short span segmental bridge

8/6/2019 Short Span Segmental Bridge

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Presents an overview of the design features andconstruction techniques used in some recent pre-cast prestressed short span segmental bridges inCzechoslovakia.

in the webs. Because of the relatively lowsegment depth, no work in the low cells ofthe box girders w as possible. Therefore, alltendons were anch ored in the upper part ofthe section. The tendons were formed bysix, 7-wire strands 0.612 in. (15.5 mm) indiameter anchored by their own anchoragesystem. The tendons are situated along theentire length of the bridge in both the up-per and lower faces of the sup erstructure.Since the stresses in the tendons after lossesare lower than permissible stress levels, incases of unexpected overloading, the pres-

tressing steel can act similar to mild steelreinforcement, thus limiting the width ofcracks in the joints. Note that the C zecho-slovakian Bu ilding C ode prohibits the useof unbon ded tendons.

OVERPASSESOverpass b ridges for field and forest roads

represent a specialized group of bridges.They are cha racterized by the fact that theright angle of crossing and the width be-tween railings are always constant and their

Fig. 1. Bridges (DS-W type) in Prague.PCI JOURNALJJanuary-February 1986 107


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Fig. 2. Bridge (DS -T type) across the Rokytka R iver in Prague .

span s vary only slightly. This mak es it pos-sible to standard ize them as w hole bridgesof variable spans.

The bridges were first designed as con-tinuous beams or slant-leg portal frames (seeFig. 4) with classical abutments. After ob-taining additional design experience, theabu tments were replaced with precast endbeams and prestressed tie rods. This newdesign then served as a model for designinga standardized bridge. Varying the spans ofthe bridge was made possible by eithershortening or omitting central or end seg-me nts (see Fig. 5).

The superstructure is assembled withsegments of double cell, trapezoidal box crosssections (Fig. 6 ) and end cross beams. Thesegments, with widths of 21 ft (6.5 m), arecast simultaneously with fascia, and end crossbeams with short wings. Diaphragms aredesigned in segments above the interme-

diate supports. The weight of typical seg-men ts is 17 tons. Fig. 7 show s a continuousbox girder bridge.

The bridge deck is supp orted by precastcolumns. Since the intermediate supportsare very slender, the stab ility of the b ridgeis secured in the transverse d irection w ithtwo tie rods su pporting the end cross beam(see Fig. 9).

The arrangem ent of the prestressing steelresulted from the designed method of as-sembly. During erection of the central span,the draped tendons placed in the webs weregradually tensioned. Next, the side spanswere erected and the straight tendons placedin the deck segments were tensioned. Thesetendons were then secured with continuoustendons after erection was completed (seeFig. 10a).

Construction of both continuous beam orslant-leg portal frame bridges is generally

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Fig. 3. Overpass bridge across the Highway Prague-Brno.

0

Fig. 4. Erection of side spans of overpass b ridge.

the same. Fig. 10 shows the erection se-quence of the latter bridge type. The cen-tral part of the superstructure is erectedsymmetrically from the middle of the bridgeon staging. Erection then begins with theplacemen t of the central segmen t onto thetop of four jacks. Next, the second and thirdsegments are temporarily prestressed to thefirst segmen t, supported by jacks, and post-tensioned by the draped tendons. Finally,the jacks supporting the first segment areremoved and the procedure is repeated (seeFig. 10b) until the segm ents reach the tem-porary towers situated near the slant legs.

The segm ents of the side span s are thenerected, forming cantilevers on ea ch side ofthe temporary towers (see Figs. 4 and 1 0c).Each segment is first temporarily pre-stressed to the already assembled structureand the straight tendons are positioned.Wh en the last segments are supported, the

end cross beams are temporarily pre-stressed and the continuous tendons aretensioned. After tensioning, the erected su-perstructure is dropped onto the interme-diate supports so that the static arrangementof the structure corresponds to the three-span continuous beam. The precast sup-ports are adjusted to the deck (Fig. 10d),the pockets in the foundations are cast, andtemporary towers are removed.

The continuous beam is also erected in asimilar way; however, the intermediatetowers are not necessary since they areformed by the piers. The erection of thebridge structure itself is carried out within2 weeks.

Since 1974, more than 40 overpasses ofthis type have been built and constructionof other structures is in progress. To d ate,the structural performance of all such bridgeshas b een excellent.

PCI JOURNAL/January-February 1986 09


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E D5300 6200 650x I I I I o0 250 500 75TTTt f _150;0) ( 500) ( 1 5 0 ; O )0

f_Ii0O_l400 2700=3000 L1100-1400I 800Fig. 5. Overpass bridges: (a) elevation of slant leg portal frame; (b) elevation of a continuous beam; (c) cross section.Note: 1 cm = 0.3937 in.


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1^ -t140 13U+ 143 2 143 1604_ _ T- X60 '

Fig. 6. Cross section of segment. Note: 1 cm = 0.3937 in.

Fig. 7. C ontinuous box girder bridge.PCI JOURNAL/January-February 1986 1 1


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Fig. 8. S lant leg po rtal frame.

Fig. 9. Slant leg portal frame End cross beam and tie rods.112


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a2'

(b 1

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(d

Fig. 10. System of prestressing tendons and erection sequence: (a) draped andstraight tendons; (b) continuous tendons; (c) erection of central span; (d) erection ofside spans; (e) adjustment of precast supports.

FOOTBRIDGE OVER THETRAMLINE IN BRNO

In planning the overpass crossing over thehigh speed tramline in the city of Brno, itwas n ecessary to design a footbridge with avery small horizontal curvature radius of 148ft (45 m) and variable elevation (see Fig. 11).Despite the complexity of the bridge's ge-ometry, it was possible to build the struc-ture using segmental construction.

The two-span footbridge was designedusing segments and end cross beams w hichwere man ufactured in the same forms as theabove mentioned overpass structures. The

side supports were formed by precast col-um ns. The column s of the lower end of thebridge are fixed while the columns of theupper end are hinged. The intermediatesupport is formed by a very slender octag-onal precast column (see Fig .15) which issituated eccen trically to the logitudinal a xisof the bridge with respect to the redistri-bution of torsional mom ent.

The requirement to use existing manu-facturing equipment affected the arrange-men t of tendon s and the erection process.For this reason, the deck was post-ten-sioned using continuous tend ons that weretensioned after the superstructure was

PCI JOURNAUJanuary-February 1986 113


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erected. The segments w ere placed contin-uous ly from the lower end of the structureto its upper end in successive cantilever.The static effects were controlled by tem-porary towers that supported each succes-sive segment.

Erection began with placement of the firsttwo segments on jacks situated on a shortstaging (see Fig. 12a). After post-tensioning(Fig. 13), the inside jacks were rem oved andthe end cross beam w as prestressed to them.The remaining segmen ts were then erectedusing the ca ntilever meth od of progressiveplacement (see Figs. 12b through 12d).During the erection process, the seg-

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o1Fig. 12. E rection sequence.PCI JOU RNA L/January-February 1 98 6

ments were gradually prestressed using 2x 2 continuou sly coupled cables placed inthe top flange. Since the h ydraulic jacks onlysupported the erected segment during itserection (inducing a very small force), thesuperstructure was stressed by only nega-tive mom ent load wh ich was later balancedduring post-tensioning. After erecting all thesegments an d the end cross beam (see Fig.12e), the pockets in the foundations werecast. Then, the superstructure was post-tensioned by the continuous tendons andthe temporary towers were removed. Theentire erection sequence can be seen in Fig.12.

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Fig. 13 . Post-tensioning of the first two segm ents.

Fig. 14. Erection of the end cross beam.116


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Fig. 15. Finished structure.

The footbridge was erected precisely inthe designed shape without any shims orconcrete joints. These excellent results en-abled progressive placemen t techn ology tobe u sed for other types of bridges.

DS-W BRIDGESThe DS-W b ridges are designed w ith box

girders transversely connected by the topflange (see Fig. 16 ). The shape of the struc-ture was designed on the basis of detailedstructural analysis' which showed the suit-ability of this type of structure for bridgesof various widths, skew crossings, and cur-vatures. This type of construction is alsobeing used to design bridges with complexgeometry conditions.

The box beam s consist of segments withthe dimensions shown in Fig. 17. The endsof the bridges are always perpendicular whilethe intermediate supports may be skew.These supp orts are formed by slender pre-cast column s connected to the deck by con-crete hinges. The hinge in the longitudinal

direction of the bridge m ak es rotation of thesuperstructure possible, and rotation in thetransverse direction ensures stability (see Fig.18).

Diaphragms are designed only in piersegmen ts. The typical segment w eight is 20tons wh ile the weight of the pier segm entsis 25 tons.

Originally, the first bridge structures w eretransversely connected only by post-ten-sioning recognizing the p ressure reserve inthe joints. Becau se of the relatively high liveto dead load ratio, the amount of prestress-ing was qu ite large.

For this reason, full-scale tests were car-ried out on the connection between twosegments. The test segments were sup-ported under their webs and then the 3.3ft (1 m) wide joint between flanges was rein-forced w ith mild reinforcing steel and cast.The segments were loaded not only by aconcentrated force placed on the deck , butalso by different positions of the supportscorresponding to the rotations and deflec-tions of the bridge girders.



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ED1 166 1 Q- - - 3 7 LL - '65 _

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CmC D0C DwCO0)

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mox 404t44 1407Fig. 17. Segment of DS-W type bridge: (a) cross section; (b) longitudinal section. Note: 1 cm = 0.3937 in.

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Fig. 18 . C onstruction of DS-W type bridge in Brno-Reckovice, C zechoslovakia.

The loading was repeated in several cyclesand finally the ultimate strength of the con-nection w as determined. The cracks w hichdeveloped u nder service load were smallerthan 0.08 in. (0.2 mm ) and they closed im-mediately after unloading. Also, sufficientultimate strength was found to exist overthe entire connection area.

For this reason, girders with an axial dis-tance of less than 21 ft (6 .5 m) are connectedonly with mild reinforcing steel. Segmentswith larger axial distances are connected w ithmild reinforcing steel and post-tensioning toreduce the secondary effects due to creepand sh rinkage of the concrete.

During erection, the box girders are con-tinuously post-tensioned using two systemsof tendons w hich distribute loading duringboth erection and service life of the bridge(see Fig. 19). Straight tendons are placed inthe deck slab and are u niformly distributedalong the deck width and anchored at theslab stiffening n ear the joints.

On e group of tendons (Cables B ) is situ-ated only near the supports, while the sec-ond group (C ables C) is gradually tensionedand coupled along the entire length of thebridge. The function of these tendon s du r-ing erection is to balance the neg ative mo-ment in the cantilever and, during placementof additional segments, produce uniformcompression of the intermed iate spans.

Also during erection, the draped tend ons(Cables A) are gradua lly tensioned from theface of the segmen ts which are being assem-bled.

The segmen ts are manufactured in formsdescribed in Ref. 3. Segment geom etry wasdetermined by a computer program andma nufactured according to the design levell ine and real measured values of segmentswh ich had previously been cast.

The superstructure itself is erected incantilever from one end of the bridge to theother. The segm ents are p laced in positioneither by a mobile crane which moves onland or by a portal crane moving on a tracksituated along both sides of the erectedbridge p ortion (see F ig. 21). Stresses in theassembled cantilever are reduced by u singone or two temporary towers to distributeloads.

The erection sequence is illustrated in F ig.20 and show s the assembly of the end andfirst intermediate spans. Sp an length is 8 2and 98 ft (25 an d 30 m ), respectively.

The end span is erected in three stages:1. The first segment is placed on the

abutment and temporary tower. After an-choring to the abutment, the second andthird segments are erected and prestressedwith Cables C (Fig. 20a). Then, the thirdsegment is supported and C ables A are ten-sioned.


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construction drectionAs^At3


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ED________ED _JLJ1HHIN.LiLED____Fig. 20. Erection sequence.

2. This stage includes th e erection of anadd itional three segments in cantilever (Fig.20b), the supporting of the sixth segmentand removing the support from the thirdsegment.

3. During the final stage, the last threesegments of the end span are erected incantilever after the rectification of align-ment is done on the temporary supports (Fig.20c). After supporting the cantilever withthe rectification su pport, Cab les A are ten-

sioned, the temporary support under thesixth segmen t is removed, the first span isrectified in elevation, and th e concrete hingeis cast (Fig. 20d).

The erection of the intermediate spans isalso carried out in three stages:

1. The first four segments are erected (Fig.20a ). After supporting the fourth segm ent,Ca bles A are tensioned an d the reaction ofrequired intensity is induced.

2. Erection of an additional three seg-122


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Fig. 21. Erection of DS-W segme nt by a portal crane.

men ts is carried out (Fig. 20f), the supp orttower under the fourth segmen t is removedand the reaction of required intensity is in-duced.

3. Finally, the last three segments areerected (Fig. 20g), the cantilever is sup-ported by a rectification support, Ca bles Aare tensioned, the temporary support underthe seventh segment is removed and thereaction correspond ing to the given stage oferection is induced an d the concrete hingesare cast (Fig. 20h).

The magnitude of the forces of the tem-porary support is determined so that themomen t above the support may constantlyhave the same value as the mom ent whichappears after erecting the first four seg-men ts. A compu ter then determ ines static

effects and monitors the structure duringerection.

The DS -W bridges were su ccessfully de-signed for two overpasses with very smallhorizontal curves of 230 and 427 ft (70 and120 m), respectively, and for four urbanviaducts in Brno. All the structures are per-forming superbly and the correctness of thestructural and construction details has beenverified.

DS-T BRIDGESThe DS-T bridges represen t the latest type

of construction developed a nd combine theadvantages of both simple manufacture anderection with low concrete and steel con-sumption. These bridges are comprised of



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UI.A 0Rte.IFig. 22. Arrangement of DS-T type bridges: (a) elevation; (b) plan; (c) cross section. Note: 1 cm = 0.3937 in.


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3 Q5 , q 9 5O 5^2

O 340{9 5 01 8 0 0-1630lion4 7 59Q50 0g0 ^5Fig. 23. Segme nt of a DS-T type b ridge: (a) cross section; (b) longitudinal section.Note: 1 cm = 0.3937 in.

open cross section segments which areformed by two girders: the deck slab andfascia beams (see Figs. 23 and 24). Eachsegment is reinforced by a cross beam atmidspan while the deck slab is strengthenednear the faces. Segmen t depths are 5.2 and6.6 ft (1.60 and 2 m) and various segmentwidths are obtained by changing the pro-jection of the cantilevers.

DS-T bridge construction was designedon the basis of a very detailed analysis byboth the grillage idealization and finite ele-ment methods.' The analysis also showedthat DS-T construction may be ap plied forbridges with sm all horizontal curves, 8 20 ft(250 m), and for bridges with the interme-diate supports skew. Fig. 22 illustrates atypical arrangement of a DS-T structure. Theends of this type of bridge are always per-pendicular and the abutment segments arereinforced with m onolithic cross beams nearexpansion joints. On slender, intermed iatemon olithic supports, the superstructu re isplaced with the h elp of the pot bearings.

DS-T structures are gradually post-ten-sioned during erection by two systems ofcables straight and drap ed tendons inthe same way as in the previously discussed

DS-W bridges (see Fig. 19). Furthermore,a finite element ana lysis show s that the a p-proaching of tendons to girders does not af-fect distribution of normal stress from thestraight tendons. Therefore, these tend onsare uniformly distributed in the deck forsimple manu facture.

The draped tendons are placed in the gir-ders and anchored in the top part of thesection. The segments are reinforced withmild reinforcing steel in the transverse di-rection of the bridge. Transverse prestress-ing was use d in only one case, nam ely, thebridge across the Rokytka R iver in Pragu e,where the segment w idths of 6 4 ft (19.50 m)were extended by monolithic fascias.

The segments weigh a maximum of 63tons and are m anufactured in a stationaryprecasting plant using a special method ofma tch casting. In contrast to the forms de -signed by Freyssinet International whichprovide a blank end and a soft mold bottomto enable the twisting of segmen ts, the newmethod utilizes equipment formed by twomutually independent elemen ts: a fixed formand a manipulator (see Figs. 25 and 26).After stripping, the segment cast is notshifted in the contact position on mold bot-



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Fig. 24. Segments of a DS-T type bridge.

tom, but is lifted, shifted, and placed on theman ipulator. The contact joint is always onthe sam e level as the contact segmen t andis pressed to the rear, perpendicular edgeof the form. Since the form bottom is al-ways plane, twisting is obtained by turningthe contact segmen t along its longitudinalaxis.

The form itself consists of two fixed frames,outside and inside shutters, and a movab lefront end. The frames are situated underthe girders and contain two squeeze-outrollers which, after stripp ing, slightly lift thesegmen t before it is removed from the form.The required segment shape is formed byturning the m ovable front end (see Fig. 27).The man ipulator makes it possible to adjustthe contact segment into a position parallelwith the rear edge of the form and to pressthe segment to the form to stabilize thatposition in casting.

Note that the manipulator is formed bytwo frames placed on top of each other sothat the lower frame enables the lifting ofthe manipu lator while the upper frame en-ables the turn ing of the vertical axis. Turn-ing the segm ent along its longitudinal axisis achieved by using different heights ofguiding inlays shaped identical to the

squeeze-out roller heads in the form.The production equipment is furthersupplemented with a device for guiding

the segment onto the ma nipulator, a walk-way, and trucks equipped with a w inch forpulling out the tubes forming the tendonducts.

The b asic operations involved are as fol-lows:

1. Placing the reinforcing cage into theopen m old2. Adjusting the contact segment on themanipulator

3. Adjusting the movab le front end4. Adjusting segment parameters5. Forming the tendon ducts6 . Casting the segment7. Taking out the tubes forming the ten-

don ducts8 . Measu ring the segment after casting9. Form stripping

10. Moving the contact segment intostorage and placing the newly castsegmen t on the manipulator

The reinforcing cage is assemb led insidea w ooden template where the tubes for thetendon ducts and the inserts for formingpocke ts for the tendon anch ors are placed.Con crete is supplied to the m old via a con-

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Fig. 25. Forms for DS-T type segments: (a) cross section of form; (b) cross section ofcontact segme nt on m anipulator; (c) longitudinal section; (1) m ovable front end; (2)outside shutter; (3) inside shutter; (4) frame; (5 ) manipu lator; (6 ) guiding device.

veyor belt. First, one girder is filled to thelower edge of the slab, and then the other.The slab with the cross beam is then cast,followed by the fascia. Next, the concrete iscompacted by means of internal and surfacevibrators. Hardening of the concrete is ac-celerated by steaming.

During the manufacturing process, muchemphasis is placed on the precise adjust-ment of segment dimensions and on sub-sequent segment measurements after casting.The values of segment depths and lengthswhich were adjusted and measured areshown in Fig. 27. Length was m easured us-ing a slide measuring gage and theodolite;depth values were determined by level-ling. The adjusted values were set by acomputer program which determined thedimensions of manufactured segments ac-cording to the designed level line and realmeasured values in segments w hich had al-ready been cast.

The superstructure of a DS-T bridge isassembled in can tilever from one end of thebridge to the other (see Fig. 29) and re-sulting stresses are controlled by using tem-porary supports (see Fig. 30). Thearrangement of the basic assembly compo-sition is illustrated in F ig. 28 .

Segments of common spans are erectedby a special portal crane, KPJ-90, whichtravels on the girders of the deck being as-sembled. The storage of segments behindthe abutmen t is operated by an other portalcrane travelling on track s situated along theabutment wings where the first threesegments of the end span are also assem-bled.

Fig. 26. Forms for DS-T type segments.PCI J OU RN AU January-February 198 6 27


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N

Fig. 27. Segment geometry.


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Fig. 28. Erection sequence for DS-T type bridges: (a) cross section behind abutment; (b) cross section by deck; (c) view from face oferected cantilever; (d) elevation; (1) portal crane; (2) portal KPJ -90 crane; (3) service walkway; (4) temporary su pport; (5) rectificationsupport.

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During erection of common spans, theportal crane turns the segmen t and shifts itin front of the KPJ-90 cran e wh ich is trav-elling behind the a butmen t. The crane thentakes th e segm ent an d lifts it to the face ofthe erected cantilever (Figs. 31 and 32). Afterthe segment is prestressed with four tem-porary tendons, the crane returns for thenext segment. The prestressing tendons aretensioned from the service walkway, inde-

pendent of the crane.However, since the erection sequence

depends on finishing the earthwork behindthe abutment and on transporting and as-sembling the KPJ-90 crane, some bridgesmust b e erected using the portal crane onlyif the above cond itions cann ot be met.

The erection process of individual seg-ments is the same as outlined for the DS-W bridges and illustrated in Fig. 20. At

Fig. 29 . DS-T type bridge at Valasske Me zirici during construction.

Fig. 30 . Tempo rary support of DS-T type bridge at Valasske Me zirici.130


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present, four segmental bridges with a totallength of approximately 0.6 mile (1 km) havebeen constructed and bridges with a totallength of 1.3 miles (2 k m) are currently un-der construction. In add ition, the attentiongiven to precise manufacture, careful as-sembly, and the effects of creep an d shrink -age has en abled bridges 1300 ft (400 m) inlength to be assembled w ithout steel shimsor concrete joints.

CONCLUSIONLow concrete and steel consumption, easy

man ufacturing, speed of construction, andbridge aesthetics demonstrate the advan-tages of using segmental construction for bothlong and short span bridges. In addition,the positive economic benefits of precastsegmental construction are m aking futuretechnological developments successful.

Fig. 31. Portal crane KPJ-90 on deck of bridge across Rokytka River in Prague.

Fig. 32. Erection of bridge segments across the Rokytka River in Prague.



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This technology has b een ap plied for thedesign of C zechoslovakia's first cable-stayedbridge across the Elbe River, where con-struction began early in 198 5. It is our hopethat others, too, will challenge current d e-sign practices and further develop this ad-vanced technology.

ACKNOWLEDGMENTThe design systems of the bridges de-

scribed in this paper were developed byEnterprise Dopravni Stavby in Brno,Czechoslovakia.

REFERENCES1. Mu ller, Jean, "Ten Years of Exp erience in

Precast Segmental Construction," PCI JOU R-NAL, V. 20, No. 1, January-February 1975,pp. 28-62.2. Podolny, Walter, and Muller, Jean, Construc-tion and Design of Prestressed Concrete Seg-menta l Bridges, John Wiley & Sons, New York,N.Y., 1982.3. Strasky, Jiri, "Segmental Bridges DS-W, DS-T Static Analysis and Structural Design,"PhD Thesis, Technical U niversity, Brno,Czechoslovakia.

short span segmental bridge

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