A Prestressed ConcreteRailroad Bridge on

the Prairieby

Jon K. JenningsEngineerNorth Dakota Concrete

Products Co.(A Member of the Cretex

Companies, Inc.)Bismarck, North Dakota

• ying in the center of North Dakota, justa short distance north of Fort Lincoln

(where General Custer began his fatefuljourney to the Little Bighorn), the historiccity of Mandan has preserved its, Old Westlegacy, in part, through an extensive reno-vation of Fort Lincoln. Situated at theintersection of Interstate 94 and the Mis-souri River, Mandan is also known as arailroad town. Indeed, the town has one ofBurlington Northern's major switchingyards located in the heart of the city. Theconfluence of these railroad tracks and thetraffic between Fort Lincoln and Mandanhas brought about the construction of thefirst prestressed concrete railroad bridgein North Dakota.

Built in 1943, the original underpass onNorth Dakota's Highway 1806 allowed foronly two lanes of vehicular traffic, and thevertical clearance of the structure re-stricted its use by many trucks. However.the decision to replace the old structureposed several problems for the designers.

• The existing structure was located onthe mainline of the Burlington NorthernRailroad. Coal trains, many 100 units inlength or longer, used the tracks each day.Therciore, it was imperative that traffic bemaintained on three tracks at all timesduring the construction period.• Elevation changes would not he al-lowed in the new design. The site is at theend of Burlington Northern's switchingyard: large displacements of the trackswould not be feasible.• The existing underpass surface eleva-tion was only 2 ft (0.t+1 in) above theground water. To provide adequate verti-cal clearance, the new roadway had to bebuilt below the water table.• In 1989, North Dakota residents wouldhe celebrating the state's centennial an-niversary. The crowds expected for theJuly celebration could triple the normalpopulation of Mandan. and a major con-struction project would not be allowed inthe downtown area at this time.

26

The first prestressed concrete railroad bridge inNorth Dakota spans 72 ft (21.9 m) and is built on 25AASHTO Type IV girders. Completed 9 monthsahead of schedule, the structure was made ready inplenty of time for the state centennial celebration.

Fig. 1. The city of Mandan, known for its Old West atmosphere and as a railroad town,now has the distinction of having the first prestressed concrete railroad bridge in NorthDakota.

Historically, railroad structures inNorth Dakota have been built with struc-tural steel. However, after consulting withBurlington Northern officials in I)enver, itwas determined that a prestressed con-crete structure would be a viable alterna-tive. Preliminary estimates by the bridgedesign division revealed that the concretealternative was, by far, the most economi-cal solution, and the steel design wasabandoned.

The new bridge, designed for a CooperE-8U Railroad live loading, would he asingle span structure 72 fl (21.9 m) inlength. and it would allow for four lanes ofvehicular trafttc. An H f't (2.44 m) pedestrian

walkway on each side of the roadway wasalso included in the design. With 25AASH'I'O 'Type IV prestressed concretegirders, each 54 in. (1372 mm) deep, thestructure was designed to carry four rail-road tracks, one mainline and three spurtracks, for a total width of 81 11 (24.7 ml.See drawings for details.

To accommodate train traffic while thebridge was being built, two shoofly (tem-porary) structures were let to contract inthe spring of 1987. These were placed oneach side of the existing structure, withample room reserved for construction ofthe new bridge. Construction of the shoo-fly structures was completed in the fall of

PCI JOURNAL]May-June 1959 27

ABUTMENT NO.l ABU7MEf1T N0,2

ry

SURVEY -

r I F__1

11 EkrSt A r lb FACE

re'•s" 11 rw 1 I

r̂ N̂Sr 3S s ^ 1iNC'W O`CL F 1 f^48l.F, W4Ll 18r6• _ I

F

•I

MAIN LINE r

o !

TRACKI

I 1 ffr r! 1

rc TRACK 2 + r -

1 ^ Mlst. w.

1 /

E

7RACN 3 +r1 I-

Ljj

"qLt

T/1

!+ t

I r 1

+ f -

i

r 1 1

1•

r1 Ti m y f r+ +

PLAN —jSTING RETAINING MALL

TYPICAL RAIL 8 BALLAST SECTION

soIOECK REINFORCEMENT

NOT SHOWN)

RRUP9

IO 24 SP AT S0^ I2S S4"CONC. 7'BEAMS 1 Ip"

RR II PS

=BARS

SECTION AT c. BEAM

0Cz

wL

fp

co

<D

END VIEWGIRDER ELEVATION

IVCO

Fig. 2. Each of the 25 girders used in this project is 54 in. (1372 mm) deep and has aspecial thickened end block.

1987 and work on the new structure wasscheduled for the spring of 1988.

The citizens of Mandan, concerned thatthe project would interfere with the cen-tennial celebration, wanted the project tohe completed by July I, 1989. the date thecelebration was scheduled to begin; theheavy traffic occasioned by the celebra-tion would be unmanageable if it had to bedetoured around the underpass. This tighttime frame dictated a healthy incentiveclause in the contract for an early comple-tion. An armiunt of $30(K) per day would hepaid for openin g the overhead to rail trafficbefore October 15, 1988. and $5000 perday would he paid for opening NorthDakota 1806 to vehicular traffic prior toAugust 15, 1989.

Before the actual construction of the re-placement bridge could get started, thewater table had to he lowered. The watertable was approximately 10 ft (3.05 m)above the finished roadway, and severalmethods of solving the perpetual problemwere explored.

Continuous pumping of the water wasone method considered. This would requirethat 500,100 gallons (1892.5 m3 ) of waterhe pumped each day for the next 1(1(1years. However, the prospect of constantpump and pipe repairs. a required back-upsystem. and the unknown effect on thecity water supply made this solution unap-pealing. Another alternative was to use asteel sheet piling cofferdam which wouldencircle the entire underpass, but anyfaults in the system would be impossible topinpoint and repairs would he extremelydifficult and expensive.

The ultimate solution was a large con-crete ''bathtub." The structural designwas straightforward, the tub could hebuilt using conventional materials, andrepairs could he made from the sur-firce. The slab was designed to he 350 ftlong x 75 ft wide (106.7 x 22.9 ml, varyingin depth from 2 to 8 ft (0.61 to 2.44 m). The485(1 cu yds (3708 m^) of concrete sealwould he totally encased with 3743 sq yds(3130 m 2 ) of 60 mil (1.52 nim) waterproof-

;I]

Fig. 3. A large concrete "bathtub" was created to lower the existing water table. Here,workers are pouring the seal slab.

Fig. 4. The concrete seal was encased in a waterproofing system.

PCI JOURNALiMay-June 1989 31

Fig. 5. Trucks delivered girders to the jobsite. The timing of the deliveries was crucialbecause train traffic could not be delayed.

ing (Bituthene 3000 waterproofing systemwith PVC waterstops). Because of themassive pours required, the specificationscalled for a maximum concrete ternpera-ture of 6(PF (16°C) and a minimum hluce-rnent rate of l00 cu yds (76 m-1 ) per fir.

With these preliminary problems ad-dressed, and with the conditions of thecompletion times determined, the under-pass was let to contract on March 25,1988. The low bidder was Industrial Build-ers, Inc., of Fargo, North Dakota. and thecontractor was on the jobsite in less than amonth to begin construction.

Careful attention to the details and de-livery schedules of all materials helped thecontractor maintain an aggressive time-table. In addition, a hot, dry summer fa-cilitated the use of two construction shiftsto work around the clock.

The prestressed concrete girders wereproduced in Menoken, North Dakota,which is about 20 miles (32.18 km) fromthe jobsite. The 54 in. (1372 mm) girders

proved to be an excellent choice: thedepth was the minimum needed, and theweight allowed for a lower impact factorthan the comparable steel design. A spe-cial thickened end Klock was used. Provi-sions were made in the girders to attachcast-in-place diaphragms at the thirdpoints.

The detensioning strength of the con-crete. 5500 psi (3R MPa). controlled thedesign. With only two girders produced ateach setup. it was imperative that the de-sign strength be attained overnight. AType III cement with superplasticizerwas used to achieve this goal. The averagestrength attained after I day was 6282 psi(43 MPa). and the average strength at 21days was 7112 psi (49 MPa). [he pre-stresser worked 6 days each week to meetan anticipated July 15 delivery date.

On July 25, the girders were delivered tothe jobsite. Careful coordination with therailroad was required because the shooflystructures were used to situate the trucks.

32

Fig. 6. To help the cranes in the unloading process, the delivery trucks were driven on theshoofly

Fig. 7. Two cranes were used to unload each girder.

PCI JOURNALMay-June 1969 33

Fig. 8. After this photograph was taken, the final four girders were put in place. All 2fgirders were placed in position within 2 days.

:.w iE

Fig. 9. Stay-in-place forms were used to hasten deck construction. Shown here is thedeck steel in place prior to the pouring of the concrete.

34

Fig. 10. North Dakota's first prestressed concrete railroad bridge was opened to rail trafficon August 16, 1988, and the underpass was opened to vehicular traffic in November.

Two cranes were used to set each beam inplace, and all 25 girders were in placewithin the next 2 days.

Stay-in-place deck forms were used tohasten the deck construction. Within 5days, the diaphragms had been formed andpoured, the deck had been formed, and thereinforcement had been placed, ready forthe deck concrete. The deck was placed inI day. Within another 8 days. the deckhad attained a concrete strength of 5000psi (34 MPa) and the superstructure wasready for ballast and tracks.

On August 16. 1988 — only 144 daysafter the project was let — rail traffic wasback to normal in Mandan. On November12, the entire project was completed, 9months ahead of schedule. The cost of thestructure was approximately 57 million,which includes a SI.5 million bonus forearly completion and $700,000 for theshoofly structures.

The designers of this project had wanteda structure that could be constructedquickly and shalt would he maintenance

free. Prestressed concrete has proved tobe the right choice. The citizens of Man-dun, who helped drive the piling, pour theconcrete and nianufactur-e the girders,have a structure of which they can hejustifiably proud as they prepare for theircentennial celebration.

CreditsOwner: State of North Dakota and City of

Mandan, North Dakota.Engineer: North Dakota State Highway

Department. Ray Zink, chief engineer:Forest Durow. bridge engineer: ()avidLeer, design engineer: and L)oug Fer-cho, project engineer,

Contractor: Industrial Builders, Inc.,Fargo. North Dakota. WarrenDiederich. owner: and Ron Langeberg,project superintendent.

Precast Prestressed Concrete Manufac-turer: North Dakota Concrete ProductsC:o. (a Member of the Crctcx Com-panies. Inc.), Bismarck, North Dakota.

PCI JOURNAL/May-June 1989 35