The fabricator of the structural steel for the third floor tapered composite beams at the troubled Salesforce Transportation Center in San Francisco is calling for a review of the design of the beam and crane connections as part of an investigation into the cause of brittle cracks in the foundation. double beam shelves. The 80 foot long member spans Fremont Street. The establishment closed at the end of September.
At midspan, each 8-foot-deep beam wall is thickened with vertical hangers supporting the second-floor bus deck below. Suspension plates are welded to the web and cut through the bottom flange of the beam.
“I think the hanger parts are the biggest cause of the problem,” said Robert Hazleton, president of manufacturer The Herrick Corp. “We have been assured that a thorough review of the hanger design will be carried out.”
“We are all very keen to determine the cause of the [cracks] found on two [beams] at the Fremont Street transfer center,” said Mark Zabane, chief executive of Transbay owner Joint Power Authority (TJPA). We weigh everything. facts to determine how and why it happened and do not exclude the design, fabrication or installation of the incident. We are fully investigating the incident and are cooperating with the Metropolitan Transportation Commission’s independent analysis provided by the experts on the review team. ”
Double girders are very similar to bridges, suspending bus platforms and spanning streets. A structural engineer familiar with the design said suspension beam connection details are not commonly used in modern bridge construction. A bridge expert, who asked not to be named, said the hangers could be installed around the flange, rather than through it, and bolted to the wall using plates mounted on the wall.
The dual hanger part eliminates the need to cut hanger slots in a 4″ thick flange. Experts said the existing groove intersected the flange butt weld mid-span of the beam, where bending stresses are greatest.
The engineers added that connecting the hanger to the bridge also resulted in “very poor fatigue detailing” at the ends of the butt welds between the bridge and hanger. Additionally, the connection details used (and weld holes added at the construction drawing stage) were not included in the bridge specifications.
Bruce Gibbons, managing director of Thornton Tomasetti (TT), a civil engineer at the transport centre, declined to comment on the matter pending the outcome of the investigation.
Herrick’s concerns arose in response to a Dec. 13 public appearance before the TJPA board of directors by LPI engineer Robert S. Vecchio, a metallurgist who studies brittle fracture.
Vecchio said analysis and testing conducted so far “suggests that the likely cause of beam failure was cracks that developed within the radius of the beam’s weld holes prior to use.” Vecchio said LPI’s client is TJPA (through TT and Turner Construction, an enterprise). construction management company).
Shallow surface microcracks form during “thermal cutting of a weld hole in a highly hardened and brittle martensitic surface layer,” Vecchio said. Then, when butt welding flange plates, large pop-up cracks may form in two of the four flanges.
“In addition to the normal stresses encountered during the manufacturing process, flanges can be subject to rapid, low-energy failure when the beam is subjected to service loads,” Vecchio said. Ongoing material testing and finite element stress analysis will be factored into the final root cause assessment next month, he said.
Hazleton said 2 inches. 4 inches. Because of their size, shape, and flange placement, the flange holes on either side of the wall are not weld access holes. WAH is 2 x 12 inches.
“They don’t have to touch the welds, they don’t have to be ground or subjected to non-destructive testing because they’re not [WAH],” he said.
The American Institute of Steel Construction agrees. Lawrence F. Crute, associate director of AISC, said WAH is particularly suitable for “contours suitable only for composite I-walls” and “allows adjacent flange welds to run continuously, interfering with the wall.” Engineering President.
Gibbons disagrees. He said the holes in the Fremont Street flange resulted from a request for information from steel industry subcontractor Skanska USA Civil West “for the purpose of performing a beam weld.” “In this request, both the steel subcontractor and the engineer described the holes as welded holes. … Don’t worry … the steel subcontractor at the time expressed concern about the applicability of the part,” he added.
At the TJPA meeting, Gibbons proposed a plan to repair the beam. A bolt-only repair will bypass the broken flange by sandwiching it between 14-foot-long steel strips on either side of the wall.
Nadine M. Post, ENR’s architecture editor-in-chief, is an award-winning journalist with 45 years of experience covering trends, issues, innovations, controversies and challenging projects. The post has profiled many industry giants, including 10 ENR Excellence Award winners. She has covered disasters, failures and attacks, including the 1993 bombings and the destruction of the World Trade Center in 2001. Project stories include the World Trade Center renovation; Burj Khalifa 828 meters high; Disney Concert Hall in Los Angeles and Bullitt Center and Rainier Plaza towers in Seattle; In 1985, Post wrote the McGraw-Hill book for restoration architects Richard S. Hayden and Thierry V. Despont, Restoring the Statue of Liberty (1986).
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补上周–Hannah
Post time: Jun-24-2024