Short Span SegmentalBridges in Czechoslovakia

Jiri StraskyChief Design EngineerDopravni StavbyDesign and Construction EngineersBrno, Czechoslovakia

Miroslav KorenekStructural Engineer

Dopravni StavbyDesign and Construction Engineers

Brno, Czechoslovakia

R ecently, two distinct types of precastprestressed segmental bridge design

have been developed in Czechoslovakia.These designs are suitable for both urbanand highway bridges with spans 'ranging upto 150 ft (45 m). The first design (Fig. 1)consists of box segments transversely con-nected by the top flange while the secondalternative (Fig. 2) utilizes segments of open,double-tee cross sections.

The development of these bridges waspreceded by the successful construction ofseveral segmental overpasses (Fig. 3). Al-though these bridges were built under dif-ferent social and economic conditions, someof their structural features and constructionmethods have interest and applicability toprojects in other parts of the world.

The following features are common to allthe bridge structures described herein:

1. The superstructure consists of precastmatch-cast segments with a specified cubestrength of about 7000 psi (50 MPa). Short-line horizontal casting was used to produceall constant-depth segments while adjusta-ble forms allowed for both horizontal andvertical curves and variations in superele-

vation. The segments of the box cross sitions were cast in forms similar to thedeveloped by Campenon and Bernard. " 2 Tdouble-tee members were cast in forms iing a new concept. The segments are c.in one-day production cycles and the curiof the concrete was accelerated by steamithe freshly cast components.

2. To control temperature and humidvariations, deicing salts, and different ereand shrinkage ratios of the segment slyand webs, the joints between segments wecovered with epoxy resin during assembAlso, the faces of the segments were pivided with multiple keys and erectedcantilever. This made it easier for the picast segments to adjust to their proper 1sition after temporary post-tensioning.

3. The erection process was designedsuch a way that the structural configuratiof the bridge would not be affected by t

redistribution of creep caused by volumestress changes.

4. During the course of erection, theperstructure was gradually post-tensionby two systems of tendons: straight tend(placed in the deck and draped tendons plac

106

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 was possible. Therefore, alltendons were anchored 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 superstructure.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 Czecho-slovakian Building Code prohibits the useof unbonded tendons.

OVERPASSESOverpass bridges for field and forest roads

represent a specialized group of bridges.They are characterized 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

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

spans vary only slightly. This makes it pos-sible to standardize them as whole 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, theabutments 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-ments (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-ments is 17 tons. Fig. 7 shows a continuousbox girder bridge.

The bridge deck is supported by precastcolumns. Since the intermediate supportsare very slender, the stability of the bridgeis secured in the transverse direction withtwo tie rods supporting the end cross beam(see Fig. 9).

The arrangement 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

108

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

0

Fig. 4. Erection of side spans of overpass bridge.

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 theplacement of the central segment onto thetop of four jacks. Next, the second and thirdsegments are temporarily prestressed to thefirst segment, 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 segments reach the tem-porary towers situated near the slant legs.

The segments of the side spans are thenerected, forming cantilevers on each side ofthe temporary towers (see Figs. 4 and 10c).Each segment is first temporarily pre-stressed to the already assembled structureand the straight tendons are positioned.When 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 date,the structural performance of all such bridgeshas been excellent.

PCI JOURNAL/January-February 1986 109

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

Fig. 7. Continuous box girder bridge.

PCI JOURNAL/January-February 1986 111

Fig. 8. Slant leg portal frame.

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

112

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 necessary 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 whichwere manufactured in the same forms as theabove mentioned overpass structures. The

side supports were formed by precast col-umns. The columns 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 eccentrically to the logitudinal axisof the bridge with respect to the redistri-bution of torsional moment.

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

PCI JOURNAUJanuary-February 1986 113

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Fig. 11. Footbridge over tramline in Brno: (a) elevation; (b) plan; (c) cross section over upper side support;(d) cross section over intermediate support. Note: 1 cm = 0.3937 in.

erected. The segments were placed contin-uously 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 removed andthe end cross beam was prestressed to them.The remaining segments were then erectedusing the cantilever method of progressiveplacement (see Figs. 12b through 12d).

During the erection process, the seg-

b 1

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Fig. 12. Erection sequence.

PCI JOURNAL/January-February 1986

ments were gradually prestressed using 2x 2 continuously coupled cables placed inthe top flange. Since the hydraulic jacks onlysupported the erected segment during itserection (inducing a very small force), thesuperstructure was stressed by only nega-tive moment load which was later balancedduring post-tensioning. After erecting all thesegments and 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.

115

Fig. 13. Post-tensioning of the first two segments.

Fig. 14. Erection of the end cross beam.

116

Fig. 15. Finished structure.

The footbridge was erected precisely inthe designed shape without any shims orconcrete joints. These excellent results en-abled progressive placement technology tobe used for other types of bridges.

DS-W BRIDGESThe DS-W bridges are designed with 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 beams consist of segments withthe dimensions shown in Fig. 17. The endsof the bridges are always perpendicular whilethe intermediate supports may be skew.These supports are formed by slender pre-cast columns connected to the deck by con-crete hinges. The hinge in the longitudinal

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

Diaphragms are designed only in piersegments. The typical segment weight is 20tons while the weight of the pier segmentsis 25 tons.

Originally, the first bridge structures weretransversely connected only by post-ten-sioning recognizing the pressure reserve inthe joints. Because of the relatively high liveto dead load ratio, the amount of prestress-ing was quite 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 with 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.

PCI JOURNAL/January-February 1986 117

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Fig. 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. Construction of DS-W type bridge in Brno-Reckovice, Czechoslovakia.

The loading was repeated in several cyclesand finally the ultimate strength of the con-nection was determined. The cracks whichdeveloped under 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 withmild reinforcing steel and post-tensioning toreduce the secondary effects due to creepand shrinkage of the concrete.

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

One group of tendons (Cables B) is situ-ated only near the supports, while the sec-ond group (Cables C) is gradually tensionedand coupled along the entire length of thebridge. The function of these tendons dur-ing erection is to balance the negative mo-ment in the cantilever and, during placementof additional segments, produce uniformcompression of the intermediate spans.

Also during erection, the draped tendons(Cables A) are gradually tensioned from theface of the segments which are being assem-bled.

The segments are manufactured in formsdescribed in Ref. 3. Segment geometry wasdetermined by a computer program andmanufactured according to the design levelline and real measured values of segmentswhich had previously been cast.

The superstructure itself is erected incantilever from one end of the bridge to theother. The segments are placed in positioneither by a mobile crane which moves onland or by a portal crane moving on a tracksituated along both sides of the erectedbridge portion (see Fig. 21). Stresses in theassembled cantilever are reduced by usingone or two temporary towers to distributeloads.

The erection sequence is illustrated in Fig.20 and shows the assembly of the end andfirst intermediate spans. Span length is 82and 98 ft (25 and 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 Cables A are ten-sioned.

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construction drectionAs —^

At A3 A< As^A3' ( Aa A2 , -A

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Fig. 19. System of prestressing tendons.

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Fig. 20. Erection sequence.

2. This stage includes the erection of anadditional 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 support, Cables A are ten-

sioned, the temporary support under thesixth segment is removed, the first span isrectified in elevation, and the 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 segment,Cables A are tensioned and the reaction ofrequired intensity is induced.

2. Erection of an additional three seg-

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

ments is carried out (Fig. 20f), the supporttower under the fourth segment 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, Cables Aare tensioned, the temporary support underthe seventh segment is removed and thereaction corresponding to the given stage oferection is induced and the concrete hingesare cast (Fig. 20h).

The magnitude of the forces of the tem-porary support is determined so that themoment above the support may constantlyhave the same value as the moment whichappears after erecting the first four seg-ments. A computer then determines static

effects and monitors the structure duringerection.

The DS-W bridges were successfully 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 represent the latest type

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

PCI JOURNAL/January-February 1986 123

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

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Fig. 23. Segment of a DS-T type bridge: (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. Segment 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 applied forbridges with small horizontal curves, 820 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 monolithic cross beams nearexpansion joints. On slender, intermediatemonolithic supports, the superstructure isplaced with the help of the pot bearings.

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

DS-W bridges (see Fig. 19). Furthermore,a finite element analysis shows that the ap-proaching of tendons to girders does not af-fect distribution of normal stress from thestraight tendons. Therefore, these tendonsare uniformly distributed in the deck forsimple manufacture.

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 used in only one case, namely, thebridge across the Rokytka River in Prague,where the segment widths of 64 ft (19.50 m)were extended by monolithic fascias.

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

PCI JOURNAL/January-February 1986 125

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

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

The form itself consists of two fixed frames,outside and inside shutters, and a movablefront end. The frames are situated underthe girders and contain two squeeze-outrollers which, after stripping, slightly lift thesegment before it is removed from the form.The required segment shape is formed byturning the movable front end (see Fig. 27).The manipulator 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 manipulator while the upper frame en-ables the turning of the vertical axis. Turn-ing the segment 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 further

supplemented with a device for guidingthe segment onto the manipulator, a walk-way, and trucks equipped with a winch forpulling out the tubes forming the tendonducts.

The basic operations involved are as fol-lows:

1. Placing the reinforcing cage into theopen mold

2. Adjusting the contact segment on themanipulator

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

don ducts8. Measuring the segment after casting9. Form stripping

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

The reinforcing cage is assembled insidea wooden template where the tubes for thetendon ducts and the inserts for formingpockets for the tendon anchors are placed.Concrete is supplied to the mold via a con-

126

Fig. 25. Forms for DS-T type segments: (a) cross section of form; (b) cross section ofcontact segment on manipulator; (c) longitudinal section; (1) movable front end; (2)outside shutter; (3) inside shutter; (4) frame; (5) manipulator; (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 measured 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 which had al-ready been cast.

The superstructure of a DS-T bridge isassembled in cantilever 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 Fig. 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 abutment is operated by another portalcrane travelling on tracks 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 JOURNAUJanuary-February 1986 127

N

Fig. 27. Segment geometry.

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 support; (5) rectificationsupport.

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During erection of common spans, theportal crane turns the segment and shifts itin front of the KPJ-90 crane which is trav-elling behind the abutment. The crane thentakes the segment and 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 be erected using the portal crane onlyif the above conditions cannot 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 Mezirici during construction.

Fig. 30. Temporary support of DS-T type bridge at Valasske Mezirici.

130

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 km) are currently un-der construction. In addition, the attentiongiven to precise manufacture, careful as-sembly, and the effects of creep and shrink-age has enabled bridges 1300 ft (400 m) inlength to be assembled without steel shimsor concrete joints.

CONCLUSIONLow concrete and steel consumption, easy

manufacturing, 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 making 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.

PCI JOURNAL/January-February 1986 131

This technology has been applied for thedesign of Czechoslovakia's first cable-stayedbridge across the Elbe River, where con-struction began early in 1985. It is our hopethat others, too, will challenge current de-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. Muller, Jean, "Ten Years of Experience in

Precast Segmental Construction," PCI JOUR-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-mental 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 University, Brno,Czechoslovakia.

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