Officials OK repairs to $2 billion transit center
David Wagman | January 23, 2019San Francisco's Metropolitan Transportation Commission (MTC) approved a plan to repair damage to a $2 billion transit center.
The facility opened last summer and was forced to close in September after fissures were discovered in steel beams on a deck used by commuter buses above Fremont Street.
The $2 billion transit center has been closed since September. Source: TJPAContractors installed a multi-level shoring system at Fremont and First streets as a temporary measure and as a root cause review and monitoring began. No additional fissures have been found. In addition, bolts in the areas around the fractured girders over Fremont Street were tested and none were found to be damaged or cracked.
Herrick Steel and Arcelor-Mittal Steel companies will supply material for the girder repair and reinforcement and in mid-January were fabricating the plates for delivery to the site.
A peer review panel is overseeing a review of building-wide shop drawings, inspection reports and design documents to determine if other inspections will be necessary before reopening the facility to the public.
The Transbay Joint Powers Authority (TJPA) closed the facility Sept. 25 after conferring with builder Webcor/Obayashi and structural engineers Thornton Tomasetti.
One of the cracks discovered soon after the transit facility opened. Source: TJPAThe initial fissure was found in the ceiling of the third-level bus deck near Fremont Street in the heart of downtown San Francisco. Inspections showed an additional fissure on an adjacent beam. TJPA said at the time that the problem was contained to just that area of the terminal.
Consulting engineering firm LPI Inc. was hired Oct. 1, 2018, to perform a root cause assessment of the girder fractures and to remove and test the fractured sections. The firm presented its initial root cause assessment to officials in mid-December.
The assessment included sampling four girder flanges, three of which were found to have suffered full flange-width fractures. The firm said the girder fractures were caused by cracks that formed in a girder weld access-hole radii. Initially, shallow (micro) surface cracks developed during thermal cutting of the weld access holes in what the firm described as the “highly hardened and brittle martensitic surface layer.” Larger “pop-in cracks” later formed in two of the four flanges, potentially during butt welding of the flange plates.
The analysis showed the presence of black, tenacious, high-temperature oxide on both the shallow surface cracks and the larger pop-in cracks. Engineers said this confirmed that both crack types formed at what they said were elevated temperatures.
The fracture origins were located in the mid-thickness of the flange where low fracture toughness, as confirmed by toughness testing, provided little resistance to rapid, low-energy, brittle fracture.
CVN testing was performed on all flange samples at the top, quarter depth, mid-thickness, three-quarters depth and bottom. The Charpy impact test, also known as the Charpy V-notch (CVN) test, is a standardized high strain-rate test that determines the amount of energy absorbed by a material during fracture.
Engineers found that quarter-depth CVN results were consistent with project and girder plate mill certifications. They said that rapid, low-energy fracture of the flanges occurred as the girder was subjected to service loading on top of the normal residual stresses due to welded fabrication.
Hmmm , let me guess… The steel beams came from (drumroll ...) China.
In reply to #1
Considering China bought a lot of our ‘abandon’ steel mills in the 90’s, dismantle them, shipped them to China, reassembled them.
that shouldn't come as a surprise.