The 21st Century Renaissance of Tied-Arch Bridges

The Veterans Memorial Bridge (Image)

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Isambard Kingdom Brunel’s celebrated bridges—such as the Windsor Railway Bridge—are sometimes referred to as the earliest built examples of tied-arch bridges. But while Brunel’s wrought iron structures were groundbreaking and remain engineering landmarks, their primary innovation lies in their truss configurations, not in the development of the tied-arch system.

 

Images describe true-arch (top) and tied-arch (bottom) behavior. Differences in compression and tension are highlighted in blue and green.

 

 

The bowstring truss—often conflated with the tied-arch—actually has a more modest and distinctly American origin. Squire Whipple, an American engineer, is credited with inventing and patenting the bowstring truss bridge in 1841. Whipple’s design was the first to apply scientific analysis to bridge construction, using cast iron for compression members and wrought iron for tension, and incorporating a “string” (tie) to resist the outward thrust of the arch. His approach not only advanced structural understanding but also enabled rapid, economical bridge construction for projects such as the Erie Canal. Hundreds of Whipple’s bowstring truss bridges were built across the United States, and his work set the standard for canal and early railway crossings.

Thomas William Moseley, another American engineer, further advanced the form with his patented wrought-iron lattice girder bridges, contributing to the prevalence and refinement of bowstring truss designs in the United States during the mid-19th century. These American bridges, while more modest in scale than Brunel’s monumental works, align more closely with what today’s engineers recognize as tied-arch bridges, both in structural behavior and visual form.

“The important thing to know about tied arches, compared to true arches, is that they are structurally independent of the piers that support them,” says HNTB Associate Fellow and Senior Technical Advisor Natalie McCombs, P.E., S.E. “So technically, once the arch and the tie girders are in place, a tied arch can be picked up and moved around.” This has obvious advantages for constructability, as tied arches can be built near—but not obstructing—the water channels they will span and then moved into place. They may also require smaller foundations compared to true arches, as support piers don’t have to act as “thrust blocks” constraining the lateral forces that true arches generate.

The appeal of tied-arch bridges extends beyond structural efficiency. Their distinctive silhouettes create memorable landmarks that define skylines and serve as community icons. The arch form speaks to something fundamental in human aesthetics—it’s a shape that has resonated since Roman aqueducts and cathedral vaults. When properly executed, tied-arch bridges are a synthesis of form and function and succeed in gaining widespread aesthetic appeal.

Turning Point for Tied-Arch Design

By the 1980s, these once-favored structures faced increased scrutiny. Though not a tied-arch, the 1983 collapse of the Mianus River Bridge in Connecticut raised nationwide awareness of fracture-critical structures—now referred to as non-redundant steel tension members (NSTM).

In 1978, the FHWA issued Technical Advisory T 5140.4, which advised against the construction of tied-arch bridges due to several concerns, including the following:

• Tied-arch bridges, including tie girders, had been fabricated with electroslag welding that could lead to cracking in the tie girder and propagate through the entire member.

• Tied-arch bridges sometimes experienced lamellar tearing in the hanger connections to the arch rib, primarily due to high restraint in the welded connections, and other structural vulnerabilities had been observed.

• The advisory specifically noted that tied-arch structures are “one of the most non-redundant structures,” as they rely entirely on the capability of just two tie girders to accommodate the total thrust imposed by the arch ribs. This lack of redundancy, also apparent in arch hangers, means that a loss of critical components could potentially lead to instability of the structure.

• Construction challenges were identified, and T 5140.4 specifically said that fracture repairs could be “very costly, time consuming and in many cases have inconvenienced the traveling public.”

This advisory and FHWA statements in the 2000s understandably advised against new tied-arch bridge design.

However, the structural challenges tied to early tied-arch designs spurred innovation. Modern tied-arch bridges now incorporate advanced materials, refined fabrication methods and robust detailing strategies to mitigate past vulnerabilities. Redundancy has been improved through multiple load paths and enhanced connection detailing.

By the mid-2000s, confidence in the tied-arch form began to rebound. HNTB played a key role in that resurgence, with notable examples including Blennerhasset, Lake Champlain and the Amelia Earhart Bridge. Blennerhasset Bridge—which carries U.S. Highway 50 over the Ohio River from Parkersburg, W.Va., to the Ohio state line—was the first bridge in the United States to have internal redundancies, bolted tie girders and network hangers.

Engineering trends come and go in cycles. Each project demonstrates how modern analysis tools, advanced materials and innovative engineering and detailing allow today’s bridge engineers to address historical concerns about tied-arch bridges while preserving their aesthetic and structural advantages.

The Veterans Memorial Bridge: A Tied-Arch Masterpiece

The renaissance of tied-arch bridges is due to several key engineering breakthroughs, with improved redundancy being the most critical. “The historical concern was that tied-arch bridges were considered NSTM, meaning the loss of a single component could potentially cause instability of the structure,” explains McCombs. “We’ve overcome this by using internally redundant tie girders and network-hanger configurations, where hangers cross multiple times, creating redundant load paths throughout the structure.”

The Veterans Memorial Bridge connecting Little Rock and North Little Rock, Ark., features two 440-foot “basket handle” network tied-arch spans. Its opening in March 2017 (28 days ahead of schedule) was one of several examples in the modernization of tied-arch bridges, where the Arkansas Department of Transportation and HNTB successfully implemented several innovative engineering solutions to address redundancy.

In an arch bridge of any type, hangers (also called suspenders) are the structural members that connect the bridge deck to the arch ribs, transferring loads from the deck level to the arch’s top chord. Modern hangers are made of steel wire structural strand or prestressed strand. Historic hanger orientation used vertical hangers to support the deck. If repair, replacement or loss of one of the hangers would occur, the adjacent hangers would have to carry a higher percentage of load that the bridge may not have been designed for and could lead to structural instability. The network hangers significantly improve the redistribution of forces in this scenario and are considered in the design process.

The distinctive “crisscrossed” Veterans Memorial Bridge spans are an example of network hangers, a specific type of inclined hanger system where hangers intersect each other multiple times. This creates truss-like behavior within the arch plane, leading to a more-efficient distribution of forces and a higher level of redundancy and is a huge part of why the renaissance of tied arches has been as successful as it has been. “When these network hangers crisscross, it adds stiffness and redundancy,” adds McCombs. “If any one of these hangers were to be removed, the overall structure would remain stable—there are alternate paths for the load to be taken up by neighboring hangers.”

 

 

In the design of the Veterans Memorial Bridge and other recent tied-arch bridges, HNTB implemented several non-redundancy mitigation strategies. For one thing, bolting is favored over welding at key locations. “By using bolted connections at corners, we create what we call ‘internal redundancy,’” says McCombs.

When tie girders are rectangular, angles can be placed in the corner and bolted to both the flange and the web, which was done on the Amelia Earhart Bridge. For the Veterans Memorial Bridge, the tie girder was a parallelogram. A tab plate was welded to the web and then bolted to the flange. If a crack were to occur in the web, it would not propagate through the entire cross section due to the bolted connection. Additionally, the modern tied-arch bridge designs consider a scenario where the structure is analyzed for one of the tie girder flange or web plates to be fractured. In this scenario, the structure is designed to remain stable, and the network hanger system plays a key role. In historic tied-arches, the structure would likely have lost stability.

Construction of the Veterans Memorial Bridge took advantage of the self-supporting nature of tied-arch spans. Both 440-foot network tied-arch spans were pre-assembled on custom falsework towers supported by barges, located immediately downstream of the existing bridge and out of the main navigation channel. After each span was assembled at nearly its final elevation, the barge-supported arches were maneuvered into their final locations using tugboats and winch lines, then lowered onto permanent bearings by ballasting the barges. This nifty bit of just-barely offsite construction significantly minimized the closure time of the bridge and avoided prolonged disruption of road and river traffic as well as increased safety to the contractor and the public.

Form and Function in Iowa City

The “Partial Through Tied-Arch” design of the Park Road Bridge in Iowa City, Iowa, was chosen in part to accommodate aesthetic priorities of the community. In this case, the University of Iowa’s 1,800-seat Hancher Auditorium—a curved structure clad with brushed stainless steel shingles that give the venue its signature “flowing” appearance—is immediately adjacent to the bridge, and Iowa City reasonably desired that the two examples of aesthetic infrastructure complement each other.

For this reason—as well as for structural concerns—HNTB considered tied-arch solutions early in the design process. There was one challenge: ordinarily, tied-arch bridges are supported at the ends (or tips) of arch ribs to form the eponymous “tie.” But at Park Road, this would’ve meant a visually awkward rib height relative to the auditorium and the 450-foot bridge span.

Enter the partial through tied-arch design that McCombs and HNTB’s team proposed. It allowed for an economical tie girder to arch-rib height-to-span ratio, and visually, the constructed arch presents a gentler profile above the deck that sweeps in a way that’s in aesthetic harmony with the curvy Hancher Auditorium. To complete the pairing, the arch ribs and deck railings are clad and finished in materials that subtly complement and call to the Hancher’s satin-finished stainless steel.

HNTB’s tied-arch bridge design, completed in 2018, is actually the New Park Road Bridge, a replacement of the previous bridge called for as one of the two main components of the “Iowa City Gateway”—the $42 million project resolved University access issues brought to light during the devastating flood of 2008, when the Iowa River crested at a record 31.53 feet (major flood stage is 25 feet), causing more than $270 million in damage to the area.

To account for 100-year flood levels, Dubuque Street was raised 8 feet in places, mostly via innovative use of mechanically stabilized earth walls. And the new bridge, a “three-span, reinforced concrete, partial through tied-arch structure with a continuous post-tensioned tie girder supporting the deck and transverse floorbeams” is positioned 1 foot above the 200-year flood level and improves on its predecessor’s performance in that it has only two piers in the water instead of five, significantly reducing impediments to river flow during flood events.

 

 

FHWA’s concern about non-redundant tied arches on this project was mitigated by using a concrete post-tensioned tie girder. The inclusion of upper and lower courses of “tendons” (bundles of high-strength steel strand) running through the length of the tie girder introduces compression in the tie girder to resist the thrust.

And, it sure is elegant.

An Infrastructure Success Story

The proactive efforts of government agencies in identifying key vulnerabilities in some early tied-arch bridge designs—and establishing guidelines for improvement—stand as a testament to effective and forward-looking regulation. These agency initiatives highlighted valid concerns and created space for innovation. Their actions helped inspire today’s generation of visionary engineers to reimagine and elevate the tied-arch bridge, preserving its elegance and practicality while advancing safety and performance. And respond they did; engineering breakthroughs in internal redundancy and network hanger design significantly mitigated bridges previously categorized as NSTM and non-redundant aspects of bridge design practice. In the last 20 years, there have been nearly twice as many tied-arch bridges built as cable-stay bridges in the United States, because the tied-arch bridge is often more cost effective and addresses internal redundancies.

• Kansas Department of Transportation’s Amelia Earhart Memorial Bridge is a network tied-arch bridge spanning the Missouri River. A community favorite that opened in December 2012 features a main-span length of 527 feet and a 100-year design life.

• The Northaven Trail Bridge in Dallas is the only known network tied-arch bridge with a doubly curved deck and skewed supports. First built offsite, Texas Department of Transportation’s bridge then was moved into place over a single weekend via accelerated bridge construction in 2023.

• Near Secaucus, N.J., the movable swing span Portal Bridge, originally built in 1910, is being replaced by a fixed-span, two-track bridge rising 50 feet above the Hackensack River. The replacement—led by NJ Transit and Amtrak—will be one of the first network tied-arch rail bridges in the United States and will eliminate the need for movable components, reducing the risk of operational disruptions.

In some ways, too, the current crop of long-lived and striking tied-arch masterpieces vindicates the early work of Squire Whipple and Thomas William Moseley. Their innovative contributions to bridge engineering set the tone for the work of engineers today who are ensuring that tied-arch bridges can be expected to be around for centuries to come.