The Iowa River Bridge onRelocated U.S. 20A Launched Steel I-Girder StructureDAVID M. ROGOWSKI P.E. - HNTB CORPORATION, KANSAS CITY, MISSOURINORM MCDONALD P.E., RON MEYER P.E. IOWA DEPT. OF TRANSPORTATION,AMES, IOWA

ABSTRACT

This paper discusses the environmental issues and restrictions surrounding the proposed site of the Iowa RiverBridge crossing and then focuses on the difficult site accessibility issues, the proposed sequencing of thesubstructure construction and the launched construction techniques proposed for the erection of thesuperstructure units.

PROJECT DESCRIPTION

For years the portion of Highway U.S.20 throughHardin and Grundy counties of middle Iowa hasconsisted of two lane county blacktop roads weavingthrough many small communities. The primaryreason for the discontinuous alignment has been theenvironmentally sensitive Iowa Greenbelt in theheart of Iowa. The Greenbelt, which runs along thebanks of a 50-mile stretch of the Iowa River, ishome to many endangered species of plants,animals and aquatic life:

Bald eagles occupy several winter roosting areasthat provide the needed protection from winterstorms.

Several rare species of mussels, some found

only in the river waters of the Iowa Greenbeltarea, are very sensitive to purity andtemperature changes to water quality.

Natural crop sites of the Monkshood plant, a

federally protected poisonous herb, exist in theriver valley in addition to the many plant speciescommon to the wetlands of the riverbanks.

In addition to the many environmental concerns,poor sub-soils, slope instabilities and difficult siteaccessibility issues add to the complexities ofconstructing a bridge in the area.

To complete the new U.S. 20 alignment, a relocatedportion of this four lane highway would need tocross through the Greenbelt and bridge the 460 mIowa River valley thus permanently affecting one ormore of the environmentally sensitive areas of thisecosystem. Corridor studies began in the early 70showever with the evolution of new environmentalregulations came new reasons to delay the project.Finally in 1996, the site was finalized and sixalternative structure types and erection methodswere evaluated.

The Iowa Department of Transportation preferred alow profile structure to minimize the visual impact ofthis scenic area. Thus the selected alternativeconsists of a weathering steel I-Girdersuperstructure erected as two parallel 12.0 m widedeck structures each consisting of five equal spansof 92 m. Each deck structure will consist of a 230mm concrete slab with a 38 mm low-slump concretewearing course supported by a system of four 3450mm deep I-girders spaced at 3600 mm centers. Thedeck structures will be supported on cast-in-place

Artists rendering of Proposed Iowa River Bridge

reinforced concrete substructure units consisting oftwo column bents ranging in height from 20 m to 38m and founded on drilled shafts or driven H-piles.

To minimize the impacts to the environment and tofacilitate construction of the bridge, a launchederection method is proposed for the deck structures.This erection method will allow the steel portions ofthe superstructure to be constructed behind one ofthe bridge abutments and pushed (or pulled), withdeck forms and reinforcing in place, from pier to pieracross the river valley.

CONSTRUCTION DIFFICULTIES

The selected site for the new, river crossing posedmany construction challenges for the designers:

Due to the environmental concerns, numerousconstruction mitigation restrictions wereestablished and subsequently became morerestrictive as the final design of the projectprogressed. These restrictions included:

Prohibited construction activities on the westslopes (near the eagles roost) during a

Winter shut-down period betweenNovember 1st through April 15t

Monitored construction activities on the east

slopes during the period between November1st and April 15th with the possibility of ashut down at any time if it is determinedthat the noise and/or activities disrupt theroosting habits of the eagles.

Prohibited use of causeways and/or

temporary bridges to cross the river. Protection of all equipment used in the river

valley by a containment system designed toprevent fluid spills (fuel, oil etc.) fromreaching the river.

Limited construction activities which must

work around allowable zones ofdisturbance ranging from clearing andgrubbing as required at selectedconstruction access ways to selectiveclearing (with chainsaws only) and nogrubbing in zones containing protectedtrees whose eventual mature heights wouldnever exceed heights within 5 m to 10 mbelow the girders.

Defined drainage paths and/or distributionsystems to minimize the run-off down theside slopes from disturbed areas of theconstruction site, approach roadways andthe eventual final bridge. Note: Pathsand/or distribution systems are designed todirect rainwater (and road salts) to pre-constructed sediment basins on each side ofthe river.

Removal of all excavated materials including

drilling material and spoils from the drilledshaft operations to selected disposal sightsabove the limits of the river valley.

Due to the remote site location in middle Iowaand a delayed bidding schedule which will placethe contract letting for the bridge on the samedate as the adjacent grading packages, the onlyinitial access route to the site will be bytemporary improvements to existing countyroads and private drives.

Due to the tight timeframe for the bridge

construction, the restricted winter shut downperiods and the delayed letting schedules, thebridge contractor will be required to beginconstruction at the two main piers on the westslopes followed by the two main piers on theeast slopes working outward towards theabutments all while allowing the gradingcontractor to construct the approach roadwaysconcurrent to the bridge construction.

LAUNCHED ERECTION METHOD

With the anticipation of limited constructionequipment access into the river valley and with theenvironmental restrictions enforced, steel erectionfrom within the valley was ruled out by theengineers and a launched erection sequence wasselected as the method of construction. Withstringent winter shut-down restrictions on the westslopes, the bridge is designed to be launcheddownhill along the 0.5% grade from behind the eastabutment pushing with the use of hydraulic thrustpistons (or pulling with the use motors, cables andsheaves) towards the west abutment.

To facilitate the launching operations, a largelaunching pit excavated behind the east abutment isanticipated. Although the eventual depth of this pitwill ultimately be determined by the type of rollersystem utilized by the contractor, the girder depth

plus one meter is assumed for design. Althoughcontractors means and methods will also determinethe eventual length of this pit, a minimum pit lengthof 150 m (which provided sufficient backspanerection to counterbalance the cantilevered portionof girders) is assumed for design.

To determine the relative magnitude of thelaunching and braking forces, 10% and 2.5%coefficients of friction are assumed respectively. Aroller bearing manufacturer was contacted duringthe design process. Specifications for these bearingsindicate a recommended design coefficient of frictionof 5%. Thus by doubling the recommendation, aconservative over-turning force could be applied tothe substructure units as well as provided in thedesign documents. Conversely, by assuming aminimum coefficient of friction equal to half of thatrecommended, a conservative required braking forcecould be provided in the design documents.

To minimize the amount of construction equipmentrequired in the valley after launching, all deck forms,slab reinforcing, steel girders, miscellaneous bracingand the enclosed drainage system are assumed tobe launched as a complete system. Initially erectionruns indicated that the lead cantilever would deflectover 4500 mm when the loaded girders extendedthe full 92 m. However, the deflections werereduced to 3000 mm by adding a trussed launchingskid and by removing the deck forms and slabreinforcing from the cantilevered portion of thelaunch.

To accommodate the deflection of the cantilever, asystem of 275-ton hinged roller bearings at eachgirder centerline at each pier are assumed. Thetrussed launching skid is designed with the bottomchord sloping upwards a distance slightly greaterthan the calculated 3000 mm deflection. It isanticipated that this upward slope will allow thelaunching skid to land at the cantilevered end and beguided back to vertical alignment as the skid andgirders are pushed (or pulled) across the span.

To provide torsional stability to the lead cantilever, asystem of longitudinal X-bracing members areprovided at the top and bottom flanges in the centerbay of the four girder launch. The longitudinal X-bracing in conjunction with the typical lateral K-bracing and the girders form a torsionally stiff corefor each of the two deck structures launched.

A three dimensional model of the entire bridge(including substructure and superstructuremembers) was created to check the final in-placegirder and pier designs against the temporary designload conditions experienced during the launchingoperations. Numerous trial runs were attempted todevelop a launching sequence that coulddemonstrate that temporary erection stresses wouldnot govern the final in-place stresses for both thesubstructure and superstructure members. Themodel was able to accurately depict the suggestedlaunching sequence proposed in the contractdocuments and as noted below:

i. After completion of pier construction,installation of permanent pot bearings,temporary erection roller bearings andexcavation of the launching pit, begin erection ofdeck structures including the steel girders, allmiscellaneous bracing, framing for the drainagesystem, deck forms and launching skid.

ii. Begin launching the south deck structure by

pushing against a temporary reaction pier (orpulling against a permanent pier). After thecantilevered end has landed on the temporaryroller bearing, been guided back to verticalalignment and temporarily set on the permanentpot bearing to focus the dead load on the piercenterline, launch the north deck structure in asimilar fashion.

iii. Repeat this staggered launching sequence

until the both decks have been completelylaunched across all five spans.

iv. Transfer loads to permanent pot-bearings

and disassemble the launching nose.

Although provided as a Suggested ErectionSequence, the sequence shown in the plans wasmodeled and designed with the full anticipation thatthe contractor could follow the steps as noted.However, the contractor will ultimately have theflexibility to recommend alternate sequences basedon preferred means and methods.

SUMMARY

When evaluating the proposed erection methods fora structure, many factors outside the control of theengineer may govern the direction of the design. Inthe case of the Iowa River Bridge, theenvironmental, geotechnical, site accessibility, andconstruction schedule concerns played a significantrole in the decision to construct the bridge using aunique and challenging method of erection. Byproposing the launching of the steel I-girder deckstructures, the designers minimized the impacts tothe environment and developed the groundwork forconstruction of a large bridge at a site with difficultaccessibility issues. This, however, did not comewithout a steep price tag. The estimatedconstruction cost of the project is now at$21,000,000. Of this estimated value, 15% to 20%of the costs can be directly attributed to design anddetailing considerations that were added toaccommodate the environmental concerns and siteaccessibility challenges.

ACKNOWLEDGEMENTS

Iowa Department of TransportationOffice of DesignSandra Larson Deputy Director

Iowa Department of TransportationOffice of Bridge DesignGary Novey Assistant Bridge Engineer

A Launched Steel I-Girder StructurePPROJECT DESCRIPTIONCONSTRUCTION DIFFICULTIESLAUNCHED ERECTION METHODSUMMARYWhen evaluating the proposed erection methods for a structure, many factors outside the control of the engineer may govern the direction of the design. In the case of the Iowa River Bridge, the environmental, geotechnical, site accessibility, and constrACKNOWLEDGEMENTS