Case LE-CS-2026-03 · Format illustration based on publicly reported information
| Property | Salesforce Transit Center, 425 Mission Street, San Francisco, CA |
|---|---|
| Structure | 4-level, approx. 1,430-ft-long transit hub with rooftop park |
| Element | Twin built-up plate girders spanning approx. 87 ft over Fremont Street, supporting bus deck and park levels |
| Opened | August 2018; closed September 2018 after fractures found; reopened July 2019 |
| Reported damage | Brittle fractures through both bottom flanges (4-inch Grade 50 plate) at mid-span weld access holes |
| Status | Repaired with bolted cover-plate splices; parallel girders at First Street found sound and reinforced proactively |
Summary
Six weeks after opening, ceiling installers discovered a fractured bottom flange on a girder carrying the rooftop park over a city street; its twin had an identical crack. The center was closed immediately and Fremont Street shored from below — the correct emergency response to a fracture-critical finding.
The peer-reviewed investigation traced the failure to fabrication: weld access holes thermally cut into very thick flange plates left micro-cracks that were never ground out or inspected, creating ready-made crack initiators in a high-stress, low-redundancy detail.
Observed Conditions
Full-depth brittle fractures of both bottom flange plates of the twin Fremont Street girders, initiating at weld access hole corners near mid-span.
No prior visible distress: fractures were discovered by trade workers, not by monitoring — the failure mode is sudden, not progressive.
Identical girder details existed at First Street, requiring immediate verification of a second potentially critical location.
Probable Cause
Thermal (flame) cutting of weld access holes in 4-inch Grade 50 plate produced a hardened, micro-cracked surface layer.
Specified grinding/polishing of the cut surfaces and post-cut inspection (MT) of the access holes was not effectively performed or verified.
High tensile stress concentration at the access hole corner, plus thick-plate constraint (low fracture toughness through-thickness), enabled brittle crack propagation.
Hydrogen-assisted cracking from welding adjacent to the un-remediated cut surface is cited as the initiation mechanism.
Engineering Assessment
This is a fracture-critical detail: a two-girder system with no alternate load path, where one full flange fracture removes roughly half the span capacity. In such systems, fabrication quality control is not paperwork — it is the structure’s actual redundancy.
The selected repair is mechanically sound: bolted splice “sandwich” plates (filler plus cover plates each side of the web and across the flange) re-establish the tension load path around the fractured zone without introducing new field welds into already-suspect thick plate — bolting, not welding, was the right call.
Repairs & Remediation
Immediate shoring: temporary towers under both girder lines, closing the street below, before any investigation traffic loads.
Material sampling: core the fracture origin for metallurgical analysis (hardness mapping, fractography, Charpy toughness through-thickness).
Bolted cover-plate splice across the fractured region, designed for full flange tension capacity, with slip-critical high-strength bolts.
Verification of the sister location (First Street girders): UT/MT of all weld access holes; proactive reinforcement even where sound.
Fleet-wide review of every thermally cut access hole in plates over 2 inches, with grinding and MT where surface condition is unverified.
Monitoring
Strain gauges on repaired flanges during staged re-loading (shoring removal) to confirm load-path assumptions.
Periodic UT of splice regions and remaining access holes for the first inspection cycles after reopening.
Documented fracture-control plan added to the operations manual for future inspections.
Engineering Takeaway
For commercial owners the lesson is that steel failures rarely announce themselves — the building looked perfect until someone lifted a ceiling tile. Fabrication and erection records matter as much as design calculations, and any structure with non-redundant transfer girders deserves a fracture-critical inspection protocol. Our commercial assessments review the load path first: where redundancy is low, inspection rigor must be high.