A 112-year-old serpentine stretch of wooden track, switches and traffic signals regulates Chicago Transit Authority (CTA) elevated trains just north of the Belmont Red and Brown line station on the north side.

The two lines converge at these S curves, creating a longtime choke point of train traffic. During rush hour, Purple line trains, which run express up to Evanston, Ill., join the jumble on the same tracks as the Red line trains.

In hopes of preventing future spilled coffees, stumbles and uncomfortable leans, the CTA is straightening out the serpentine section with a $2.1-billion modernization that began in 2019 and is expected to be completed by the end of 2024. In addition to cutting out that S bend, joint-venture contractor Walsh/Fluor will build four new CTA Red line stations, replace 9.6 miles of 100-plus-year-old Red/Purple line track with new concrete support structures and track, add 23 miles of new traffics signals and even move a historic building 30 ft. The concrete structure will make it about 80% quieter than the wood and steel one. The project is being paid for by city tax-increment financing funds and federal grants obtained during the administration of then-Mayor Rahm Emanuel. CTA will keep running trains the entire time.

A bracing frame will be installed on the outside of the Vautravers Building to lock everything together. Wolfe House Movers will then jack and move the whole building.
Photos by Jeff Yoders/ENR
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“During construction, we’ll rebuild two tracks at a time while we’re running Red and Purple service on the remaining two tracks,” says Chris Bushell, CTA’s senior vice president for infrastructure, who is in charge of the largest project (cost-wise) in the agency’s history. Regarding the four new stations, Bushell said Stage A of construction will go from May 16 to late 2022, and then Stage B will start and continue until 2024. In Stage A, construction will happen on the east half of the tracks while train service runs on the west side. Then in Stage B, “we’ll flip that,” Bushell says.

A “straddle bent” design was necessary due to limited availability of support pier locations for the new, 45-ft-high elevated flyover structure in Lakeview. The Walsh-Fluor-led design-build team, including Stantec and EXP, selected and designed as the most viable solution a steel box supported at each end by a concrete column, spanning approximately 80 ft between columns “straddling” track structure below. The steel box was designed in accordance with fracture-critical provisions and includes internal member redundancy of tension elements.

“Longitudinal bridge girders are supported by, and ‘integral’ with, the steel box—this allows vertical clearance requirements to be met while minimizing the profile of this new structure,” says Bill Amrhein, Stantec principal and CTA Red/Purple line modernization structural design lead.

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Flying By

天桥已经实质上已经完成，预计将在今年年底之前使用，但是还有另一条曲线要导航。CTA购买了16座建筑物，以创建三个街区，5.7亿美元的天桥和湖景社区的轨道重建，但其中一栋仍然存在。分包商Wolfe House和建造Movers的127年历史的住宅结构将于8月从其基础上升起，并搬到West 30英尺，以使轨道对齐。Walsh-Fluor船员已经为三层楼，六街大楼的新基础挖掘了新的基础。

“To secure the building’s exterior, a bracing frame will be installed on the outside to lock everything together. Wolfe House Movers will install large jacking beams through the building’s foundation,” says David Shier, project executive for the Walsh Group. “Then they will slide smaller ‘needle’ beams throughout the perimeter of the foundation walls to ensure the load is evenly distributed.”

Shier says that once the jacking frame is completed, a unified jacking system will be used to lift the building off its foundation. A series of slide beams and push rams will then be used to move the building west, and then south. The building will never actually drop from its elevation. To ensure adequate bearing on the new foundation, the building will first be temporarily secured in its final location and then the new concrete foundations will be poured and placed underneath it. Walsh-Fluor had a design study done prior to construction to affirm that a move was feasible with the structure’s current integrity. The team has been working on reinforcing it since the project began in 2019. CTA plans an interior renovation once the project is completed.

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混凝土解决方案

布什尔说，新的混凝土柱相距约100英尺，而现有的一百年钢柱相距约50英尺。他说，设计决定是建筑友好的。

“That greater span enables us to assemble this quicker,” he says. “As you can imagine, drilling those holes [each concrete support is drilled 80-90 feet to bedrock] is pretty difficult and time consuming. The fewer of those that we have to do, the better for the schedule and budget performance we need on the project.”

As Stage A progresses in 2021, a gantry system will be brought in to install track structure of the Red/Purple line in a longitudinal way that Bushell says will minimize impacts to the adjacent community. Track pieces will be precast off site and brought in so that there will be minimal fabrication of the structural components in the tight spaces of Uptown and Edgewater.

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Shier说：“由于物流的严格，选择了龙门机系统。”“通常，建造混凝土桥梁会更加困难。此方法最适合紧密空间。它允许建筑与主动轨道相邻，这对整个建筑持续时间有重大好处。”

红线上的CTA的劳伦斯，阿盖尔（Argyle），贝尔温（Berwyn）和布莱恩·莫（Bryn Mawr）站都被Exp设计的新的符合ADA兼容车站所取代。Walsh-Fluor建造了较小的临时站，供车手在2021年5月在劳伦斯·布林·莫尔（Lawrence-Bryn Mawr）的施工开始之前，从布莱恩·莫尔（Bryn Mawr）和阿盖尔（Argyle）登机。Shier说，该项目计划在2024年底完成。贝尔蒙特（Belmont）和霍华德街（Howard Street）红线北部终点站之间23号轨道里程的新信号系统仍需要与新的轨道结构一起安装，一旦龙门系统今年晚些时候到达。

“We have worked to coordinate the project with CTA to make a plan to get it all done—but it is, ultimately, a very big job,” Shier says.