based on its examination of two of the extracted rods, the team found that:

  • The anchor rods failed as a result of hydrogen embrittlement, resulting from the applied tensile load and from hydrogen that was already present and available in the rod material. The root cause of the failures was from "higher than normal susceptibility of the steel to hydrogen embrittlement."
  • The steel rods comply with the "basic mechanical and chemical requirements of ASTM A354 grade BD."
  • The metallurgical condition of the steel was "less than ideal." There was a large difference in hardness from center to edge and high local hardness near the surface. The material exhibits low toughness and marginal ductility. All of these factors combined made the anchor rods susceptible to failure.

"It is not uncommon to perform a second heat treatment," the report said. "However, in this case, given what is now known about the poor quality of the 2008 rod material, the second heat treatment may have further hardened and strengthened the material and contributed to the rods' susceptibility to hydrogen embrittlement."

Avoiding 'Significant Destruction'

The committee determined that 96 of the rods that were installed and embedded into a pier cap and are in-line with the vertical columns of the pier cannot be replaced because that would require "significant destruction of the pier cap."

Toll Bridge Program Oversight Committee

A recent rendering shows how the steel saddle option will sit on top of the base of the shear keys and apply vertical post-tensioning force.

Seeking an alternative fix, the Oversight Committee assessed three retrofit designs, resulting in unanimous approval of a steel saddle retrofit option. The steel saddle meets all design requirements and "applies direct preload to the lower housing via the radial forces that are developed from the main vertical post-tensioning force being applied as intended in the original design."

Although this method requires more detailed fabrication, the installation will be easier and require less drilling of the concrete beam.

The saddles will sit on top of the base of the shear keys, with steel tendons inside that spread down either side of the concrete cap beam. Workers have been chipping concrete on either side of the shear keys to make room to place the saddles, drilling into the cap beam to allow steel tendons to pass through, and cutting into the face of the cap beam to provide a better connection with the new concrete that will encase the tendons.

Long-Term Concern

As for the other 2,210 rods, the committee found none to have failed or show evidence of hydrogen embrittlement.

steel saddle retrofit
Bay Area Toll Authority

Crews work from the pier platform, drilling the concrete cap beam in preparation for the bolt fix.

However, there is concern about whether the remaining rods will be susceptible to long-term stress corrosion. To assess that risk, five tests were deployed: in-situ hardness test (Test I), Rockwell hardness test (Test II), Charpy V-Notch test for toughness and chemical composition (Test III), and two accelerated stress corrosion cracking tests (Townsend Test IV and Raymond Test V).

According to the report, Tests I, II and III confirmed that the rods have a low risk for near-term hydrogen embrittlement failures, but there are a number of rods that exhibit surface hardness in excess of the point at which there is increased risk for stress corrosion cracking under sustained high tension.

Further stress corrosion testing is underway as part of Tests IV and V that will provide data for analysis and remediation of the rods.

The Blame Game

Several responsible parties were identified for different aspects of the investigation's findings:

  • Caltrans, the owner and operator;
  • T.Y. Lin International/Moffatt & Nichol Design Joint Venture, the engineer of record;
  • American Bridge/Fluor Joint Venture, the contractor for the superstructure; and
  • Kiewit/FCI/Manson Joint Venture, the contractor for the E2/T1 marine foundation.

Randy Rentschler, a spokesman for the Metropolitan Transportation Commission and Bay Area Toll Authority, said Caltrans would seek undetermined monetary damages from T.Y. Lin International/Moffatt & Nichol and American Bridge/Fluor, The Sacramento Bee reported.

Rentschler said in March that there was $300 million in contingency funds built into the original budget, and that fixing the bolts would not make a dent in that amount.

Before the new span can open to traffic, a four-day full closure of the existing bridge will be necessary to complete construction work at the Oakland touchdown and Yerba Buena Island tunnel to transition traffic from the old span to the new one.

A public briefing on the investigation will take place at 10 a.m. PT today (July 10) at the Metropolitan Transportation Commission offices in Oakland, CA. The meeting will also be audio-cast online.

   

Tagged categories: Bridges; Corrosion; Department of Transportation (DOT); Galvanized steel; Steel

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