旧金山陷入困境的Salesforce Transit Center的三楼锥形,内置的盘子大梁的钢制制造商呼吁进行Girder-Hanger连接设计评论,这是探针的一部分桥梁弗里蒙特街的80英尺长的成员。该设施于9月下旬关闭。

在中跨处,每个8英尺深的小梁网都用垂直衣架板增厚,以支撑下面的二楼巴士甲板。衣架板焊接到网络上,并通过梁的底部法兰插入插槽。

“I believe the hanger detail is the largest contributing factor to the problem,” says Robert Hazleton, president of fabricator The Herrick Corp. “We have been assured that a thorough review of the hanger design is part of the investigation.”

Mark Zabaneh, executive director for the owner, Transbay Joint Powers Authority (TJPA), says, "we are all very eager to determine the cause of the [fractures] discovered on two [girders] at the transit center at Fremont Street, repair the girders and safely reopen the transit center. We are weighing all of the facts as we go about determining how and why this happened and have not ruled out design, fabrication or installation. We are fully investigating this incident as well as cooperating with the independent analysis" being provided by experts from the Metropolitan Transportation Commission’s design review panel."

The twin girders are much like bridges in that they hang a bus deck and span a street. The girders’ hanger connection detail would normally not be used in modern bridge construction, says a structural engineer familiar with the design. The hanger could have been framed around—not through—the flange and connected with bolts to the web using plates framing into the web, says the bridge expert, who declines to be identified.

A double hanger detail would have avoided cutting a hanger slot into the 4-in.-thick flange. The existing slot intersects the butt-weld joint in the flange at the girders’ midspans, where there are maximum flexural stresses, says the expert.

The connection of the hanger plate to the web also produces a “very poor fatigue detail” at the end of the web-to-hanger butt weld, adds the engineer. Further, the connection detail used, with a weld hole added during the shop-drawing phase, is not covered in the bridge specification.

Bruce Gibbons, a managing principal of transit center structural engineer Thornton Tomasetti (TT), declined to comment on the connection until after the investigation is complete.

赫里克(Herrick)的担忧是对12月13日的公开演讲的回应,由LPI首席执行官工程师Robert S. Vecchio(冶金学家)调查脆性断裂。

根据Vecchio的说法,迄今为止进行的分析和测试“暗示了梁骨折的可能原因是在服用前在Girder Weld-Weld-Access-Hole-Radii中形成裂缝。”根据Vecchio的说法,LPI的客户是TT和Turner Construction Co.的TJPA。

Vecchio说,在“在高度硬化和脆弱的马氏体表面层中的焊缝访问孔的热切割过程中,浅表面微裂缝”说。然后,在法兰板的对接焊接过程中可能在四个法兰中的两个中形成较大的弹出裂纹。

“Rapid low-energy fracture of the flanges occurred as the girder was subjected to service loading, on top of the normal stresses due to fabrication,” said Vecchio. Material testing and finite-element stress analyses underway will be considered in the final root-cause assessment expected next month, he added.

Hazleton says the 2-in. by 4-in. flange holes on either side of the web are not weld access holes, based on their size, shape and flange location. WAHs would have been 2 in. by 12 in.

“They were not required to access a weld and not ground or subjected to nondestructive testing because they are not [WAHs],” he says.

The American Institute of Steel Construction concurs. WAHs are specifically applicable “only to the profile of the web” in a built-up I-shape and “provided to allow access to make the weld of the adjacent flange continuous past the interfering web,” says Lawrence F. Kruth, AISC’s vice president of engineering.

Gibbons disagrees. The holes in the Fremont Street flanges were proposed as a result of a request for information by the steel subcontractor, Skanska USA Civil West, “for the purpose of executing the welding of the girders,” he says. “In that RFI, the holes were described as weld access holes by both the steel subcontractor and engineer. … No concerns … regarding the suitability of this detail were raised by the steel subcontractor at the time,” he adds.

At the TJPA meeting, Gibbons proposed a girder fix. The bolt-only repair would bypass fractured flanges by sandwiching them, on both sides of the web, between 14-ft-long steel splints.