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Replacing Amelia

by Frank Blakemore, P.E., and Natalie McCombs, S.E., P.E. | Jul 25, 2014
A steel network tied arch serves as the successor to a 1930's steel truss honoring the famous female flyer.

Many know the story of Amelia Earhart, best known for being the first female to fly solo across the Atlantic Ocean and later attempting a flight around  the world at age 40. But likely few know the pioneering  pilot was a native of Atchi- son, Kan., where a bridge named after her flew traffic over the Missouri River for 74 years.

The  steel through truss, built as a Works Progress Administration (WPA) project in 1938, had  been  nursed  through nearly  10 rehabilitations before the decision was made to replace it in  2002.  Its shoulderless, narrow roadways  fell short of 21st century needs. And the condition of its deck trusses worried the owners.Because the bridge was a beloved, historic structure,   both  the  Kansas  and  Mis- souri Departments of Transportation (KDOT and MoDOT) knew  the  community would have a significant role in selecting the new design. In fact, citizens already had made one thing clear: The new bridge would be made of steel, like its predecessor.

Steel Defines Different Designs

KDOT hired HNTB Corporation and AMEC in 2007 for final design of the project, with HNTB as lead bridge designer. HNTB performed preliminary design on several alternatives and presented  KDOT and MoDOT with the  two most cost-effective design alternatives: a steel through truss span and a steel tied-arch  span with network hangers. In addition to an arch design being a community favorite, the design also came in under the truss span’s cost estimate. The tied arch won and the design was set. To cover the project’s $59.4 million cost, Kansas would contribute $30.6 million and Missouri would kick in $28.8 million.

Contractor Archer Western began  construction  in  June  2009  with  a  targeted   completion date of 2011, but historic flooding would suspend not  once but twice before the bridge  officially opened to traffic this fall.

The new U.S. 59 Amelia Earhart  Bridge, a four-lane, ample- shouldered network tied-arch bridge, rests just 78 ft south of the old bridge. Approximately 2,546 ft long, the structure consists of 2,019 ft of 78-in. NU (Nebraska University) prestressed concrete I-girder approach spans and a 527-ft steel tied-arch main span.

The new bridge has a 100-year design life and is capable of handling the 20-year traffic projections of 12,400 vehicles a day, twice the  capacity of the  previous bridge. The former  bridge, kept open to traffic during construction, will be closed once traf- fic is shifted to the new bridge and will be demolished next year. Steel suppliers, including  Nucor-Yamato, SSAB   and Arcelor-Mittal, shipped approximately 2,280 tons of Grade  50 A709 steel and 145 tons of Grade  36 A709 steel to fabricator, Industrial  Steel  Construction. Fracture-critical F3  steel  was used  for  tension  members  and  T3  steel  was designated  for bending members.

When it came to constructability, the  steel design facilitated construction in three ways:

  • Falsework was kept to a minimum.
  • The contractor, Archer Western, had the option of constructing  the main arch span off-site, floating it in and lifting it into  place or constructing the  span over the river.  (Because  Archer  Western  had  erection   towers from a previous project, the span was built on-site as it was the more cost-effective option.)
  • For the on-site  option,  crews could cantilever the steel erection  over the  navigation  channel,  which kept  the river open to traffic during construction.

Steel also offered a greater span length. The  main arch span length of 527 ft, an economical design not possible with concrete, was necessary due to the heightened potential for scour caused by the older, adjacent bridge and a railroad bridge just upstream. Steel also made it possible to achieve the required  52-ft vertical clearance over the river. (Concrete might have been applicable here but only if used in a more expensive cable-stay design.)

In terms of aesthetics, several aspects of the new bridge are references to either the original truss or to Amelia Earhart. The X-bracing between the ribs was chosen to provide a visual connection to characteristics of the earlier historic truss form. In addition, pier caps exhibit the rounded wing shapes of the planes Amelia flew in her day. At the community’s request,  designers added high-intensity beam luminaires to the arch portals, echoing the previous bridge’s aesthetics. These twin beams of light cast an eternal  gaze into the night  sky and symbolize Earhart’s passion  for  flying. Additionally,  LEDs  (light-emitting diodes) with changeable colors were provided along the top edge of the arch ribs to commemorate holidays, special events, and draw attention to the structure. Light poles and light fixtures were also selected by the community to tie in with the historic downtown.

 

A Springboard for Innovation

The  designers  were  able  to  introduce   several  safety and cost-saving innovations to the steel tied-arch  design:

Creating  internal  redundancy.  Two main force-carrying components  exist in the tied-arch  system:

  1. The arch ribs follow a parabolic curve, rising 90 ft above the driving surface for a span-to-height ratio of 5.83. The arch rib is a 4-ft wide by 4-ft, 6-in.-high welded box section, which allows for internal  inspection of the arch rib and upper hanger connections.
  2.  The  tie  girder  is a 4-ft-wide  by 6-ft-high  bolted  box section  between  the  ends of the  arch ribs and contains the  lower  hanger  connections. The  bolted  box section increases safety by internal redundancy.

 

 Because tie girders carry tension and a loss of these members would result in catastrophic structural failure, tie girders are classified as fracture-critical. This weakness prompted a Federal Highway Administration (FHWA)  advisory in 1978, recommending  tie  girders   have  redundant  tie  members. Since that advisory, few tied-arch  spans have been designed until recently.

Engineers on the project addressed the FHWA advisory’s concern by separating each tie girder plate. To create the necessary internal redundancy,  the flanges were bolted  to the webs, using 8-in. by 8-in. angles. If a crack were to occur in either flange or web, it would not continue through the adjacent plates and result in loss of the entire section.

Network hanger   system   increases   redundancy.   During the  preliminary  design  phase, designers  compared  a network hanger system to a vertical hanger system and discovered the network hanger system provided increased redundancy, improved public safety and offered a 3% cost savings.

A network  hanger  system increases redundancy  by connecting  two  hangers  to  the  tie  girder  at  the  same  point and angling them  away from each other  (Figure 1), so they attach to different locations on the arch rib. Using this technique, forces from a hanger loss are distributed to the adjacent hanger, and the tie girder still is supported at the hanger location.

The hangers used on the network arch of the new bridge are ASTM A586 pre-stretched bridge strand.

Knuckles  are critical  to  thrust  transfer.  The knuckle  is where the tie girder and arch rib join, a critical component of a tied-arch  design. Because the flanges in this region are discontinuous, the web plates are the critical link and must be able to transfer the arch thrust force to the tie girder. For the Amelia Earhart Bridge, the knuckle consists of 2¼-in. web plates on each side of the arch rib.

With  the  knuckle  connections’  complex  geometry,  using angles to connect the tie girder flanges to the knuckle web plate was not an option. Instead, crews welded a vertical tab plate to the top and bottom flange plates to allow bolting to the knuckle web plate. Because the knuckle is capable of carrying the entire tensile force in the web, shear lag effects had to be considered in the transition  to the tie girder box shape.

Stringers framed into floorbeams address clearance restrictions. One of the biggest initial design challenges was securing the vertical clearance of approximately 52 ft over the river. The  grade on the Atchison side of the new bridge was fixed because of an existing intersection, so designers had to reduce the superstructure depth as much as possible. To achieve this, they framed the stringers into the floorbeams so that the top of the floorbeam and the top of the stringer align (Figure 2). By comparison, a typical arch floor system uses stringers running atop the floorbeam.

In coordination with the framed-in stringer concept, designers detailed the stringer-to-floorbeam connections  with slotted holesin one ply of the flange connection plate and the web (Figure 3). The  slotted holes occur at every other  floorbeam  and allow the structure  to elongate (bolts are only finger-tight)  during erection and slab placement without inducing axial forces into the stringers under  dead load. This  allows the arch to deflect into shape with the placement of the deck. After the majority of the slab placement occurs, bolts in the slotted connections  are fully tightened.

The resulting floor system of the new Amelia Earhart Bridge consists of floorbeams spaced at 15-ft intervals, corresponding to the location of the hangers  with stringers  spaced 8 ft, 3 in. apart. The intermediate floorbeams are 6-ft I-beam sections at the tie girder,  with the  top  flange following  the  deck’s slope. They are connected with 24-in. rolled beam stringers. The end floorbeam is a welded box section.

The new Amelia Earhart Memorial Bridge is the gem of the Atchison skyline— a signature  structure  that  pays homage  to its precursor  and serves as a tribute  to one of America’s greatest pilots.

Owners

Kansas Department of Transportation and Missouri Department of Transportation

Structural Engineer

HNTB Corporation, Kansas City

General Contractor

Archer Western, Chicago

Steel Team

Fabricator

Industrial Steel Construction, Inc., Gary, Ind. (AISC member/AISC certified Fabricator/NSBA member)

Steel Detailer

Tensor Engineering, Indian Harbour Beach, Fla. (AISC member/NSBA member)