Creating a Signature Arch Cable-stayed Bridge for Downtown Miami

A six-arch cable-stayed bridge along the critical Interstate 95 connector route will serve as the signature feature for a five-mile highway reconstruction now under way as part of the City of Miami’s expansive Connecting Miami project.

Karsten Baltzer, WSP USA managing director for construction services, heads part of the team that represents the Florida Department of Transportation (FDOT) and Miami-Dade Expressway Authority on the $818 million project.

Formally known as the Interstate 395/State Route 836/Interstate 95 Design-Build Project, Connecting Miami will enhance three highway corridors in downtown Miami and include the signature bridge that will cross over NE 2nd Avenue and Biscayne Boulevard, redefining the Miami skyline with its six sweeping arches.

This is familiar territory for Baltzer, who brings to the project experience working on two of the five longest bridges in the world when they were constructed—the Jiangyin Yangtze River Bridge in Jiangsu, China, with a main span of 4,544 feet; and Denmark’s Great Belt Bridge, featuring a main span of 5,328 feet. Before joining WSP in 2020, he’d also been a lead engineer on other iconic bridges spanning the waterways of San Francisco Bay, Tacoma Narrows and Turkey’s Bosporus Strait.

While Miami’s I-395 bridge won’t break records for suspended span length, the urban bridge features a complex spider-shaped array of arches from which cables will hold up the roadway and will redefine downtown Miami’s skyline.

“It also breaks new ground in the use of auger cast piles to support foundations,” Baltzer said. “This is one of the first times auger cast piles have been used for a road bridge in the United States, so it’s a challenging new method.”

As part of the construction and engineering inspection consultant team for the public transportation agencies Baltzer and his team administer, monitor and inspect the work of the design-build team: Archer Western-de Moya Joint Venture.

WSP’s responsibility includes, among other things, ensuring that every aspect of construction, including hundreds of auger cast piles—3-foot-wide holes drilled 140-feet deep and filled with rebar and concrete—are built to last at least the 100-year design life of the infrastructure project.

“The main challenge with the auger cast piles was to maintain proper temperature as the concrete cured,” Baltzer said. “Too hot, which is a constant concern in Miami, and the concrete doesn’t cure properly.”

The piles were successfully completed in 13 months, and then came the more conventional but still massive task of casting the concrete footings. It is the largest concrete placement in FDOT’s history.

After constructing coffer dams to keep out groundwater and tying 1.7 million pounds of steel rebar, the project team began pouring 5,200 cubic yards of 8,000 psi concrete into the 140-foot x 68-foot x 14-foot central pier footing on Friday, July 23.

“Five concrete plants in the region were involved and four pump trucks were used to place the concrete into the foundation,” Baltzer said.

By Sunday morning, 33 hours later, the pour was complete. But the job of maintaining proper cure temperatures in the giant slab continued for the next seven days. Six miles of cooling pipes were placed in the concrete as it was cast.

“Seven large chillers provided cold water that circulated through the pipes,” Baltzer added. “To control uniform temperature during curing, wireless thermometers communicated the temperatures throughout the footing to operators.”

The cooling systems, as well as the testing methods used on concrete samples to measure chloride resistance, are standard construction processes, according to Baltzer. A more customized approach was needed to ensure concrete flowed perfectly into the lowest area of the footing, where rebar was tied so thickly that only inches of space remained between rebar.

“The builders specified a custom concrete mix, then used two mockups to make sure it flowed correctly between all those spaces,” he said.

Similar fine-tuning of concrete chemistry is needed to build the six arches. These elegantly curved structures will give the bridge its signature look.

“Using cable stays, they’ll bear much of the bridge’s weight and make it possible to replace the forest of concrete columns holding up the current bridge with fewer piers,” Baltzer said. “It’s an innovation that will create new connections between parts of downtown Miami and free up space for new parks, walkways and other urban amenities.”

But the arches are no walk in the park to build.

“The six arches have 345 segments that will be cast in our precast yard, then brought on-site and assembled with cranes,” Baltzer said.

To achieve adequate strength in the arches, rebar is tied in even more thickly than in the footings.

“To make sure we can get concrete into all the areas between the rebar in the congested sections, we’re going to use self-consolidating concrete,” Baltzer explained.

Because of the elliptical shape of the arches, he said each segment will meet its neighboring segments at slightly different angles. That means the forms for all 345 segments must be adjusted to create the precise angle specified for each segment.

“Similarly, the anchors for the stay cables must be placed at precise locations and angles,” Baltzer added. “So, it’s no wonder that WSP has a team of inspectors working full-time in the casting yard, responsible for ensuring the casting is done correctly.”

Connecting Miami is targeting completion by the Fall of 2024, creating additional capacity along I-395 with three through lanes in each direction and separate connector ramps for traffic.

When the I-395 signature bridge and the rest of the project is opened to traffic, it may not attract global attention like some of Baltzer’s previous prominent projects. All that matters to his team, though, is that it provides value to the community that will depend upon its reliability for years to come. 

“We have the satisfaction of knowing that we helped create an important infrastructure facility that will benefit generations of Miami residents and visitors every single day for a century—at least,” he said.