Controlling Twist in PrecastSegmental Concrete Bridges

John E. BreenThe Nasser I. AI-Rashid Chairin EngineeringThe University of Texas at AustinAustin, Texas

SynopsisDiscusses several precast seg-

mental box girder bridges where sub-stantial undesired twists were ob-served during erection.

Comparison of casting yard dataand erection measurements indicatedthe excessive twist problem originatedduring geometry control in the castingyard.

The sensitivity of normal controlprocedures is illustrated and recom-mendations for a check procedure tocontrol such twists is given.

The decade from completion of theCorpus Christi Bridge in 1973 to

finishing of the Linn Cove Viaduct in1983 showed the emergence of precastsegmental box girder bridges as a majorconstruction form in North America.

During this period many highly infor-mative publications 1 -14 appeared in theengineering literature and providedguidance to both the designer and con-tractor in the basics and details of thistype of construction.

Several of these publications allude toproblems in geometry control which re-sulted in "important geometric imper-fections" on certain projects. 7 ' Unfortu-nately, relatively little detailed infor-mation has been given. The importantlessons which can be learned from suchproblems need to be emphasized with-out an attempt to assign blame to anyindividual involved in these projects.

The persistent nightmare for both de-signers and contractors involved intheir first precast segmental bridge proj-ect is that somehow their gracefulstructure growing in balanced fashionfrom the central pier (Figs. la and 1b)will fail to meet up smoothly with itsmate which is approaching it from thenext pier. Unlike cast-in-place canti-lever construction, the basic geometry

86

for match cast precast segmental bridgesis set in the precast yard.

While shimming can be used as a lastresort to change the course of an errantcantilever, it is highly undesirable andsometimes ineffective.' 2 More drasticcorrections have been made using wetjoints. L2 Proper geometry control andattention to erection procedures shouldmake such recourse unnecessary.

Unfortunately, errors can he made andsubstantial mismatches at closure haveoccurred (Fig_ . lc). Such closure errorsare difficult to overcome and conceal.With ingenuity, serious errors havebeen overcome by forming and castingwhole transition units in situ (Fig. Id).

In reality, there are two very differentorders of magnitude of concern ongeometry control. While it is extremelyimportant to maintain careful controlduring segment production and erec-tion, the box girder bridge system ismore forgiving of some types of errors.

Fig. 1 a. Balanced cantilever erection usinga launching truss.

Fig. 1 b. Precast segmental box girder bridge being erected in balanced cantileverfashion using a crawler crane.

PCI JOURNAL'July-August 198587

Fig. 2 illustrates the relative stiffnessesof an actual box girder bridge systemwith approximately 200 ft (60 m) spansresulting in 100 ft (30 m) balanced can-tilevers during construction.

Fig. 2a indicates that if the tips at oneend had to be forced into a mating posi-tion through a 1 in. (25 mm) vertical de-flection, relatively low forces of 40 kips(178 kN) in each web would produce the

Fig. i c. Closure error between two cantilever arms of a precast segmental box girderbridge.

Fig. 1 d. Remedial measures to correct for closure errors.

88

P P -40 < 40e(178kN)

---_ ] e (25 mm.) 25 mm)

0) Canfilever Tip -- Vertical Stiffness

PT = 540" 540" {2400kN)

(25mm)T i ^Yz5mm.lL

b) Cantilever Tip - Torsional ( Twist) Stiffness

Fig. 2. Relative stiffnesses of a typical box girder bridge system.

required deflection. Such forces can beeasily developed using strongback sys-tems between the two cantilevers suchas shown in Fig. 3a.

Somewhat larger but similar mag-nitudes of forces are required to providelateral or horizontal adjustments withsimple equipment as shown in Fig. 3b.In fact, many box girder bridges areclosed using final adjustments of thismagnitude without overall damage tothe bridge system.

Such locked-in forces are reduced bycreep and do not affect the ultimate ca-pacity of the section. In many cases pierfixity can be released1° and lesser forcesare required.

In contrast, Fig. 2h indicates that ifthe tips have an unwanted twist and atorque is required to displace each tip 1in. (25 mm) in a twist mode, the requiredforces on the webs are 540 kips (2400kN). The basic torsional stiffness of thebox girder system, which is such an

asset in resisting load upon completion,makes it virtually impossible to com-pensate for twist type geometry errorsthrough application of closure forces,Forces of the magnitude shown in Fig.2b could produce substantial shear andtorsion damage to the structure.

There are some cases where attemptshave been made to forcibly twist suchunits with resulting damage to shearkeys and webs. Thus, it is extremely im-portant that geometry control proce-clures specifically ensure twist control.Many widely publicized procedureshave ignored or incorrectly portrayedthe need for such controls. Podolny andMuller' have strongly urged that suchtwist checks be an implicit part of thegeometry control.

In studying problems which have oc-curred in several major bridges, the au-thor became aware of the appreciablesensitivity of the geometry control sys-tem in short line match cast construction

PCI JOURNAUJuIy-August 1985 89

Fig. 3a. Supplementary "strongback" beams with threaded rods spanning betweencantilever tips to align tips vertically prior to closure placement.

Fig. 3b. Diagonal pulling device between cantilevers to align tips horizontally prior toclosure placement.

90

VerticalElevation SegmentProfile Length Costing Curve

PredictedCeformotions

Desired FinalProfile

SpanPier Midspar

a) Costin g Curve

p E E

ISEGMENT 1 i SEGMENT

Desired

2

ActualJoint I JointJoint

A g C

b) Match Costing

Fig. 4. Basis of match casting in segmental construction.

to certain kinds oferrors. The purpose ofthis paper is to illustrate that sensitivityand point out the relatively simple con-trol procedures that can be used tominimize twist errors in segment pro-duction.

MATCH CAST PROCEDURESThe basics of match casting have been

thoroughly described by otherss,e 7 , bo buta brief description will be given here sothat the reader who is not familiar withthis widely used process will better un-derstand the problem.

In any completed bridge there is a de-sired final elevation or grade profile asshown in Fig. 4a. The profile may in-clude horizontal curvature, vertical

curvature, and varying superelevation;however, only the vertical curvatureprofile of one web is shown here forsimplification. This desired final grademust be adjusted to determine the de-sired profile for the actual casting of thesegments.

The adjustments should consider de-formations upon erection due to the ef-fects of dead load, as well as the effectsof prestressing forces and losses, creep,shrinkage, and relaxation. Consideringthese effects and the sequential natureof erection, a theoretical casting curve asshown in Fig. 4a is determined.

This curve represents the desiredprofile along each web during casting sothat the pieces will be initially de-formed in a pattern opposite to the de-

PCI JOURNAL/July-August 1985 91

formations that will take place uponerection. Thus, the bridge cast, so as tomeet the casting curve, should have thedesired profile after initial erection de-formations have occurred.

This, naturally, assumes that seg-ments will be correctly fabricated. Theeffect of very small systematic differ-

ences in segment lengths along the topand bottom fibers could produce ex-treme deflection variations at spancenterlines. This sensitivity has beeneliminated by the use of match casting.'

As shown in Fig. 4b, Segment 1 wouldbe cast first. Assume that an error wasmade in casting Segment 1 so that the

OuTSIOE FORMWORK

INSIDE FORMWORK

ELEVATION

: : : i

PLAN

ELEVATION

PLAN

Fig. 5. Match casting systems (from Ref. 6).(a) Schematic of long line casting system.

92

actual segment is a prism ABE'D. If thesegment is used as the face of the formfor casting Segment 2, and if the correctelevation requirements are set for theline DE'F, when Segment 2 is cast itwill directly compensate for the previ-ous error. The overall construction willbe correct in spite of a potentially seri-ous error in the distance DE = DE'.

There are two general procedures forproducing match cast segments. In the"long line" concept shown in Fig. 5a, abase form is set for an entire cantileverspan at the desired grade indicated bythe casting curve of Fig. 4a.

Match casting of all units for a can-tilever span would proceed and gener-ally segments would not be moved untilmost of the span had been cast. In thisprocedure, the entire span can be seenwith all units in their correct relation.There is minimal danger of twistgeometry control errors because thebasic geometry is apparent when theform soffit is set.

The long line procedure was used verysuccessfully in casting the variable

depth segments for the Kentucky RiverBridge .6 A modified long line in whichabout one-half of the cantilever spansegments were cast before relocation onthe soffit was used in the pioneer Cor-pus Christi Bridge.'

In contrast, in the `short line" conceptshown in Fig. 5b, all segments are castin a level position in one set locationagainst a very stiff fixed bulkhead. Thenewly cast segment is then moved toform the end form of the next segment tobe cast. The newly cast segment iscarefully positioned and adjusted usingvarious jacks to develop the proper hori-zontal, vertical, and twist angles with re-spect to the casting bed in order to en-sure the correct geometrical relationshipbetween these two pieces as called forby the casting curve.10

The bottom and side forms are thenpositioned to span between the stifffixed end bulkhead and the previouslymatch cast segment. The forms shouldbe flexible enough to warp as requiredyet still have the rigidity to hold thefluid concrete in correct position. The

Fig. 5 (cont.) Match casting systems (trom Hes. b).(b) Schematic of short line match casting system.

PCI JOURNALJJufy-August 1985 93

Casting Machine

Fixed TargetIiil

Rigid Fixed Bulkhead

Segment inCasting Machine

w ••

Previously CostSegment in Match

•! Casling Position

Geometric Control Points• Elevation Pins+ Centerline Wires

Fixed InstrumentTransit Stotion

Level

Fig, 6. Short line method geometric control.

other previously cast specimens havebeen moved to a storage location and nooverall physical check of the relations ofall pieces for a span is obtained until thesegments are erected.

Geometric control in the short linemethod is provided by insertion of ref-erence pins and wires into the freshconcrete of the segment being cast. Thelocations are generally near the segmentfront and rear edges over the webs andcenterline as shown in Fig. 6. After thefresh concrete hardens, the elevations ofthe pins are carefully read using sur-veying instruments and the centerlinelocations are scribed into the centerlinewires before the segments are separatedor moved.

Once the survey is complete, thesegments are then separated and thenewly cast segment is moved into thematch cast position and adjusted in al-

titude to cast the next segment in thespan. It is carefully set into position atthe appropriate angles to become theend form and the cycle begins again.

On many projects it has been commonto read the pin elevations using a rodequipped with either V/32 in. divisions or1 mm divisions. Thus, the smallest scalereading corresponds to 0.032 — 0.040in. (0.8 — 1.0 mm). In complex geom-etry cases, the use of an Invar rod withhundredth of a foot increments and ac-curate interpolation to the nearest one-thousandth of a foot using a parallelplate micrometer is recommended toprovide readings in the 0.012 in. (0.3mm) range.

In actuality, with the distances,climatic conditions, and instrument ac-curacies involved, such precision is dif-ficult to attain and maintain on a con-sistent basis unless very careful checksare included. It is highly recommendedthat independent sets of readings bemade by two different crews and cross-checked before the segments aremoved. Once the segments are sepa-rated, no further geometric check ispossible. All control must rely on the ac-curacy of these "initial readings."

The control process is further illustrat-ed in Fig. 7. Fig. 7a shows the desiredrelation along one web of three seg-ments of a span with respect to their lo-cations and desired elevations of thecasting curve. Subsequent addition ofother segments along with the effects ofprestressing forces, creep, shrinkage,etc., will result in segments moving intothe final desired position.

Segment 1 will be firmly attached tothe pier. Segment 2 needs to be cast insuch a fashion that when erected andbefore deforming it will initially makethe angle a, with respect to Segment 1.Segment 3 must be cast in such a waythat it makes the angle a5 with respect toSegment 2.

In every case the bulkhead end topsurface and the longitudinal centerlineof the segment being cast in the casting

94

ReferenceLine

a) Casting Curve Positionin Structure

Instrument A' B . f xReference Line t Fixed Bulkheod

O

b) Costing Segment 1

InstrumentReference Line A° El C' D' X

O— Fixed Bulkhead

i Z

c) Casting Segment 2

Instrument '^' —"p ^'^F xReference Line

O Fixed Bulkheadz O`ra3

d) Casting Segment 3

Fig. 7. Vertical elevation geometric control duringcasting operations.

machine are in the horizontal plane atthe elevation set by the fixed bulkhead(X). Fig. 7b shows the level casting ofSegment 1.

After initial curing, the control eleva-tions of the pins (A', B') are read andSegment 1 is then moved to the matchcasting position shown in Fig. 7c. Thevalues of the control elevations A" andB" are determined by consideration ofthe desired relations between A, B, C,and D and the actual cast relations of A',B', and X.

After Segment 2 is cast and has hard-ened, the relative relations of the twosegments are determined by carefulmeasurement of A", B", C' and D'. Seg-

ment I then goes to storage as shown inFig. 7d. Segment 2 is set into its desiredconfiguration using the control eleva-tions C" and D" which are based on thedesired geometry between Segments 2and 3 considering the actual geometry ofall previously cast segments.

Note that any error in measurement ofan angle would result in a closure errorproportional to the distance of the jointfrom the centerline of the pier. Thus,segments near the piers are particularlycrucial.

The procedure can operate verysmoothly. The success attained in eon-trol of the complex geometry of the LinnCove Bridge shown in Fig. 8 indicates

PCI JOURNALIJuly-August 1985 95

that proper control measures will ensureshort line casting success. This particu-lar bridge involved reverse curves andrapid superelevation changes whichfurther complicate geometry control. Acombination of precise, checked mea-surements and careful evaluation ofgeometry including large scale plots inwhich three dimensional checks aremade provided proper control.

SEGMENT TWIST CONTROLIn order to clarify the proper geomet-

ric control procedures required to pro-duce a bridge with the desired twistprofiles, it is necessary to first definecertain terms:

• Superelevation (S) is defined at aspecific longitudinal station along thebridge as the difference in vertical ele-vations of the top surface directly abovethe center of the inner and the outerwebs (see Fig. 9), Note that since the topsurface is always assumed to be astraight line at any transverse section,the intersection of the top surface and

longitudinal centerline of the unit beingcast is assumed to be the center of rota-tion regardless of the amount of super-elevation or the change in supereleva-tion. Thus, each web is displaced one-halfofthe superelevation in opposite di-rections. The rotation produces a hori-zontal offset in the base form.

• Superelevation Angle (Q) is definedat a specific longitudinal station alongthe bridge as the superelevation (S) di-vided by the horizontal distance be-tween the center of the inner and outerwebs (L$). Thus, Q = SIL K (see Fig. 9).

• Twist Angle (y) is defined as atransverse rotation which takes placebetween two specific Iongitudinal sta-tions along the bridge and is the changein superelevation angle between thosestations (see Fig. 10).

• Twist (T) is defined as a simplifiedterm similar to superelevation in whichthe twist angle between two longitudi-nal sections (y) is multiplied by thehorizontal distance between the centerof the inner and outer webs (La) so thatthe effective surface warping can he ex-

Fig. 8. Complex geometry at Linn Cove Bridge.

96

Fig. 9. Definition of superelevation S and superelevation angle a-.

pressed by a unique linear dimensionrather than an angular dimension. Thus,T = yLs (see Fig. 10).

The twist in a segment can be acci-dental due to errors in casting or can bedeliberate when there is supposed to bea change in superelevation along thebridge. When changes in superelevationoccur there must he a difference intransverse slope from segment front torear face introduced into the segments.As shown in Fig. Ila, the top surface ofasegment with such a twist actually be-comes a warped surface. The edge ad-jacent to the fixed bulkhead is alwayscast level.

The previously cast match casting po-sition segment (conjugate unit) must berotated about its longitudinal axis toprovide the desired twist angle, y, inaddition to being rotated about the axisof intersection with the next segment toprovide the desired camber. As shownin Fig. lib, the top surface of such asegment has a very complex geometry ascompared to the case with no twist.

The difference between the front andrear face transverse slopes is a measureof the twist in a particular segment. Withvariable superelevation, this twistshould match the superelevation changedesired in the segment. With flat or con-stant superelevation sections, the twist

should be zero. Many box girder spansare designed to have either no superele-vation or a constant superelevationwithin the span.

In either case there is no desired twistso that the segments may be cast with aflat fixed bulkhead and with the matchcast unit in the appropriate orientationto produce the desired horizontal andvertical curves but with the match castunit in a transversely flat position(y = 0). If all segments are cast this waythey can be rotated uniformly uponerection to provide the desired constantsuperelevation. However, as indicatedby Bender and Janssen, 1 ° counteractinghorizontal or vertical curvature may berequired in determining the castingcurve.

Even when the theoretical twist iszero, there is a danger that due to errorsduring fabrication, placement or curing,the actual twist in a segment may not hezero. In every segment the final surveyresults taken just before separation ofthe match cast units must be carefullyexamined to determine the actual as castsegment twist T1.

A suggested procedure in which thesurveying values are given in terms offoresights from a reference plane isshown in Fig. 12. As shown, the twist ineach segment can be easily determined

PCI JOURNAUJuIy-August 1985 97

^x=4

0.995- Sfotion y

^

/pia i p optQy = 50

Same SegmentTwisted .010

5

.010f10

5Direction of Fall

Instrument Reference Plane2.000

1.000All nolues are Station x 'instrument *oresignts

1.000 1.060

^Ua'0 ` 4 cD1Tr Y

! 000 .Station y

rxy= Q,-a.y=4

S1.000-1.000=0 ` 5y=1.000-1.000=0

No Twist Values

(a) Segment with no twist.

V y61.8

Note : Cimensions may beassumed in ony consislontunit system. Values shownfor Illustration only.

Instrument Reference Plane2.000

All values oreInstrument foresights 1.000

Station xI.oao

f.00s

5x't.000-I,o0C-0Sy = 1.005 -0.995=0.010

y'^ y = Qx - Q = 0-0.001 = -0.001T y.> L B = 1-0.0015(101 =-0.010

(b) Segment with twist angle y and twist T.

—k

(c) End view of twisted segment.

Fig. 10. Definition of segment twist.

98

n st, z T p'^,S pS ON/SCGO^NS ^T a^ '

az ^- n

o^ i1 -zT

CONJUGATE UN17

END BU LKHEAD

(a) Isometric view with variable superelevation(from Ref. 7).

C B

No Twist

EL. A=DC EL. B4C

LA

Twist EL (B-A) - (C-D)

(b) Complications with twist.

Fig. 11. Short line casting — Variable superelevation.

from the survey foresights made in themated position in the casting yard. Notethat the twists are determined betweensimilar locations on adjacent segmentsin order to include joint effects.

The algebraic summation of the ascast twists for all segments of a can-tilever span should be determined

(T = ET {), continuously plotted for com-parison with the desired twist, and ap-propriate corrections made when settingup for each match casting to controltwist tendencies. Since twist errors aremore difficult to compensate for inerection than either vertical or horizon-tal alignment errors, priority should be

PCI JOURNAUJuly-August 1985 99

S - S

'!Mini Angle -n- t3 = - McL 8

• B LB

Segment ITtaiet = S - SMC - IFS(D) - FS(A)1 - rFS( DI) - FS(A1)1

i=N

Cantilever Dist ° ^ Segment 1 twists

i-o repo

O ^G^G O1 ^^'C

Station

Instrument— ---------- — Plane

Foresight D Foresight A

a

D 5 Q^ CostForesight D Concrete

D1 LB A

(C) 5 G'NC I Foresight Ai(

^n^ch

Al CaRCre es^

()

Fig. 12. Determination of segment twist directly from castingyard measurement.

given to casting adjustments to elimi-nate twisting tendencies.

SENSITIVITY OF CONTROL

PROCEDURES

The practical implementation of thegeometry control procedures in matchcasting relies on the casting curves de-scribed previously (shown in Fig. 4a)and the computation of the actual "ascast" geometry based on the surveyingmeasurements made before separationof the two match cast segments. Using

linear algebraic relations or graphicalplots to a large scale, the actual as castprofile of the segments cast to date canbe computed.

It is recommended that the as castprofiles be plotted to careful scale on thedesired casting curve as shown in Fig.13. The as cast profile will normallyvary around the desired line. Theamount of variation at any particulartime must be considered in choosing theelevations to which the previously castsegment is to be tilted so as to obtain thedesired geometrical configuration tobring the cantilever span close to the

100

Bolt-Line ABProfile

Veil icalElevation

I Segment Span Distance IPier Midspan

Fig. 13. Practical utilization of casting curves.

desired casting curve.The extreme sensitivity of the casting

curves to very small errors in measure-ment of the initial segments has notbeen fully appreciated. The angle be-tween any two segments must be pro-jected to the end of the cantilever spansto determine its full effect on deflectionof the closure segment. Very often thelength of the pier segments is shortenedto minimize lifting weight which wouldotherwise be excessive because of theheavy diaphragms.

In addition, in many cases the moresensitive central units are cast first whenthe construction crews are just begin-fling to become familiar with the processand are more prone to error. Fig. 14ashows the as cast computed profilesfor bolt lines AB and DC for an ap-proximately 100 ft (30 m) cantilever ofan actual bridge.

The calculated profiles were deter-mined by linear algebraic extrapolationformulas from the initial elevationreadings made on the match cast hard-ened segments before separation. In thisparticular span the desired supereleva-tion in the first seven segments wasvariable. This is why the bolt line ele-vations on AB and DC differ so much.

After the seventh segment the differ-ence in the elevations between the two

bolt lines remains approximately con-stant as there was to be a constantsuperelevation in the remainder of thespan. Both of these calculated as castcasting curves were close to the desiredcasting curves upon which the precasterbased his segment casting decisions.

There was some discrepancy in theinitial segments but corrections werequickly made and the units were man-ufactured with an apparent close matchto the planned profile. It will be shownsubsequently that, when the units wereactually erected, substantial unwant-ed twists were apparent. Extreme dif-ficulty was experienced in closing thespans.

What has not generally been ap-preciated is the extreme sensitivity ofthe chain of measurements and calcula-tions which must be made to control thisprocess. In Fig. 14b the calculated pro-files shown in Fig. 14a are shown alongwith a fictitious dashed line curve.

This dashed line curve illustrateswhat happens to the computed eleva-tions along bolt line DC if an initial sur-veying error was made on the pier piecewhich resulted in the initial elevation ofD being taken as'//a in. (0.8 mm) higherthan actual and the initial elevation of Cbeing taken as 'hs in. (0.8 mm) lowerthan actual.

PCI JOURNAL/July-August 1985 101

VerlicalElevationI„a2 -

(mm.f

-I^

Fig. 14a Computed bolt line profiles from casting yard measurements.

50

25

0

125)

x

^cx ^x/

^xk \

x x\

0 Ac 6

vertical 0 x 5 6 7 8 9 10 II 12 13 14 Segments AlongElevation Pier x Midspan Span

z 25tMrin Bolr DC^g^ x

Line

-50•

t-501 t

-75 ^^r—Bolt DC Linebut with Arbitrary1552 (0 $ mm } error on elevationof D end C of Pier Piece

-i00oll ABLine

- 125 100} •

Fig. 14b. Sensitivity of computed bolt line profiles to measurement errors.

102

DACs

Along

Surveying errors of this order of mag-nitude are certainly possible since theyrepresent the least level of readingsometimes used in plants. While theymight be detected when the piece isbeing put in the match cast position, it isnot always likely.

Such measurement errors would re-sult iu false calculated rotation of thepier segment and cause the piece to beset in an incorrect match cast position.The entire cantilever would then be ro-tated downward. Because the pier seg-ment in this bridge was only aboutone-half the length of the other seg-ments and because of the long length ofthe cantilever, the angular magnificationis very high.

The very small assumed discrep-ancies in pier segment readings result inabout 14 in. (38 mm) change in the cal-calculated elevation ofthe web DC at mid-span. The arbitrarily assumed initialreading error is certainly at the mostcritical location but is not even a worstcase in terms of magnitude, given thefield conditions ordinarily experienced.

Closure operations to overcome suchdiscrepancies would be in the realm ofthe possible given the vertical deflec-tion stiffness shown in Fig. 2a. As a lastresort there would be a possibility ofshimming corrections in the field iferection surveys indicated a consistenttendency for such an error during erec-tion.10

However, the actual problem is verymuch more serious when twist is con-sidered. The difference in elevations ofthe deck above the webs at any trans-verse section of the bridge is thesuperelevation. The second differenceor change in superelevation betweentwo transverse sections of the bridge isthe twist. The desired change insuperelevation in any segment can bedetermined from the planned differ-ences in elevations along the bolt lines.

Fig. 15 shows the desired cumulativetwist or change in superelevation calledfor in the plans for the same cantilever

span as previously shown in Fig. 14.The apparent twists in the actual as castunits are plotted based on the seconddifferences from the calculated eleva-tions of bolt lines AB and DC as deter-mined by linear algebraic extrapolationfrom the individual segment mea-surements taken before separation.

It can be seen that after an initial poorstart the cumulative twist was appar-ently corrected and the final values areindicated to be very close to those de-sired. In reality, such was not the case.

Fig. 16 indicates the extreme sen-sitivity of twist calculations based onextrapolated elevations to small sur-veying or calculation errors. Again, tak-ing a very arbitrary ± '/32 in. (0.8 mm)measurement error in bolt line DC ofthe pier segment as previously illus-trated in Fig. 14b, the cumulative twistis calculated as the sum of the seconddifferences in elevation between thetwo bolt lines. On this basis, the com-puted value of the twist is greatlychanged as shown by the lower dashedcurve of Fig. 16.

In reality, a much simpler and moreaccurate measurement of the twist isavailable. As previously shown in Fig.12, the twist in each segment can besimply and directly determined from theforesights made in the mated position inthe casting yard. The cumulative twist issimply the algebraic sum of the indi-vidual segment twists.

The small measurement errors in piersegment elevations assumed in thisexample would actually only introduce atwist change in the entire span of 2/32 in.(1.6 mm) since the cumulative twist re-flects only the algebraic summation oftwists in each segment, Because of thegeometric extrapolation of the angularpier error to all segments when verticaldeflections are extrapolated, the twistdetermined from the computed ex-trapolated elevations indicates over 25times the actual effect.

This oversensitivity to computationsfor twist from the extrapolated geometry

PCI JOURNAL/July-August 1985 103

of the bolt line can work in reverse tomask errors, Even more dangerously, itcan actually result in twists being cast insegments to counteract the apparenttwists resulting from erroneous eleva-tions. Fig. 17 shows the same span. Thetwo tipper curves represent the desiredcumulative twist and the twist which theprecaster assumed he was providing byexamining his computed casting curvesplotted from calculated (extrapolated)bolt line elevations.

There is good agreement. However, asimple check was bypassed. As shownin Fig. 12, the twist for any segment canbe determined directly from the matedmatch cast segment's bolt foresights. Thesimple algebraic sum of the twists foreach segment gives the cumulative twistfor the span.

Fig. 17 also shows the actual twistmeasured in the field after erection aswell as the cumulative twist determinedby simple summation of the segment

twists as measured in the precast plantusing the relations shown in Fig. 12. Ascan be readily seen, this simple checkwould have immediately indicated thata severe problem was developing dur-ing casting.

Corrective action could be takenduring the casting phase to correct forthe unwanted twist by counter-rotatingthe next segments to eliminate the error.The simple check correctly mirrors thesubsequent chaotic field conditions.The magnitude of the as cast twist dis-crepancy over the webs as indicated isfurther magnified by the ratio of thedistance between tips and webs to resultin almost a doubling of the error at thesegment outer tips. As can be seen fromFigs. Ic and 2, errors of this magnitudein twist cannot be easily accommodated.

It is interesting to note that the patternof the discrepancy between the assumedand the actual twist shown in Fig. 17 issimilar but of much greater magnitude

125 +(toolComputed values based onelevations extrapolated from

yard measurements

^f75)

DesiredCumulative

Twist 75-1/Beres een Webs

i" l50) !32 !

I mm)50

z5) /

25 /

10 I 2 3 4 56 7 69 10 I1 12 13 14Pier Segments Along Span Midspon

Fig. 15. Desired and computed cumulative twist values.

125 1100)Computed values based onelevations extrapolated fromyard measurements

-^100 -`^last --^

DesiredCumulative

Twist 75

Between Webs/\150

/32

(mm) /50 / /

1251 //

25/ Computed values as above but with%

arbitrary ±y" (0.8mm) error on% elevation of 0 and C at pier

piece to illustrate sensitivity

00 I 23 4 56 7 8 9 10 II i2 13 l4

Pier Segments Along Span Midspan

-25

Fig. 16. Sensitivity to measurement errors of twist calculations based on extrapolatedelevation calculations.

than the hypothetically based discrep-ancy shown in Fig. 16. The purpose ofthese comparisons is not to pinpointcause but to illustrate the sensitivity ofthe commonly used procedure of con-trolling profiles by extrapolating eleva-tions along each web and neglecting thesimple twist summation check of Fig.12.

In addition to the usual control proce-dure of plotting the extrapolated mea-sured elevations on the casting curvesfor each bolt line, all control of segmen-tal precasting operations should utilize athird or cross check plot which com-pares the desired cumulative twist to theactual twist determined from the simplealgebraic summation shown in Fig. 12.This plot will illustrate simply andreadily the development of undesirabletwist trends. Corrective measures to

eliminate twists by selecting setup ele-vations which introduce correctivecounter rotations can then be appliedduring segment production.

APPLICATION TOCONSTANT

SUPERELEVATION BRIDGESIt might he assumed that such twist

check procedures are not required forlevel or constant superelevation bridgessince there should be no twist in such abridge. Figs. 18 and I9 show twist mea-surements from two different projects indifferent states. Both spans were to haveno change in superelevation and hencezero twist. Fig. 18 shows computed andmeasured values from another span ofthe bridge shown in the preceding

PCI JOURNAL/July-August 1985 105

125 {loo}

100175f

Computed values based onelevations extrapolated fromcasting yard measurements

DesiredCumulaf ive

TwisfBetween Webs

35

{mm)50

25

150) /

;25] / ^\/ Field measurement

from erection

Algebraic summolion of individualsegment twists determined fromcasting yard measurements

00 I 2 3 4 5 6 7 8 9 10 I1 12 !3 14

Pier Segments Along Span Midspon

—25

Fig. 17. Desired and measured cumulative twist values.

example. This particular span was tohave no change in superelevation andthus all segments were to be cast with-out twist (Curve A).

The calculated elevations based onthe linear extrapolation of the bolt pro-file measurements made during thematch casting process indicated a rela-tively small twist on the order of to %in. (6 to 10 mm) might develop (CurveB). During erection substantial twistwas measured (Curve C). As shown inFig, 18, when the measured twist be-tween webs was approximately 2 in. (50mm), corrective action was taken to in-sert shims in the joints to counteract thetwisting.

Note that the measured twist in thefield up to this point coincides veryclosely with the predicted twist (CurveD) as determined by the algebraic sum-mation of the individual segment twists

as found from the procedure of Fig. 12.The application of considerable

shimming was successful in this case inlimiting the twist to about half of thepotential twist cast into the specimen.Such shimming should he minimized oreliminated because of the potentialdamage to the long term integrity of thejoints. It would be far better to use atwist control check and corrective actionduring casting.

Fig. 19 is taken from a different proj-ect for a bridge with no designsuperelevation. The algebraic summa-tion of the individual segment twistsdetermined from casting yard mea-surements indicates that substantial un-dersired twists were cast into the seg-ments. Again, the lack of an effectivecheck procedure did not alert the pre-caster or the contractor to the magnitudeof the possible problem.

106

Pier0 1

0

-251-251

-50

Cumulative f-50)Twist

z -75(mm)

(-75)

-100

1-1001-125

-150 (-125)

-175

Segments Along Span3 4 5 6 7 8 9 i0 II E2 13

Midspan14

*"Desired (01\̀1 Computed values

O based on elevationsextrapoloted fromcasting yardmeasurements

Algebraic summation of -individual segment twistsdetermined from costing yardmeasurements

I

ShimmingCommenced

\ MeldMeasurements

Fig. 18. Twist measurements with effective field shimming as a control measure.

Field measurements agree very wellwith the predicted twist pattern in spiteof corrective shimming being applied atthe stages indicated to control both ver-tical elevation and twist. The shimmingmeasures used to control twist made rel-atively little difference in this span. Theineffectiveness of shimming has alsobeen reported by Harwood.'"

Note that this supposedly flat, levelcantilever had a twist between bolt lineson the webs of approximately 1' in. (32mm). A number of problems involvingjointing and shear keys on this bridgemay have been aggravated by the un-wanted twist tendencies and the need toovercome them in closure.

While the ideal procedure is to use

the algebraic summation of the indi-viduaI segment twist measurementcheck as a control during the castingprocess to eliminate this problem, thetwist check can be used to evaluate thepotential twist of already cast units priorto erection. The procedure is a simpledirect check which does not involve theextrapolation procedures involved incomputation of elevations of segmentsin short line casting. The knowledge oftwist tendencies in the units wouldallow the owner, the precaster and theerector to plan accordingly and to re-solve problems at an early stage.

In all of the bridges illustrated in Figs.14 through 19, the precasters hadelected to base setup geometry control

PCI JOURNAL/July-August 1985 107

on a series of algebraic extrapolationequations which correctly projected boltline elevations but did not require threedimensional twist consistency. The lackof a simple twist check combined withreliance on algebraic rather than threedimensional graphic controls resulted inunits being cast with substantial twistseven though in zones with no change insuperelevation.

CONCLUSIONSWith continued development and

widening usage of precast segmentalbox girder bridges, more highway de-

partments, engineering firms, contrac-tors and precasters are becoming in-volved with such construction. Whilegeometry control procedures have beenpublished in several references, thesensitivity of some elements of the con-trol process has not been widely ap-preciated.

The examples cited of problemswhich occurred in several Americanbridges during the past decade indicatethat twist can be a serious problem inshort line match casting. Because ofhigh torsional stiffness of box girders,correction of substantial twist closureerrors is very difficult. However, a rela-

30

inch

2

10N

E Pier Segments Along Span Midspan

0 0 3 4 5 6 7 8 9 10 11 12 13 14 I5 16 17^

Desired (0)

-10

Cumulotive

\_. Field Measurements

Twistrim,

-20

inch Algebraic summation ofindividual segment twistsdetermined from costing yard

_ 30 measurements

C ■ r .- Shimming

40 * s ._Twist Shimming`-a w *

-7u2inches

Fig. 19. Twist measurements from a different project.

108

tively simple check can be made duringthe match casting process which willprovide sufficient warning and informa-tion so that twist can he immediatelycontrolled.

Proper geometry control proceduresduring precasting should include boththe conventional casting curves for thebolt lines and a separate tabulation or(preferably) graphical plot (like that inFig, 17) of the cumulative twist of thesegments. Twist values should he de-termined as shown in Fig. 12 from theelevation foresights taken in the matchcast position before separation of theunits. Such a direct summation of thecumulative twists of each segment is amuch more reliable indicator of twisttendencies than are changes in crossslopes calculated from the extrapolatedbolt line elevation values of conven-tional casting curves.

Such extrapolated bolt line elevationsare overly sensitive to minor measure-ment errors in pier segments and nearthe piers. Conventional linear equa-tions and graphical solutions for webbolt line elevation curves assume planardeflections. Adding the cumulativetwist check forces the process to con-sider warping tendencies and ties thetwo web bolt line elevation curves to-gether with a simple check operation.

The most effective procedure for con-trolling twist is to eliminate any un-wanted twist tendencies for the can-tilevers during the match cast process.The major elements of such a controlprocess are:

1. Where possible, design spans forzero or constant superelevation so thatno planned twist is introduced. Thissimplifies the problem to control ofsmaller accidental twists.

2. Precision in surveying to eliminateor minimize subsequent computationalerrors. This requires highly accurateprocedures such as substantial fixed in-strument platforms, fixed referencetargets and precision leveling tech-niques to strive for measurements in the

one-thousandth of a foot (0.012 in, or 0.3mm) range,

3. Repeated, independent checks ofboth measurements and computations ofgeometry before moving segments.

4. Determination of individual seg-ment as cast twists using the relationsoutlined in Fig. 12.

5. Plotting ofa cumulative twist curvewhich gives the algebraic summation ofthe twists found in Step 4 of all indi-vidual segments in a cantilever as theyare cast.

6. Comparison at each casting stage ofdesired and actual cumulative twists.

7. Giving priority in computing set upelevations for the previously cast unit inthe match cast process to the elevationsrequired to correct twist errors by acounter-rotation. After the proper rela-tion of the bolt Iines to counteract twisterrors are determined, then the offsetsrequired for vertical deflections shouldbe computed. The match cast segment isthen positioned to these elevations inthe match casting position.

8. If any residual twist error of mag-nitude greater than what can be handledin the closure process remains after allsegments for a span are cast, as a possi-ble corrective measure, shimming of thejoints during erection with shims on thetop surfaces of keys in one web and thebottom surfaces of keys in the other webcan be attempted. Since this shimmingcan weaken the joints and offset thebenefits of the match casting process, itshould be avoided if at all possible. As alast resort during erection, an inten-tional wet joint can be cast between twoof the match cast segments. Harwoodreports` t on the use of approximately 2in. (50 mm) wide wet joints betweenseveral segments to correct alignmentand twist tendencies on a major bridgeproject. This is a difficult and costly wayto correct a problem which need notoccur. In extreme cases where a seriouserror was made in a limited number ofsegments, a corrected segment could berecast before erection. Proper attention

PCI JOURNAUJu€y-August 1985 109

to Steps I through 7 should eliminateany extraordinary corrections,

ACKNOWLEDGMENTSFew engineers are willing to share

intimate details of their problems inpublic. The author is particularly grate-ful to Mr. R. Lawrie, Mr. H. McLaren,the Centric Corporation, and the State ofIllinois for openly sharing data on con-struction problems with him.

The author is also indebted to twopioneers of segmental bridge construc-tion, M. Jean Muller and Mr. ArvidGrant, for their valuable discussions of

these problems.Dr. William Stone, now of the Na-

tional Bureau of Standards, greatly as-sisted in this study by developing acomputer program to carry out the re-quired geometry checks.

Partial support for the development ofthis article was provided by the Univer-sity Research Institute of The Univer-sity of Texas at Austin as part of a studyon Construction and Durability prob-lems in Long Span Bridges.

Finally, the author would like to thankthe PCI JOURNAL reviewers for theirconstructive comments. Hopefully, theclarity of the paper was improved in re-sponse to their comments.

NOTE: Discussion of this paper is invited. Please submityour comments to PCI Headquarters by March 1, 1986.

iui'.

REFERENCES1. Muller, Jean, "Ten Years of Experience

in Precast Segmental Construction," PCIJOURNAL, V. 20, No. 1, January-February 1975, pp. 28-61.

2. CEBIFIP Recommendations for the De-sign and Construction of ConcreteStructures, Third Edition, Cement andConcrete Association, London, England,1978.

3. PCI Committee on Segmental Constnic-tion, "Recommended Practice for Seg-mental Construction in Prestressed Con-crete," PCI JOURNAL, V. 20, No. 2,March-April 1975, pp. 22-41.

4, Precast Segmental Box Girder BridgeManual, Joint Venture: Prestressed Con-crete Institute, Chicago, Illinois, andPost-Tensioning Institute, Phoenix,Arizona, 1979.

5. Post-Tensioning Box Girder BridgeManual, Post-Tensioning Institute,Phoenix, Arizona, 1978.

6. Barker, James M., "Construction Tech-niques for Segmental Concrete Bridges,"PCI JOURNAL, V.25, No.4, July-August1980, pp. 66-68.

7. Podolny, Walter, Jr., and Muller, JeanM., Construction and Design of Pre-stressed Concrete Segmental Bridges,John Wiley & Sons, New York, N.Y.,1982.

8. Henson, H. Henrie, "Construction Dif-ficulties on 170 Vail Pass SegmentalConcrete Bridges," Prestressed ConcreteSegmental Bridges, Structural En-

gineering Series No. 6, Bridge Division,Federal Highway Administration,Washington, D.C., August 1979.

9. Breen, J. E., Cooper, R. L., and Calla-way, T. M., "Minimizing ConstructionProblems in Segmentally Precast BoxGirder Bridges," Research Report121-6F, Center for Highway Research,The University of Texas at Austin, Au-gust 1975.

10. Bender, Brice F., and Janssen, H.Hubert, "Geometry Control of PrecastSegmental Concrete Bridges," PCIJOURNAL, V. 27, No. 4, July-August1982, pp. 72-86.

11. Mathivat, Jacques, The Cantilever Con-struction of Prestressed ConcreteBridges, John Wiley & Sons, New York,N.Y. 1983.

12. Harwood, Allan C., "I-205 ColumbiaRiver Bridge—Design and ConstructionHighlights," PCI JOURNAL, V. 27, No.2, March-April 1982, pp. 56-77.

13. Tai, James C., and Lo, George K., "1-205Over Columbia River Bridge_ GeometricControl for Cast-in-Place and PrecastSegmental Box Girder Construction,"Transportation Research Record 871,1984.

14. Nair, R. Shankar, and Iverson, James K.,"Design and Construction of theKishwaukee River Bridges," PCIJOURNAL, V. 27, No. 6, November-December 1982, pp. 22-47.

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PCI JOURNAL'July-August 1985 i ll