十多年来，建筑界一直关注建造一条新的520号浮桥（世界最长）的努力，该桥将西雅图连接到华盛顿湖的东部。但是在四月份盛大开业之后，在闪亮的新桥上的关注，道路高高20英尺以上，浮动浮桥已经消失了。现在，工作人员将注意力转移到拆除旧桥上。

Located mere feet to the south of the new bridge, the 7,580-ft-long concrete pontoon bridge may no longer have a purpose in Seattle, but the pontoons will live on in projects worldwide. Dismantling the bridge has become a priority as part of the larger $2.38-billion bridge project expected to wrap up before this winter, with a tricky detensioning job a critical part of the process.

“Once we opened the new bridge, we immediately started the decommissioning process to remove the old bridge from the lake,” says Dave Becher, Washington State Dept. of Transportation director of construction for the bridge replacement and HOV program. “The [old] bridge is a vulnerable structure, and it puts the new bridge at risk. It is very critical for us to get the old bridge removed before the storm season at the end of the year so we don’t have to worry about it being damaged or damaging the new bridge.”

但是，由于外部张紧后张开桥梁的长度，因此将31个浮桥拆分进行转售和重新利用构成了挑战。

Joint venture Kiewit-General-Manson, working with Northstar Demolition for the removal of the approaches and road decks, called on Schwager Davis Inc. to devise a way to deal with that post-tensioning dilemma.

“It is a mixed bag of bonded and unbonded post tensioning,” says Jacob Myer, design engineer for SDI. “It is really unique that you would be detensioning that and that these spans are the longest in the world.” The two styles of post-tensioning incorporates both independent free movement of strands in some places and not in others.

With 24 ft of stored energy in the tendons, Myer says the challenge was devising a way to release the energy without setting off a chain reaction with the worst-case outcome: sinking pontoons.

SDI总裁Guido Schwager说：“ 520号州际公路的浮动桥退役是我职业生涯中最复杂的张紧后挑战。”

Beginning To End

挑战始于1963年，当时常绿点浮桥的交通为7,500英尺以上。但是，它确实在1997年增长了，当时地震改造提高了处理77 mph的持续风能，而不是通过给出32座新的外部高密度聚乙烯树脂管包裹的桥梁，而不是57英里 /小时。典型的肌腱长度可能跑约400英尺，但是520号州际公路上的16个肌腱延伸3,200英尺，另外16英尺延伸3,600英尺，增加了500 psi的轴向压缩，以保持桥梁的混凝土ponto骨在一起。

When it came time to pull the old bridge apart, confronting the grouted and external tendons posed the trickiest of challenges. Having them be both grouted and external meant cutting or mechanically releasing one strand simply transferred the load into the bonded whole, yet with one fewer strand holding. Once four strands were cut, the remaining strands would eventually fail and release up to 615,000 lb of force and retract the remaining tendon length “like a lethal rubber band,” SDI says.

“It is really unique that you would be detensioning bonded and unbonded,” Myer says. “And with the lengths of these spans being (among) the longest in the world, you have 24 ft of stored energy in these tendons, which is a tremendous amount.”

To find a solution, Kiewit-General-Manson analyzed several scenarios. One idea involved using robots to cut the tendons to remove the danger to humans. Another idea included removing the HDPE pipe from a section of the tendon within a hollow pontoon, clamping over each end of the section with high-tension bars and taking the force of the small section between the clamps.

与propo SDI确定有问题sals. The robot scenario would likely create such a whipping force in the cut tendons that it would break through pontoon walls and sink both concrete and robot. The second plan seemed more realistic, but there were still significant issues. Not knowing the condition and strength of the existing grout and relying solely on friction between a clamp device offered the chance for the same type of catastrophic failure, but this time with humans instead of robots in the line of danger once a tendon reached its breaking strength of 58 kips.

“我们如何在不引发链反应的情况下控制这种存储的能量的释放？”迈尔说。“我们经历了多种不同的方案，涉及如何控制它。”

迈尔（Myer）和他的团队设计了一种基于测序的新解决方案，他称之为“我们见过的最独特的后张紧态度应用”。该计划涉及在安装两个相距约10英尺的重粘养设备之前，在肌腱跨度中心附近卸下75英尺的HDPE管和灌浆，并通过高强度杆连接。有了该安全措施，机组人员可以一次机械地消除，切割，reanchor和Retension One Strand。

The end result led to lifting off post-tensioning, transferring it under load to high-strength bars and ultimately letting that force down: a “non-typical” response to the solution. Using this process, the force in the tendon temporarily increased by 7% but never exceeded safe working loads. After all strands were re-anchored and retensioned, the load transferred to the high-strength rods that could be safely released hydraulically, one strand at a time.

“Many simple operations combine to make up a complex, yet effective solution,” Myer says.

Testing Before Detensioning

不确定灌浆会如何反应，Myer建立了一个180英尺的模型，以在加利福尼亚州Lathrop的Con-Fab California Corp.的铸造床中测试该过程。迈尔说。“这是一个很大的未知。”

The team devised a backup plan if the condition of the tendons turned out to be far different than expected. Once SDI arrived on the water, it analyzed the jobsite conditions, which included pontoons needing the removal of water and the discovery of cables with retrofitted wall clamps not listed in the retrofit drawings. Those clamps could have posed a binding point that could snag the incremental release of force if left unchecked. As crews removed the HDPE casing, it revealed sections of tendon with voids—air pockets with no grout—and moisture-filled soft grout. Even with these issues, Myer says there was no corrosion issues and that the conditions of the strands were almost perfect, allowing the original plan to continue unhindered.

七人组合完成了五月至7月之间脱位的工作，在150 kips的自上而下，与创建桥时相同的压力过程，但仅在相反的情况下进行。工作人员从桥的中央开始，朝岸边工作。

环境挑战

In order to protect the Lake Washington environment, crews had debris nets in place to capture any pieces that fell into the water.

As SDI worked to detension the pontoons, the above water work continued. The project team removed the east and west transition spans, reports Greg Meadows, WSDOT State Route 520 floating bridge and landings manager. To pick off the spans and remove them from the bridge, crews created a floating island and used a derrick barge to assist in the process, which will include the recycling of all the steel from the old structure.

梅多斯说：“迄今为止，在此过程的早期，每个计划的过渡跨度和第一部分的去除几乎已经消失了。”“我们制定了一个很好的拆除计划。”

Along with the pontoon work and above water removals, decommissioning the old State Route 520 bridge includes the elimination of all underwater obstacles—including columns that proceed into the lake bed and footing areas—that could pose a risk for boaters and wildlife.

While the creation of the new bridge required that all staging on the north side not interfere with the old bridge, dismantling the old bridge has all staging and work on the south side to remain clear of the new bridge. “This was a much greater challenge for work on the new bridge,” Meadows says. “For the old bridge, working from one side or the other does not make much of a difference.”

With the new bridge nearby, however, Becher says it does require crews to pull pontoon sections off and dismantle them elsewhere to steer clear of the new bridge.

“The real key is to maintain separation between the old and new bridge,” Meadows says.

一旦拆除了桥，工作人员将努力维修海岸线。在西雅图的一侧，新桥的对齐与旧桥接近，而麦地那东侧的枢纽将成为一条景点，使游客有机会在湖上望去。