Structural engineers have recommended repairs to the $2 billion Transbay Transit Center in San Francisco, which was forced to close in late September after cracks were discovered in some structural elements.

The facility had opened only weeks earlier and was heralded as a major improvement in San Francisco’s public transit system. The Transbay Joint Powers Authority (TJPA) closed the facility Sept. 25 after conferring with builder Webcor/Obayashi and structural engineers Thornton Tomasetti.

September 25 image of the fissure located on flange on the bottom portion of the beam that spans over Fremont Street. Source: TJPA.orgThe 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 October 1 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. A final report is expected in January. (View the engineers' presentations.)

Flange-Width Fractures

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.

Artist's view of the Transbay Transit Center with San Francisco Bay in the background. Source: TJPA.orgCVN testing was performed on all flange samples at the top, 0.25 depth, mid-thickness, 0.75 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 0.25 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.

Repair Plan

Plans to restore the bottom flange to its original design capacity were presented by engineering firm Thornton Tomasetti. The plan calls for workers to grind the flame-cut surface of the web hangers to a smooth surface, and then perform a magnetic particle test. Bolted cover plates then will be installed to replace the flanges. The repair is expected to be localized to the fracture area only.

Thornton Tomasetti said that although the cracked girders were left with reduced section, they still had sufficient capacity to support the building’s dead loads and occupancy loads. The actual forces measured in the girders were less than what was calculated using normal design procedures, the engineering firm said.

Analysis showed that the girders deflected 0.75 inches to 1 inches after cracking. The amount of load shed from the girder was up to 10% and that hanger loads reduced after cracking. Adjacent beams and columns were not overstressed, but the firm said it planned to test the integrity of girder-bolted connections as a precaution.