1. Report No.

VTRC 91-IRl2. Government 40c•••&0" No.

Technical Report Documentation Page

3. Reelp'ent'. Cata'og No.

5. Report Da'e4. nUeandSubllUe Field Instrumentation and Measured Responseof the 1-295 Cable-Stayed BridgeInterim Report on Construction Period Strains inCable Stays...... .... I. Performing Organization "eport No.

7. AuthorC ••

Thomas T. Baber, Furman W. Barton, W. T. McKeel, Jr.I. Performl", Or,anlzatloft Name and Addre••

Virginia Transportation Research CouncilBox 3817 University StationCharlottesville, VA 22903

10. Work Unit No. (TRAIS)

11. Contr.' or Grant No.

12. 8po..orlng Agency Name anet Addre••

Virginia Department of Transportation1401 E. Broad St.Richmond, VA 23219

15. Supplementary Not••

11. Ab.tr..'

Interim Report14. Spo..orlng Agency Code

During the construction of the 1-295 cable-stayed bridge, a number ofthe stays on the main span- cantilever were instrumented with electricalresistance strain gages mounted directly on the wires of the seven-wirestrands making up the stay cables. Measurements were taken beginningbefore tensioning and continuing until completion of the cantilever.Expected changes in the strains caused by lifting of segments and sub­sequent stay tensioning activities were observed. During the multi­strand tensioning activities on the instrumented stays, significantvariability in the strain behavior of the individual strands was observed,a fact that should be considered in the design of cable-stay systems andtensioning methods.

17. Key Worda

Cable-stayed bridges, strains, staycables, cantilever construction

11. DII."bullon S.atement

II. Securl'Y Cia If. (0' .hI. reporU

Form DOT F 1700.7 (8-72)

20. Sec..l.y Cia••". (0' thl. p.ge.

Reproduction of completed page authorized

21. No. of 'age. 22. Price

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25?

FIELD INSTRUMENTATION AND MEASURED RESPONSEOF THE 1-295 CABLE-STAYED BRIDGE

INTERIM REPORT ON CONSTRUCTION PERIOD STRAINSIN CABLE STAYS

Thomas T. BaberFaculty Research Scientist

Furman~ BartonFaculty Research Scientist

w. T. McKeel, Jr.Senior Research Scientist

(The opinions, findings, and conclusions expressed in thisreport are those of the authors and not necessarily

those of the sponsoring agencies.)

Vu-ginia Transportation Research Council(A Cooperative Organization Sponsored Jointly by the

VIrginia Department of Transportation andthe University ofVrrginia)

In Cooperation with the U.S. Department ofTransportationFederal Highway Administration

Charlottesville, Virginia

January 19.91VTRC 91-IR1

253

260BRIDGE RESEARCH ADVISORY COMMITTEE

C. A NASH, Chairman, Suffolk District Administrator, VIrginia Department ofTransportation

W. T. MCKEEL, Executive Secretary, Senior, Research Scientist, VIrginia Transpor­tation Research Council

J. E. ANDREWS, Assistant State Structure and Bridge Engineer, VIrginia Depart­ment of Transportation

G. W. BOYKIN, Suffolk District Materials Engineer, VIrginia Department ofTrans­portation

T. W. EATON, Management Services Division, VIrginia Department of Transporta­tiOD

L. L. MISENHEIMER, Staunton District Bridge Engineer, VIrginia Department ofTransportation

R. H. MORECOCK, Fredricksburg District Bridge Engineer, Virginia Departmentof Transportation

C. NAPIER, Structural Engineer, Federal Highway Administration

W. L. SELLARS, Lynchburg District Bridge Engineer, Virginia Department ofTransportation

F. G. SUTHERLAND, State Structures & Bridge Engineer, VIrginia Department ofTransportation

L. R. L. WANG, Professor, Department of Civil Engineering, Old Dominion Univer­sity

FIELD INSTRUMENTATION AND MEASURED RESPONSEOF THE 1·295 CABLE-STAYED BRIDGE

INTERIM REPORT ON CONSTRUCTION PERIOD STRAINSIN CABLE STAYS

Thomas T. BaberFaculty Research Scientist

Furman~ BartonFaculty Research Scientist

w. T. McKeel, Jr.Senior Research Scientist

INTRODUCTION

Background

In June 1986, investig~tors at the VIrginia Transportation Research Councilbegan implementing a plan for instrumenting the 1-295 cable-stayed bridge duringthe construction phase with the intent of making continued field measurements ofresponse during the in-service phase. Originally, the plan was to instrument decksegments, piers, and pylons on the south side of the bridge using electrical resis­tance strain gages mounted on dummy reinforcing bars, cable stays using electricalresistance strain gages mounted directly on the stay cables, and thermocouples. Alltransducers were to be connected to an automatic data acquisition system. This in­strumentation installation has been completed. The data obtained are under analy­sis and will be the subject of future reports.

The overall objective of this study is to evaluate the behavior and response ofthe 1-295 cable-stayed bridge over the James River near Richmond, VIrginia. Thisinvestigation and program ofinstrumentation seek to address a number of ques­tions regarding bridge response for which only limited experimental data are avail-able. .

In the summer of 1987, the Federal Highway Administration (FHWA) re­quested that additional instrumentation be installed to monitor stresses in the ma­jor components of the bridge during the construction period. The additional workwas initially concerned with data measurements on the north side of the bridge andprimarily required instrumentation of the deck segments with mechanical straingages and instrumentation of selected cable stays with electrical resistance straingages. A separate work plan covering this supplemental investigation was sub-

261

262

mitted in November 1987 and approved in December 1987. As construction hadjust begun on the north cantilever at the time of the FHWA request, investigatorsimmediately began to implement the additional instnunentation in late July 1987.

Objectives

This report documents the work performed on one particular phase of this in­vestigation from July 1987 through Apri11989. As of that date, the bridge erectionwas nearing completion but construction was still in progress on the south cantile­ver of the main span and closure of the main span had not taken place.

The specific objectives addressed in this report are those initiated by theFHWA in their 1987 request for additional instnunentation. Specifically, this re­port focuses on strain data obtained from electrical resistance strain gages on se­lected cable stays on the north cantilever during the erection of the north cantilever.

Description of Bridge

The 1-295 bridge is a segmentally erected, precast, post-tensioned, cable­stayed box girder bridge that consists of 31 individual spans, including approachspans. The approach spans are each 150-ft-Iong precast box girder segments withexternal post tensioning continuous over 6 spans constructed by the span-by-spanmethod.

The portion of the structure that was the focus of this investigation consistedof the central 7-span continuous section, which includes the 630-ft main span overthe river and the 3 approach spans on either side. An elevation sketch of this por­tion of the bridge is shown in Figure 1. The middle 5 spans of the bridge, includingthe main span, are supported by 26 cable stays arranged in a single plane harp con­figuration and emanating from two pylons, one on either side of the river. Cable­stay forces are transferred to the twin box girders through precast delta frame as­semblies located between the girder segments at each stay location as shown in thecross-section sketch of Figure 2. Figure 3 illustrates the detail of the cross sectionat one of the main pier/pylon locations. The main span over the river was con­structed as two cantilevers extending from the two piers located adjacent to the py­lons and made continuous by a midspan closure pour. Typical main span segmentsare 10 ft long and weigh approximately 70 tons.

Forces from the first cable stay, stay 81, are transferred to a delta frame lo­cated 40 ft from the center line of the pylon on both the main and back spans, withsubsequent stays being spaced at 20-ft intervals. This configuration is illustratedschematically in Figure 4. Details of the cable stay assembly are shown in ~igures

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5 and 6. The instnunentation plan for the cable stays, along with other elements ofthe bridge, is discussed by Baber et ale (1988).

Project Status

At this writing, construction on the bridge is complete. All transducers pro­posed in both the original and supplemental work plans have been installed, andthe data acquisition system is operational. However, the focus of this report is ondata gathered during the construction of the north cantilever.

STAY-CABLE INSTRUMENTATION

Electrical Resistance Strain Gages-North Side Stay Cables

A major concern in the long-term performance of a cable-stayed bridge is thesatisfactory fatigue performance of the stay cables. As designed, the stay-cable sys­tem for the 1-295 bridge has between 72 and 90 high-strength steel, seven-wirestrands per sta~ The tendons are encased in a polyethylene pipe and grouted witha portland cement mortar to provide corrosion protection. '1b minimize the amountof live load stress taken by the anchorages, the anchorage zone is filled with a steelbaWepoxy grout mixture as the first stage of grouting.

Since the strains in the stay cables are one of the major response quantitiesof interest, direct instrumentation of the stays appeared to be the most desirableapproach, after various alternatives were considered (Baber et al., 1988). Original­ly, the investigators were asked to install electrical resistance strain gages on 3 ofthe 13 cable stays (stays 82, 87, and 813). However, since this type of gage installa­tion is identical with that used on the south side as part of the original instrumen­tation, and since very little information could be found in the literature pertainingto field instrumentation of cable stays, it was decided to instrument additional stays(81 and 85) to allow the problems associated with the field application to be workedout and to provide additional backup in the event gages were lost. This proved pru­dent since both eventualities occurred.

On the north side, foil electrical resistance strain gages were mounted direct­lyon the wires in the strands of stays 81, 82, 85, 87, and 813. '1b provide for a reli­able determination of average strains in the cables, and to provide some indicationof strain distribution throughout the cable cross section, a number ofgages wereplaced on each stay cable. After initial experimentation with different lead wireand dummy gage configurations, it was decided to use a temperature-compensatingdummy gage, mounted on a short length of prestressing wire identical with the ca­ble-stay strand and located adjacent to the active gage in the cable-stay duct. The

9

269

active and dummy gages were independently wired with two wire leads. On stays81 and 82, eight gages were installed, two on each of four strands. There appearedto be a significant tendency for some strands to shift positions or twist during ten­sioning, causing loss of gages, so it was decided to only use one strain gage on eachstrand for subsequent stays. On stay 85, four gages were installed on four separatestrands. Eight gages were installed on stays 87 and S13, but on eight differentstrands. Sketches of the stay instrumentation are shown in Figures 7 and 8 andfurther details of the procedure used for mounting, wiring, and waterproofing thegages were discussed by Baber et ale (1988).

All of the gages on all stay cables were independently wired to maximize reli­ability under field conditions, with all averaging of data to be done during the anal­ysis phase. The lead wires from the active and dummy gages were led out of thepolyethylene pipe through the expansion joint prior to tensioning of the sta)'. Onthe north side of the bridge, these lead wires were then run to switch and balanceunits where strains were read manually using a strain indicator unit.

Gage Installation Difficulties and Suggested Solutions

The gage installation design was selected to provide tensioning, constructionperiod, and service life data. Since the variety of data to be gathered could beobtained only over an extended time period, it was necessary to plan the gage in­stallation for maximum durabilit)'. Hence, the investigators chose to use epoxyadhesives. This choice created severe problems in installing the gages, as the straingage grade epoxy adhesive chosen will not cure below 70°F and requires at least 6hr to cure under those conditions. Accelerated curing could be achieved, but only byusing a heating blanket that locally raises the temperature significantl~ Underfield conditions, either very low temperatures that made artificial heating a necessi-

. tyor very high ambient temperatures that reduced the pot life of the epoxy wereroutinely encountered. The situation was further complicated by the relativelyshort period of time available to complete the instrumentation. Typically, the inves­tigators would be able to access the stay cables in position within the polyethyleneducts no more than 24 hr before tensioning. In several instances, the investigatorscould not obtain confirmation of the date of installation of a cable stay until roughly1 day before the stay was actually pulled through the duct.

A possible solution for this problem would be to install the gages twice. Us­ing a quick-acting adhesive, such as cyanoacrylic glue, a quick installation could beobtained prior to tensioning. This set of gages, which would have limited durability,could be used to provide estimates of the strains during tensioning. Subsequent totensioning, when more time is available for curing slow adhesives, a second instal­lation could be undertaken. This installation would be calibrated with the first setof gages and would then replace those gages for subsequent readings. This ap­proach was not followed during the present project, but this method might be suit­able for future installations of this type.

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A second difficulty was recognized fairly late in the project and was resolvedfor the final installation of gages for the live load studies. In all early applicationst

. separate two-wire leads were used for both the active and dummy gages. In thetensioned configuration, it was quite difficult to find a secure place for the dummygages to be located where they would not be disturbed and'damaged by the groutingactivities. Consequently, a revised lead wire configuration was developed, as shownin Figure 9. A single three-wire lead was used. The jacketing was stripped off overa length of approximately 2 in near the active gage end. One of the leads was cut atthis point and connected to the dummy gage that was mounted on a length of pre­stressing strand approximately 1 in long. This dummy gage was then waterproofedin place using a layer of epoxy, a layer of polysulfide waterproofing, and a carefullyapplied wrapping of vinyl electrical tape. The end of the three-wire lead was thenstripped and prepared for attachment to the active gage in the field. The wire thathad been attached to the dummy gage was twisted together with a second lead wireand attached to one terminal of the active gage. The remaining lead was attachedto the other active gage terminal. The resulting temperature-compensating gageinstallation required only a single three-wire jacketed conductor, and the dummygage was located securely in line, with the lead attached to the active gage. Thisinstallation method appeared to be superior to the original installation method usedduring the final grouting stage.

Electrical Resistance Strain Gages-South Side Stay Cables

Techniques for instrumenting the stay cables were developed using the staycables on the north cantilever. Based on the experience gained during this instnt­mentation, a consistent procedure for instrumenting the stays was employed withthe south side stays, which consisted of mounting foil-resistance strain gages direct­lyon the wires in the strands making up the stay cable. Even though this processhad been practiced with the north side stays, problems were still encountered.

Three stay cablest stays 82,87, and S13, were instrumented with electricalresistance strain gages with eight gages installed on each stay. Each gage was in­stalled on a different wire in the strand in order to provide for a reliable determina­tion of average strains in the cable. Access to the stay was by means ofthe expan­sion joint provided in the polyethylene pipe as shown in Figure 7, and lead wiresfrom the gages exited through this expansion joint directly to the data acquisitionsystem located in the box deck segment. This required embedding lead wires forsplicing through the concrete superstructure into a blockout in the segment.

Data Acquisition-8outh Side Stays

The data acquisition system, manufactured by the John Fluke Company, usesa Hellos main controller to communicate with a number of individual remote

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scanning units, each ofwhich is located near an instrumented section in an extend­er chassis. The Helios controller, and an associated personal computer, are locatedin a specially designed and fabricated cabinet, which is provided with complete en­vironmental controls and which also serves to provide security for the equipment.

The lead wires from strain gages on the stay cables were connected either tothe Helios or to one of four extender chassis. Each extender chassis has its own AIDconverters, and the digitized data from each extender chassis are transmitted to theHelios controller via RS422 cables. The Hellos then sends the data to the control­ling personal computer where it is logged into Lotus 1-2-3-formatted data files forsubsequent analysis. Specially designed software permits data sampling and re­cording at regular intervals as prescribed by the investigators. Further details ofthe data acquisition system were discussed by Baber et ale (1988).

RESULTS AND DATA ANALYSIS

Scope

In this section, results obtained primarily from electrical resistance straingages on the stay cables of the north side are presented and discussed. Installationof all gages has been completed, including additional strain gages mounted on stays87 and 813 of the north half of the bridge specifically for live load strain measure­ments.

Strains in North Cantilever Stay Cables

General Discussion

This section S11mmarizes the constnlction strain variations measured in thenorth cantilever cable stays. All strain readings on the north side of the bridgewere taken manually using a switch and balance unit and strain indicator. On sev­eral of the cable stays, thermocouples were located at a number of locations aroundthe stay to provide the investigators with thermal information. The investigatorsencountered several problems in acquiring the data on the north side stay cables,including loss of gages during tensioning, shifting of the slack stays prior to tension­ing, thermal changes induced by solar heating and wind cooling on the leads, andsensitivity of the strain indicator and switch and balance units to the connections ofthe individual leads. With the exception of the loss of gages during tensioning, all ofthese problems were resolved; thus many of these problems should not be encoun­tered with the south side data collection and during subsequent live load studies.

IS

275

276The scheduling and timing of all strain readings were important. This was

particularly true for the strains in the stay cables. As with the gage installations,scheduling readings also proved difficult to arrange for a number of reasons. Ideal­ly, strain readings should be obtained immediately before and after major construc­tion activities such as tensioning a cable, installing a girder post tensioning, andlifting a segment. In addition, the investigators considered it desirable to recordstrain readings on a regular basis, preferably at least once a week, even ifno con­struction activities had taken place. When the instrumented stay was being ten­sioned, it was planned to take regular readings during the tensioning procedure.Although most of these objectives were achieved, a number of nagging problems didcause difficulties with the data collection.

The physical distance from Charlottesville to the bridge site often made aquick response to construction activities difficult. Also, the need for frequent datacollection necessitated that readings be taken by a number of different personnel.The wiring and strain indicator system appeared to be quite sensitive to a variety offactors. In particular, electrical noise caused by construction generators appearedto lead to some fluctuation in the readings. Moreover, the gages appeared sensitiveto quite small changes taking place on the bridge. In addition, temperaturechanges induced by solar heating and wind cooling appeared to change the readingssomewhat. This latter effect was largely eliminated by the installation of dummygages in each stay cable. The data, once recorded, were entered into Lotus files on apersonal computer for subsequent analysis.

A large amount of data was collected, as there were five instrumented stays,and typically, after the expected loss of some gages during tensioning, roughly fivegages per stay. All active gages were read every time a significant change tookplace on the bridge. It was found to be impractical to take gage readings after everychange of the structure since this would have required one of the investigators to beon the bridge to take cable-stay readings essentially all the time. Thus, it was de­cided to take readings immediately before and immediately after tensioning of astay cable. These readings could be taken fairly efficiently and tended to reflect themajor response variables. The "before tensioning" readings showed the influence ofthe newly hung girder segments and delta frame, whereas the "after tensioning"readings revealed the strain changes in all active stays induced by tensioning asingle additional stay.

For present purposes, it appeared to be most desirable to plot the data in aform that would reflect the time-varying nature of the strains during the construc­tion period. After some preliminary analyses, it was decided to plot strain or strainincrement as a function of time to illustrate changes in strain in the stay cables at­tributable to stay tensioning, segment lifting, and other construction activities.Ideally, this simple presentation also permits an evaluation of stay-cable responseduring the actual tensioning of the instrumented stay.

The data to be presented and discussed subsequently are shown plotted inFigures 10 through 46. Typically, each graph covers a I-month period except incertain cases where a break in the data made a different time scale more'

16

convenient. The vertical axis is labeled as average relative strain. The readingsare relative in the sense that the zero strain level was lost during the study: Sincethe zeros were lost in several cases, the measured "absolute" strains ceased to havesignificance. 1b accommodate for this problem, the various strain readings recordedover time using different instruments were normalized with respect to the same ref­erence. Thus, except as noted, the strain readings plotted in the various figuresshould not be interpreted to be absolute. In most instances, the changes in strainattributable to a particular event are of greatest interest, and these data were ob­tained without significant problems.

All readings were taken with a single strain indicator. The individual gageswere connected to several switch and balance units, which were connected to thegage lead wires and left in the field, protected by metal boxes. Initially, when stays1 and 2 were instrumented, the switch and balance units on those gages were ad-justed to read zero strain in the slack state. Several strain rea'dings were taken onstays 1 and 2 using those initial settings with no dummy gages. The initial read­ings taken during tensioning appeared to be reasonably stable. It was subsequentlydiscovered that the wiring scheme was very sensitive to thermal changes, primarilycaused by resistance changes in the lead wires. 1b eliminate this problem, dummygages were installed on each of the switch and balance units. The data were subse­quently stabilized, but unfortunately, the zero reading was lost when the wiringscheme was changed. Stay 5 was initially wired with a single dummy gage, whichcontinued to function throughout the erection period. Consequently, the zero read­ings established using the switch and balance unit on the stay 5 gages are consi­dered to be good. Considerable shifting of the strands was observed before and dur­ing tensioning, however. Stay 7 also had a single dummy gage installed thatfunctioned adequately during tensioning, and for some time thereafter. Thatdummy gage failed and had to be replaced during the tensioning of stay 9. Conse­quently, after that dummy gage was replaced, all strain readings on stay 7 were rel­ative rather than absolute. In spite of these difficulties, valuable data were ob­tained. In particular, the changes in strain during a tensioning or segment-liftingactivity appeared to be measured relatively consistently by all gages on a singlestay cable and the changes are of absolute, rather than relative, significance.

Proper evaluation of the cable-stay strain data requires a knowledge of exact­ly what events took place within a particular time frame. For this purpose, an ac­curate and up-to-date construction schedule was maintained by the research team.The major construction activities are labeled on the graphs of the cable stay straindata to facilitate the interpretation of the changes in strain. Care should be takenin interpreting the data, however, since the changes in strain indicated on thegraphs may be attributable to a number of construction activities that occurredwithin a very short period of time. These will be pointed out in the discussion of theindividual stay data that follows. The data that follow reflect strains measured instays 1, 2,5, 7, and 13 on the north side from the time each stay was pulled andtensioned until the last stay, stay 13, was installed in December 1988. For conve­nience, data recorded from gages on each stay are presented and discussed individ­ually for each stay.

17

278

Stay 1 Data

Figures 10 through 19 depict the average measurements of changes in strainin stay cable 1 from the period March 16, 1988, when stay number 1 was initiallytensioned, until December 1988 when the last stay on the north side, stay 13, wasinstalled. Except for the initial data in Figure 10, which reflects strain measure­ments taken before dummy gages were added, each figure represents strains re­corded during a particular month. As noted previously, the addition of the dummygage caused an indeterminate shift in the baseline. A rough correction is possiblebased on the last reading before the addition of the dummy gages, but this correc­tion was not considered desirable, or necessary, since the changes in strains causedby subsequent events are of primary interest. The relative strains plotted representthe average of all active gages on the stay. Variations in these gages are discussedin a subsequent section. Examination of the data in these figures indicates severalinteresting features worth noting.

On this particular stay, the strands were pushed through the polyethyleneducts individually, rather than being pulled through as a group. The gages were in­stalled aasoon as the strands were in place within the ducts and prior to tension­ing. Thus, it was possible to take readings continuously during the tensioning pro­cess. As shown in Figure 10, five average strain readings were recorded while thestay was being tensioned. The five values plotted on the graph correspond to aver­age strain values of 0, 288, 523,1,960, and 2,485 microinches per inch. There issome uncertainty in the intermediate readings, as the tensioning process was con­tinuous and the pressure in the pretensioning jacks continued to vary while thereadings were being taken. Based on laboratory studies using identical gages andprestressing strand segments, when the gages are mounted on an angle on individ­ual wires, the effective modulus of elasticity is on the order of 33 million psi, thenthe total tension imparted in the stay was approximately 82,000 psi, which com­pares favorably with the initial target tension of 82,500 psi listed in the plans.

Approximately 2 weeks later, stay 2 was installed and tensioned and, asshown in Figure 10, this caused a decrease in the strain in stay 1 of approximately380 microinches per inch. On April 19, stay 3 was tensioned and, consistent withearlier observations, the strain in stay 1 was again reduced by approximately 360microinches per inch. This same pattern was observed on all subsequent stay ten­sioning except that the strain reduction diminished with the distance of the ten­sioned stay from stay 1.

The data from Figures 11 through 19 also indicate that erection of additionalsegments caused slight increases in the average strain readings, although these in­creases were generally not significant. This is not seen in Figure 10, when the addi­tion of segments 5 and 6 was accompanied by an apparent decrease in stay force. Inthis instance, the addition of temporary and permanent post tensioning to the topslabs appears to have counteracted the weight of the added segments. This resultwas not observed on any other stay, which is also logical, since stay 1 is close to thepiers and temporary post tensioning at this stage extended beyond the piers.

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Subsequent construction activities included eventual removal of the temporary posttensioning, a lengthy period of construction stoppage, and the restressing of stays 1through 4. During this entire period, the strain changes measured were well withinthe expected levels.

Stay 2 Data

Average strain data from stay 2 are provided in Figures 20 through 29. Thisstay was installed one strand at a time, as was stay 1, and was tensioned on April 1,1988. The average change in strain introduced by the tensioning process is indi­cated in Figure 20, based on the average of four gages. From the time the jackswere in place and set to load until the tensioning was completed, a period of lessthan 15 min, there was an increase in strain of approximately 2,150 microinchesper inch. This corresponds to an increase of almost 2,500 microinches per inch mea­sured on stay 1 when it was tensioned. As was observed with stay 1, tensioning ofsubsequent stays produced a slight decrease in the strain of stay 2, as may be seen

. in Figures 21 through 29. These reductions in strain are relatively small, usuallyon the order of300 to 400 microinches per inch or les8.

From these figures, it also appears that the erection of additional segmentsbetween stay 2 and stay 3 caused very slight decreases in the strain in stay 2. Asdiscussed previously, erecting additional segments was not the only activity that oc­curred between the strain data recordings that produced the strain incrementplotted. For example, when a segment is added, a significant portion of the load issupported by the post tensioning in the deck, both by the tendons in the ducts in thedeck slab itself and in the Diwidag bars. Moreover, some post tensioning was addedsoon after the cable-stay stressing operations. Ifone takes into account the load im­parted by the post tensioning, which is comparable to the load in the stay cables,the decrease in strain in the stay cables attributable to erecting more segments ap­pears reasonable. It also appears that this effect would not be observed as thelength of the cantilever increases.

Stay 5 Data

The strain data for stay 5 are presented in Figures 30 through 35. In thisparticular case, it was possible to record strain data from the zero reference levelprior to tensioning all the way up to the maximum, including the strain at the max­imum pressure prior to fixing the anchors. This peak reading was not available forthe previous stays. As may be seen in Figure 30, the increase in average strain as aresult of the tensioning, from zero load until the jacks were set, was approximately2.260 microinches per inch, which compares with values for stay 1 and stay 2 of2,485 and 2,150 microinches per inch, respectivel~ However, as may also be seen inthe figure, the maximum strain increase during the tensioning process was actuallyslightly more than 2,800 microinches per inch, which would correspond to a stressduring initial tensioning of approximately 92,000 psi, with a tie off stress of approx­imately 74,000 psi. Although this value is somewhat low relative to the design

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stress, it is consistent with the relatively high level ofstrand shifting observed dur­ing the tensioning procedure. This point will be discussed further.

As was noted for stays 1 and 2, the tensioning of subsequent stays resulted ina slight reduction in strain in stay 5. However, the erection of additional segmentsin general produced a slight increase in the average strain in stay 5. This would beexpected but was contrary to what was observed for stays 1 and 2. This is not unex­pected, as stay 5 is located much further from the pier and temporary post tension­ing added at this point did not extend all the way back to the pier. In fact, sometemporary post tensioning over the pier was removed prior to the tensioning of stay6.

The strain record for stay 5 was uninterrupted for the entire erection period,thus permitting the total strain variation during the erection to be estimated. Theaverage reduction in strain measured from the time that stay 6 was tensioned untilthe north cantilever was complete was found to be only approximately 200 microin­ches per inch. As with the earlier stays, the strain readings were reasonably uni­form over the entire construction period, indicating no unusual or unexpected loadsoccurring in the stays.

Stay 7 Data

Average relative strain r~adings in stay 7 as a function of time are shownplotted in Figures 36 through 39. In this particular stay, there was some uncer­tainty in the initial zero reading but it was possible to determine that the averageincrease in strain in the instrumented wires as a result of tensioning the stay wasapproximately 2,300 microinches per inch. The maximum increase in averagestrain during the tensioning process just prior to setting the anchors was approxi­mately 3,100 microinches per inch, which was a somewhat larger increase that wasobserved for stay 6.

One month after installation of the gages on stay 7 and tensioning of the stay,there was a discontinuity in the strain data 8S a result of the failure of the dummygage during the tensioning of stay 9. A replacement dummy gage was installed,and subsequent strain data should be consistent with that recorded earlier beforethe failure, but there is always the possibility of a slight shift.

In stay 5 it was noted that the erection of additional segments beyond thestay caused modest increases in the average strain in the stay, whereas slight re­ductions in average strain were observed for stays 1 and 2 when additional seg­ments were added. From Figures 36 through 39, it is obvious that the addition ofsegments beyond stay 7 produced significant increases in average strain in stay 7.For example, the erection of the next two deck segments and the delta frame, justprior to the installation of stay 8, resulted in a strain increase in stay 7 of almost2,400 microinches per inch, a much more pronounced effect than observed with pre­vious stays. A similar but smaller increase occurred in stay 7 with the erection ofthe next two segments, but the erection of segments beyond stay 9 appeared to have

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no effect on the average strain in stay 7. These readings were taken during the3-week period preceding the failure of the dummy gage, so it may wen be that thelarge strain changes observed during this period may be attributable to drift of thedummy gage, leading to failure.

Examination of the strain data for stay 7 indicated that the tensioning ofsubsequent stay cables always produced a slight reduction in the average strain instay 7. This observed behavior is identical with that noted with all previous stays.However, it is also noted from the data plotted in the figures that, unlike earlierstay cables, the subsequent construction activities associated with additional staysand additional segments resulted in a net increase in the average strain in stay 7 ofapproximately 2,000 microinches per inch, whereas previously instrumented staysshowed a net reduction in average strain from tensioning to completion of the canti­lever. If the large changes in average strain that occurred preceding the dummygage failure are discounted, there was a net decrease in gage strain of roughly 600microinches per inch subsequent to the installation of stay 9, which is much moreconsistent wi~h the remaining data.

Stay 13 Data

Stay 13 was the last stay cable in the north cantilever, and thus the straindata from this stay are limited. The average relative strain data for stay 13 areshown in Figure 40 for the month of December 1988. Because of the constructionscheduling and operation, it was not possible to install the gages prior to stay ten­sioning and, hence, no data are available relating to the increaSe in average strainduring the tensioning process. However, the limited data available do indicate thatthe gages are operational, and useful data should be recorded in the future.

Individual Strand Data

The strain data presented thus far for the five instrumented stay cables havebeen in the form of average relative strains. It is of considerable interest to ex­amine the range of strains observed during similar construction activities in asingle 8ta~ Nominally, it would appear that the strain changes should all be virtu­ally identical. It was decided to conduct a more detailed analysis of the strain datafor the individual strands of stay 5 since the instrumentation on this stay was morereliable and was associated with fewer problems than with other stays.

The strain data recorded from the four gages on the individual strands ofstay 6 are presented in Figures 41 through 47. At the time the stay was tensioned,it was apparent that there was considerable variability in the strains measured inthe different strands. In fact, the range of readings during tensioning was 10 largethat the investigators questioned initially whether all of the gages were functioningproperl~ Once the initial tensioning had been applied, however, the individualstrands behaved in the same manner, as evidenced by the almost identical strainincrements recorded for the different strands. The technician taking the readings

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reported that, during tensioning, a considerable amount of noise was heard emanat­ing from the strands. The investigators had already observed during gage instal­lation that there was considerable sag in some of the strands and that a number ofthe strands were twisted around or pinched by other strands. Thus, it was con­cluded that the observed range of strains had, in fact, occurred. It is felt that theprincipal ractors contributing to the large range of observed strains during tension­ing include the presence of initial strains caused by strand slippage in some of thestrands and the variation in slack in the various strands. It was concluded that thedifference in strain readings from the individual strands would seem to indicatethat the individual strands are not stressed to the same level.

This tentative conclusion was further supported by examination of the straindata obtained during tensioning of the other stays. These data are given in Table 1.It was contemplated to present the mean and variance of the data, but there did notappear to be a large enough set of data points for this approach to be meaningful.Consequently, the mean strain value, the difference between the largest and small­est value, and the percentage of the range relative to the mean are given. It is seenfrom the data in Table 1 that a 25 to 30 percent variation in the strains measured,relative to the mean, ocurred with all stays instrumented during tensioning andthat stay 5, for which considerable shifting of the strands during tensioning was ob­served, had more than 83 percent variation. This suggests that the tensioning pro­cedure may lead to a significant range of strains in the individual strands and thatsome strands could be overstressed and others understressed. The relative consis­tency of the data during subsequent events lends further credence to this sugges­tion.

Table 1

Strain nata Obtained During Stay Tensioning

Stay Number

1 2 6 7

Date tensioned 3/16188 4f}J88 7/19/88 9128188Mealured strains 2857 2120 1786 2662

2611 2030 3061 27662100 201. 2986 28212471 2030 1190 ~7

2MO 2338

Average strain ~ 2147 2250 2507Strainranp 767 626 1871 77.Strain range ~ 30~ 26% 83% 31~

Average strain +Target strain • 99.~" 86.6~ 100.3~

60

CONCLUSIONS

This report documented the electrical resistance strain gage measurementson the cable stays taken as part of an ongoing research project on the field instru­mentation of the 1-295 cable-stayed bridge. Analysis of the data presented is stillunderway. However, based on the presented data, it is possible to draw some con­clusions concerning the measured responses of the cable stays.

1. Stay-cable gage readings taken before and after stay tensioning reflectedchanges in strains in the instrumented cables relatively consistent withthe expected strains. There appeared to be some tendeny to underesti­mate the strain during initial tensioning.

2. The strains measured in the stay-cable strands being tensioned displayedconsiderable variability, consistent with the different degrees of slacknessand twist of the individual strands.

3. The strain increments measured in the stay-cable strands during eventssubsequent to initial tensioning were considerably more consistent thanthe values obtained during tensioning and reflected the expected behav­ior of the strands during these events.

4. The measured strains in the stay cables were influenced by temporaryand permanent post tensioning as well as by the installation of subse­quent stays and the lifting of segments.

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REFERENCE

Baber, T. T.; Hilton, M. H.; McKeel, W. T., Jr.; and Barton, F. W. (1988). Field moni­toring on the 1-295 bridge over the James River: Instrumentation installationa.nd construction period studies. Charlottesville: VIrginia Transpo~ationRe­search Council.

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