On a Beautiful Tilt

On a Beautiful Tilt

Detroit’s new Mexicantown Bagely Street  Pedestrian Bridge is the first cable-stayed bridge in the state and part of Michigan’s $230 million  I-75  Gateway  Project.  The  two-span,  cable stayed structure  crosses 10 ramps  and roadways, including  bothI-75 and I-96, and provides a vital link between the east and west sides of Detroit’s Mexicantown community.

The  total  bridge  length  is 417 ft, with a main span of 276 ft and a back span of 141 ft. The forestays are arranged in a fan con- figuration and are inclined in both the longitudinal and transverse directions. The bridge features a unique asymmetrical design, witha selected look of a single cable plane. A single 155-ft-tall inclined pylon provides the upper  support  for the  cables, which form an eccentric plane and are anchored  at the lower end to a tapered, trapezoidal, single-cell steel box girder.

The  back span balances the forces imposed by the forestays and anchors into a deadman/abutment. The welded steel, trapezoidal box girder carries the variable-width deck slab. The project incorporates five tuned  mass dampers  to control  vibration  of the  bridge  super- structure.  Each portion  of the  project,  including  abutments,  entry plazas, barriers, and fencing employs architectural  finishes with three- dimensional variations, and is therefore  highly stylized aesthetically.

The bridge lies on a tangent horizontal alignment. The western span expands from 15 ft, 3 in. to 21 ft, 6 in. while the shorter eastern span widens even more dramatically, from 21 ft, 6 in. to 34 ft. The pedestrian  walkway entrance  and exit grades of the vertical profile are at 5% grades and are connected  by a 200-ft crest vertical curve whose midpoint is located near the pylon. The minimum vertical clearance to the closest underlying roadway is 16 ft, 103⁄8 in. at the eastern abutment.

The structural  system—a single-cell box girder superstructure— is supported  at the westerly forespan by stay cables anchored  eccentrically to the girder shear center  at the northern girder web. The eastern back span is self-supporting and also transmits compression forces introduced by the westerly forestays to the east abutment.

The eccentric cable loading on the single box girder system produces torsion and lateral thrust in the girder and this is resisted by upward, downward, and lateral bearings at the west abutment and tension linkages and vertical and lateral bearings at the pylon. This figure shows that both vertical and lateral bearings are used to resist torsion. The pylon linkage and bearing system also allows translation produced mostly by thermal affects along the longitudinal axis of the bridge.

The concrete  and steel pylon is eccentric  in two directions and also tapers in two directions  from its base to its top. The  foundation,  at the base of the pylon, resists gravity loads primarily by a cluster of piles located at the line of action of the pylon. An extension of the foundation to the north helps to resist overturning loads created primarily by wind and live load effects.

Construction Activities and Scheduling

This bridge required a detailed erection manual and geometric control plan prepared by a specialty erection engineer.   The   erection   manual   outlined   62 individual stages for completing the bridge and closely followed the proposed erection  plan conceptualized  by the design engineer and included in the contract  documents.

To ensure  that  survey  discrepancies  would  be  mini- mized  and  resolved  quickly, the  project  team  agreed  to coordinate  all surveys. The  erection  engineer,  as part  of the erection  manual and ongoing  computations, provided target coordinates  and elevations for key points and eleva- tions,  including  at the  pylon  stay housing,  at temporary shoring,  at box girder  splices, along the  box girder  deck and at all stay cable connection points.

The contractor and the  owner/engineer closely monitored   the   geometry   throughout  construction.  In   one instance, the temporary guys were adjusted to correct the location of the pylon stem; however giving credit to the accuracy of the erection  engineer’s analysis, the geometry largely agreed with the predictions.

The steel  box girder  erection  scheme  required  three falsework towers to support  the west span before the stay cables were  installed.  The contract  documents  included camber  values to account  for these  three  temporary  sup- ports. Two additional  falsework towers  were provided  at each side of the pylon to support  the girder  prior  to the deck pour  engaging  the  vertical and lateral  bearings  and link plates at that  location. The contractor opted to complete the top of the pylon strut after placement of the steel box girder to mitigate  tolerance  and fit-up requirements of the girder itself and the many support  elements.

The first major step in the bridge’s construction was to build the abutments and the pylon. To help maintain alignment and provide support during construction and prior to installation of the stay cables and pylon post- tensioning, the pylon was temporarily guyed with four guys at two vertical levels. At each level, guys extended transverse and longitudinal to the bridge axis to maintain pylon stability and provide support in all directions.

Robert B. Anderson, P.E., is a senior structural engineer for URS Corporation, Tampa, Fla., and has been involved in the planning, design and construction of bridges for more than 21 years. Bob’s experience includes the design of five major cable-stayed bridges, several major freeway-to-freeway interchanges, as well as numerous water crossing structures.  Mike Guter, P.E., is a project manager for URS Corporation in Grand Rapids, Mich. He has worked on a variety of transportation projects since beginning his career in 1993, including roles in both construction and design. Victor Judnic, P.E., is a senior construction engineer for the Michigan Department of Transportation. His experience includes the design and construction of harbors, roads and bridges over the past 21 years.

Construction started  at the east abutment, which serves as the bridge abutment;  an earth anchor wall for five stay cables anchoring  the front span of the bridge; and an architectural plaza that transitions from the pedestrian bridge to a much larger non-structural plaza area. This abutment also was used as the temporary anchor point for the east temporary pylon guys.

Additionally the east abutment provided the fixity for the steel tub girder, with a diaphragm cast integrally  with the girder. The eastern end of the girder was temporarily supported on bearings to allow for beam rotation during the deck pour. The temporary bearings ultimately were encapsulated in concrete and the integral abutment connection was made complete.

The back span is fully supported by the east abutment and the pylon strut. The east abutment earth anchor wall is constructed on a 6-ft by 6-ft, 11-in. concrete     grade    beam that was integrally tied to the remainder of the abutment with steel reinforcement. The stay cables are anchored with steel forgings connected to post-tensioned anchor rods that include an end plate poured into the grade beam. A structural and architectural wall extends up from  the grade  beam,  supporting, hiding and protecting the anchor rods. This wall has aesthetic treatments including bush-hammered and board-formed surfaces.

The west abutment was the next substructure element to be constructed. A more conventional abutment, it includes three pot bearings supporting the steel box girder vertically (both upward and downward to prevent torsional rotation) and transversely.

On to the Superstructure

The east abutment earth anchor wall plate, pylon stay cable housing, and steel box girders are shop fabricated steel components. The earth anchor plate forms a base for the five east stay cable anchor blocks. This assembly was cast into the concrete earth anchor wall. Post-tensioned bars extend from the face of this plate into a concrete beam that sits underneath the earth anchor wall.

The pylon stay cable housing  is 30 ft, 6 in. in height  and forms the northern half of the top pylon stem. The stay housing includes five 2½-in.- thick plates with two pinholes  on the west side and one pinhole  on the south  side for connection to the  clevis-type stay cable anchorages. The placement of the  stay housing  was controlled  by levelling  nuts  on  18, 1½-in. anchor bolts poured into the pylon stem.

The box girder  remains  a constant  width from the west abutment  to just west of the pylon, at which point it begins to widen to its largest width at the east abutment. The top flange of the box girder  also varies from being solid across the top, in areas of high torsion, to being split into two flanges on top of each web. In areas where the top flange spanned from web to web, plates were added to provide bending strength to support the concrete  deck placement.  In addition, tuned  mass dampers  (see sidebar) installed between the west abutment  and the pylon required top flange openings to be covered with plates after installation.

Phase  2 (see construction phase diagram)  shows the  next major  construction  step involving the assembly of the steel box girder on both falsework and permanent supports.  Temporary support elevations were determined as part of the erection manual. Top of beam elevations were determined and checked against the anticipated elevations (which included the girder camber accounting for later deflections), and then screed rail elevations were established and the deck slab was cast in typical fashion.  Because fencing  fabrication  and  installation took longer  than expected, the contractor elected to temporarily  ballast the bridge with a uniform load consist- ing of wide-flange steel beams from their stockpile.

The most complex part of the deck slab construction was developing a unique deck forming system to construct the large overhangs (see diagram, Phase 3). The contractor devised a formwork system suspended beneath the bridge and hung from the box girder flanges. HP12×53 sections were the primary members spanning from flange to flange and overhanging each side from which forms were supported.  The hangers counteracted torsional forces of the asymmetrical deck overhang by using steel plates as beams extending across the top of the box girder. Uplift forces were counteracted at the center of the beam with a welded tie- down at the solid top flange and cable tie-down at the open flange sections of the box girder. The overhang forming was set approximately ½-in. high to account for the anticipated deflections of the overhang during the deck pour. To maintain the freeway opening date of July 4, 2009, the deck slab was poured during February 2009, an unusual occurrence this far north.

Phase 4 involved the installation and stressing of stays in a balanced fashion at both the west and east sides of the pylon. Each of the 15 stay ends (10 in the forespan connected to girder and five connected to the east abutment back stay) had a targeted force. Some of the stays required only a single jacking operation  within the erection manual sequence,  while  others  required  two  jacking  operations at different  stages within  the  erection  manual  sequence. During the jacking operations and upon completion, the geometry  of the system was verified. As the installation of the permanent stays progressed, the temporary guys were removed. Also, the stressing of the stays caused a decrease and eventual lift-off at the temporary falsework supports. At Stages  47, 53, 54, and  60 of the  erection  engineer’s detailed  construction sequence,  the  vertical  pylon  post- tensioning  was installed and stressed in stages.

Working with the Stays

The  stay cables consist  of galvanized  steel  wire rope comprising structural  wire (ASTM 586) with a hot-dipped galvanized Class A coating  for the inner  wires and Class C  coating  for  the  outer  wires. One end  of the  cable at the  pylon  used  a pin  and  clevis anchorage  system. The forestays used a threaded spanner  nut  anchorage  system at the  girders. The back stays were anchored at the  east abutment  with a steel anchor block casting and shims that were tensioned with four anchor rods each embedded into the abutment  mass. The clevises and other hardware were manufactured in a lost sand casting process (ASTM 148). The stay cable socket-to-strand connection was accomplished by splaying the wire rope and pouring molten zinc into a conical shaped space. The sockets are designed and attached to develop 110% of the breaking strength of the cable. All sockets were proof tested to 55% of the breaking strength of the cables. The lengths of the cables were determined by survey following completion of the  pylon and erection of the girder. Tolerance was provided in the threaded socket length to account for construction tolerance and temperature compensation.

Because  galvanized  wire  rope  is  able to wick moisture  within  the  cables, painting of the stay cables was included  in the contract   to  prevent  the  ingress  of  water and  corrosive   elements.   However   painting  subsequently   was  eliminated   due  to concerns  about  the  long-term effects  of locked  moisture  and  oils  on  the  coating system.  Serrated nuts were added at the low cable anchorage at the west girder and weep holes to the anchor Wt’s attached to the girder webs to facilitate the draining of water from the cable bottoms.

The first step in field installation is to unwind the cables from wooden shipping spools. Because of their length weight, this can be an awkward operation and requires special attention to avoid unwinding the spools too fast. To protect the galvanized coating on the cables, they are laid out on a protected   surface. The  cables  are  then lifted  by two  cranes  and  attached  at  the pylon stay cable housing, then  attached  at the girder or abutment.

For the Bagley Street Bridge’s western forestays, workers installed a threaded stressing rod into the anchor socket, along with a temporary extension rod, which allowed stressing by a single center-hole jack. Once the proper tension was achieved, a ring nut was spun tight.

At the east abutment, a stay cable anchorage block was positioned   through four threaded   anchor   rods.  These   rods were stressed to  a prescribed  tension  by four   center-hole   jacks   bearing   against nuts on the rods and the anchorage  block. Once the tension was achieved, shims were installed between the stay cable anchorage block and the base of the abutment earth anchor wall. Final tensioning of the anchor rods, with the shims in place and preventing the cables from  being  further  loaded, was done to a high load level to ensure the shims remain in compression.

Calibrated jacks were used to stress the cables.  However, the stay cable supplier used a secondary “tensionometer” to monitor the stay cable tension in all the cables after each was stressed. This device accepts inputs of cable density and length and has an accelerometer sensor that is able to mea- sure the primary frequency of a cable. The cable is forced to vibrate by field personnel. With these vibration inputs,  the  tensionometer  outputs  a measured force obtained by classical equations that are programmed into the equipment.

Finishing Up

The bridge finishing works shown in Phase 6 include the final steps  involved in the construction of the bridge. These included verification of cable forces and girder   and  pylon  geometry;   installation of  the  architectural   fence;  installation  of a cable tie at the forestays to reduce stay cable  vibrations;  and  final  tuning  of  the tuned mass dampers.

Other  tasks included  in the  bridge  fin- ishing works were:

• Removal  of steel  ballasting  and  construction of concrete barrier rail                   
• Sandblast finishing of the pylon, deck overhangs, and barrier rail
• Installation of modular expansion joint at the west abutment
• Concrete benches  at  the  pylon  and east abutment  and decorative concrete treatments
• Completion of lightning   grounding system
• Installation and aiming of decorative lighting
• Grouting of   east   abutment    post- tensioned stay cable anchor rods
• Installation  of architectural edge plates and fence mesh covering  at east abutment

The Mexican town Bagely Street  Pedestrian Bridge opened May 5, 2010. It was part of a Cinco de Mayo festival organized by the Southwest Detroit Business Association and the Detroit Consulate of Mexico in salute of 200 years of Mexican independence.

A project  of  this  magnitude  relies  on contributions   from    many    individuals; therefore, the authors also wish to acknowledge and give their sincere appreciation  to Bob  Jones  and  Josh  Goldsworthy  (Walter  Toebe Construction), Dave  Rogowski (Genesis Structures),  Eric Morris and Ken Price  (HNTB),  Jerry  Clodfelter   (CBSI), Jorge  Suarez  (Michael  Baker  Corp.)  and Peter Bugar (URS).

Owner

Michigan department of transportation

Architect

Van tine/Guthrie studio of Architecture, Northville, Mich.

Design Engineer

HNTB Corporation, Chicago

(AISC Member)

Construction Inspection Engineer

URS Corporation, Grand Rapids, Mich., and Tampa, Fla.

Specialty Construction Inspection

Michael Baker Corporation, Pittsburgh

Steel Detailer

Tensor engineering, Indian Harbour

Beach, Fla. (AISC and NISD Member)

Steel Fabricator

Industrial Steel Construction, Gary, Ind. (AISC and NSBA Member)

Erection Engineering

Genesis structures, Kansas City, Mo. (AISC Member)

General Contractor

Walter Toebe Construction, Wixom, Mich.

Structural Software

Lusas Bridge

Stay Cable System Suppler

CBSI, Houston