Hello Everyone: More information gained from engineering magazines and photographs (structural drawings not public). Over Fremont and First Streets, the lower bus deck is essentially a suspended bridge—hung from two 8-foor deep girders at the upper park level. Somewhat unusual for a suspension structure is the single chord (vertical column) that hangs the lower bus deck from each girder.

SKYRISE: Transbay Transit Center Engineer Describes Design Innovations and Challenges

De Oliveira expanded on the specific elements of the building: “On the roof level, the beam that runs the perimeter of the structure, the spandrel beam, there’s a little gap of unsupported girder, and in the event of an earthquake, what will happen is each one of the architecturally exposed steel trees rocks back and forth, and that short segment of steel girder is intended to yield in shear and flexure. The trees are comprised of those steel pipes and steel castings and they remain predominantly elastic, so that the girder yields up and down and absorbs the energy input into the building by the earthquake.”

The Y-shaped tree columns, at the building exterior, are primarily bracing elements that have flexibility. At the bridges, the two upper girders carry most of the load of the park deck and the suspended bus deck. The single chord (vertical column), at the midpoint of each girder, is heavily loaded. Suspended structures do move. And the Y-shaped tree columns allow movement. Ultimately, the lesson here may be that simplicity is best. The bridge design seems overly complex. The simple approach would have been a truss bridge, where the bridge’s entire height (from park level down to bus level) acts as a single “beam” or “tube”.

The vertical column does a lot of work—all by itself. No redundancy. If the bus level moves differentially from the park level, there’s much stress at one point. Differential movement could be caused by differences in stiffness, deflection, vibration, oscillation, lateral forces, wind, uplift, suction, pressures, dynamic loading, thermal expansion, soil settlement…. Because both bridges in the project are designed similarly, one would expect the same stresses. Just food for thought. Regards, Howard Wong, AIA

Video shows beam cracks, description of general structural system
Salesforce Transit Center in San Francisco could be closed for another week

It looks simple, but it’s not. Complexity of Transbay Transit Center raises risks

At that midway point, where each girder meets a single column that in turn supports the bus deck below, is where each of the cracks occurred. “Those two girders are working hard, spanning a good length and bearing a heavy load,” Panian said. “And the place where it’s expected to carry the most load is where it is cracked.” What may have helped prevent disaster is that the two girders don’t hold the span in place all by themselves. They’re paralleled by smaller, more conventional beams on either side that connect to the transit center structure on the east and west.

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